Question 1

What is the freezing point of pure water on the Celsius temperature scale under standard atmospheric pressure?

Accepted Answer

Introduction / Context: The freezing point of water is one of the most fundamental reference temperatures in science and everyday life. It is used as a calibration point for thermometers and as a benchmark in weather reports and physical experiments. On the Celsius scale, the freezing point of pure water at standard atmospheric pressure was originally chosen as one of the fixed points for defining the scale. This question checks whether you can recall the correct value of this temperature in degrees Celsius. Given Data / Assumptions: The substance is pure water, not a solution containing salts or impurities. Freezing point refers to the temperature at which water changes from liquid to solid. Standard atmospheric pressure is approximately 1 atmosphere. The options include several temperatures on the Celsius scale, such as 0, 32 and 100 degrees Celsius. Concept / Approach: The Celsius temperature scale was historically defined so that the freezing point of water is 0 degrees Celsius and the boiling point is 100 degrees Celsius at standard atmospheric pressure. Although modern definitions use the Kelvin scale and more precise reference points, these values remain accurate enough for everyday and school level use. At 0 degrees Celsius under one atmosphere pressure, pure water and ice coexist in equilibrium. Therefore, the freezing point of water in degrees Celsius is 0, not 32, which is the freezing point on the Fahrenheit scale. Step-by-Step Solution: Step 1: Recall that on the Celsius scale, the freezing point of pure water is defined as 0 degrees Celsius. Step 2: The boiling point of water on the same scale is 100 degrees Celsius at standard atmospheric pressure. Step 3: The value 32 degrees is associated with the freezing point of water on the Fahrenheit scale, not on the Celsius scale. Step 4: Negative values such as minus 32 or minus 42 degrees Celsius are far below the freezing point and represent temperatures where water has long been solid. Step 5: The temperature 100 degrees Celsius represents the boiling point of water at one atmosphere and not the freezing point. Step 6: Therefore, the correct freezing point of water in Celsius is 0 degrees Celsius. Verification / Alternative check: If you place a mixture of ice and pure water in a well insulated container at normal atmospheric pressure and insert a thermometer, it will read close to 0 degrees Celsius while both phases coexist. This experiment confirms that the phase change between water and ice occurs at that temperature on the Celsius scale. Weather forecasts frequently refer to 0 degrees Celsius as the temperature at which water freezes on roads and surfaces. Science textbooks universally state that the freezing point of water is 0 degrees Celsius in standard conditions, confirming that this is the correct value. Why Other Options Are Wrong: Option B, 32 degrees Celsius, is incorrect because 32 degrees is a warm temperature; water is liquid and feels warm, not freezing, at this temperature. Option C, minus 42 degrees Celsius, and option D, minus 32 degrees Celsius, are far colder than the freezing point and would represent conditions where ice is already well formed. Option E, 100 degrees Celsius, is the boiling point of water at standard pressure, not the freezing point. Only option A matches the accepted freezing point of pure water on the Celsius scale. Common Pitfalls: A common confusion arises from mixing the Celsius and Fahrenheit scales, especially because in Fahrenheit the freezing point of water is 32 degrees and the boiling point is 212 degrees. Students sometimes mistakenly pick 32 degrees Celsius due to this mix up. To avoid this, remember that 0 and 100 are the freezing and boiling points on the Celsius scale, while 32 and 212 are the corresponding points on the Fahrenheit scale. Keeping this mapping in mind helps you quickly pick the correct temperature in exam questions and everyday reasoning. Final Answer: The freezing point of pure water on the Celsius scale at standard atmospheric pressure is 0 degree Celsius.

Question 2

According to VSEPR theory, what is the molecular geometry (shape) of interhalogen compound IF7 (iodine heptafluoride)?

Accepted Answer

Introduction / Context: Valence Shell Electron Pair Repulsion (VSEPR) theory is used in chemistry to predict the shapes of molecules based on the repulsion between electron pairs around a central atom. Interhalogen compounds, where halogens bond with one another, provide interesting examples of unusual geometries. IF7, iodine heptafluoride, is a classic case often mentioned in textbooks. This question asks you to identify the correct molecular geometry of IF7 according to VSEPR theory, testing your knowledge of advanced molecular shapes. Given Data / Assumptions: The compound is IF7, which means iodine bonded to seven fluorine atoms. Iodine is the central atom and forms seven I–F bonds. We assume that iodine in this compound has no lone pairs in its valence shell, just bonding pairs. The options include several possible geometries: pentagonal bipyramid, square pyramid, trigonal bipyramid, octahedral and tetrahedral. Concept / Approach: According to VSEPR theory, the shape of a molecule is determined by the arrangement of electron pairs around the central atom that minimises repulsion. Seven bonding pairs and no lone pairs around a central atom correspond to an AX7 type configuration. The geometry that minimises repulsions for seven electron pairs is a pentagonal bipyramidal arrangement, where five atoms form a planar pentagon around the central atom and two atoms occupy axial positions above and below that plane. Therefore, IF7 is predicted to have a pentagonal bipyramidal geometry, not square pyramidal, trigonal bipyramidal, octahedral or tetrahedral geometries, which correspond to other coordination numbers. Step-by-Step Solution: Step 1: Count the valence electrons on iodine. As a group 17 element, iodine has seven valence electrons. Step 2: In IF7, iodine forms seven covalent bonds with seven fluorine atoms, using all seven of its valence electrons in bonding. Step 3: Because each bond uses one electron from iodine, there are seven bonding pairs and no lone pairs on the central iodine atom. Step 4: In VSEPR notation, this gives an AX7 type, where A is the central atom and X represents a bonded atom. Step 5: For seven electron pairs, the geometry that minimises repulsions is pentagonal bipyramidal: five fluorine atoms arranged in a planar pentagon around iodine and two fluorine atoms along an axis perpendicular to that plane. Step 6: Other geometries listed correspond to different numbers of electron pairs: trigonal bipyramidal corresponds to AX5, octahedral to AX6 and tetrahedral to AX4. Step 7: Therefore, the correct molecular geometry for IF7 is pentagonal bipyramid. Verification / Alternative check: Structural studies, such as X ray diffraction, confirm that IF7 has seven equivalent I–F bonds and that five of these are in one plane forming a pentagon, with two additional fluorine atoms occupying axial positions. This matches the pentagonal bipyramidal arrangement predicted by VSEPR theory. Advanced inorganic chemistry texts specifically list IF7 as the standard example of a molecule with AX7 electron pair geometry. By comparing with other known geometries, such as PF5 (trigonal bipyramidal) and SF6 (octahedral), you can see that additional bonding pairs require expanding the coordination number to seven, which yields the pentagonal bipyramidal shape. Why Other Options Are Wrong: Option B, square pyramid, corresponds to AX5E type geometries, such as BrF5, where there are five bonds and one lone pair. Option C, trigonal bipyramid, is the geometry for AX5 molecules like PF5 with five bonding pairs. Option D, octahedral, describes AX6 molecules such as SF6, with six bonding pairs. Option E, tetrahedral, corresponds to AX4 molecules like CH4 with four bonding pairs. None of these geometries involve seven bonding pairs with no lone pairs on the central atom. Only option A, pentagonal bipyramid, matches the seven coordinate arrangement of IF7. Common Pitfalls: Students sometimes struggle with geometries beyond the most common shapes (linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral) and may guess octahedral or trigonal bipyramidal out of habit. Another pitfall is miscounting electron pairs or forgetting to consider lone pairs on the central atom. To avoid these errors, systematically count valence electrons, determine the number of bonding and lone pairs and then match the AXn or AXnE notation with the known VSEPR geometries. Remembering that IF7 is the textbook example of a pentagonal bipyramidal molecule can also help fix this shape in memory. Final Answer: The molecular geometry of IF7 (iodine heptafluoride) according to VSEPR theory is Pentagonal bipyramid.

Question 3

In agriculture, lime (such as calcium oxide or calcium carbonate) is sometimes applied to soil mainly in order to achieve which effect?

Accepted Answer

Introduction / Context: Soil chemistry plays a crucial role in agriculture because it affects nutrient availability and plant growth. Farmers sometimes add lime, usually in the form of calcium oxide or calcium carbonate, to their fields. This practice is known as liming. Understanding why lime is added helps connect basic acid base chemistry with practical farming methods. This question asks for the main reason lime is applied to soil, focusing on whether it increases acidity, improves porosity, restores nitrates or changes alkalinity. Given Data / Assumptions: Lime refers here to calcium containing materials such as quicklime (calcium oxide) or limestone (calcium carbonate) used in agriculture. Many soils become acidic over time due to factors such as acid rain, fertilisers and leaching of basic ions. Plant nutrient availability and microbial activity are affected by soil pH. The options describe possible effects of adding lime: changing acidity, porosity, nitrates or nutrients like phosphate. Concept / Approach: Calcium oxide and calcium carbonate are basic substances. When added to acidic soil, they react with excess hydrogen ions and acidic components, thereby raising the pH and making the soil less acidic and more alkaline. This process neutralises soil acidity and makes many nutrients more available to plants. Although liming can influence soil structure and indirectly affect porosity and nutrient cycling, its primary chemical purpose is to correct soil acidity. It does not directly restore nitrate content, nor is it mainly used to supply phosphate. Therefore, the best description is that lime increases soil alkalinity and neutralises excess acidity. Step-by-Step Solution: Step 1: Recognise that lime used in agriculture is often calcium carbonate or calcium oxide, both of which are basic in nature. Step 2: Acidic soils have a low pH and contain excess hydrogen ions and acidic compounds that can limit plant growth and nutrient availability. Step 3: When lime is added to acidic soil, it reacts with these hydrogen ions, forming water and neutral salts, thereby raising the soil pH. Step 4: Increasing soil pH from strongly acidic to a more neutral or slightly alkaline range improves conditions for many crops and beneficial soil organisms. Step 5: The main purpose is not simply to make soil more porous; while structure can improve, this is a secondary effect. Step 6: Lime does not directly add nitrate ions to the soil; nitrates come from nitrogen fertilisers and microbial activity. Step 7: Lime also does not significantly supply phosphate; phosphate fertilisers are used when additional phosphorus is needed. Step 8: Therefore, the key effect of liming is to increase soil alkalinity and neutralise excess acidity. Verification / Alternative check: Agricultural extension manuals and soil science textbooks explicitly state that the primary purpose of liming acidic soils is to raise soil pH and neutralise acidity. Recommended lime application rates are often based on soil pH measurements and buffer capacity. Farmers observe improvements in crop yield when excessively acidic soils are corrected with lime. While some secondary benefits such as improved soil structure can occur, these are not the main reason for applying lime. The consistent emphasis on pH correction across multiple sources confirms that increasing alkalinity and neutralising acidity is the correct answer. Why Other Options Are Wrong: Option A, increasing soil acidity, is the opposite of what lime does; lime is basic and reduces acidity. Option B, making the soil more porous only, ignores the primary role of pH correction; any porosity change is secondary. Option C, restoring nitrates, is incorrect because lime does not supply nitrogen; nitrates are provided by nitrogen fertilisers or biological nitrogen fixation. Option E, supplying phosphate nutrients, is wrong because lime is not a phosphate fertiliser; compounds like superphosphate or diammonium phosphate are used for phosphorus. Only option D correctly describes the main purpose of liming as increasing alkalinity and neutralising excess soil acidity. Common Pitfalls: Students sometimes see lime mentioned in both construction and agriculture and may confuse its various uses, such as in mortar, with its role in soil. Another pitfall is to assume that any soil amendment automatically adds nutrients like nitrogen or phosphorus. To avoid these mistakes, remember that lime is primarily an acid neutralising agent. Its main job in soil is to adjust pH, not to act as a direct nutrient fertiliser. Associating the word lime with basicity and pH correction helps you correctly answer similar questions about soil management and agriculture. Final Answer: Lime is applied to soil mainly to Increase the alkalinity of the soil and neutralise excess acidity.

Question 4

In the natural carbon cycle, which of the following paths shows carbon moving from plants to animals and then back to the atmosphere?

Accepted Answer

Introduction / Context: The carbon cycle describes how carbon moves continuously between the atmosphere, plants, animals, and other parts of the Earth system. In school level environmental chemistry, students are expected to know how carbon dioxide from the air is taken up by plants, transferred to animals through feeding, and then returned to the atmosphere by respiration and decay. This question focuses on recognising a realistic path that carbon can follow between plants, animals, and the atmosphere. Given Data / Assumptions: Carbon in the atmosphere is mainly present as carbon dioxide gas. Green plants take in carbon dioxide from the atmosphere during photosynthesis. Animals obtain carbon by eating plants or other animals. Both plants and animals release carbon dioxide back to the atmosphere during respiration. The correct option should show a complete path from plants to animals and finally back to the atmosphere. Concept / Approach: In the simplest land based carbon cycle, carbon dioxide from the atmosphere is fixed by plants into sugars and other organic molecules. When animals eat these plants, the carbon becomes part of their bodies. Through respiration, both plants and animals break down food molecules and release carbon dioxide back into the air. A realistic path that begins in plants, passes through animals, and ends in the atmosphere should follow this biological logic and include the return of carbon to the atmosphere as the final step. Step-by-Step Solution: Step 1: Identify the plant step. Plants are the main producers that fix atmospheric carbon into biomass through photosynthesis. Step 2: Identify the animal step. Animals eat plants, so carbon moves from plant tissues into animal tissues. Step 3: Identify the atmosphere step. During respiration and decay, carbon returns to the atmosphere as carbon dioxide. Step 4: Match the sequence. The path Plant ----> Animal ----> Atmosphere fits this complete flow from plant to animal to air. Verification / Alternative check: If we imagine a leaf on a tree, carbon in that leaf may later be eaten by a herbivore such as a cow. Inside the cow's body, the carbon enters cells and is used for energy. When the cow respires, it releases carbon dioxide to the atmosphere. This real life story clearly follows the order plant, then animal, then atmosphere. The option Plant ----> Animal ----> Atmosphere exactly matches this realistic description, which verifies our choice as correct. Why Other Options Are Wrong: Option A (Animal ----> Plant ----> Atmosphere) is incorrect because plants do not normally get their carbon directly from animals; they take carbon dioxide from the air. Option C (Atmosphere ----> Plant ----> Animal) shows a real path but stops at the animal and does not include return to the atmosphere, so it is incomplete for this question. Option D (Plant ----> Atmosphere ----> Animal) is not realistic because animals do not take carbon directly from the atmosphere; they eat plants or other animals. Common Pitfalls: Students sometimes forget that the carbon cycle is a loop and focus only on movement into living organisms. Another common mistake is to think animals breathe in carbon for food, which is incorrect. Animals breathe in oxygen and get carbon mainly by eating. Keeping a simple story in mind, where plants make food from air, animals eat plants, and both release carbon dioxide back to air, helps to avoid these confusions. Final Answer: The correct carbon path from plants to animals and back to the atmosphere is Plant ----> Animal ----> Atmosphere.

Question 5

Which of the following polysaccharides contains a chemically modified monosaccharide unit in its structure?

Accepted Answer

Introduction / Context: This biochemistry related chemistry question asks about polysaccharides, which are long chains of monosaccharide units. Some polysaccharides are made only from simple glucose units, while others contain monosaccharides that have been chemically modified, for example by adding nitrogen containing groups. Understanding which polysaccharide contains such a modified sugar helps students distinguish between different structural carbohydrates found in plants, animals, and fungi. Given Data / Assumptions: Polysaccharides listed are starch, chitin, glycogen, and cellulose. Starch and glycogen are storage polysaccharides. Cellulose and chitin are structural polysaccharides. A modified monosaccharide means a sugar unit that has an additional functional group such as an acetyl amide group compared to simple glucose. Concept / Approach: Starch, glycogen, and cellulose are primarily polymers of glucose molecules. The glucose residues may be linked in different ways, but the basic monosaccharide is still glucose. Chitin, in contrast, is made from N acetylglucosamine units. N acetylglucosamine is a derivative of glucose where one hydroxyl group is replaced by an acetylated amino group. Because of this extra nitrogen containing modification, chitin is said to contain a modified monosaccharide. Therefore, we look for the polysaccharide whose building blocks are not plain glucose but a chemically altered form. Step-by-Step Solution: Step 1: Recall the composition of starch. Starch is a polymer of alpha glucose units and does not usually contain modified sugars. Step 2: Recall the composition of glycogen. Glycogen is a highly branched polymer of alpha glucose, similar to starch but found in animals. Step 3: Recall the composition of cellulose. Cellulose is a linear polymer of beta glucose units forming plant cell walls. Step 4: Recall the composition of chitin. Chitin consists of repeating N acetylglucosamine residues, which are glucose units modified by an acetylated amino group. Step 5: Conclude that chitin is the only option containing a modified monosaccharide. Verification / Alternative check: Biology and chemistry textbooks describe chitin as the main component of the exoskeletons of arthropods and the cell walls of fungi. They explicitly state that the monomer is N acetylglucosamine rather than simple glucose. In contrast, starch and glycogen are energy storage polymers of glucose, while cellulose is a structural polymer of unmodified beta glucose. This cross check confirms that chitin is unique among the listed polysaccharides in containing a chemically modified sugar unit. Why Other Options Are Wrong: Option A: Starch is made from alpha glucose units and does not require a modified sugar to form its structure. Option C: Glycogen is also built from alpha glucose units, similar to starch, and functions as animal energy storage. Option D: Cellulose is composed of beta glucose units and is not based on a nitrogen containing sugar derivative. Common Pitfalls: Students sometimes think that any structural polysaccharide must contain a modified sugar, which leads them to consider cellulose alongside chitin. Another confusion is between the names glucosamine and glucose; although they sound similar, glucosamine and N acetylglucosamine have extra functional groups. Remember that chitin is special for containing nitrogen and is often described as a nitrogenous polysaccharide, which helps to separate it from purely glucose based polymers such as starch and cellulose. Final Answer: The polysaccharide that contains a modified monosaccharide unit is chitin.

Question 6

In the three dimensional tertiary structure of a protein, which of the following interactions is not directly responsible for maintaining that tertiary structure?

Accepted Answer

Introduction / Context: Proteins fold into complex three dimensional shapes that are crucial for their biological functions. This folding is organised into levels of structure: primary, secondary, tertiary, and sometimes quaternary. Tertiary structure refers to the overall three dimensional arrangement of a single polypeptide chain. The question asks which type of interaction is not directly responsible for stabilising this tertiary folding pattern, so we must link each option to the correct structural level. Given Data / Assumptions: Tertiary structure describes the overall three dimensional shape of a single protein chain. Possible stabilising forces listed are hydrogen bonds, ionic bonds, peptide bonds, and hydrophobic interactions. Primary structure is defined by the covalent peptide bonds between amino acids. Tertiary structure depends mainly on interactions among R groups (side chains) and sometimes backbone atoms. Concept / Approach: Peptide bonds are strong covalent bonds that join amino acids end to end and determine the linear primary sequence. Once this sequence exists, the chain can fold. Tertiary structure is then stabilised by weaker interactions between side chains, including hydrogen bonds, ionic or salt bridges, hydrophobic clustering, and sometimes disulfide bonds. Therefore, peptide bonds are essential for primary structure but are not considered the direct drivers of tertiary folding. The other listed interactions act between side chains at different positions in the chain and directly contribute to the folded three dimensional shape. Step-by-Step Solution: Step 1: Identify the role of peptide bonds. Peptide bonds link amino acids in a linear chain and define the primary structure of a protein. Step 2: Identify forces responsible for tertiary structure. Hydrogen bonds form between polar side chains or between side chains and backbone. Ionic bonds occur between positively and negatively charged side chains. Hydrophobic interactions cluster non polar side chains inside the protein, away from water. Step 3: Decide which interaction is not directly responsible for tertiary structure. Because peptide bonds simply make the chain and do not rearrange to create tertiary folding, they are not classified as tertiary stabilising interactions. Verification / Alternative check: Textbooks on biochemistry commonly draw a clear distinction between primary structure, which is the amino acid sequence joined by peptide bonds, and higher levels of structure, which arise from side chain interactions. When describing tertiary structure, they explicitly mention hydrogen bonding, hydrophobic interactions, ionic interactions, van der Waals forces, and sometimes disulfide bridges, but they do not list peptide bonds as part of tertiary stabilisation. This confirms that peptide bonds belong to primary structure, not directly to tertiary. Why Other Options Are Wrong: Option A: Hydrogen bonds between side chains or backbone atoms are central to stabilising both secondary and tertiary structures. Option B: Ionic bonds or salt bridges between charged side chains help maintain the protein fold and are important tertiary interactions. Option D: Hydrophobic interactions drive non polar side chains to cluster in the interior of the protein, a key factor in tertiary folding. Common Pitfalls: Learners sometimes assume that any bond in a protein, including peptide bonds, must contribute directly to all levels of structure. This leads them to overlook the specific roles of different types of interactions. Another mistake is to confuse secondary and tertiary levels, thinking hydrogen bonds only apply to alpha helices and beta sheets, when in reality they also stabilise tertiary folds. Remember that peptide bonds build the chain, while side chain interactions shape the final fold. Final Answer: The interaction that is not directly responsible for tertiary structure is the peptide bond.

Question 7

When two hydrogen atoms combine to form a hydrogen molecule (H2), what type of bond is formed between the two hydrogen atoms?

Accepted Answer

Introduction / Context: This basic chemistry question focuses on the type of chemical bond that holds two hydrogen atoms together in a hydrogen molecule, H2. Bond type recognition is fundamental for understanding how atoms share or transfer electrons to reach more stable electronic configurations. Hydrogen provides a simple example of covalent bonding, which is widely used to illustrate the concept of electron sharing. Given Data / Assumptions: Each hydrogen atom has one electron in its outer shell. The stable configuration for hydrogen resembles the helium configuration with two electrons in the first shell. In H2, two hydrogen atoms combine to form a diatomic molecule. We must classify the bond as hydrophobic, ionic, covalent, or hydrophilic. Concept / Approach: In a covalent bond, atoms share one or more pairs of electrons. For hydrogen, sharing one pair of electrons between two atoms allows each atom to feel as if it has two electrons in its outer shell. Ionic bonds, in contrast, involve transfer of electrons and formation of oppositely charged ions, which does not happen between identical hydrogen atoms. Hydrophobic and hydrophilic describe interactions with water, not the basic chemical bond inside a molecule. Therefore, the bond in H2 is a simple single covalent bond formed by sharing an electron pair. Step-by-Step Solution: Step 1: Consider the electron configuration of a hydrogen atom. Each hydrogen atom has 1 electron in the first shell and wants 2 electrons for stability. Step 2: Consider what happens when two hydrogen atoms approach each other. They can share their electrons so that both atoms effectively have access to 2 electrons. Step 3: Identify the bond type. Sharing of one pair of electrons between the two atoms is the definition of a single covalent bond. Step 4: Compare with other bond types. No electron transfer occurs, so the bond cannot be ionic. Verification / Alternative check: Lewis dot structures show each hydrogen atom with one dot representing one electron. When they form H2, the two dots become a shared pair between the symbols H and H. This is a classic textbook example of a covalent bond. Additionally, both atoms are identical and have equal attraction for the shared electrons, confirming that the bond is a non polar covalent bond. This matches the general explanation found in basic chemistry texts for the hydrogen molecule. Why Other Options Are Wrong: Option A: Hydrophobic describes the tendency of non polar groups to avoid water and does not name a bond between atoms. Option B: Ionic bonding involves electron transfer and formation of ions, which is not possible between two identical hydrogen atoms in this case. Option D: Hydrophilic describes an attraction to water and is not a type of chemical bond linking two atoms. Common Pitfalls: Some learners confuse terms used for interactions in biology, such as hydrophobic and hydrophilic, with fundamental chemical bond types. Others think any strong interaction might be ionic, but ionic bonds always involve ions with full charges. Remember that hydrogen molecule H2 is the simplest example of a covalent bond formed by equal sharing of a pair of electrons between two identical atoms. Final Answer: The two hydrogen atoms in H2 are joined by a Covalent bond.

Question 8

In normal oxy acetylene welding, which gases are commonly used to produce the high temperature flame needed for welding metals?

Accepted Answer

Introduction / Context: Welding is a common industrial process that joins metals using heat. One traditional and widely taught method is gas welding, specifically oxy acetylene welding. In this technique, the flame is produced by burning a fuel gas in oxygen, achieving temperatures high enough to melt steel. Understanding which gases are used in standard welding equipment is an important part of applied chemistry and engineering awareness. Given Data / Assumptions: The question refers to normal welding using a gas flame, typically meaning oxy acetylene welding. Common fuel gases include acetylene, hydrogen, and others, but not all are standard for normal welding. The options list combinations of oxygen, acetylene, nitrogen, and hydrogen. We assume basic industrial practice where the most common setup is required. Concept / Approach: In oxy acetylene welding, acetylene (C2H2) is used as the fuel gas because it produces a very hot flame when burned in oxygen, reaching temperatures around 3000 degrees Celsius or more. Oxygen is supplied from a high pressure cylinder to support complete combustion of acetylene. Nitrogen is relatively inert and would not support such a hot flame, and while hydrogen can be used for some specialised welding or cutting processes, the classic and most widely taught gas welding combination is oxygen with acetylene. Step-by-Step Solution: Step 1: Identify the standard welding method implied. The phrase normal welding by gas usually refers to oxy acetylene welding. Step 2: Recall the gases used in oxy acetylene welding. Acetylene is the fuel gas, and oxygen is the supporter of combustion. Step 3: Compare with the options. Option A lists oxygen and acetylene, exactly matching the standard pair. Other options include nitrogen or hydrogen combinations that are not typical for standard oxy acetylene welding. Verification / Alternative check: Practical welding manuals and vocational training material describe the gas welding torch as having two supplies: one from an oxygen cylinder and the other from an acetylene cylinder. The flame produced is specifically called an oxy acetylene flame. While other fuel gases such as liquefied petroleum gas or hydrogen may be used for specialised tasks, they are less common in basic metal welding instruction. This confirms that the correct answer must include only oxygen and acetylene. Why Other Options Are Wrong: Option B: Acetylene and nitrogen would not give a strong welding flame because nitrogen does not support combustion in the same way oxygen does. Option C: Oxygen, hydrogen, and acetylene together is not a standard combination and would complicate flame control, so it is not used as the normal welding setup. Option D: Oxygen and hydrogen can produce a flame, but this oxy hydrogen flame is less common for general metal welding and is not usually referred to as normal welding gas in basic chemistry questions. Common Pitfalls: Students may be tempted to select any option that includes oxygen, forgetting that a specific fuel gas must also be chosen. Another mistake is thinking nitrogen participates in combustion because it is a major component of air, but in reality nitrogen is largely inert. Remember the familiar phrase oxy acetylene welding to fix in mind that the classic combination is oxygen plus acetylene. Final Answer: The gases used in normal oxy acetylene welding are Oxygen and Acetylene.

Question 9

Proteins from different sources differ from one another primarily because of what feature of their polypeptide chains?

Accepted Answer

Introduction / Context: This question tests understanding of what gives each protein its unique identity. Proteins are made from only about twenty standard amino acids, yet there are thousands of different proteins in living organisms, each with its own structure and function. The key reason for this diversity lies in the order in which amino acids are arranged along the polypeptide chain, known as the primary structure. Recognising this concept is central to biochemistry and molecular biology. Given Data / Assumptions: Proteins are long chains of amino acids connected by peptide bonds. The options mention peptide bonds, sugar sequences, nucleotides, and amino acid sequences. Proteins are not made directly from sugars or nucleotides; these belong to carbohydrates and nucleic acids respectively. Peptide bonds are of the same chemical type in most proteins. Concept / Approach: The primary structure of a protein is the specific sequence of amino acids from the amino terminus to the carboxyl terminus. Even small changes in this sequence can alter the way a protein folds into its secondary and tertiary structure and thus change its activity. The chemical nature of a peptide bond is essentially the same in all proteins. Proteins do not consist of long chains of sugars or nucleotides; such chains form polysaccharides and nucleic acids. Therefore, the main feature that makes one protein different from another is the particular order of amino acids in its polypeptide chain. Step-by-Step Solution: Step 1: Identify the basic building blocks of proteins. Proteins are polymers of amino acids joined by peptide bonds. Step 2: Consider the nature of peptide bonds. The peptide bond is a standard amide linkage and does not change its chemistry from one protein to another. Step 3: Examine the role of sequences. Different proteins have different orders of amino acids, which determine their shape and function. Step 4: Compare with the options. Only the option mentioning amino acid sequence correctly describes what varies between proteins. Verification / Alternative check: Biology texts describe how the sequence of amino acids is coded by the sequence of nucleotides in DNA. Mutations change the DNA sequence, which can change the amino acid sequence and thereby change the protein. Experimental techniques such as peptide sequencing and mass spectrometry focus on determining the exact order of amino acids to distinguish proteins. These examples confirm that amino acid sequence is the key distinguishing feature of each protein molecule. Why Other Options Are Wrong: Option A: Peptide bonds are chemically similar in all proteins; they do not differ in type from protein to protein. Option B: Proteins are not defined by sequences of sugar molecules; that description fits polysaccharides. Option C: Nucleotides are the building blocks of DNA and RNA, not of proteins, so this statement does not apply. Common Pitfalls: Students sometimes confuse different classes of biological macromolecules and may think that all biopolymers are built from nucleotides or sugar units. Another pitfall is to focus on peptide bonds because they are often mentioned during protein synthesis, but the bond type is the same in all proteins. Remember the simple idea that a protein is defined by its amino acid sequence, just as a word is defined by the order of letters. Final Answer: Proteins differ from one another mainly because the sequence of amino acids in the polypeptide chain differs from protein to protein.

Question 10

In human nutrition, which group of vitamins is correctly identified as water soluble vitamins?

Accepted Answer

Introduction / Context: Vitamins are essential micronutrients that are divided into water soluble and fat soluble groups. This classification affects how they are absorbed, transported, stored, and excreted in the human body. Knowing which vitamins are water soluble is important for understanding dietary requirements, toxicity risks, and deficiency symptoms, and this topic frequently appears in general science and biology examinations. Given Data / Assumptions: Vitamins B complex and C are commonly taught as water soluble vitamins. Vitamins A, D, E, and K are commonly taught as fat soluble vitamins. The options group different combinations of these vitamins. We must select the option that includes only water soluble vitamins. Concept / Approach: Water soluble vitamins dissolve in water, are not stored in large amounts in the body, and excess amounts are usually excreted in urine. Fat soluble vitamins require fat for absorption and can be stored in body tissues. Vitamin C and the B group (including B1, B2, B3, B6, B12, folic acid, and others) are water soluble. Vitamins A, D, E, and K are fat soluble and are stored in the liver and fatty tissues. Therefore, the correct option must include B and C without grouping them with fat soluble vitamins A, D, or K. Step-by-Step Solution: Step 1: Identify the water soluble vitamins. Vitamin C and all B group vitamins are water soluble. Step 2: Identify the fat soluble vitamins. Vitamins A, D, E, and K are fat soluble. Step 3: Check each option. Option A lists B and C, both water soluble. Option B mixes K and D, which are fat soluble, with C. Option C lists A, D, and K, all fat soluble. Option D mixes several fat soluble and water soluble vitamins together. Verification / Alternative check: Standard nutrition tables and biology textbooks state that deficiencies of water soluble vitamins develop more quickly because the body does not store them in large quantities. They also point out that toxicity from water soluble vitamins is less common than from fat soluble vitamins. These sources consistently classify vitamin C and the B complex as water soluble. Options that include A, D, or K cannot be correct for the category of exclusively water soluble vitamins, confirming Option A as the right choice. Why Other Options Are Wrong: Option B: Vitamins K and D are fat soluble, so this set is not purely water soluble. Option C: Vitamins A, D, and K are all fat soluble vitamins, not water soluble ones. Option D: This mixture includes both fat soluble and water soluble vitamins and therefore does not match the requirement. Common Pitfalls: A common mistake is to assume that vitamin C is the only water soluble vitamin and forget about the B group. Another confusion is that many learners remember the fat soluble group by the phrase ADEK but still misplace vitamin C. Keeping a simple rule in mind that only B and C are water soluble makes it easier to classify vitamins correctly in objective questions. Final Answer: The vitamins that are water soluble are Vitamins B & C.

Question 11

In basic chemistry, water (H2O) is correctly classified as which of the following types of substance?

Accepted Answer

Introduction / Context: This question tests a fundamental classification in chemistry: the difference between elements, molecules, and compounds. Water is one of the most familiar substances, and students must be able to describe it correctly using these terms. The key idea is that a single substance can be both a molecule and a compound at the same time, so understanding how the definitions overlap is important. Given Data / Assumptions: Water has the chemical formula H2O. Hydrogen and oxygen are elements. Molecule is defined as two or more atoms chemically bonded together. Compound is defined as a pure substance formed when atoms of different elements combine in a fixed ratio. We assume standard school level definitions of these terms. Concept / Approach: An element is a substance made of only one type of atom, such as pure hydrogen gas or pure oxygen gas. A molecule refers to a group of two or more atoms held together by covalent bonds; these atoms may be of the same element or different elements. A compound is a pure substance made of atoms of different elements bonded in a fixed ratio. Because water consists of two hydrogen atoms and one oxygen atom covalently bonded, each water particle is a molecule. Since these atoms are of different elements in a fixed 2:1 ratio, water is also a compound. Therefore, both the terms molecule and compound apply to water. Step-by-Step Solution: Step 1: Check whether water is an element. Water contains hydrogen and oxygen, so it is not a single element. Step 2: Check whether water is a molecule. H2O consists of three atoms bonded together, so it is definitely a molecule. Step 3: Check whether water is a compound. Because it contains different elements (H and O) in a fixed ratio, water is a compound. Step 4: Combine the correct descriptions. Water is both a molecule and a compound, so the answer must include both. Verification / Alternative check: Chemistry textbooks usually list water among common molecular compounds. They explain that all compounds whose particles are discrete molecules, such as water, methane, and carbon dioxide, can be called both molecules and compounds. Only substances made from one element, such as O2 or N2, are molecules but not compounds. Since water always appears with both hydrogen and oxygen, it fits both definitions. This confirms that the most precise description from the options is the one that includes both terms. Why Other Options Are Wrong: Option A: Element is incorrect because water contains more than one type of atom. Option B: Molecule alone is incomplete; although true, it does not fully reflect that water is also a compound. Option C: Compound alone is also incomplete; water is a compound whose smallest unit is a molecule. Common Pitfalls: Learners often think that being a compound and being a molecule are mutually exclusive categories, so they pick only one label. Another common mistake is to confuse water with hydrogen or oxygen gases and call it an element. Remember that a molecular compound like water is both a molecule and a compound, while an elemental molecule like O2 is only a molecule and not a compound. Final Answer: Water is correctly described as Both B & C, that is, it is both a molecule and a compound.

Question 12

In atomic chemistry, the chemical reactivity of an atom mainly arises from which of the following features?

Accepted Answer

Introduction / Context: This question examines why some atoms are very reactive while others are relatively inert. In basic atomic theory, the electrons in the outermost, or valence, shell play the key role in chemical bonding and reactivity. Atoms seek stable electron configurations, often resembling noble gases, and the presence of unpaired electrons in the valence shell creates a strong tendency to gain, lose, or share electrons. Understanding this idea is crucial for predicting chemical behaviour across the periodic table. Given Data / Assumptions: Valence electrons are the electrons in the outermost shell of an atom. Unpaired electrons are single electrons in orbitals that are not part of an electron pair. The options mention features such as energy differences between shells and distances from the nucleus. We assume the standard Bohr and quantum mechanical models taught at school level. Concept / Approach: Chemical bonds form when atoms interact through their valence electrons. Atoms with completely filled valence shells, such as noble gases, are usually unreactive. Atoms with incomplete shells, especially when there are unpaired electrons, tend to form bonds to achieve a more stable arrangement. While overall energy levels and distances from the nucleus influence these processes, the direct cause of typical chemical reactivity is the presence of available unpaired valence electrons that can be shared, gained, or lost. Step-by-Step Solution: Step 1: Consider noble gases. Noble gases have full valence shells with paired electrons and show very low reactivity. Step 2: Consider halogens and alkali metals. Halogens have one unpaired electron in the valence shell and are highly reactive non metals. Alkali metals have one valence electron that can be easily lost, making them highly reactive metals. Step 3: Relate this to unpaired electrons. In both cases, unpaired or easily unpaired electrons in the valence shell drive reactions. Step 4: Compare with the given options. The option that directly mentions unpaired electrons in the valence shell best matches this explanation. Verification / Alternative check: If we examine elements in the same period, those with half filled or nearly empty valence shells are often more reactive than those with filled shells. For example, fluorine and sodium are both very reactive due to one electron short of a full shell or one electron more than a stable configuration. Neon, with a full valence shell and no unpaired electrons, is almost inert. This pattern is consistent across the periodic table and supports the idea that unpaired valence electrons are the main source of reactivity. Why Other Options Are Wrong: Option B: Energy differences between shells exist for all atoms and influence spectra, but they do not by themselves explain why one atom is more chemically reactive than another. Option C: The sum of potential energies of all shells is an abstract quantity and does not directly indicate reactivity in a simple way. Option D: The average distance of the outer shell from the nucleus affects ionisation energy but does not alone determine whether an atom has unpaired electrons available for bonding. Common Pitfalls: Students may focus too much on general energy diagrams and forget that chemical reactions involve specific electrons that can form or break bonds. Another mistake is to think that any atom with many electrons is automatically more reactive, which is not true for heavy noble gases. Keeping in mind that reactivity depends on the availability of unpaired valence electrons helps to explain both highly reactive and inert elements in a unified way. Final Answer: The reactivity of an atom mainly arises from the existence of unpaired electrons in the valence shell.

Question 13

Which statement is true for all atoms that are anions in basic atomic chemistry?

Accepted Answer

Introduction / Context: This question checks the basic definition of an anion in chemistry. Atoms and ions are distinguished by their numbers of protons and electrons. A neutral atom has equal numbers of protons and electrons. When an atom gains or loses electrons, it becomes an ion and carries a net charge. Anions are negatively charged ions, and the question asks which statement must always be true for any anion, regardless of which element it comes from. Given Data / Assumptions: An anion is a negatively charged ion. Protons carry positive charge and are located in the nucleus. Electrons carry negative charge and surround the nucleus. Neutrons are neutral and do not affect the net charge. The atomic number of an element equals the number of protons and does not change for ions of that element. Concept / Approach: The net charge on an ion is determined by the difference between the number of protons and electrons. For a neutral atom, protons and electrons are equal. An anion has a negative charge, which can only happen if there are more electrons (negative charges) than protons (positive charges). The number of neutrons can vary between isotopes but does not contribute to charge. Therefore, the universal property of all anions is an excess of electrons over protons, regardless of how many neutrons are present. Step-by-Step Solution: Step 1: Recall the definition of an anion. An anion is a negatively charged ion formed when an atom gains one or more electrons. Step 2: Relate charge to numbers of protons and electrons. Net charge = number of protons minus number of electrons. For the charge to be negative, electrons must outnumber protons. Step 3: Evaluate the statements. The only statement that must always hold is that the atom has more electrons than protons. Verification / Alternative check: Consider a chloride ion, Cl-. Neutral chlorine has 17 protons and 17 electrons. As Cl-, it has gained one extra electron, giving 17 protons and 18 electrons. This extra electron creates the negative charge and demonstrates that the anion has more electrons than protons. The same logic applies to oxide O2-, sulphide S2-, and other common anions. In each case, negative charge arises from extra electrons compared with the number of protons, confirming the general rule. Why Other Options Are Wrong: Option A: The number of protons compared with the neutral atom of the same element does not change; only the number of electrons changes, so this statement is false. Option B: Saying the atom has more protons than electrons would describe a positively charged ion or cation, not an anion. Option C: Having more neutrons than protons does not determine charge at all, so this cannot define an anion. Common Pitfalls: Students sometimes confuse cations and anions, mixing up which one gains or loses electrons. A simple memory aid is that anion starts with the letter a like in the word add, because an anion has added electrons. Confusing the role of neutrons is another issue; they can vary in number for isotopes but have no impact on charge. Always connect the sign of the charge directly with the balance between protons and electrons. Final Answer: For all anions, the true statement is that the atom has more electrons than protons.

Question 14

When iron rusts in moist air, should this process be classified as a chemical change or a physical change?

Accepted Answer

Introduction / Context: Rusting of iron is one of the most familiar real life examples used to distinguish between chemical and physical changes. A physical change affects the form of a substance but does not create a new substance, while a chemical change produces one or more new substances with different properties. The question asks whether rusting should be treated as chemical or physical, so we must examine what happens to iron at the molecular level when it rusts. Given Data / Assumptions: Rust forms when iron reacts with oxygen and water from the environment. The main product is hydrated iron(III) oxide, commonly called rust. Rust has different colour, texture, and properties compared with pure iron. Physical changes are usually reversible by simple physical methods, while chemical changes are often difficult to reverse. Concept / Approach: During rusting, iron atoms combine with oxygen and water molecules to form new compounds. This process involves breaking old bonds and forming new chemical bonds, which is the hallmark of a chemical reaction. The original metallic iron is converted to a flaky, brittle oxide that has different properties, such as reduced strength and conductivity. Therefore, rusting cannot be viewed as mere physical change like melting or cutting; it is a chemical change, more specifically a slow redox reaction and an example of corrosion. Step-by-Step Solution: Step 1: Identify what happens to iron during rusting. Iron reacts with oxygen and water to form iron oxides, especially hydrated iron(III) oxide. Step 2: Check whether a new substance is formed. Rust is chemically different from metallic iron in both composition and properties. Step 3: Compare with definitions. Chemical change involves formation of new substances, while physical change does not. Step 4: Conclude that rusting is a chemical change. Verification / Alternative check: One way to verify is to ask if the process is easily reversible by a simple physical method. Melting or freezing water can be reversed by changing temperature, so these are physical changes. In contrast, once iron has rusted, simply changing temperature or pressure cannot restore metallic iron. Removing rust requires either strong chemical treatment or complete replacement of the rusted portion. This difficulty of reversal and the change in composition both confirm that rusting is a chemical change. Why Other Options Are Wrong: Option B: Physical change is incorrect because rusting does not merely change the form; it creates new chemical substances. Option C: Both A and B cannot be correct because the process clearly fits the definition of a chemical change and not a typical physical change. Option D: None of the above is not valid since chemical change is a correct and standard description of rusting. Common Pitfalls: Some learners think that any change that happens slowly or can be scraped off must be physical, which is not accurate. Others confuse rusting with temporary surface tarnishing. Remember that rusting involves actual chemical reaction between iron, oxygen, and water, and it leads to loss of metallic properties. Keeping the rule in mind that new substances with new properties mean chemical change will help avoid these errors. Final Answer: Rusting of iron is a Chemical change.

Question 15

In classical chemical terminology, what is the Latin derived chemical name for the element Gold (symbol Au)?

Accepted Answer

Introduction / Context: This question relates to the historical naming of elements in chemistry. Many element symbols come from Latin names rather than modern English names. Gold is one such element, and its symbol Au does not match the English word gold. Knowing the Latin name helps students understand periodic table symbols and also appears in questions on chemical nomenclature and history of science. Given Data / Assumptions: The English name of the element is gold. The chemical symbol for gold in the periodic table is Au. Several Latin style names are given as options. We assume standard historical names used in chemistry. Concept / Approach: The symbol Au originates from the Latin word aurum, which was used in alchemical and early chemical literature for gold. Other metals such as silver (Ag from argentum) and iron (Fe from ferrum) also have Latin derived symbols. The options include names that sound similar to Latin words for different elements. We must identify which one corresponds to gold. Aurum is the traditional Latin name for gold, so that option is correct. Step-by-Step Solution: Step 1: Recall the symbol for gold. Gold has the chemical symbol Au in the periodic table. Step 2: Recall the Latin basis of Au. The symbol Au comes from the Latin word aurum. Step 3: Match the Latin name with options. Option C lists Aurum, which clearly corresponds to Au. Step 4: Verify that other options refer to different elements. Verification / Alternative check: In lists of element names and symbols, gold is shown as Au from aurum, silver as Ag from argentum, and iron as Fe from ferrum. The option Argentinum resembles argentum, which is connected to silver, not gold. The term ferrus resembles ferrum or ferrous, linked with iron. Selenium is a different non metallic element with symbol Se. Only aurum matches both the language and the symbol Au assigned to gold, confirming this choice. Why Other Options Are Wrong: Option A: Argentinum suggests a relationship to argentum, the Latin name for silver, whose symbol is Ag, not Au. Option B: Ferrus looks like a variant of ferrum or the adjective ferrous, both connected to iron, which has the symbol Fe. Option D: Selenium is the English and Latin derived name for a different element with symbol Se and has nothing to do with gold. Common Pitfalls: Students sometimes guess based on similarity to English words without recalling the link between symbols and Latin names. Another mistake is to confuse gold and silver because both are precious metals. Remembering a few popular Latin names, such as aurum for gold, argentum for silver, and ferrum for iron, helps in quickly answering such symbol related questions. Final Answer: The Latin derived chemical name for gold is Aurum.

Question 16

In a chemical reaction, what is meant by the term product in contrast to reactant?

Accepted Answer

Introduction / Context: Understanding the basic language of chemical equations is essential for studying chemistry. Every reaction is described using reactants and products. Reactants are the starting substances, while products are the new substances formed. This question directly asks for the definition of a product in the context of a chemical reaction, which is a core concept in school level chemistry. Given Data / Assumptions: Chemical reactions are written in the form: Reactants → Products. Reactants are present at the beginning of the reaction. Products are present at the end of the reaction. The options present different possible descriptions of a product, some of which actually describe reactants or catalysts. Concept / Approach: A product is defined as a substance that is generated when reactants undergo a chemical change. This may involve breaking bonds, forming new bonds, and rearranging atoms. Products usually have different properties from the reactants. In a chemical equation, products are written on the right hand side of the arrow. Catalysts and solvents may participate in the process but are not normally called products unless they are chemically changed and appear on the right side of the equation as new substances. Step-by-Step Solution: Step 1: Recall the layout of a chemical equation. Reactants are written on the left side, products on the right side of the arrow. Step 2: Define reactant. A reactant is a substance present at the start that takes part in the reaction and may be consumed. Step 3: Define product. A product is a substance that is formed as a result of the reaction and appears at the end. Step 4: Compare with the given options to identify the one matching this definition. Verification / Alternative check: Consider the reaction: 2H2 + O2 → 2H2O. Hydrogen and oxygen gases are present at the start and are reactants. Water is not present initially but appears after the reaction as a new substance with different properties; therefore water is the product. In an acid base neutralisation reaction such as HCl + NaOH → NaCl + H2O, the salt and water are products. In each case, products are clearly the substances formed by chemical change, confirming the definition used in the correct option. Why Other Options Are Wrong: Option B: This describes a reactant, not a product, because it is present at the start and is consumed. Option C: A catalyst speeds up a reaction without being consumed and is not usually called a product unless it is chemically changed. Option D: A solvent dissolves reactants and may remain unchanged, but it is not normally classified as a product in the reaction equation. Common Pitfalls: Students sometimes confuse reactants and products because both appear in the equation. Another common error is to think that anything present in the reaction mixture at the end is a product, including unchanged catalysts or solvents. Remember that products are new substances created by the reaction, while reactants are the starting substances that undergo change. Looking at which side of the arrow a substance appears on in a balanced equation helps avoid confusion. Final Answer: In a chemical reaction, a product is a substance that is formed as a result of the chemical reaction.

Question 17

In plant mineral nutrition, which of the following elements is classified as a micronutrient rather than a macronutrient?

Accepted Answer

Introduction / Context: Plants require many mineral elements for healthy growth, and these are divided into macronutrients and micronutrients. Macronutrients are needed in relatively large amounts, while micronutrients are required only in trace quantities. This question asks you to identify which listed element is a micronutrient in standard plant nutrition, a topic commonly covered in biology and agricultural science. Given Data / Assumptions: Macronutrients usually include nitrogen, phosphorus, potassium, calcium, magnesium, and sulphur. Micronutrients include iron, manganese, zinc, copper, boron, molybdenum, chlorine, and nickel. The options given are Mg, Cl, P, and Ca. We assume normal school level classification of essential plant nutrients. Concept / Approach: Macronutrients are required by plants in larger concentrations and are often the main ingredients in fertilisers. Magnesium, phosphorus, and calcium all appear on the list of macronutrients. Magnesium is part of chlorophyll and enzyme activation, phosphorus is crucial for ATP and nucleic acids, and calcium is important for cell wall stability. Chlorine, although essential, is needed in much smaller amounts and is grouped with micronutrients. Therefore we must look for the element among the options that typically appears in the micronutrient list, which is chlorine (Cl). Step-by-Step Solution: Step 1: Classify Mg. Magnesium is a macronutrient because it is a central part of chlorophyll and is required in significant amounts. Step 2: Classify P. Phosphorus is also a macronutrient, important in energy transfer and nucleic acids. Step 3: Classify Ca. Calcium is a macronutrient, needed for cell walls and membrane function. Step 4: Classify Cl. Chlorine is required in very small amounts, making it a micronutrient. Verification / Alternative check: If we examine tables of essential elements for plants, chlorine is always listed under micronutrients together with iron, zinc, and other trace elements. Fertilisers are often labelled with N, P, and K as primary macronutrients, and sometimes Ca and Mg appear as secondary macronutrients. Chlorine is rarely highlighted because plants need only trace amounts. This consistent grouping across reference sources confirms that Cl is the only micronutrient in the set of options. Why Other Options Are Wrong: Option A: Mg (magnesium) is a macronutrient because it is needed in relatively large quantities in plant tissues. Option C: P (phosphorus) is a primary macronutrient, commonly represented in fertiliser labels. Option D: Ca (calcium) is also required in larger amounts as a secondary macronutrient, especially for cell wall formation. Common Pitfalls: Learners sometimes assume that any element present in small letters in a question is a trace element, which is not a reliable rule. Another confusion arises because chloride deficiency is less commonly discussed than nitrogen or phosphorus deficiency, so students may forget its classification. Remember that magnesium, phosphorus, and calcium are major plant nutrients, while chlorine, though essential, is needed only in traces and therefore counts as a micronutrient. Final Answer: The element that is a micronutrient is Cl, chlorine.

Question 18

According to the Aufbau Principle in atomic structure, how are electrons added to orbitals as an atom is built up?

Accepted Answer

Introduction / Context: The Aufbau Principle is a guiding rule in atomic chemistry used to determine the electron configuration of atoms. It explains the order in which electrons occupy atomic orbitals as the atomic number increases. Understanding this principle is essential for writing correct electron configurations, predicting valence electrons, and explaining periodic trends such as atomic size and ionisation energy. Given Data / Assumptions: Electrons occupy orbitals of different energy levels and sublevels (s, p, d, f). The Aufbau Principle provides a general filling order based on increasing energy. The options include correct and incorrect descriptions of how electrons are added. We assume standard introductory chemistry treatment of the Aufbau Principle. Concept / Approach: The word Aufbau comes from German and means building up. The Aufbau Principle states that electrons enter the lowest energy orbitals available first before any electrons occupy higher energy orbitals. This leads to the familiar order of filling, such as 1s, 2s, 2p, 3s, 3p, 4s, and so on, following the diagonal rule. Electrons do not fill orbitals randomly or based on colour or symmetry. Instead, their arrangement minimises the total energy of the atom, leading to the most stable configuration. Step-by-Step Solution: Step 1: Recall the main statement of the Aufbau Principle. Electrons occupy orbitals starting from the lowest energy available and then move to higher energy orbitals. Step 2: Interpret what lowest level or sublevel means. It refers to orbitals like 1s, then 2s, then 2p, and so on, in order of increasing energy. Step 3: Compare with the options. Option A clearly states filling from the lowest possible level or sublevel. Options B, C, and D describe conditions that are not part of established quantum rules. Verification / Alternative check: When writing electron configurations for elements from hydrogen to neon, we fill 1s, then 2s, then 2p. For potassium, we fill up to 3p and then 4s before 3d because 4s has slightly lower energy than 3d. These patterns are all predicted by the idea that electrons fill lower energy orbitals first. This consistent behaviour across the periodic table confirms that the Aufbau Principle focuses on energy level order, not random filling or symmetry based rules, and matches the description in Option A. Why Other Options Are Wrong: Option B: Colour of orbitals is not a meaningful physical property in this context; electrons do not choose orbitals by colour. Option C: Electrons do not occupy orbitals in a random order; their arrangement follows predictable energy based rules. Option D: While final configurations may show patterns, electrons are not added simply to produce symmetrical diagrams; energy minimisation is the real guiding principle. Common Pitfalls: Students sometimes confuse the Aufbau Principle with Hund rule or the Pauli Exclusion Principle. Another pitfall is to misremember the filling order and place electrons into 3d before 4s for some elements. Using the diagonal rule diagram and remembering that Aufbau emphasises lowest energy first helps keep these ideas straight. The key phrase to remember is that electrons fill from lowest energy level or sublevel upwards. Final Answer: According to the Aufbau Principle, electrons will be added to an atom starting from the lowest possible level or sublevel.

Question 19

According to Brønsted–Lowry acid–base theory, what is the conjugate base formed when H2O donates a proton?

Accepted Answer

Introduction / Context: This question applies Brønsted Lowry acid base theory, which defines acids as proton donors and bases as proton acceptors. When an acid loses a proton, the species that remains is called its conjugate base. Water is amphoteric, meaning it can act as either an acid or a base. The question asks specifically for the conjugate base of H2O when it donates a proton, a common concept in acid base equilibrium problems. Given Data / Assumptions: H2O is neutral water. When H2O donates a proton (H+), it loses one hydrogen ion. The remaining species must have one fewer hydrogen and carry a negative charge. The options include NH3+, OH-, H2O, and a statement suggesting all of them. Concept / Approach: According to Brønsted Lowry theory, an acid base conjugate pair differs by exactly one proton. If H2O acts as an acid and donates one proton, the equation can be written as H2O → H+ + OH-. The species formed after loss of H+ is OH-, the hydroxide ion. Therefore, OH- is the conjugate base of H2O in this context. Ammonium ion NH4+ and ammonia NH3 form another conjugate acid base pair, but NH3+ is not the species formed from water. H2O itself is the original acid in this situation, not its own conjugate base. Step-by-Step Solution: Step 1: Write the acid dissociation of water. H2O can donate a proton: H2O → H+ + OH-. Step 2: Identify the species before and after proton donation. Before donation: H2O as the acid. After donation: OH- is left behind. Step 3: Apply the conjugate base definition. Conjugate base = acid minus one proton, so the conjugate base of H2O is OH-. Verification / Alternative check: Consider the autoionisation of water: 2H2O ⇌ H3O+ + OH-. In this equilibrium, one water molecule acts as an acid and donates a proton to another water molecule, which acts as a base. The donor becomes OH-, and the acceptor becomes H3O+. Thus, OH- is always observed as the result of water losing a proton. Many acid base tables list conjugate pairs such as H2O and OH-, further confirming that hydroxide ion is the conjugate base of water. Why Other Options Are Wrong: Option A: NH3+ is not formed from H2O and is not the conjugate base of water; it would be associated with a different acid base system. Option C: H2O is the original acid in this case, not the species formed after losing a proton, so it cannot be its own conjugate base. Option D: All the above is impossible because only OH- matches the definition of the conjugate base of H2O. Common Pitfalls: Learners sometimes confuse conjugate acid and conjugate base pairs or forget that they differ by exactly one proton. Another mistake is to think that any negatively charged species can be a conjugate base of water, which is not correct. Always write the simple equation showing the acid donating a proton and then identify what remains. For water, this process clearly leads to the hydroxide ion as the conjugate base. Final Answer: When H2O donates a proton, its conjugate base is OH-.

Question 20

In carbohydrate chemistry, a disaccharide is formed by the chemical bonding of which types of sugar units?

Accepted Answer

Introduction / Context: Carbohydrates are classified based on the number of sugar units present. Monosaccharides are single sugar units, disaccharides contain two sugar units, and polysaccharides contain many units. This question asks specifically how a disaccharide is formed, which is important for understanding common sugars such as sucrose, lactose, and maltose, all of which are widely discussed in basic biology and chemistry. Given Data / Assumptions: A monosaccharide is the simplest sugar, for example glucose or fructose. A disaccharide consists of exactly two monosaccharide units joined together. A polysaccharide consists of many monosaccharide units. The options offer different combinations of mono and polysaccharides. Concept / Approach: Disaccharides are formed when two monosaccharides react together in a condensation reaction, usually releasing a molecule of water and forming a glycosidic bond. For example, glucose plus fructose forms sucrose, glucose plus galactose forms lactose, and two glucose molecules form maltose. Polysaccharides such as starch or cellulose are much larger and are built by repeated addition of monosaccharides, not by joining two polysaccharides. Therefore, the core definition of a disaccharide must be based on two monosaccharides joined together. Step-by-Step Solution: Step 1: Recall the definition of monosaccharide. A monosaccharide is a single sugar unit such as glucose, fructose, or galactose. Step 2: Recall the definition of disaccharide. A disaccharide is formed when two monosaccharide molecules join through a glycosidic linkage. Step 3: Consider polysaccharides. Polysaccharides are long chains of many monosaccharides and not involved in forming disaccharides directly. Step 4: Match the correct description with the options. Verification / Alternative check: Textbooks commonly show equations like glucose + fructose → sucrose + water to illustrate disaccharide formation. They also present lactose as glucose plus galactose and maltose as glucose plus glucose. Each example supports the rule that disaccharides consist of two monosaccharides. There is no standard example where a disaccharide is formed by bonding two polysaccharides or a monosaccharide with an entire polysaccharide, so the alternative descriptions do not match the definition. Why Other Options Are Wrong: Option B: Two polysaccharides joined together would form an even larger polysaccharide, not a disaccharide. Option C: One monosaccharide and one polysaccharide would again produce a complex carbohydrate, not a disaccharide of exactly two units. Option D: All the above cannot be correct because only the first option matches the definition of a disaccharide. Common Pitfalls: Students sometimes think that any combination of sugar molecules might be called a disaccharide as long as it involves a bond, but the name specifically refers to two sugar units. Confusing the number of units in di and poly can also cause mistakes. A simple memory aid is that mono means one, di means two, and poly means many, and these prefixes directly tell you how many sugar units are present in the carbohydrate. Final Answer: A disaccharide is formed by the chemical bonding of two monosaccharides.

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