Question 1

Which law of chemical combination explains the formation of both carbon monoxide and carbon dioxide from the elements carbon and oxygen?

Accepted Answer

Introduction / Context: The laws of chemical combination describe how elements combine to form compounds in fixed quantitative relationships. When the same two elements form more than one compound, specific patterns appear in the mass ratios of the combining elements. The formation of carbon monoxide and carbon dioxide from carbon and oxygen is a classic example used to illustrate one particular law. This question asks you to identify which law of chemical combination explains this situation. Given Data / Assumptions: The elements involved are carbon and oxygen. Two different compounds are formed from the same pair of elements, namely carbon monoxide and carbon dioxide. Mass ratios of oxygen that combine with a fixed mass of carbon are different for each compound. We assume standard textbook statements of the laws of chemical combination. Concept / Approach: The law of multiple proportion states that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in simple whole number ratios. In this example, carbon monoxide and carbon dioxide both contain carbon and oxygen but in different mass ratios. For a fixed mass of carbon, the mass of oxygen in carbon dioxide is approximately twice that in carbon monoxide, giving a simple whole number ratio close to 1:2. This behaviour is exactly what the law of multiple proportion describes, so that is the correct law for this question. Step-by-Step Solution: Step 1: Note that carbon monoxide and carbon dioxide are formed from the same two elements. Step 2: Recognise that they have different compositions. Carbon monoxide has the formula CO, while carbon dioxide has the formula CO2. Step 3: For a fixed mass of carbon, compare the mass of oxygen in CO and CO2 and find that the ratio is approximately 1:2. Step 4: Recall that the law of multiple proportion deals with simple whole number ratios in mass when two elements form several compounds. Step 5: Match this pattern to the definition of the law and select the law of multiple proportion. Verification / Alternative check: You can cross check by briefly revisiting the other laws. The law of conservation of mass states that total mass is conserved in a chemical reaction, which does not specifically address different compounds from the same elements. The law of definite proportions applies to one compound at a time and says that a compound always contains the same elements in the same proportion by mass. The law of reciprocal proportions involves three elements forming two different compounds in a particular ratio. None of these exactly describe the situation of carbon forming both carbon monoxide and carbon dioxide with oxygen, where simple ratios of mass for a fixed element are compared across compounds. Only the law of multiple proportion matches this description. Why Other Options Are Wrong: The law of conservation of mass is a general principle about total mass in a reaction, not about multiple compounds of the same elements. The law of definite proportions explains fixed composition of a single compound, not the relationship between several compounds formed from the same elements. The law of reciprocal proportions concerns three elements and the ratios in which they combine, which is not the case here. The law of constant gaseous volume is not a standard law of chemical combination at this level and does not apply to this example. Common Pitfalls: Students often confuse the laws of definite proportions and multiple proportion because both involve mass ratios. A helpful memory tip is that definite proportions focuses on one compound, whereas multiple proportion compares several compounds formed from the same pair of elements. Whenever you see more than one compound mentioned, think of the law of multiple proportion. Careful reading of the problem usually clarifies which law is required. Final Answer: The formation of carbon monoxide and carbon dioxide from carbon and oxygen is explained by the law of multiple proportion.

Question 2

In cell biology and biochemistry, which enzyme complex is primarily responsible for phosphorylating ADP to form ATP?

Accepted Answer

Introduction / Context: Adenosine triphosphate, usually written as ATP, is often described as the energy currency of the cell. Many cellular processes depend on ATP, and understanding how ATP is generated is central to both biology and biochemistry. One of the key reactions is the phosphorylation of ADP to form ATP. This question asks you to identify the enzyme complex that carries out this reaction in processes such as oxidative phosphorylation and photophosphorylation. Given Data / Assumptions: ADP stands for adenosine diphosphate. ATP stands for adenosine triphosphate. The reaction of interest is ADP plus inorganic phosphate forming ATP. We assume a basic understanding of cellular respiration and energy transfer. Concept / Approach: During oxidative phosphorylation in mitochondria and during photophosphorylation in chloroplasts, ATP is produced by a specialised enzyme complex known as ATP synthase. This enzyme uses the energy stored in a proton gradient across a membrane to add an inorganic phosphate group to ADP, forming ATP. Other enzymes like DNA polymerase or RNA polymerase are involved in nucleic acid synthesis, not direct ATP formation from ADP in the electron transport chain or light reactions. Therefore, ATP synthase is the correct answer. Step-by-Step Solution: Step 1: Recall that ATP is produced from ADP and inorganic phosphate in mitochondria and chloroplasts. Step 2: Remember that this process is strongly linked to a proton gradient across a membrane. Step 3: Identify the enzyme complex that uses this proton motive force to catalyse ATP formation as ATP synthase. Step 4: Compare this role with the roles of DNA polymerase and RNA polymerase, which synthesize nucleic acids. Step 5: Recognise that glycogen phosphorylase breaks down glycogen and lactate dehydrogenase is involved in anaerobic metabolism, not direct ATP synthesis from ADP. Verification / Alternative check: Most textbooks on cell biology describe the inner mitochondrial membrane as housing ATP synthase complexes that appear like small knobs or stalked particles. These structures allow protons to flow back into the matrix and couple this flow to the formation of ATP from ADP and inorganic phosphate. The same type of enzyme is also present in chloroplast thylakoid membranes. No other enzyme listed among the options has this central role in ATP generation through chemiosmosis. This confirms that ATP synthase is the correct choice. Why Other Options Are Wrong: DNA polymerase is responsible for synthesising DNA during replication and does not directly phosphorylate ADP to ATP. RNA polymerase synthesises RNA from a DNA template and is unrelated to ATP synthase activity in membranes. Glycogen phosphorylase participates in glycogen breakdown by releasing glucose phosphate units but does not catalyse the specific reaction from ADP to ATP. Lactate dehydrogenase converts pyruvate to lactate under anaerobic conditions and helps regenerate NAD plus, not produce ATP from ADP in the electron transport chain. Common Pitfalls: A common error is to select an enzyme that contains the word phosphorylase, assuming it deals with phosphorylation of ADP. However, phosphorylase enzymes usually add phosphate to organic molecules like sugars, not to ADP in the chemiosmotic system. Another pitfall is to confuse enzymes involved in energy pathways with those that directly generate ATP. Keeping in mind that ATP synthase is specifically linked to chemiosmotic ATP production will help you answer such questions correctly. Final Answer: The enzyme complex that phosphorylates ADP to form ATP in oxidative phosphorylation and photophosphorylation is ATP synthase.

Question 3

Among the following carbohydrates, which one is correctly classified as a polysaccharide?

Accepted Answer

Introduction / Context: Carbohydrates are broadly divided into monosaccharides, disaccharides and polysaccharides based on the number of simple sugar units they contain. Understanding these categories is important in both biology and chemistry because structure strongly influences function. This question asks you to identify which of the listed substances is a polysaccharide, that is, a carbohydrate made from many repeating sugar units joined together. Given Data / Assumptions: Fructose, glucose and galactose are common simple sugars. Sucrose is a common table sugar formed by combining two monosaccharides. Cellulose is a structural component of plant cell walls. We assume knowledge of basic carbohydrate classification. Concept / Approach: Monosaccharides are the simplest carbohydrates, such as glucose, fructose and galactose, and contain a single sugar unit. Disaccharides, such as sucrose, lactose and maltose, contain two sugar units. Polysaccharides are long chain carbohydrates made up of many monosaccharide units linked together and include substances like starch, glycogen and cellulose. Cellulose is a major structural polysaccharide in plants and is composed of many glucose units linked by specific glycosidic bonds. Therefore, among the options, cellulose is the polysaccharide. Step-by-Step Solution: Step 1: Classify fructose as a monosaccharide because it contains just one simple sugar unit. Step 2: Classify glucose as another monosaccharide, widely used as an energy source in cells. Step 3: Recognise sucrose as a disaccharide formed from glucose and fructose units. Step 4: Recall that cellulose is a polymer made from many glucose units and serves as a plant structural material. Step 5: Conclude that cellulose is the only polysaccharide among the given options. Verification / Alternative check: If you consult a typical classification table, you will see that monosaccharides include glucose, fructose and galactose, disaccharides include sucrose and lactose, and polysaccharides include starch, glycogen and cellulose. Cellulose consistently appears under the polysaccharide category. No standard reference lists fructose, glucose or sucrose as polysaccharides, because they contain only one or two sugar units. This cross check confirms the identification of cellulose as the correct answer. Why Other Options Are Wrong: Fructose is a single sugar unit and therefore a monosaccharide, not a polysaccharide. Glucose is also a monosaccharide, widely important for energy metabolism, but it is not a polymer of many sugars. Sucrose is composed of two monosaccharide units, glucose and fructose, so it is classified as a disaccharide. Galactose, similar to glucose and fructose, is a monosaccharide and does not meet the definition of a polysaccharide. Common Pitfalls: Some students may confuse sugar types because several names sound similar or because they are all common in the diet. Remember that the key to identifying a polysaccharide is the presence of many repeated sugar units, not just one or two. Linking each name to a typical example or function can help: glucose for energy, sucrose as table sugar and cellulose as plant fibre. Using these associations makes it easier to place each carbohydrate into the correct category. Final Answer: The carbohydrate that is correctly classified as a polysaccharide is cellulose.

Question 4

Which of the following substances is not a chemical compound but instead a mixture or alloy?

Accepted Answer

Introduction / Context: In chemistry, it is important to distinguish between pure compounds and mixtures. Compounds have a fixed chemical formula and definite proportions of elements that are chemically bonded, whereas mixtures and alloys contain components that are physically combined and not in a fixed ratio. This question asks you to identify which of the listed substances is not a chemical compound but rather a mixture, specifically an alloy. Given Data / Assumptions: Brass is a commonly used metallic material in industry and household items. Quick lime, marble and soda ash all have well defined chemical formulas. We assume standard definitions of compound, mixture and alloy. The options mix both chemical compounds and a metallic mixture. Concept / Approach: A chemical compound has a fixed composition and can be represented by a chemical formula. Examples include calcium oxide, calcium carbonate and sodium carbonate. An alloy is a homogeneous mixture of metals or of a metal with other elements, and its exact composition can vary within certain limits. Brass is a classic example of an alloy, typically made from copper and zinc in variable proportions. It does not have a single fixed chemical formula, so it is considered a mixture rather than a compound. Therefore, among the options, brass is the correct choice as the substance that is not a chemical compound. Step-by-Step Solution: Step 1: Identify quick lime as calcium oxide, which has the fixed chemical formula CaO. Step 2: Identify marble as primarily calcium carbonate, with the formula CaCO3. Step 3: Recognise soda ash as sodium carbonate, with the formula Na2CO3. Step 4: Recall that brass is an alloy of copper and zinc with variable composition and no single fixed formula. Step 5: Conclude that brass is not a pure compound but a mixture and therefore the correct answer. Verification / Alternative check: When you look up the composition of brass, you will find ranges such as 60 percent copper and 40 percent zinc or other similar proportions. The exact ratio can change depending on the type of brass and its intended use. In contrast, CaO, CaCO3 and Na2CO3 have fixed stoichiometric ratios of their elements. This difference in compositional flexibility clearly shows that brass is a mixture, while the others are compounds. The additional option calcium chloride, with formula CaCl2, is also a compound, which reinforces that brass is the only mixture in the list. Why Other Options Are Wrong: Quick lime is calcium oxide, a compound with a definite formula and fixed ratio of calcium and oxygen. Marble, although a natural material, is mainly calcium carbonate and is considered a compound in introductory chemistry contexts. Soda ash refers to sodium carbonate, which has the fixed chemical formula Na2CO3 and is a pure compound. Calcium chloride is another well defined compound with formula CaCl2, not a mixture or alloy. Common Pitfalls: Students may mistakenly think that natural materials like marble are mixtures, but in basic textbook treatment they are often considered primarily as examples of specific compounds. The more important clue here is the term alloy. If you remember that an alloy is a mixture of metals and that brass is a common alloy, the question becomes straightforward. Always look for whether a substance has a fixed formula when deciding if it is a compound or a mixture. Final Answer: The substance that is not a chemical compound but is instead a mixture or alloy is brass.

Question 5

Which of the following molecules are polar and therefore have a net dipole moment?

Accepted Answer

Introduction / Context: Molecular polarity is a key concept in chemistry that helps explain solubility, boiling point, intermolecular forces and many physical properties. A molecule is polar if the distribution of electron density is uneven and the individual bond dipoles do not cancel out, resulting in a net dipole moment. This question asks which among SO2, CH2Cl2 and PCl3 are polar. Correctly identifying the shape and symmetry of each molecule is essential to determining whether it is polar. Given Data / Assumptions: SO2 consists of one sulfur and two oxygen atoms. CH2Cl2 consists of one carbon, two hydrogen atoms and two chlorine atoms. PCl3 consists of one phosphorus and three chlorine atoms. We assume standard valence shell electron pair repulsion theory for molecular shapes. Concept / Approach: Polarity depends on both bond polarity and molecular geometry. Individual bonds may be polar if atoms have different electronegativities. If the molecule is symmetric, bond dipoles may cancel and the molecule can still be nonpolar. SO2 has a bent structure, not linear, so its bond dipoles do not cancel and it is polar. CH2Cl2 has a tetrahedral arrangement but different atoms around the carbon (two hydrogens and two chlorines), so the bond dipoles do not fully cancel and the molecule is polar. PCl3 has a trigonal pyramidal shape due to one lone pair on phosphorus, which prevents complete cancellation of bond dipoles, again giving a net dipole moment. Thus all three molecules are polar. Step-by-Step Solution: Step 1: For SO2, apply valence shell electron pair repulsion theory and note the presence of a lone pair on sulfur that leads to a bent geometry. Step 2: Recognise that in a bent molecule with polar sulfur oxygen bonds, the bond dipoles cannot cancel completely. Step 3: For CH2Cl2, recall that the molecule has a tetrahedral shape but different surrounding atoms, so the polar carbon chlorine bonds are not symmetrically opposed. Step 4: For PCl3, identify the trigonal pyramidal shape caused by a lone pair on phosphorus and accept that the polar phosphorus chlorine bonds produce a net dipole. Step 5: Conclude that SO2, CH2Cl2 and PCl3 are all polar molecules and choose the option that includes all of them. Verification / Alternative check: An additional verification is to consider physical properties and typical textbook classifications. SO2 is often mentioned as a polar molecule responsible for certain atmospheric phenomena. CH2Cl2 is known to be polar and is often used as a polar organic solvent. PCl3 is also recognised as polar due to its pyramidal structure. None of these molecules have a fully symmetric geometry that would allow complete cancellation of bond dipoles. These supporting facts confirm that all the listed molecules are polar. Why Other Options Are Wrong: Options that name only one of the molecules as polar ignore the polarity and geometry of the others. The option stating none of the molecules are polar conflicts with both molecular shape analysis and standard textbook information. Since each molecule separately has a net dipole, any answer that excludes one or more of them is incomplete or incorrect. Common Pitfalls: A common mistake is to assume that any molecule with a tetrahedral or trigonal structure is nonpolar, without checking the actual atoms involved or the presence of lone pairs. Another pitfall is to focus only on electronegativity differences and forget to consider geometry, which decides whether bond dipoles cancel. Drawing simple three dimensional sketches and marking bond dipoles can help you avoid these errors and confidently determine molecular polarity. Final Answer: All three molecules listed, SO2, CH2Cl2 and PCl3, are polar, so the correct answer is all of the above.

Question 6

Given the electronegativities calcium 1 point 0, fluorine 4 point 0, hydrogen 2 point 2 and sodium 0 point 93, which pair of atoms has the highest electronegativity difference?

Accepted Answer

Introduction / Context: Electronegativity is a measure of how strongly an atom attracts shared electrons in a chemical bond. The difference in electronegativity between two bonding atoms helps predict whether a bond will be nonpolar covalent, polar covalent or ionic. Larger differences usually indicate more ionic character. This question gives electronegativity values for calcium, fluorine, hydrogen and sodium and asks you to identify the pair of atoms that has the highest electronegativity difference. Given Data / Assumptions: Electronegativity of calcium is 1 point 0. Electronegativity of fluorine is 4 point 0. Electronegativity of hydrogen is 2 point 2. Electronegativity of sodium is 0 point 93. We assume simple subtraction of values to find electronegativity differences. Concept / Approach: To find the pair with the highest difference, we subtract the electronegativity values for each given pair and compare the results. The pair with the largest numerical difference will have the most polar or most ionic bond character. Pairs of identical atoms, such as F and F, will have a difference of zero. We therefore expect that a metal with very low electronegativity combined with fluorine, the most electronegative element, will yield the highest difference. Step-by-Step Solution: Step 1: For F and F, difference equals 4 point 0 minus 4 point 0, which is 0. Step 2: For Ca and F, difference equals 4 point 0 minus 1 point 0, which is 3 point 0. Step 3: For H and F, difference equals 4 point 0 minus 2 point 2, which is 1 point 8. Step 4: For Na and F, difference equals 4 point 0 minus 0 point 93, which is 3 point 07. Step 5: Compare the values and see that 3 point 07 for Na and F is the highest electronegativity difference. Verification / Alternative check: We can verify our result qualitatively. Sodium is a highly electropositive metal with very low electronegativity, while fluorine is the most electronegative element in the periodic table. The bond between sodium and fluorine in sodium fluoride is strongly ionic, which is consistent with a very large electronegativity difference. Calcium is also a metal with low electronegativity, but it is slightly higher than that of sodium, so the calcium fluorine difference is a bit smaller. This qualitative reasoning matches the numerical calculation and confirms that sodium and fluorine have the greatest difference. Why Other Options Are Wrong: The pair F and F has no difference in electronegativity because it consists of identical atoms. Calcium and fluorine have a large difference, but it is smaller than the sodium fluorine difference. Hydrogen and fluorine have a moderate difference that leads to a polar covalent bond but not the largest difference among the given pairs. The pair hydrogen and sodium shows a difference between a non metal and a metal, but this difference is still less than that between sodium and fluorine. Common Pitfalls: Students sometimes choose calcium and fluorine due to the clear contrast between a metal and an extremely electronegative non metal, without carefully calculating the exact values. Another pitfall is to overlook that sodium has slightly lower electronegativity than calcium, making the sodium fluorine difference slightly larger. Always perform the simple subtraction for all options instead of relying purely on intuition. Final Answer: The pair of atoms with the highest electronegativity difference is sodium and fluorine, that is Na and F.

Question 7

For hydrogen chloride gas, which type of intermolecular force is the dominant specific interaction in addition to general dispersion forces?

Accepted Answer

Introduction / Context: Intermolecular forces are the attractions between molecules that influence boiling points, melting points and many physical properties of substances. Hydrogen chloride, HCl, is a common example used to illustrate polar covalent molecules and their intermolecular forces. This question asks you to identify the dominant specific intermolecular force in HCl, apart from the general dispersion forces present in all molecules. Given Data / Assumptions: Hydrogen chloride has the chemical formula HCl. The bond between hydrogen and chlorine is polar due to a difference in electronegativity. Hydrogen bonding requires hydrogen bonded to highly electronegative atoms such as fluorine, oxygen or nitrogen. All molecules exhibit dispersion forces, but the question focuses on the characteristic additional force. Concept / Approach: HCl is a polar covalent molecule because chlorine is significantly more electronegative than hydrogen, which creates a permanent dipole. Polar molecules experience dipole dipole interactions, where the positive end of one molecule is attracted to the negative end of another. Hydrogen bonding is a special, stronger type of dipole dipole interaction that occurs only when hydrogen is directly bonded to fluorine, oxygen or nitrogen. In HCl, hydrogen is bonded to chlorine, not to F, O or N, so conventional hydrogen bonding is not present. Therefore, the dominant specific intermolecular force, in addition to dispersion forces, is ordinary dipole dipole interaction. Step-by-Step Solution: Step 1: Recognise that chlorine is more electronegative than hydrogen, making the HCl bond polar. Step 2: Recall that polar molecules exhibit dipole dipole forces due to the attraction between partial charges on different molecules. Step 3: Check whether HCl can form hydrogen bonds by verifying if hydrogen is bonded to fluorine, oxygen or nitrogen. Step 4: Conclude that since hydrogen is bonded to chlorine instead, typical hydrogen bonding does not occur in HCl. Step 5: Select dipole dipole interaction as the main specific intermolecular force beyond dispersion forces. Verification / Alternative check: Comparing boiling points gives an additional check. Substances that show strong hydrogen bonding, such as water, have much higher boiling points than similar sized molecules without hydrogen bonding. HCl has a lower boiling point than water despite having a similar number of electrons, which suggests that strong hydrogen bonding is absent. Textbook discussions also classify HCl as a polar molecule with dipole dipole forces, rather than as a classic hydrogen bonded system like HF, H2O or NH3. This supports the conclusion that dipole dipole interaction is the correct answer. Why Other Options Are Wrong: Hydrogen bonding is not typically assigned to HCl in school level chemistry because the hydrogen is not bonded to fluorine, oxygen or nitrogen. Only dispersion forces would ignore the clear permanent dipole in HCl, so it is incomplete as a description of its intermolecular forces. Claiming all the options apply equally is incorrect because hydrogen bonding and ionic lattice attractions do not accurately describe HCl as a simple polar covalent gas. Ionic lattice attraction refers to the forces in ionic solids such as sodium chloride, not to a covalent molecule like HCl under normal conditions. Common Pitfalls: Students may be tempted to choose hydrogen bonding whenever they see hydrogen combined with a more electronegative atom, without checking whether it is one of fluorine, oxygen or nitrogen. Another pitfall is to think that all polar molecules must involve hydrogen bonding. Remember that dipole dipole forces are more general and occur in many polar substances, while hydrogen bonding is a special stronger case with very specific requirements. Final Answer: The dominant specific intermolecular force in hydrogen chloride, apart from dispersion forces, is dipole dipole interaction.

Question 8

For the following data, which solution is the most concentrated, that is, has the greatest mass of solute per millilitre of solution?

Accepted Answer

Introduction / Context: Concentration of a solution expresses how much solute is present in a given quantity of solvent or solution. One simple way to compare concentrations is to look at the mass of solute per unit volume of solution, often in grams per millilitre. This question gives several combinations of mass and volume and asks you to determine which solution is the most concentrated based on mass per millilitre comparison. Given Data / Assumptions: Option A: 6 grams of solute in 10 millilitres of solution. Option B: 20 point 5 grams of solute in 50 millilitres of solution. Option C: 30 grams of solute in 120 millilitres of solution. Option D: 2 point 6 grams of solute in 2 millilitres of solution. Option E: 10 grams of solute in 25 millilitres of solution. We compare concentration as mass of solute divided by volume of solution. Concept / Approach: To identify the most concentrated solution, we convert each description to grams of solute per millilitre of solution by dividing the mass by the volume. The solution with the largest grams per millilitre value is the most concentrated. There is no need to convert to other units, because all masses are in grams and all volumes in millilitres, so we can compare the ratios directly. Step-by-Step Solution: Step 1: For option A, concentration equals 6 divided by 10, which is 0 point 6 grams per millilitre. Step 2: For option B, concentration equals 20 point 5 divided by 50, which is approximately 0 point 41 grams per millilitre. Step 3: For option C, concentration equals 30 divided by 120, which is 0 point 25 grams per millilitre. Step 4: For option D, concentration equals 2 point 6 divided by 2, which is 1 point 3 grams per millilitre. Step 5: For option E, concentration equals 10 divided by 25, which is 0 point 4 grams per millilitre. Comparing all results, option D has the highest concentration at 1 point 3 grams per millilitre. Verification / Alternative check: As an alternative check, notice that in option D the mass of solute is already more than the mass in some other options, but the volume is much smaller. Intuitively, putting 2 point 6 grams into only 2 millilitres should give a thicker or more concentrated solution than placing 6 grams into 10 millilitres or 20 point 5 grams into 50 millilitres. The numerical calculations confirm this intuition, since 1 point 3 grams per millilitre is clearly larger than all other computed values. Why Other Options Are Wrong: Option A gives 0 point 6 grams per millilitre, which is less than 1 point 3 grams per millilitre. Option B yields about 0 point 41 grams per millilitre, again lower than the value for option D. Option C has only 0 point 25 grams per millilitre, the lowest concentration among the options considered. Option E gives 0 point 4 grams per millilitre, which is still smaller than the concentration of option D. Common Pitfalls: A common mistake is to look only at the absolute mass of solute or only at the volume and not at the ratio. For example, some might think that 30 grams in 120 millilitres must be more concentrated than 2 point 6 grams in 2 millilitres simply because the mass is larger, ignoring the much larger volume. Always compare concentrations by dividing mass by volume. Writing down the ratios in decimal form, as done here, makes it clear which solution is most concentrated. Final Answer: The most concentrated solution is the one with 2 point 6 grams of solute in 2 millilitres of solution, corresponding to option D.

Question 9

According to standard nomenclature, what is the correct systematic name for the compound with formula PbO?

Accepted Answer

Introduction / Context: Systematic nomenclature in inorganic chemistry is designed to provide clear information about the oxidation state of metals in compounds. Lead can exist in more than one oxidation state, most commonly plus two and plus four. The formula PbO indicates a particular ratio of lead to oxygen and therefore a particular oxidation state. This question asks you to choose the most appropriate systematic name for PbO, based on standard naming rules. Given Data / Assumptions: The compound has the formula PbO. Oxygen usually has an oxidation state of minus two in binary oxides. Lead is a metal that can show oxidation states plus two and plus four. We use a naming system that clearly indicates the oxidation state of the metal. Concept / Approach: In PbO, oxygen contributes a total charge of minus two since there is one oxygen atom with oxidation state minus two. To balance this charge, lead must have an oxidation state of plus two. In the systematic naming convention, the metal name is followed by a Roman numeral in brackets indicating the oxidation state, or equivalently by a clear verbal description such as lead two oxide. Traditional names use plumbous for lead plus two and plumbic for lead plus four. Therefore, PbO is lead plus two oxide, sometimes called plumbous oxide, while lead plus four oxide or plumbic oxide would describe PbO2, not PbO. Step-by-Step Solution: Step 1: Assign the oxidation state of oxygen in PbO as minus two, based on its usual value in oxides. Step 2: Set up the total charge balance. Since the compound is neutral, the oxidation state of lead plus the oxidation state of oxygen must sum to zero. Step 3: Solve for the oxidation state of lead: x plus minus two equals zero, so x equals plus two. Step 4: Note that this oxidation state is represented in systematic naming by describing the compound as lead two oxide. Step 5: Reject names that correspond to lead plus four or that do not specify the oxidation state precisely for a metal with variable valency. Verification / Alternative check: As an alternative check, look at the formula for another common oxide of lead, PbO2. In that compound, two oxygen atoms provide a total of minus four charge, so lead must be plus four, and the compound is described as lead four oxide or plumbic oxide. This comparison clearly shows that plumbic oxide refers to PbO2, not PbO. Therefore, the compound PbO, with lead in the plus two state, is accurately named lead two oxide under the chosen naming convention. Why Other Options Are Wrong: Plumbic oxide and lead four oxide correspond to the oxide where lead is in the plus four oxidation state, that is PbO2, not PbO. Lead oxide without specifying the oxidation state is ambiguous, because lead can form both PbO and PbO2, so this name is not fully systematic. Lead dioxide is another common name associated with PbO2 rather than PbO and therefore does not describe the given formula correctly. Common Pitfalls: Students often choose general names that omit the oxidation state, leading to ambiguity whenever a metal forms more than one type of oxide. Another frequent error is mixing up plumbous and plumbic or confusing the formulae PbO and PbO2. A good strategy is always to calculate the oxidation state explicitly and then construct the name so that this value is clearly indicated. This habit helps avoid mistakes in questions involving variable valency metals. Final Answer: The correct systematic name for PbO, with lead in the plus two oxidation state, is lead two oxide.

Question 10

In an electrochemical or electronic cell, the relationship between a cathode and an anode primarily involves the movement of which particles?

Accepted Answer

Introduction / Context: This general chemistry and physics question focuses on the basic idea of how cathodes and anodes are related inside cells and circuits. Whether we talk about an electrochemical cell, a vacuum tube, or a diode, the central relationship between cathode and anode is described by the movement of charged particles. Understanding that electrons move between these electrodes is fundamental to grasping how current flows and how devices like batteries and rectifiers work. Given Data / Assumptions: - We are dealing with a cathode and an anode inside some electrical or electrochemical system. - The options mention electrons, neutrons, protons, and diodes as possible answers. - We assume ordinary cells or tubes where current is carried in a circuit by mobile charge carriers. Concept / Approach: In most practical systems, electricity is associated with the movement of electrons. In an electrochemical cell, oxidation and reduction take place at the anode and cathode respectively, and electrons travel through the external circuit from the anode to the cathode. In a vacuum tube or diode, electrons are emitted by the cathode and are attracted to the anode. Neutrons are neutral and do not take part in ordinary electrical conduction. Protons are usually fixed in atomic nuclei and do not move through wires or vacuum tubes to form common electric currents. A diode is a device that itself contains a cathode and an anode, not the particle that moves. Step-by-Step Solution: Step 1: Recall that in most circuits, electric current in metals and vacuum tubes is carried by electrons, which are negatively charged particles. Step 2: In an electrochemical cell, oxidation at the anode releases electrons, which then travel through the external wire toward the cathode where reduction occurs. Step 3: In a vacuum tube or diode, the heated cathode emits electrons that are attracted toward the positively charged anode. Step 4: Because electrons are the mobile charge carriers linking cathode and anode, the relationship between these two electrodes is primarily defined by the flow of electrons. Step 5: Neutrons and protons do not normally move between electrodes in such devices, and diodes are components, not particles. Verification / Alternative check: If you think of a simple battery circuit, electrons leave the negative terminal (which acts as the anode in a galvanic cell) and travel through the wire to the positive terminal (the cathode). Textbooks describe current direction and potential differences in terms of electron flow between these two electrodes. In vacuum tubes, diagrams clearly show electron beams leaving the cathode and striking the anode. These consistent examples confirm that electrons are the particles whose movement defines the relationship between cathode and anode. Why Other Options Are Wrong: - Neutrons: These neutral particles reside in atomic nuclei and do not move through circuits to create electric current under normal conditions. - Protons: Protons are positively charged but are tightly bound in the nucleus and do not typically flow between electrodes in standard electrical or electrochemical setups. - Diodes: A diode is a component that itself contains a cathode and an anode. It is not the moving entity that connects them. The question asks about particles, not devices. Common Pitfalls: Students sometimes confuse conventional current direction with the actual motion of electrons. Conventional current is defined as flowing from anode to cathode, while electrons physically move from cathode to anode in many devices. This can cause confusion about what actually travels between the electrodes. It is helpful to remember that, regardless of direction conventions, the real mobile charged particles in common circuits are electrons, not neutrons or protons. Final Answer: Electrons are the particles whose movement links the cathode and anode in cells and electronic devices.

Question 11

In a discharge tube experiment, cathode rays are composed of which fundamental particles?

Accepted Answer

Introduction / Context: This question comes from the early history of atomic structure and experimental physics. J J Thomson and other scientists studied cathode rays in evacuated discharge tubes long before electrons were widely accepted as fundamental particles. By observing how these rays behave in electric and magnetic fields, they concluded that cathode rays are streams of tiny negatively charged particles, which we now call electrons. Recognising that cathode rays are made of electrons is essential for understanding how the electron was discovered. Given Data / Assumptions: - The setting is a gas discharge tube with a cathode and an anode at low pressure. - Cathode rays are observed travelling from the cathode toward the anode. - The options mention electrons, protons, neutrons, and positively charged ions as possible constituents. - We assume standard school descriptions of cathode ray experiments. Concept / Approach: Cathode rays originate at the negatively charged cathode in a discharge tube when a high voltage is applied. The important clues about their nature come from experiments showing that they are deflected by electric and magnetic fields in a way that indicates a negative charge. They also produce fluorescence when they strike certain materials and can spin a small paddle wheel, showing that they have mass and momentum. These observations are consistent with a beam of negatively charged particles with a very small mass, which are electrons. Protons and positive ions are positively charged, so they would be deflected in the opposite direction. Neutrons are neutral and would not be deflected by electric fields at all. Step-by-Step Solution: Step 1: In a discharge tube, when voltage is applied, rays are seen to travel from the cathode (negative electrode) toward the anode (positive electrode). Step 2: When an electric field is applied across the path of these rays, they are deflected toward the positive plate, which shows that they carry negative charge. Step 3: When a magnetic field is applied, the rays are also deflected in a manner consistent with negatively charged particles moving through the field. Step 4: The rays can cause mechanical effects, such as spinning a lightweight paddle wheel, indicating that the rays consist of particles with mass, not just energy like light. Step 5: From the measured charge to mass ratio, scientists concluded that these particles are much lighter than atoms and are identical regardless of the gas used. They were named electrons. Verification / Alternative check: Standard textbooks describe how J J Thomson measured the charge to mass ratio of cathode ray particles and found the same value for all metals and gases tested. This universality pointed to a fundamental particle, the electron. Furthermore, the charge on the electron determined later by Millikan agreed with the values inferred from cathode ray experiments. No evidence suggested that cathode rays consisted of protons, neutrons, or positive ions, confirming that they are streams of electrons. Why Other Options Are Wrong: - Protons: These are positively charged particles found in atomic nuclei. A beam of protons would be deflected in the opposite direction under an electric field compared to cathode rays. Why Other Options Are Wrong (continued): - Neutrons: Neutrons carry no charge, so they would not be deflected by electric fields and would not match cathode ray behaviour. - Positively charged ions: These are atoms that have lost electrons. A beam of such ions is called an anode ray or positive ray, not a cathode ray, and behaves differently in fields. Common Pitfalls: One common confusion is between cathode rays and anode rays. Anode rays are streams of positive ions that travel in the opposite direction and were studied by Goldstein. Another pitfall is thinking that any ray in a tube must be made of light, whereas cathode rays clearly show particle like properties with mass and charge. Remembering that cathode rays originate at the cathode and behave like negatively charged particles helps keep the concepts clear. Final Answer: Electrons are the particles that make up cathode rays in a discharge tube experiment.

Question 12

Which structural description correctly represents ATP (adenosine triphosphate), the main energy currency molecule in cells?

Accepted Answer

Introduction / Context: This biochemistry oriented question asks about the structure of ATP, which stands for adenosine triphosphate. ATP is often called the energy currency of the cell because it stores and transfers energy for many biochemical reactions. To understand how ATP works, it is important to know its structural components. ATP is a nucleotide made from three phosphate groups, a five carbon sugar (ribose), and a nitrogenous base (adenine). Recognising this arrangement is essential for topics such as cellular respiration and metabolism. Given Data / Assumptions: - The molecule in question is ATP, adenosine triphosphate. - The options mention combinations of phosphate groups, types of sugar, and nitrogenous bases. - We assume standard biological definitions where adenosine is adenine plus ribose, and triphosphate refers to three phosphate groups. Concept / Approach: A nucleotide has three main parts: a nitrogenous base, a pentose sugar, and one or more phosphate groups. In ATP, the base is adenine, the sugar is ribose, and there are three phosphate groups linked in a chain. The term adenosine refers specifically to adenine plus ribose. Attaching three phosphates to adenosine gives adenosine triphosphate. In contrast, deoxyribose is the sugar found in DNA nucleotides, and thymine is a base used in DNA but not in ATP. Therefore, the correct description must include three phosphate groups, ribose, and adenine together. Step-by-Step Solution: Step 1: Recall that ATP stands for adenosine triphosphate. Adenosine itself is composed of adenine plus ribose. Step 2: The word triphosphate indicates that there are three phosphate groups attached in series to the 5 prime carbon of the ribose sugar. Step 3: Therefore, the structure of ATP can be described as three phosphate groups attached to a ribose sugar, which in turn is attached to the nitrogenous base adenine. Step 4: Option that mentions three phosphate groups attached to a ribose sugar attached to adenine matches this description exactly. Step 5: Options involving deoxyribose or thymine describe components more typical of DNA structure, not ATP. Verification / Alternative check: Standard biology diagrams show ATP with a chain of three phosphate groups (often labelled alpha, beta, and gamma), a ribose sugar in the centre, and adenine attached to the sugar. This is often drawn with the phosphates to the left, the sugar in the middle, and the adenine base to the right. Textbooks consistently describe ATP as a ribonucleotide, reinforcing that the sugar is ribose rather than deoxyribose. No recognised form of ATP uses thymine or deoxyribose, which confirms that the only correct structural description is three phosphate groups attached to a ribose sugar attached to adenine. Why Other Options Are Wrong: - Three phosphate groups attached to a deoxyribose sugar attached to adenine: This would describe a deoxyribonucleotide triphosphate used in DNA synthesis, not ATP, which is a ribonucleotide. Why Other Options Are Wrong (continued): - Three phosphate groups attached to a ribose sugar attached to thymine: Thymine is a DNA base and is not part of ATP, which always contains adenine as its base. - None of the above: This is wrong because one of the options correctly matches the known structure of ATP. Common Pitfalls: Students sometimes confuse ribose and deoxyribose or mix up ATP with deoxyribonucleotide triphosphates such as dATP, which are used in DNA replication. Another common error is to associate thymine with any nucleotide based question, forgetting that ATP specifically uses adenine. To avoid confusion, remember that ATP is a ribonucleotide (so it contains ribose) and that its name starts with adenosine, which signals the presence of adenine as the base. Final Answer: Three phosphate groups attached to a ribose sugar attached to adenine correctly describes the structure of ATP.

Question 13

Which of the following compounds contains both ionic bonds and covalent bonds within its overall structure?

Accepted Answer

Introduction / Context: This inorganic chemistry question focuses on recognising compounds that contain more than one type of chemical bond. Many ionic compounds that include polyatomic ions have ionic bonding between ions and covalent bonding inside the polyatomic ion itself. Being able to identify both ionic and covalent bonds in a single formula helps learners understand structures of salts such as sodium nitrate, calcium sulphate, and ammonium chloride, all of which combine these bonding types. Given Data / Assumptions: - The compounds listed are sodium nitrate (NaNO3), calcium sulphate (CaSO4), and ammonium chloride (NH4Cl). - Each compound contains at least one polyatomic ion and a metal or cationic group. - We assume typical valence structures where polyatomic ions are held together by covalent bonds and combine ionically with counter ions. Concept / Approach: An ionic bond forms between positively charged cations and negatively charged anions. Covalent bonds form when atoms share electron pairs, as in molecules and polyatomic ions. In compounds like NaNO3, CaSO4, and NH4Cl, there is an ionic bond between the metal cation (or ammonium cation) and the polyatomic anion or monatomic anion. Inside many of these ions, such as nitrate (NO3 minus) and sulphate (SO4 2 minus), the nonmetal atoms are linked by covalent bonds. Similarly, the N H bonds inside the ammonium ion (NH4 plus) are covalent. By analysing the ions present, we can see that all three compounds combine ionic and covalent bonding. Step-by-Step Solution: Step 1: Consider sodium nitrate, NaNO3. Sodium exists as Na plus and nitrate as NO3 minus. The attraction between Na plus and NO3 minus is ionic. Step 2: Within the nitrate ion NO3 minus, nitrogen and oxygen atoms are bonded through shared electrons, forming covalent N O bonds. Step 3: Consider calcium sulphate, CaSO4. Calcium exists as Ca2 plus and the sulphate ion is SO4 2 minus. The electrostatic attraction between Ca2 plus and SO4 2 minus is ionic. Step 4: Inside the sulphate ion SO4 2 minus, sulphur and oxygen atoms are connected by covalent bonds. Step 5: Consider ammonium chloride, NH4Cl. The ammonium ion NH4 plus is bonded ionically to chloride ion Cl minus. Step 6: Within NH4 plus, nitrogen is covalently bonded to four hydrogen atoms through N H single bonds. Step 7: Because each compound contains ionic interactions between ions and covalent bonds within its polyatomic ion or cation, all of them contain both types of bonding. Verification / Alternative check: Most inorganic textbooks illustrate the structures of nitrate and sulphate as polyatomic ions where atoms are connected by lines representing covalent bonds. The formulas NaNO3 and CaSO4 are always discussed as ionic salts composed of these ions. Similarly, ammonium salts such as NH4Cl are presented as containing the covalently bonded NH4 plus ion paired with an anion. Therefore, reference structures support the idea that all three substances have both ionic and covalent bonding, confirming that All of the above is the correct answer. Why Other Options Are Wrong: - Sodium nitrate (NaNO3): On its own, this option is correct in containing both bond types, but choosing only this compound ignores the fact that the others also fit the description. Why Other Options Are Wrong (continued): - Calcium sulphate (CaSO4): This compound also has ionic bonding between Ca2 plus and SO4 2 minus and covalent bonding within SO4 2 minus, so it is not unique. - Ammonium chloride (NH4Cl): This also contains ionic bonding between NH4 plus and Cl minus and covalent N H bonds inside NH4 plus, so it is not the only correct choice. Common Pitfalls: Students sometimes assume that a compound is either ionic or covalent but cannot be both. This is a misconception because many salts with polyatomic ions have a combination of bond types. Another mistake is to focus only on the metal nonmetal nature of the formula and overlook the internal structure of the polyatomic ion. To avoid such errors, always break complex formulas into ions and examine the bonding inside each ion as well as the bonds between ions. Final Answer: All of the above compounds contain both ionic bonds between ions and covalent bonds within their polyatomic species.

Question 14

Among the following elements, which atom has the largest atomic radius in its ground state?

Accepted Answer

Introduction / Context: This periodic table question tests understanding of trends in atomic radius. Atomic radius is a measure of the size of an atom. In general, atomic radius increases as you move down a group and decreases as you move from left to right across a period. Using these trends, we can compare elements such as helium, neon, cesium, and francium to determine which has the largest atomic radius. Given Data / Assumptions: - The elements to compare are Neon (Ne), Francium (Fr), Helium (He), and Cesium (Cs). - We consider neutral atoms in their ground states. - We use general periodic trends in atomic size without worrying about small irregularities in measured values. Concept / Approach: Atomic radius generally increases as we go down a group because additional electron shells are added, making atoms larger. It generally decreases across a period from left to right because increasing nuclear charge pulls the electrons closer to the nucleus. Helium and neon are small noble gases in the first and second periods, respectively. Cesium and francium are alkali metals in groups at the left side of the table and in lower periods, with francium located below cesium. Therefore, among the given choices, francium is expected to have the largest atomic radius. Step-by-Step Solution: Step 1: Identify the group and period of each element. Helium is in period 1, group 18. Neon is in period 2, group 18. Cesium is in period 6, group 1. Francium is in period 7, group 1. Step 2: Recall the trend down a group. As you go down group 1 from lithium to sodium, potassium, rubidium, cesium, and francium, the atoms become larger due to additional electron shells. Step 3: Helium and neon are near the top of the periodic table, with very small atomic radii because they have few electron shells and high effective nuclear charge for their small size. Step 4: Cesium is already a very large atom in period 6, but francium lies below it in period 7, so francium has one more electron shell than cesium, making it even larger. Step 5: Comparing all four elements, francium, the bottom member of group 1, has the largest atomic radius among the choices. Verification / Alternative check: Periodic table charts that show atomic radius trends typically colour or size elements to indicate relative atomic size. These diagrams highlight that alkali metals at the bottom left, especially francium, have the largest radii. In contrast, noble gases at the top right have some of the smallest atomic radii. Looking at such charts confirms that francium is larger than cesium and significantly larger than helium or neon. Even if experimental data for francium are limited due to its radioactivity, general trends and theoretical calculations agree that francium has the largest radius among the listed elements. Why Other Options Are Wrong: - Neon: Neon is a noble gas in period 2 and has a relatively small atomic radius compared with heavier elements in lower periods. Why Other Options Are Wrong (continued): - Helium: Helium is in period 1 and has one of the smallest atomic radii in the periodic table, so it cannot be the largest among these options. - Cesium: Cesium is very large, but francium lies below cesium in the same group and therefore has a larger radius than cesium. Common Pitfalls: Students sometimes mistakenly think that heavier elements always have smaller radii due to stronger nuclear attraction, forgetting the dominant effect of additional shells. Another source of confusion is mixing up atomic radius with ionic radius, which follows slightly different patterns. To avoid errors, remember the simple rule that atomic size increases down a group and decreases across a period, which places the largest atoms at the bottom left of the periodic table, near francium. Final Answer: Francium has the largest atomic radius among the given elements.

Question 15

Place these hydrocarbons in order of decreasing boiling point (highest to lowest): 1. Paraffin wax 2. Octadecane 3. Hexane 4. Neohexane 5. Ethane.

Accepted Answer

Introduction / Context: This organic chemistry question tests understanding of how molecular size and branching affect boiling points of hydrocarbons. Boiling point trends are important for predicting physical properties, separating mixtures using distillation, and understanding how structure influences intermolecular forces. The compounds listed include paraffin wax (a mixture of long chain alkanes), octadecane (C18H38), hexane (C6H14), neohexane (2,2 dimethylbutane), and ethane (C2H6). We must order them from the highest boiling point to the lowest. Given Data / Assumptions: - Hydrocarbons considered: paraffin wax, octadecane, hexane, neohexane, and ethane. - All are non polar molecules where London dispersion forces dominate intermolecular attractions. - Paraffin wax is a general term for a mixture of long chain alkanes with higher molar mass than octadecane. - We assume normal atmospheric pressure for comparing boiling points. Concept / Approach: In non polar hydrocarbons, boiling point mainly depends on the strength of London dispersion forces. These forces increase with molar mass and with surface area. Longer, unbranched chains have higher boiling points than shorter or more highly branched chains. Therefore, the longest chains boil at the highest temperatures, and the smallest, most branched molecules boil at the lowest. Paraffin wax contains very long chains and so has the highest boiling point. Octadecane, a long unbranched C18 alkane, comes next. Among C6 isomers, linear hexane has a higher boiling point than its highly branched isomer neohexane. Ethane, the smallest molecule listed, has the lowest boiling point. Step-by-Step Solution: Step 1: Recognise that paraffin wax is a mixture of high molar mass long chain alkanes, typically longer than C18, so it has very strong London dispersion forces. Step 2: Octadecane is a straight chain C18 alkane, which has a high molar mass but is generally somewhat smaller than paraffin wax components, so its boiling point is lower than that of paraffin wax but still quite high. Step 3: Hexane is a straight chain C6 alkane with a moderate boiling point. It is much smaller than octadecane and paraffin but larger than ethane. Step 4: Neohexane (2,2 dimethylbutane) is a highly branched isomer of C6H14. Branching reduces surface area and lowers boiling point compared with the straight chain isomer hexane. Step 5: Ethane is the smallest molecule listed, with only two carbon atoms. It has the weakest London dispersion forces and therefore the lowest boiling point. Step 6: Combining these comparisons, the decreasing order of boiling point is Paraffin > Octadecane > Hexane > Neohexane > Ethane. Verification / Alternative check: Data tables show that highly branched alkanes have lower boiling points than their straight chain isomers, and that boiling points increase with carbon number. For example, hexane boils at a higher temperature than its isomer neohexane, and both boil well above ethane. Similarly, heavy paraffin fractions in petroleum distillation have the highest boiling ranges. This experimental information confirms that the correct order places paraffin first, followed by octadecane, then hexane, neohexane, and finally ethane at the lowest boiling point. Why Other Options Are Wrong: - Paraffin > Octadecane > Neohexane > Hexane > Ethane: This incorrectly places neohexane above hexane, ignoring the effect of branching in lowering boiling point. Why Other Options Are Wrong (continued): - Hexane > Paraffin > Octadecane > Neohexane > Ethane: This incorrectly suggests that hexane boils higher than paraffin and octadecane, which contradicts both theory and data. - None of the above: This is incorrect because one provided order exactly matches the correct reasoning based on molar mass and branching. Common Pitfalls: Students sometimes think that any C6 compound must have a similar boiling point, forgetting the strong effect of branching. Others may under estimate the impact of very long chains found in paraffin wax. Another common error is to focus only on molecular weight and ignore branching altogether, leading to incorrect ordering among isomers. To avoid these mistakes, always remember that, for alkanes, boiling point increases with chain length and decreases with increased branching. Final Answer: Paraffin > Octadecane > Hexane > Neohexane > Ethane is the correct order of decreasing boiling point.

Question 16

In the periodic table, which of the following elements has chemical properties most similar to those of beryllium?

Accepted Answer

Introduction / Context: This question tests the concept of periodicity, specifically how elements in the same group share similar chemical properties. Beryllium is an alkaline earth metal in group 2 of the periodic table. Elements in the same group typically have the same number of valence electrons and therefore display similar chemical behaviour. By comparing the group positions of manganese, calcium, and helium, we can determine which element is most similar to beryllium. Given Data / Assumptions: - Beryllium is the reference element whose properties we want to match. - Options include manganese, calcium, both manganese and calcium, and helium. - We assume a standard long form periodic table where group 2 contains alkaline earth metals. Concept / Approach: Elements in the same vertical column or group have the same valence electron configuration and hence very similar chemical properties. Beryllium (Be) is in group 2 (IIA), the alkaline earth metals. Calcium (Ca) is also in group 2, below magnesium. Manganese (Mn) is in group 7 (VIIB) among the transition metals and has very different chemistry from beryllium. Helium (He) is a noble gas in group 18 and is chemically inert compared with beryllium. Therefore, we expect calcium, the group 2 element, to have properties most similar to beryllium. Step-by-Step Solution: Step 1: Note that beryllium has atomic number 4 and belongs to group 2, the alkaline earth metals. Step 2: Calcium has atomic number 20 and is also in group 2, directly below magnesium. It has two valence electrons, like beryllium. Step 3: Manganese has atomic number 25 and is located in the transition series, group 7, which gives it complex d block chemistry unlike the s block alkaline earth metals. Step 4: Helium has atomic number 2 and is a noble gas in group 18, with a full shell and very low chemical reactivity. Step 5: Since group membership is the best guide to similar chemical behaviour, calcium, as a fellow group 2 alkaline earth metal, is most similar to beryllium. Step 6: The option Both A and B cannot be correct because manganese is not similar to beryllium in chemical properties. Verification / Alternative check: Textbooks describe typical reactions of alkaline earth metals such as beryllium, magnesium, and calcium, including formation of oxides, hydroxides, and salts with a +2 oxidation state. Calcium and magnesium behave in patterns similar to beryllium, though beryllium also shows some unique behaviour. Manganese, on the other hand, displays many oxidation states and forms coloured transition metal complexes, unlike the simple s block chemistry of beryllium. Helium is largely inert. These comparisons confirm that calcium is the element whose chemistry is closest to beryllium among the options. Why Other Options Are Wrong: - Manganese: This is a d block transition metal with very different chemistry from the s block alkaline earth metals. It is not similar to beryllium in standard reactions. - Both A and B: Since manganese is not similar to beryllium, this combined option cannot be correct. Why Other Options Are Wrong (continued): - Helium: As a noble gas, helium is chemically inert and does not share the reactive metallic behaviour of beryllium at all. Common Pitfalls: A frequent mistake is to focus on approximate atomic size or mass rather than group position. Another error is to assume any metal might have similar properties, ignoring the role of valence electrons. Always use the periodic table groups to identify similar behaviour, since group members have the same outer electron configuration and show comparable chemical reactions. This makes it straightforward to connect beryllium with other group 2 metals such as calcium. Final Answer: Calcium is the element whose properties are most similar to those of beryllium.

Question 17

Rust, commonly seen on iron objects, is formed when iron reacts mainly with which component of moist air?

Accepted Answer

Introduction / Context: This everyday chemistry question asks about the formation of rust on iron. Rusting is a classic example of corrosion and oxidation that students encounter frequently, from rusty gates to old tools. The process occurs when iron reacts with oxygen in the presence of water or moisture. Knowing that oxygen is the main reacting element helps learners connect real world observations to basic redox chemistry. Given Data / Assumptions: - The metal involved is iron, which forms rust over time when exposed to air and moisture. - Options list carbon, oxygen, hydrogen, and all of the above as possible reactants. - We assume typical atmospheric conditions where air contains oxygen and water vapour. Concept / Approach: Rust is primarily a mixture of hydrated iron oxides, often written in simplified form as Fe2O3·xH2O. The key chemical change is oxidation of iron metal to iron(III) ions by oxygen. Water acts as a medium and participates in hydration but is not the main element combining with the iron in the new compound. Carbon is not involved in rust formation under normal conditions, and hydrogen from water does not directly form a stable iron hydride in this context. Thus, the element that reacts with iron to form rust is oxygen. Step-by-Step Solution: Step 1: Recognise that rust is a type of iron oxide, which means iron has combined with oxygen. Step 2: In moist air, oxygen dissolves in the thin layer of water covering the metal surface and reacts with iron atoms. Step 3: The iron is oxidised, losing electrons and forming iron(II) and then iron(III) ions, while oxygen is reduced. Step 4: These iron ions combine with oxygen and water molecules to form hydrated iron oxides, the flaky reddish brown material we call rust. Step 5: Carbon is present in some steels and in atmospheric carbon dioxide, but it does not directly form rust with iron. Step 6: Hydrogen is part of water but does not form a major solid compound with iron in the normal rusting process. Verification / Alternative check: Chemistry textbooks typically write simplified equations for rusting such as 4Fe + 3O2 + xH2O gives 2Fe2O3·xH2O. These expressions emphasise that oxygen from air is essential, and rust does not form in the complete absence of oxygen. Experiments show that iron immersed in oil or kept in an oxygen free environment does not rust, even if water is present. This evidence confirms that oxygen is the central element reacting with iron to form rust. Why Other Options Are Wrong: - Carbon: Although carbon can alloy with iron to form steel, it is not the main element combining with iron during the rusting process in air. Why Other Options Are Wrong (continued): - Hydrogen: Hydrogen is part of water but does not form the main solid product of corrosion on iron under normal conditions. The visible rust is an iron oxide, not an iron hydride. - All of the above: Since neither carbon nor hydrogen play the primary role in rust formation, this combined option is incorrect. Common Pitfalls: Some learners incorrectly think that water alone causes rust, forgetting the crucial role of oxygen. Others may confuse carbon in steel with atmospheric reactions and assume carbon must be part of rust. To keep the concept clear, remember that rust is a type of iron oxide, so it must involve iron and oxygen, with water acting as a necessary medium and hydrator but not as the elemental reactant that defines the compound. Final Answer: Oxygen is the element in moist air that reacts with iron to form rust.

Question 18

After using and cleaning a clinical thermometer, it should be placed in which medium to keep it hygienic and safe for the next use?

Accepted Answer

Introduction / Context: This everyday science and health question deals with the safe handling of clinical thermometers. Clinical thermometers come into contact with the human body, usually under the tongue, in the armpit, or in the rectum. To prevent the spread of infection, proper cleaning and storage are important. After cleaning, the thermometer should be placed in an appropriate medium that helps maintain hygiene, which is usually some type of sanitizer or disinfectant solution, or its protective case after disinfection. Given Data / Assumptions: - The thermometers in question are clinical thermometers used on patients. - It is assumed that the thermometer has already been wiped or rinsed to remove visible contamination. - The options suggest sanitizer, ice cold water, mercury, or none of the above as possible storage media. - We assume basic school level health and hygiene guidelines. Concept / Approach: Devices that contact body fluids or mucous membranes must be cleaned and then disinfected or sanitised to reduce the risk of transmitting germs between users. Simply dipping a thermometer in plain water or mercury does not reliably kill microorganisms. Modern practice recommends using alcohol based sanitizers or other appropriate disinfectant solutions to clean the thermometer, and then storing it safely. The most reasonable choice among the options, therefore, is sanitizer or a disinfectant solution, which actively reduces microbial load. Step-by-Step Solution: Step 1: After usage, a clinical thermometer may have traces of saliva, sweat, or other body fluids on it. Step 2: Basic cleaning removes visible contaminants, but microorganisms may still be present. Step 3: To prevent infections, the thermometer should be exposed to a substance that kills or inactivates these microbes. Alcohol based sanitizers or disinfectant solutions are commonly used for this purpose. Step 4: Ice cold water may help cool the thermometer or bring down the mercury column, but plain water does not provide disinfection. Step 5: Mercury is a toxic metal and is not used as a cleaning or storage medium for safety reasons. The mercury inside the thermometer is sealed; the outer surface should not be placed in open mercury. Step 6: Therefore, the best choice for hygienic storage after cleaning is to place the thermometer in a sanitizer or disinfectant solution, or in its case after disinfection. Verification / Alternative check: Health education materials often recommend wiping clinical thermometers with cotton soaked in alcohol or another approved disinfectant after each use. Professional healthcare settings follow similar protocols using sanitizing wipes or solutions. These procedures underscore that disinfection, not just cooling, is the goal after cleaning. The presence of alcohol or other germ killing agents is what makes a sanitizer suitable for this purpose, confirming that sanitizer or disinfectant is the correct medium among the options given. Why Other Options Are Wrong: - Ice cold water: This can lower the temperature of the thermometer and may help reset it, but plain water does not reliably kill germs, so it is not sufficient for hygiene. Why Other Options Are Wrong (continued): - Mercury: Mercury is toxic and hazardous. The mercury inside a thermometer is sealed; immersing the thermometer in mercury is unsafe and does not serve a useful hygienic purpose. - None of the above: This is incorrect because sanitizer or disinfectant solution is a standard and appropriate choice for hygienic handling of clinical thermometers. Common Pitfalls: A common misconception is that simply rinsing with cold water is enough to make medical instruments safe. Another error is confusion about the role of mercury in thermometers, leading some learners to think mercury itself might be used to clean or store the device. To avoid these misunderstandings, it is helpful to remember that disinfection requires a chemical that kills or inactivates microbes, which is what sanitizers and disinfectants are designed to do. Final Answer: Sanitizer or disinfectant solution is the appropriate medium for placing a clinical thermometer after cleaning to keep it hygienic.

Question 19

In the periodic table, group 7A (group 17) elements are best described as which of the following?

Accepted Answer

Introduction / Context: This periodic table question concerns group 7A, also labelled group 17 in the modern numbering system. The elements in this group include fluorine, chlorine, bromine, iodine, and astatine. Collectively they are known as the halogens. Understanding their reactivity compared with other metals and nonmetals is important for predicting chemical reactions and for classifying elements based on their position in the periodic table. Given Data / Assumptions: - Group 7A (group 17) elements are under discussion. - Options describe them as most reactive metals, least reactive nonmetals, most reactive nonmetals, or least reactive metals. - We assume standard periodic table behaviour, where halogens are nonmetal elements near the right side of the table. Concept / Approach: Group 7A elements are nonmetals and are collectively called halogens. They have seven valence electrons and need just one more electron to achieve a noble gas configuration. This makes them highly reactive, especially with metals, from which they readily gain an electron to form halide ions. Among nonmetals, the halogens are particularly reactive, with fluorine being the most reactive nonmetal in the periodic table. They are not metals at all, so any option describing them as metals, whether most or least reactive, is incorrect. Therefore, the proper description is that they are the most reactive nonmetals. Step-by-Step Solution: Step 1: Identify group 7A elements. These include fluorine, chlorine, bromine, iodine, and astatine. Step 2: Recognise that all of these elements are nonmetals under normal conditions, not metals. Step 3: Consider their electron configuration. Each halogen has seven valence electrons and tends to gain one electron to complete an octet. Step 4: This strong tendency to gain an electron makes halogens highly reactive, especially with metals that easily lose electrons. Step 5: Fluorine, in particular, is known as the most reactive nonmetal, and the other halogens are also very reactive compared with most nonmetals. Step 6: Therefore, the best description for group 7A is that it contains the most reactive nonmetals. Verification / Alternative check: Chemical reactivity trends show that as you move left to right across a period, nonmetals become more reactive up to the halogens, and then noble gases are largely unreactive. Halogens vigorously react with alkali metals to form salts such as sodium chloride and potassium bromide. Laboratory demonstrations, such as the reaction of chlorine with sodium, show vigorous behaviour. Metals of group 1 and group 2 are highly reactive metals, but those are on the left side of the table, not in group 7A. These patterns confirm that group 7A is composed of very reactive nonmetals. Why Other Options Are Wrong: - The most reactive metals: Group 7A elements are nonmetals, not metals, so this option cannot be correct. Why Other Options Are Wrong (continued): - The least reactive nonmetals: Halogens are among the most reactive nonmetals, especially fluorine and chlorine, so describing them as least reactive is the opposite of correct. - The least reactive metals: Again, group 7A elements are not metals at all, making this option inapplicable. Common Pitfalls: One common mistake is to mix up groups that are very reactive. Students sometimes remember that both alkali metals and halogens are highly reactive and may incorrectly think group 7A contains reactive metals. Another confusion arises from using outdated group labels without recalling which elements are in each group. To avoid this, always associate group 1 with alkali metals and group 7A or 17 with halogens, the most reactive nonmetals. Final Answer: The most reactive nonmetals is the correct description of the elements in group 7A of the periodic table.

Question 20

Which of the following substances is held together by metallic bonds between its atoms?

Accepted Answer

Introduction / Context: This question focuses on recognising metallic bonding among different types of substances. Metals have a characteristic type of bonding where valence electrons are delocalised and shared among many metal atoms, forming a lattice with a sea of electrons. Identifying which formula corresponds to a metallic substance helps students differentiate between molecular compounds, ionic compounds, and metallic elements. Given Data / Assumptions: - The options include glucose (C6H12O6), mercury (Hg), water (H2O), and sodium chloride (NaCl). - We assume standard bonding descriptions: metallic bonding in elemental metals, covalent bonding in molecular compounds, and ionic bonding in salts. Concept / Approach: Metallic bonds occur in elemental metals and alloys, where metal atoms release valence electrons into a shared electron cloud. This electron sea model explains properties such as electrical conductivity, malleability, and luster. Mercury is an elemental metal and, even though it is liquid at room temperature, its atoms are bound by metallic bonding. Glucose and water are molecular compounds held together internally by covalent bonds and between molecules by intermolecular forces. Sodium chloride is an ionic compound, where ions are held together by electrostatic attraction, not metallic bonding. Step-by-Step Solution: Step 1: Recognise that Hg is the chemical symbol for mercury, a metal that is liquid at room temperature but still metallic in nature. Step 2: In mercury, positively charged Hg2 plus like centres are surrounded by a cloud of delocalised electrons, forming metallic bonds throughout the sample. Step 3: C6H12O6 is the formula for glucose, an organic molecule whose atoms are connected by covalent bonds. Between glucose molecules, only weak intermolecular forces operate. Step 4: H2O is water, a molecular compound formed by covalent bonds between hydrogen and oxygen. Intermolecular hydrogen bonding holds water molecules together, not metallic bonding. Step 5: NaCl is an ionic solid where Na plus and Cl minus ions are arranged in a lattice and held together by ionic bonds, not metallic bonds. Step 6: Therefore, the only substance in the list that contains metallic bonds is mercury, Hg. Verification / Alternative check: You can verify this by thinking about physical properties. Mercury is a good conductor of electricity and has metallic luster, which are typical properties of metals due to metallic bonding. Glucose and water are nonconductors in solid or pure liquid forms and do not show metallic properties. Sodium chloride conducts electricity only when molten or in solution, due to mobile ions, but in solid form it is a typical ionic crystal. These observations support the conclusion that metallic bonding occurs only in mercury among the given options. Why Other Options Are Wrong: - C6H12O6 (glucose): This is a covalently bonded organic molecule with no metallic bonding present. Why Other Options Are Wrong (continued): - H2O (water): Water molecules are held together by covalent bonds internally and hydrogen bonding between molecules, not by metallic bonding. - NaCl (sodium chloride): This compound has an ionic lattice, with electrostatic attraction between Na plus and Cl minus ions, which is different from the electron sea model of metallic bonding. Common Pitfalls: Students sometimes think that any compound containing a metal symbol might have metallic bonds, but this is not correct. Once a metal forms ions and combines with nonmetals, the compound is usually ionic. Metallic bonds exist in elemental metals and alloys where metal atoms share delocalised electrons. Another confusion comes from the fact that mercury is a liquid at room temperature, leading some learners to forget that it is still a metal. Remember that being liquid does not change its bonding type; it remains metallic. Final Answer: Hg (mercury) is the substance whose atoms are held together by metallic bonds.

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