Question 1

To check whether a sample of drinking water contains a gamma emitting radioactive isotope, which instrument would be most appropriate to use?

Accepted Answer

Introduction / Context: Environmental monitoring and nuclear safety require reliable detection of radioactive contamination in water, food and air. Gamma emitting isotopes are especially important because gamma rays are highly penetrating and can pose health risks even when the radioactive material is not ingested in large amounts. Instruments such as Geiger counters and scintillation counters are used to detect this radiation. The question asks which device would be suitable for confirming the presence of a gamma emitting isotope in drinking water. Given Data / Assumptions: The suspected contaminant is a gamma emitting isotope dissolved in drinking water. Options include microscope, scintillation counter, lead plate, spectrophotometer and thermometer. We assume access to common radiation detection instruments used in nuclear science. The goal is to detect gamma radiation, not to examine cells or measure temperature. Concept / Approach: Gamma rays are high energy photons that can be detected using detectors that convert radiation energy into electrical signals or light pulses. A scintillation counter uses a scintillating material that emits small flashes of light when struck by gamma rays. These flashes are collected and converted into electrical pulses, allowing sensitive measurement of low levels of gamma radiation. A microscope is used for magnifying small objects, not for detecting radiation. A lead plate can block or shield radiation but does not measure it. A spectrophotometer measures light absorption in chemical solutions, and a thermometer records temperature. None of these substitute for a dedicated gamma radiation detector like a scintillation counter. Step-by-Step Solution: Step 1: Identify the type of radiation involved, which is gamma radiation from a gamma emitting isotope.Step 2: Recall that radiation detectors such as Geiger Muller counters and scintillation counters are used to detect ionising radiation.Step 3: Recognise that a scintillation counter is specifically designed to detect gamma rays by converting their energy into light and then into electrical signals.Step 4: Evaluate the other devices. A microscope provides visual magnification; it cannot detect radiation.Step 5: A lead plate can block gamma rays but cannot measure their intensity or confirm their presence in water.Step 6: A spectrophotometer measures optical absorbance and is not sensitive to gamma rays.Step 7: A thermometer records temperature and has no relation to radiation detection.Step 8: Conclude that the scintillation counter is the appropriate instrument. Verification / Alternative check: Practical radiation surveys of water samples often use scintillation counting, especially liquid scintillation counting when beta emitters are involved and gamma spectroscopy when gamma emitters are present. In training material for radiological protection, scintillation counters are mentioned as sensitive detectors for gamma rays and sometimes for alpha and beta particles with suitable scintillators. This strongly supports their role as the device of choice in the scenario described. Why Other Options Are Wrong: Option a, microscope, can reveal microscopic organisms or suspended particles but not gamma rays. Option c, lead plate, is useful as shielding, not measuring equipment. Option d, spectrophotometer, is used to measure light absorption at specific wavelengths for chemical analysis, not for ionising radiation. Option e, thermometer, measures temperature and does not provide any information about radioactivity. Common Pitfalls: One mistake is to think that any scientific instrument could somehow detect all phenomena. Another is to confuse optical spectroscopy with nuclear detection. Students may also focus on the word spectrum in spectrophotometer and assume it is equivalent to gamma spectroscopy, which actually uses different detectors. To avoid these errors, remember that gamma rays require specialised radiation detectors such as scintillation counters or Geiger Muller counters, not ordinary optical or mechanical instruments. Final Answer: To confirm whether drinking water contains a gamma emitting isotope, the most suitable instrument is a scintillation counter.

Question 2

At standard atmospheric pressure (about 760 mmHg), what is the freezing point of pure water when expressed on the Kelvin scale?

Accepted Answer

Introduction / Context: Temperature scales such as Celsius and Kelvin are used throughout physics and chemistry. Knowing key reference points, such as the freezing and boiling points of water, on both scales is essential for converting temperatures and understanding thermodynamic data. This question asks specifically for the freezing point of water at standard atmospheric pressure when expressed in Kelvin. Given Data / Assumptions: Standard atmospheric pressure is approximately 760 mmHg (1 atm). Pure water freezes at 0 degrees Celsius at this pressure. The Kelvin scale starts at absolute zero, with 0°C corresponding to 273 K approximately. The question requires the freezing point in Kelvin, not in degrees Celsius. Concept / Approach: The Celsius and Kelvin scales are related by a simple offset. The relationship is T(K) = T(°C) + 273 (more precisely, 273.15). The freezing point of water at 1 atm is defined as 0°C on the Celsius scale. Adding 273 gives the equivalent temperature in Kelvin. Therefore, the freezing point is about 273 K. In most school level chemistry, 273 K is accepted as the reference value for the freezing point of water. Step-by-Step Solution: Step 1: Recall the standard freezing point of pure water: 0°C at 1 atm (760 mmHg).Step 2: Use the conversion formula between Celsius and Kelvin: T(K) = T(°C) + 273.Step 3: Substitute T(°C) = 0 into the formula.Step 4: T(K) = 0 + 273 = 273 K.Step 5: Check the options and identify 273 K as the correct value. Verification / Alternative check: Similarly, the boiling point of water at 1 atm is 100°C, which corresponds to about 373 K. The difference between freezing and boiling points is 100 degrees on both scales, because Kelvin and Celsius have the same step size, only a different zero point. This consistency further confirms that 0°C must correspond to 273 K and not to any of the other values given in the options. Why Other Options Are Wrong: Option a, 0 K, is absolute zero, the lowest possible temperature, not the freezing point of water. Option c, 273 C, confuses the scale and implies an extremely high temperature, hotter than boiling water. Option d, minus 273 C, is extremely cold and corresponds roughly to 0 K, again not the freezing point. Option e, 100 K, is much colder than water's freezing point and would correspond to about minus 173°C, which is not correct for the freezing of water at standard pressure. Common Pitfalls: Students sometimes forget whether they should add or subtract 273 when converting between Celsius and Kelvin. Another mistake is to think that 0°C equals 0 K, which is incorrect because the Kelvin scale is shifted downwards by 273 degrees. To avoid these mistakes, remember that Kelvin is always greater than Celsius by about 273 and that absolute zero is at 0 K, which corresponds to about minus 273°C. Final Answer: At 760 mmHg, the freezing point of pure water on the Kelvin scale is 273 K.

Question 3

When two neutral atoms form a chemical bond to create a molecule, what actually happens at the level of electrons?

Accepted Answer

Introduction / Context: Chemical bonding explains how atoms join together to form molecules and compounds. Whether the bond is ionic, covalent or metallic, it is always related to the behaviour of electrons, especially the outermost valence electrons. Understanding this helps explain why some elements form salts, others form molecules and how stable structures like water, sodium chloride and methane arise. This question asks what fundamentally happens when two atoms form a chemical bond. Given Data / Assumptions: We are considering typical chemical bonds between atoms, not nuclear reactions. The atoms are neutral before bonding. Options mention fusion, electron sharing or transfer, proton transfer and attraction of like charges. We assume basic knowledge of ionic and covalent bonding models. Concept / Approach: In ionic bonding, one atom transfers one or more valence electrons to another, forming positive and negative ions that attract each other. In covalent bonding, atoms share pairs of electrons to fill their outer shells. In both cases, it is the electrons that move or are shared, not protons. Protons are located in the nucleus and do not normally move from one atom to another in ordinary chemical reactions. Fusion or changing the nucleus belongs to nuclear reactions, not chemical bonding. Like charges repel, not attract, so bonds cannot be formed by attraction of like charges. Step-by-Step Solution: Step 1: Recall that valence electrons in the outermost shell determine bonding behaviour.Step 2: In an ionic bond, such as in sodium chloride, a sodium atom loses an electron to a chlorine atom, forming Na+ and Cl−, which then attract each other.Step 3: In a covalent bond, such as in water or methane, atoms share electron pairs so that each achieves a stable configuration similar to a noble gas.Step 4: In both cases, electrons are either transferred or shared; protons remain in the nuclei and do not move between atoms.Step 5: Consider the options and choose the one that mentions sharing or transfer of electrons for stability.Step 6: Option b correctly states that atoms share electrons or transfer electrons to achieve more stable configurations. Verification / Alternative check: You can test this by thinking of simple Lewis dot diagrams. When you draw NaCl, you show one electron moving from Na to Cl. When you draw H2, O2 or H2O, you show pairs of dots representing shared electrons between atoms. There is never a diagram where protons are moved or shared, and no diagram shows atoms physically fusing into one larger atom during ordinary chemical bonding. These representations match the idea of electron transfer or sharing as the core of bonding. Why Other Options Are Wrong: Option a describes nuclear fusion, which changes the identity of elements and requires extremely high energies, not ordinary chemical bonding. Option c incorrectly mentions transfer or sharing of protons, which does not occur in typical chemical reactions. Option d speaks of like charges attracting, which contradicts basic electrostatics where like charges repel. Option e suggests atoms lose all their electrons and become bare nuclei, which does not happen under normal chemical conditions and would not lead to stable molecules. Common Pitfalls: Students sometimes confuse chemical bonding with nuclear reactions because both are studied in similar chapters. Another pitfall is to focus only on one type of bond, such as covalent, and forget that ionic bonds involve electron transfer, not sharing. To avoid confusion, always remember that chemical bonds involve valence electrons moving or being shared to allow atoms to achieve more stable electron configurations. Final Answer: When two atoms form a chemical bond, they share electrons or transfer electrons in order to reach more stable electron configurations.

Question 4

In basic chemistry, which term refers to a pure substance that cannot be broken down into simpler substances by ordinary chemical reactions?

Accepted Answer

Introduction / Context: Classifying matter into elements, compounds, molecules and mixtures is one of the first tasks in chemistry. An element is a fundamental chemical substance, while compounds and mixtures are formed by combining elements in different ways. This question asks which term describes the simplest form of matter in the sense that it cannot be decomposed into simpler substances by ordinary chemical means. Given Data / Assumptions: We are comparing the meanings of element, molecule, atom, compound and mixture. Ordinary chemical reactions are considered, not nuclear reactions. We look for the term that defines a basic chemical substance. Atoms are recognised as the smallest units of elements that retain their properties. Concept / Approach: An element is a pure substance composed of only one type of atom. It cannot be chemically decomposed into simpler substances. A compound consists of two or more different elements chemically combined in a fixed ratio. A molecule is a group of atoms bonded together and may belong to an element, such as O2, or a compound, such as H2O. An atom is the smallest unit of an element that retains its chemical identity, but the term that describes the pure substance itself is element. A mixture is a physical combination of substances that can be separated by physical methods. Step-by-Step Solution: Step 1: Consider the definition of an element: a pure substance consisting of only one kind of atom and which cannot be further chemically broken down.Step 2: Recognise that compounds like water can be decomposed into hydrogen and oxygen by chemical methods such as electrolysis.Step 3: Understand that a molecule is a structural unit and can refer to molecules of elements (for example O2) or compounds (for example CO2), so it is not the category of simplest substance.Step 4: Note that an atom is a single particle of an element, but the question asks for the simplest form of matter as a pure substance, which is the element itself.Step 5: Mixtures can always be separated by physical or simple chemical processes into their components.Step 6: Conclude that element is the correct term for a pure, chemically indivisible substance. Verification / Alternative check: In the periodic table, each entry corresponds to an element such as hydrogen, oxygen or iron. Textbooks define an element as the simplest chemical substance that cannot be decomposed by ordinary chemical means. They then define compounds as substances formed when elements chemically combine. This hierarchy confirms that element is the correct answer for the simplest form of matter as a chemical substance. Why Other Options Are Wrong: Option b, molecule, is a structural unit and not always a basic substance; for example, a water molecule contains two elements. Option c, atom, refers to the smallest particle of an element but the category of pure substance is the element. Option d, compound, can be chemically broken down into elements. Option e, mixture, can be separated by physical means and is clearly not the simplest form of matter. Common Pitfalls: Students sometimes confuse atom and element, using the terms as if they were identical. While closely related, an element is a macroscopic substance made of many atoms of the same type, whereas an atom is an individual particle. Exam questions that ask about pure substances that cannot be decomposed usually expect the term element, not atom. Keeping this distinction clear helps avoid wrong choices in classification questions. Final Answer: The simplest form of matter that cannot be decomposed by ordinary chemical reactions is an element.

Question 5

When milk turns sour due to the action of bacteria, which organic acid is mainly produced and responsible for the sour taste?

Accepted Answer

Introduction / Context: Food chemistry examines how chemical changes affect taste, texture and safety of food. When milk is left at room temperature, bacteria convert some of the lactose sugar into an organic acid that makes the milk sour. Recognising which acid is produced helps connect everyday observations with basic biochemical processes and is a common question in general knowledge and chemistry exams. Given Data / Assumptions: Milk contains lactose, a milk sugar, and proteins such as casein. When milk sours, lactic acid bacteria grow and ferment the lactose. The options include lactic acid, acetic acid, tartaric acid, butyric acid and citric acid. We assume ordinary souring of milk due to natural fermentation, not contamination by unusual organisms. Concept / Approach: Lactic acid bacteria, such as Lactobacillus species, convert lactose into lactic acid in anaerobic conditions. Lactic acid reduces the pH of milk and causes the proteins to coagulate, forming curd. Acetic acid is associated with vinegar, tartaric acid is found in grapes and tamarind, citric acid in citrus fruits, and butyric acid in rancid butter. Therefore, lactic acid is the characteristic acid produced when milk turns sour through normal bacterial fermentation. Step-by-Step Solution: Step 1: Recall that lactose is the main sugar present in milk.Step 2: Understand that lactic acid bacteria ferment lactose under suitable conditions, such as warmth and time.Step 3: During fermentation, lactose is converted to lactic acid, which lowers the pH of milk.Step 4: The drop in pH causes milk proteins to coagulate, leading to curd formation and a sour taste.Step 5: Compare with acetic acid, which gives the sharp taste of vinegar, not the typical sourness of curdled milk.Step 6: Note that tartaric and citric acids are associated with fruits, not with milk fermentation.Step 7: Recognise that butyric acid contributes to the smell of rancid butter, which is different from ordinary sour milk.Step 8: Conclude that lactic acid is the main acid produced when milk becomes sour. Verification / Alternative check: Dairy technology and microbiology texts consistently state that yoghurt and curd are produced by lactic acid fermentation. The bacteria used in starter cultures are often named lactic acid bacteria because lactic acid is their main metabolic product when fermenting sugars. This strong association between milk and lactic acid further confirms that lactic acid is the correct answer. Why Other Options Are Wrong: Option b, acetic acid, is typical of vinegar from fermented alcohol, not fresh sour milk. Option c, tartaric acid, is common in grapes and tamarind. Option d, butyric acid, is present in rancid butter and has a strong unpleasant odour but is not the main cause of ordinary milk souring. Option e, citric acid, is dominant in lemons and other citrus fruits, not produced in significant amounts during milk fermentation. Common Pitfalls: Students may mix up acids associated with different sour tastes, thinking that all sourness must come from the same acid. It helps to link each acid with its typical source: lactic with milk, acetic with vinegar, tartaric with tamarind or grapes and citric with citrus fruits. Remembering these associations can quickly guide you to the correct answer in food chemistry questions. Final Answer: When milk turns sour, the main acid produced by bacterial fermentation is lactic acid.

Question 6

Each water molecule can form hydrogen bonds with neighbouring water molecules. What is the maximum number of hydrogen bonds that a single H2O molecule can form?

Accepted Answer

Introduction / Context: Hydrogen bonding in water explains many of its unusual properties, such as high boiling point, high surface tension and the open structure of ice. In a three dimensional network, each water molecule can both donate and accept hydrogen bonds. This question asks for the maximum number of hydrogen bonds a single water molecule can form at one time with its neighbours. Given Data / Assumptions: A water molecule has the formula H2O. Oxygen has two lone pairs of electrons in water. Each hydrogen attached to oxygen can participate in hydrogen bonding. We assume an ideal hydrogen bonding network such as in ice. Concept / Approach: Hydrogen bonding requires a hydrogen atom covalently bonded to a highly electronegative atom (such as oxygen, nitrogen or fluorine) and a lone pair on another such electronegative atom. In water, each molecule has two hydrogen atoms that can donate hydrogen bonds and two lone pairs on oxygen that can accept hydrogen bonds. Therefore, a single water molecule can form up to two hydrogen bonds as a donor and two as an acceptor, giving a maximum of four hydrogen bonds in a tetrahedral arrangement. Step-by-Step Solution: Step 1: Draw or imagine the Lewis structure of water, with oxygen in the centre, two O–H bonds and two lone pairs on oxygen.Step 2: Each O–H bond can donate a hydrogen bond to the lone pair of an oxygen atom on a neighbouring water molecule.Step 3: Each lone pair on the central oxygen can accept a hydrogen bond from a hydrogen atom on neighbouring water molecules.Step 4: There are two hydrogens and two lone pairs, so water can donate two and accept two hydrogen bonds.Step 5: Add these to find the maximum total: 2 donor bonds + 2 acceptor bonds = 4 hydrogen bonds.Step 6: This matches the tetrahedral coordination seen in ice, where each water molecule is surrounded by four neighbours. Verification / Alternative check: In solid ice, crystallographic studies show that each water molecule is tetrahedrally coordinated to four others via hydrogen bonds. In liquid water, the network is more dynamic but still tends to approach four hydrogen bonded neighbours on average. Textbook diagrams of hydrogen bonding in water emphasise this fourfold pattern, reinforcing the conclusion that the maximum number is four. Why Other Options Are Wrong: Option a, one hydrogen bond, ignores the fact that water has two hydrogens and two lone pairs. Option b, three hydrogen bonds, underestimates the capacity of the two lone pairs and two hydrogens. Option d, two hydrogen bonds, would be correct for molecules that can act only as donors or only as acceptors, but water can do both. Option e, five, exceeds the number of hydrogen bond donors and acceptors available on a single water molecule. Common Pitfalls: A frequent misunderstanding is to think that only the hydrogens can form hydrogen bonds and forget the role of lone pairs, leading to the answer two. Another pitfall is to count all possible near neighbours in the liquid rather than the maximum bonding sites on a single molecule. To avoid these mistakes, focus on the structure of water: two O–H bonds and two lone pairs translate directly into four possible hydrogen bonds. Final Answer: The maximum number of hydrogen bonds that one water molecule can form is 4, two as a donor and two as an acceptor.

Question 7

Acid precipitation (acid rain) in the environment is mainly caused by a combination of which gaseous pollutants released into the atmosphere?

Accepted Answer

Introduction / Context: Acid precipitation, commonly called acid rain, is an important environmental issue. It damages forests, lakes, buildings and cultural monuments. Acid rain forms when certain gases in the atmosphere react with water vapour to produce acidic solutions that fall as rain, snow or fog. Understanding which gases are mainly responsible helps connect industrial emissions and vehicle exhausts with environmental impacts. Given Data / Assumptions: The phenomenon is acid precipitation with pH values lower than normal rain. Options list various combinations of gases and environmental components. We assume typical industrial and urban pollution sources. We consider gases that form strong acids when dissolved in atmospheric moisture. Concept / Approach: The major contributors to acid rain are sulfur dioxide (SO2) and nitrogen oxides (NO and NO2). These gases react with oxygen and water in the atmosphere to form sulfuric and nitric acids. Carbon dioxide dissolves in water to form weak carbonic acid, which contributes slightly to natural acidity of rain, though SO2 and nitrogen oxides are the dominant anthropogenic causes. Other components such as dust, saline or helium do not play the major acid forming role in this context. Therefore, the best answer is the combination that includes sulfur dioxide, carbon dioxide and nitrogen oxides. Step-by-Step Solution: Step 1: Identify sulfur dioxide as a major industrial pollutant released from burning coal and oil containing sulfur.Step 2: Recognise that nitrogen oxides are produced in vehicle exhaust and power stations due to high temperature combustion.Step 3: In the atmosphere, SO2 can be oxidised to SO3 and then react with water to form sulfuric acid, H2SO4.Step 4: Nitrogen oxides can be converted to nitric acid, HNO3, via a series of reactions with oxygen and water.Step 5: Carbon dioxide dissolves in rainwater to form carbonic acid, H2CO3, which contributes to natural mild acidity.Step 6: Together, these acids lower the pH of precipitation and cause acid rain.Step 7: Compare with the options and see that only option d explicitly mentions sulfur dioxide, carbon dioxide and nitrogen oxides. Verification / Alternative check: Environmental science sources explain that unpolluted rain has a pH of around 5.6 due largely to carbonic acid from dissolved carbon dioxide. In polluted regions, pH can fall to 4 or lower because of additional strong acids formed from SO2 and NOx. This supports the idea that sulfur dioxide and nitrogen oxides, with some contribution from carbon dioxide, are the key gases involved in acid precipitation. Why Other Options Are Wrong: Option a mentions hydrogen, oxygen and saline, which are not the main acid forming pollutants. Option b describes general components like water, soil and air but not specific acid forming gases. Option c confuses respiration and seawater, which do not directly cause abnormal acid rain in the way industrial emissions do. Option e includes methane, ozone and helium, which are important in other aspects of atmospheric chemistry but are not the primary causes of acid precipitation. Common Pitfalls: Students sometimes believe that any gas released by human activity must contribute equally to acid rain or that carbon dioxide alone is responsible. While carbon dioxide affects climate, acid precipitation is primarily driven by sulfur and nitrogen oxides. Another pitfall is to overlook nitrogen oxides and focus only on sulfur emissions. A clear mental association between acid rain and SO2 plus NOx helps avoid these mistakes. Final Answer: Acid precipitation is mainly caused by a mix of sulfur dioxide, carbon dioxide and nitrogen oxides released into the atmosphere.

Question 8

Among the noble (rare) gases present in Earth's atmosphere, which one is the most abundant by volume?

Accepted Answer

Introduction / Context: The noble gases, sometimes called rare gases, include helium, neon, argon, krypton and xenon. Although they are chemically inert, some of them are present in significant amounts in the atmosphere. Knowing which noble gas is most abundant in the air is a common general knowledge and chemistry question that connects the periodic table to the composition of the atmosphere we breathe. Given Data / Assumptions: The noble gases considered are xenon, helium, neon, argon and krypton. We are talking about Earth's atmosphere at ground level. Abundance refers to volume percentage in air, not abundance in the universe. We assume standard atmospheric composition. Concept / Approach: In Earth's atmosphere, nitrogen and oxygen are the major components, followed by argon at about 0.93 percent by volume. The concentrations of other noble gases are much smaller: neon, helium, krypton and xenon exist only in trace amounts. Although helium is more abundant in the universe as a whole, in Earth's atmosphere argon dominates among noble gases. Therefore, argon is the most abundant rare gas in the air around us. Step-by-Step Solution: Step 1: Recall the approximate composition of dry air: about 78 percent nitrogen, 21 percent oxygen and around 1 percent other gases.Step 2: Among these other gases, argon makes up nearly 0.93 percent by volume.Step 3: Helium and neon are present only at parts per million levels, which is far less than argon.Step 4: Krypton and xenon are even rarer, present only in tiny trace amounts.Step 5: Therefore, when comparing noble gases in the atmosphere, argon is clearly the most abundant.Step 6: Match this conclusion with the options and select argon (Ar). Verification / Alternative check: Atmospheric composition tables from reference books list argon as the third most abundant gas in dry air, after nitrogen and oxygen. Helium, neon, krypton and xenon are listed below with much smaller volume percentages. While helium is very abundant in stars, that is a different context from the air we breathe. This confirms that argon is the correct answer for Earth's atmosphere. Why Other Options Are Wrong: Option a, xenon, is extremely rare in the atmosphere and is present only in trace amounts. Option b, helium, escapes Earth's gravity over time and is present at much lower levels than argon. Option c, neon, is also a trace gas. Option e, krypton, is even less abundant than neon. None of these noble gases approach the nearly 1 percent volume concentration that argon has in air. Common Pitfalls: Students may confuse cosmic or stellar abundance with atmospheric abundance and choose helium because it is abundant in the universe. Another pitfall is to remember the word rare gas and assume that all noble gases must be equally rare. In reality, argon is not rare in our atmosphere, even though it is chemically inert. Keeping the distinction between different contexts of abundance helps avoid these errors. Final Answer: The most abundant noble gas in Earth's atmosphere by volume is argon (Ar).

Question 9

Trichloroacetaldehyde (chloral, CCl3CHO) is subjected to the Cannizzaro reaction with aqueous NaOH. The product mixture contains sodium trichloroacetate (CCl3COONa) and which other organic compound?

Accepted Answer

Introduction / Context: The Cannizzaro reaction is a well known organic redox reaction in which a non enolisable aldehyde disproportionates in the presence of a strong base. One molecule of the aldehyde is oxidised to a carboxylate, while another is reduced to an alcohol. Trichloroacetaldehyde, also called chloral, undergoes this reaction. Understanding which products are formed tests your knowledge of reaction mechanism and the patterns of oxidation and reduction in carbonyl chemistry. Given Data / Assumptions: Substrate: trichloroacetaldehyde, CCl3CHO (chloral). Reagent: aqueous sodium hydroxide, NaOH. The reaction is stated to be a Cannizzaro reaction. One product is sodium trichloroacetate (CCl3COONa). We must identify the other organic product from the options. Concept / Approach: In a Cannizzaro reaction, two molecules of an aldehyde without alpha hydrogen atoms react in base. One molecule is oxidised to a carboxylate salt, while the other is reduced to a primary alcohol. For trichloroacetaldehyde, the oxidation product is trichloroacetic acid salt, CCl3COONa. The corresponding reduction product is the alcohol where the aldehyde group CHO becomes CH2OH, giving 2, 2, 2 trichloroethanol, CCl3CH2OH. Other options such as chloroform or trichloroethanol with incorrect carbon chain length do not match the Cannizzaro pattern. Step-by-Step Solution: Step 1: Write the structure of trichloroacetaldehyde: CCl3CHO, where the aldehyde carbon is attached to a trichloromethyl group.Step 2: In the Cannizzaro reaction, one molecule of the aldehyde is oxidised to the carboxylate: CCl3COO−, giving sodium trichloroacetate with Na+.Step 3: The other molecule is reduced, where the aldehyde CHO group is converted to a primary alcohol CH2OH.Step 4: The reduction product therefore has the formula CCl3CH2OH, which is known as 2, 2, 2 trichloroethanol.Step 5: Check the options and locate 2, 2, 2 trichloroethanol as option b.Step 6: Verify that chloroform (CHCl3) or trichloromethanol do not correspond to a simple reduction of the aldehyde group of chloral. Verification / Alternative check: You can think of the Cannizzaro reaction in terms of oxidation state changes at the carbonyl carbon. In the aldehyde, the carbonyl carbon has a certain oxidation level. Oxidation to the carboxylate increases the oxidation state, while reduction to an alcohol decreases it. Structural logic then dictates that the carbon chain length remains the same. Since the aldehyde carbon in trichloroacetaldehyde is part of a two carbon skeleton (CCl3CH=O), the alcohol must also have two carbons, which fits 2, 2, 2 trichloroethanol, not a one carbon compound like trichloromethanol or chloroform. Why Other Options Are Wrong: Option a, chloroform, CHCl3, lacks the hydroxyl group and does not come from direct reduction of the aldehyde function. Option c, trichloromethanol, would have only one carbon and is not the straightforward reduction product of a two carbon aldehyde. Option d, 2, 2, 2 trichloropropanol, would have three carbons and also does not match the starting skeleton. Option e, ethanol, lacks the trichloromethyl group and is unrelated to the chlorinated aldehyde starting material. Common Pitfalls: Students sometimes forget that in the Cannizzaro reaction both oxidation and reduction occur on the same aldehyde skeleton, so the carbon chain length of the alcohol and carboxylate products must match that of the starting aldehyde. Another pitfall is to confuse the Cannizzaro reaction with haloform reactions, which do produce chloroform from certain carbonyl compounds. Keeping these reaction types separate and focusing on carbon skeleton conservation in Cannizzaro reactions helps avoid confusion. Final Answer: Along with sodium trichloroacetate, the Cannizzaro reaction of trichloroacetaldehyde with NaOH produces 2, 2, 2 trichloroethanol as the other organic product.

Question 10

The tragic "Minamata disease" outbreak in Japan was caused by environmental contamination with which toxic heavy metal compound?

Accepted Answer

Introduction / Context: Minamata disease is a famous example of industrial pollution leading to severe human health consequences. It occurred in Minamata Bay, Japan, where local residents suffered from neurological symptoms after consuming contaminated seafood. Understanding which metal caused this disaster highlights the dangers of heavy metal pollution and the need for strict environmental controls. Given Data / Assumptions: The disease in question is Minamata disease. It was associated with industrial pollution of coastal waters. Residents were exposed through seafood consumption. Options list heavy metals such as lead, mercury, cadmium, zinc and arsenic. Concept / Approach: Minamata disease was caused by methylmercury compounds discharged into the bay by a chemical factory. Microorganisms converted inorganic mercury into methylmercury, an organic form that bioaccumulates and biomagnifies in aquatic food chains. People who ate fish and shellfish from the bay accumulated high levels of methylmercury, leading to severe neurological damage. Other heavy metals like lead, cadmium, zinc and arsenic can cause different syndromes but are not responsible for Minamata disease. Step-by-Step Solution: Step 1: Recall that Minamata disease is specifically associated with mercury pollution in Minamata Bay.Step 2: The discharge came from a factory that produced acetaldehyde using mercury as a catalyst.Step 3: The mercury released to the environment was converted to methylmercury by microorganisms.Step 4: Methylmercury accumulated in fish and shellfish, which were part of the local diet.Step 5: Consumption of these contaminated foods led to symptoms including numbness, tremors, vision and hearing problems and, in severe cases, paralysis and death.Step 6: Among the options, only mercury fits this well documented history. Verification / Alternative check: Environmental case studies and public health histories consistently describe Minamata disease as methylmercury poisoning. The name has become a symbol of mercury related environmental disease. Lead poisoning leads to conditions like lead colic and encephalopathy, cadmium to itai itai disease, and arsenic to various skin and systemic disorders, but not to Minamata disease. These cross references confirm mercury as the correct answer. Why Other Options Are Wrong: Option a, lead, is associated with neurological and developmental problems, especially in children, but the famous Japanese pollution case involving lead is different. Option c, cadmium, is linked to itai itai disease in Japan, a separate environmental disaster. Option d, zinc, is an essential trace element and rarely causes such dramatic toxicity in environmental settings. Option e, arsenic, causes skin lesions and cancers, particularly in groundwater contamination cases, but is not the cause of Minamata disease. Common Pitfalls: Students may confuse different named diseases related to heavy metal pollution, such as Minamata disease (mercury) and itai itai disease (cadmium). Another pitfall is to focus only on the fact that heavy metals are toxic without remembering which specific metal corresponds to each historical case. Associating Minamata disease with methylmercury and itai itai disease with cadmium helps keep these topics clear. Final Answer: Minamata disease was caused by environmental contamination with mercury compounds, specifically methylmercury in seafood.

Question 11

Which of the following pairs of atoms is most likely to form a polar covalent bond due to a moderate difference in electronegativity while both remain non metals?

Accepted Answer

Introduction / Context: Chemical bonds can be broadly classified as ionic, covalent or metallic, with covalent bonds further divided into polar and non polar types. Polarity depends on the difference in electronegativity between bonded atoms. Understanding which combinations of atoms are likely to form polar covalent bonds is essential for predicting molecular properties such as solubility and dipole moments. This question asks you to select the pair that forms a polar covalent bond. Given Data / Assumptions: Options include H with Br, H with H, Na with Br, N with N and Na with Cl. We assume Pauling scale electronegativity trends. Elements on the left of the periodic table like sodium tend to form ionic bonds with halogens. Pairs of identical non metals usually form non polar covalent bonds. Concept / Approach: A polar covalent bond arises when two non metals share electrons but one is more electronegative, pulling the shared electron pair closer and creating partial charges. If the electronegativity difference is very large and one atom is a metal while the other is a non metal, the bond tends to be ionic. If there is no difference, as in identical atoms, the bond is non polar covalent. Hydrogen and bromine are both non metals with a moderate electronegativity difference, so the H–Br bond is polar covalent. Sodium with bromine or chlorine involves a metal and a non metal and leads to ionic bonds. Hydrogen with hydrogen and nitrogen with nitrogen form non polar covalent bonds because the atoms are identical. Step-by-Step Solution: Step 1: Identify which pairs involve two non metals. H with Br, H with H and N with N are non metal pairs, while Na with Br and Na with Cl involve a metal and a non metal.Step 2: Recognise that metal non metal pairs like NaBr and NaCl are typically ionic, not covalent, due to large electronegativity differences.Step 3: Consider H–H and N–N bonds. In each case, the two atoms are identical, so electronegativity difference is zero.Step 4: Zero electronegativity difference leads to non polar covalent bonds.Step 5: Consider H–Br. Both are non metals, but bromine is significantly more electronegative than hydrogen.Step 6: The electronegativity difference is moderate, so the shared pair is pulled closer to bromine, giving partial charges and a polar covalent bond.Step 7: Therefore, the best example of a polar covalent bond among the options is H and Br. Verification / Alternative check: Bond classification guidelines often state that an electronegativity difference between about 0.4 and 1.7 typically results in a polar covalent bond, while values above this range often correspond to ionic bonds and values near zero to non polar covalent bonds. The H–Br electronegativity difference falls into the polar covalent range. For Na–Cl or Na–Br, the difference is much larger, which is characteristic of ionic bonds in salts like sodium chloride and sodium bromide. Why Other Options Are Wrong: Option b, H and H, consists of identical atoms, so the H–H bond is non polar covalent. Option c, Na and Br, tends to form NaBr, an ionic compound. Option d, N and N, gives an N≡N bond in nitrogen gas, which is non polar covalent. Option e, Na and Cl, forms ionic NaCl, not a polar covalent molecule. Common Pitfalls: Students sometimes assume that any difference in electronegativity, even between metal and non metal, means polar covalent, forgetting that very large differences lead to ionic bonds. Others forget that identical atoms must share electrons equally, giving non polar bonds. To avoid these mistakes, remember the rough rule of thumb: identical atoms give non polar covalent bonds, moderate differences between non metals give polar covalent bonds, and large differences between metals and non metals give ionic bonds. Final Answer: The pair most likely to form a polar covalent bond is H and Br, which form a polar H–Br bond.

Question 12

In solution chemistry, the symbol "M" written after a concentration value (for example, 0.10 M HCl) most commonly stands for which unit of concentration?

Accepted Answer

Introduction / Context: In solution chemistry, different units describe how much solute is present in a given amount of solvent or solution. Laboratory bottles, textbooks and exam questions often use symbols such as M, m and N to denote different concentration units. Knowing what each symbol stands for is essential for correctly interpreting data and performing stoichiometric calculations involving solutions. This question focuses on the meaning of the capital letter M. Given Data / Assumptions: The symbol M appears after numerical values, such as 0.10 M HCl. We are asked what unit of concentration M represents. Options mention molarity, molality, normality, mole fraction and mass percent. We assume standard usage in introductory chemistry courses. Concept / Approach: Molarity is defined as moles of solute per litre of solution. It is denoted by the symbol M, so 1 M means 1 mole per litre. Molality is moles of solute per kilogram of solvent and is often denoted by a lower case m. Normality uses N and refers to equivalents per litre. Mole fraction is a ratio without units, and mass percent is expressed as a percentage. Therefore, when you see a capital M following a number and a formula, it indicates molarity. Step-by-Step Solution: Step 1: Recall that a 1 M solution contains 1 mole of solute in each litre of final solution.Step 2: Note that chemists write this as 1 M, pronounced "one molar".Step 3: Recognise that the symbol for molality is lower case m, not capital M.Step 4: Understand that normality uses N, not M.Step 5: Mole fraction is typically represented by the symbol x and has no units.Step 6: Mass percent is given as a percentage and does not use the letter M as a unit symbol immediately after a number.Step 7: Therefore, M stands for molarity, moles of solute per litre of solution. Verification / Alternative check: Look at labels on standard laboratory solutions, such as 0.1 M NaOH. Manuals and calculations that use these solutions explicitly state that molarity equals moles per litre. Problems involving dilution also use the equation M1V1 = M2V2, where M represents molarity. These repeated uses in curriculum and laboratory practice confirm that M is the symbol for molarity. Why Other Options Are Wrong: Option b, molality, uses a different symbol and depends on mass of solvent, not on solution volume. Option c, normality, refers to equivalents per litre and uses N. Option d, mole fraction, is a ratio and not typically labelled with M. Option e, mass percent, is expressed in percent and does not use M as a symbol for the concentration unit. Common Pitfalls: A common confusion is between molarity and molality because their names sound similar. Another pitfall is misreading a lowercase m as M or vice versa in handwritten notes. To avoid mistakes, remember that molarity uses the capital letter M and involves volume of solution, while molality uses lowercase m and involves mass of solvent. Keeping these distinctions clear helps ensure accurate interpretation of solution data. Final Answer: In chemistry, the symbol "M" after a concentration value stands for molarity, meaning moles of solute per litre of solution.

Question 13

In the compound ammonia (NH3), what is the oxidation number of the nitrogen atom?

Accepted Answer

Introduction / Context: Assigning oxidation numbers is a basic skill in inorganic chemistry and redox reactions. Oxidation numbers help chemists keep track of electrons gained or lost in reactions and identify oxidising and reducing agents. Ammonia, NH3, is a familiar compound, and determining the oxidation number of nitrogen in NH3 is a standard example used in teaching the rules for oxidation states. Given Data / Assumptions: The compound is ammonia, NH3. We assume the usual convention that hydrogen has oxidation number +1 when bonded to non metals. Ammonia is a neutral molecule, so the sum of oxidation numbers must be zero. We denote the oxidation number of nitrogen by x. Concept / Approach: The general rule is that for a neutral molecule, the sum of oxidation numbers of all atoms equals zero. Hydrogen usually has oxidation number +1 when bonded to non metals and minus 1 when bonded to metals. In NH3, nitrogen is the central non metal and each hydrogen is attached to it. Thus, each hydrogen is assigned +1. If the oxidation number of nitrogen is x, then x plus three times 1 must equal zero because the molecule is neutral. Solving this simple equation gives the oxidation number of nitrogen. Step-by-Step Solution: Step 1: Write the formula for ammonia: NH3.Step 2: Assign the usual oxidation number for hydrogen in a compound with a non metal, which is +1.Step 3: Let the oxidation number of nitrogen be x.Step 4: Use the rule that the sum of oxidation numbers in a neutral molecule is zero.Step 5: Set up the equation: x + 3*(+1) = 0.Step 6: Simplify the equation to x + 3 = 0.Step 7: Solve for x: x = -3.Step 8: Conclude that the oxidation number of nitrogen in NH3 is -3. Verification / Alternative check: As an additional check, consider related nitrogen compounds. In ammonium ion, NH4+, nitrogen still has oxidation number -3 because 4*(+1) + x = +1 gives x = -3. In nitrate ion, NO3−, nitrogen has a positive oxidation state, showing that nitrogen can have a range of oxidation numbers from -3 to +5. This variation is consistent with its position in group 15 and supports the idea that -3 is the lowest common oxidation state for nitrogen in hydrides like ammonia. Why Other Options Are Wrong: Option a, -1, would give a total of -1 + 3 = +2, which does not sum to zero for a neutral molecule. Option b, +1, would give +1 + 3 = +4, again not zero. Option c, 0, would give 0 + 3 = +3, which is not correct. Option e, +3, would make the sum +3 + 3 = +6, far from zero. Only -3 makes the sum of oxidation numbers in NH3 equal to zero. Common Pitfalls: Students sometimes confuse formal charge with oxidation number or forget that hydrogen usually has +1 oxidation state when bonded to non metals. Another mistake is to think that nitrogen should always have a positive oxidation number because it is a non metal. In fact, nitrogen is more electronegative than hydrogen, so in NH3 it takes electrons and has a negative oxidation state. Carefully applying the sum of oxidation numbers rule avoids these errors. Final Answer: In ammonia, NH3, the oxidation number of nitrogen is -3.

Question 14

Among the following gemstones, which substance is a pure chemical element rather than a compound or mineral mixture?

Accepted Answer

Introduction / Context: This general chemistry and general knowledge question checks whether the learner can distinguish between a pure chemical element and a compound when examples are presented as familiar gemstones. Many gemstones that people see in jewellery are actually minerals, which are naturally occurring chemical compounds, while a few, such as diamond, are almost entirely one element. Understanding this idea builds intuition about how elements, compounds, and mixtures are related in everyday life. Given Data / Assumptions: - Four gemstones are listed as options: Topaz, Ruby, Diamond, and Sapphire. - The task is to identify which choice is made of a single chemical element throughout its crystal structure. - We assume normal natural samples, not artificial composites or impurity heavy stones. Concept / Approach: The key idea is that an element is a pure substance that contains only one type of atom, such as carbon or gold. A compound is made when atoms of different elements combine in fixed proportions, such as aluminium oxide or aluminium silicate. Many gemstones are crystalline forms of such compounds. Diamond is a crystalline form of carbon, so every lattice point is occupied by carbon atoms that are covalently bonded. In contrast, Topaz, Ruby, and Sapphire are different mineral compounds containing several elements. So we simply link each gemstone to its underlying chemical composition and pick the one that is a pure element. Step-by-Step Solution: Step 1: Recall that diamond is a well known crystalline form of the element carbon where each carbon atom is bonded tetrahedrally to four other carbon atoms in a giant covalent lattice. Step 2: Topaz is a silicate mineral. Its idealised composition can be written as a compound of aluminium, silicon, oxygen, and sometimes fluorine and hydroxyl groups, so it clearly contains more than one element. Step 3: Ruby is a red variety of the mineral corundum, which is mainly aluminium oxide with chromium impurities. Aluminium oxide is a compound that contains aluminium and oxygen atoms. Step 4: Sapphire is also a variety of corundum and is again primarily aluminium oxide with small amounts of other elements as impurities that give colour. Step 5: Only diamond among the given options has a crystal structure that consists of a single element, carbon, throughout the lattice. Verification / Alternative check: A quick way to verify the answer is to remember simple textbook examples. Diamond and graphite are always presented as allotropes of carbon, which is a classic element. In contrast, standard chemistry tables list Topaz, Ruby, and Sapphire under silicate or oxide minerals, not under elemental substances. Reference diagrams and gemstone guides also confirm that Topaz is a silicate mineral and Ruby and Sapphire are aluminium oxide varieties, while diamond is elemental carbon. Why Other Options Are Wrong: - Topaz: It is a mineral compound that contains aluminium, silicon, oxygen, and often fluorine or hydroxyl groups, so it is not a single element. - Ruby: It is made mostly of aluminium oxide with chromium impurities, therefore it is a compound and not a pure element. - Sapphire: It is another variety of aluminium oxide with different trace elements, again containing more than one element. Common Pitfalls: Learners sometimes assume that any shiny stone used in jewellery must be an element and may confuse metals like gold or platinum with transparent gemstones. Another common mistake is to think that because a stone looks uniform and pure to the eye, it must be chemically pure as an element. It is also easy to forget that minerals such as Topaz, Ruby, and Sapphire are detailed in mineralogy as compounds made from multiple elements. Careful reading of chemistry textbooks and mineral tables helps avoid such errors. Final Answer: Diamond is the only gemstone listed that is a pure chemical element in crystalline form, while the others are mineral compounds.

Question 15

In the structure of a typical nitrogenous base found in DNA or RNA, which one of the following elements is not a constituent of the base ring itself?

Accepted Answer

Introduction / Context: This biology and chemistry crossover question focuses on the basic composition of nitrogenous bases in nucleic acids. Nitrogenous bases are essential parts of DNA and RNA, and they have specific elements in their ring structures. Understanding which elements are present in bases, and which are present only in other parts of the nucleotide, helps learners clearly separate the concepts of base, sugar, and phosphate groups in nucleic acid structure. Given Data / Assumptions: - The question asks about elements in a typical nitrogenous base, such as adenine, guanine, cytosine, thymine, or uracil. - The options are Carbon, Oxygen, Phosphorus, and Nitrogen. - We assume standard biological molecules as found in DNA and RNA in living organisms. Concept / Approach: A nitrogenous base is an organic molecule that usually consists of one or two fused rings made from carbon and nitrogen atoms. Some bases also contain oxygen atoms as part of functional groups such as carbonyl or hydroxyl groups. However, the phosphate group that links nucleotides together is separate from the base and sugar and contains phosphorus and oxygen. Therefore, phosphorus is a key element of the phosphate backbone but is not part of the base ring itself. We simply recall the structures of purine and pyrimidine bases and identify which element is absent from those rings. Step-by-Step Solution: Step 1: Recognise that nitrogenous bases include adenine, guanine, cytosine, thymine, and uracil, which are either purines or pyrimidines. Step 2: In both purine and pyrimidine rings, the backbone of the ring is made from carbon atoms with nitrogen atoms incorporated at specific positions, so carbon and nitrogen are definitely present in bases. Step 3: Some bases, such as cytosine, thymine, and uracil, also contain oxygen atoms in their functional groups, so oxygen can occur within the base structures. Step 4: The phosphate group that links neighbouring nucleotides in DNA and RNA contains phosphorus, but that phosphate group is attached to the sugar and is not part of the nitrogenous base ring itself. Step 5: Hence phosphorus is not a constituent of the nitrogenous base, even though it is present elsewhere in the nucleotide structure. Verification / Alternative check: If you sketch or review standard textbook diagrams of nucleotides, you will always see three labelled parts: a nitrogenous base, a pentose sugar, and a phosphate group. The base ring structures show only carbon, nitrogen, hydrogen, and sometimes oxygen; there is no phosphorus within the base ring. The phosphate group, drawn as PO4, is where phosphorus appears. This visual confirmation from diagrams is a robust way to verify that phosphorus is not part of the base itself. Why Other Options Are Wrong: - Carbon: Carbon atoms form the main skeleton of the purine and pyrimidine rings, so carbon is an essential element in every nitrogenous base. - Oxygen: Several bases, such as cytosine, thymine, and uracil, contain oxygen in carbonyl or other functional groups, so oxygen can be part of a nitrogenous base structure. - Nitrogen: Nitrogen is fundamental to nitrogenous bases, giving them their name and forming part of the ring structure and hydrogen bonding sites. Common Pitfalls: A common confusion is between the nitrogenous base and the entire nucleotide. Many learners remember that DNA and RNA have phosphate groups and therefore think phosphorus must be in every part of the molecule. Another mistake is to assume that since phosphorus is in the backbone, it is also in the base. To avoid this, always remember that the base is a distinct part attached to the sugar, while the phosphate group is separate and contains the phosphorus. Final Answer: Phosphorus is not a constituent of the nitrogenous base ring and instead appears only in the phosphate group of the nucleotide.

Question 16

Which of the following gemstones is uniquely made of a single chemical element throughout its crystal lattice structure?

Accepted Answer

Introduction / Context: This question again links everyday gemstones with basic chemistry concepts about elements and compounds. Learners are asked to recognise which gemstone is built entirely from one element rather than from multiple elements in a compound. Such questions help bridge practical examples with the periodic table and improve the ability to classify substances correctly as elements, compounds, or mixtures. Given Data / Assumptions: - Four choices are presented: Platinum, Diamond, Emerald, and Ruby. - The task is to select the gemstone that is a crystal of a single element rather than a chemical compound. - We assume typical natural forms as used in jewellery and standard chemistry references. Concept / Approach: An element is a pure substance consisting of only one kind of atom. A gemstone may be an element, like diamond which is pure carbon, or a compound, like emerald and ruby, which are mineral compounds containing several different elements. Platinum is a metallic element used in jewellery, but it is usually considered as a metal rather than a transparent gemstone. Among the listed options, diamond is the classic and widely accepted example of a gemstone that is a pure element. The approach is to recall the composition of each material and then choose the one that is an elemental crystal. Step-by-Step Solution: Step 1: Consider platinum. Platinum is indeed a chemical element and a precious metal often used in rings and jewellery, but it is normally not classed as a gemstone, rather as a metal. Step 2: Diamond is a crystalline allotrope of carbon, in which carbon atoms are covalently bonded in a three dimensional network. Every lattice point hosts a carbon atom, so the crystal is a pure element. Step 3: Emerald is a variety of the mineral beryl, which is a compound containing beryllium, aluminium, silicon, and oxygen, with chromium or vanadium impurities giving the green colour. Step 4: Ruby is a red variety of corundum, which is aluminium oxide with chromium impurities, so it clearly contains more than one element. Step 5: Among the given options, diamond is both a gemstone and a pure element, so it is the correct choice. Verification / Alternative check: You can confirm this by consulting mineralogy or chemistry references. Diamond is always listed as an allotrope of carbon under elemental forms, while emerald and ruby appear as specific mineral compounds, not as elements. Platinum is listed as an element but appears under metals rather than gems. Therefore, if the question focuses on gemstones, diamond naturally stands out as the correct answer that fits both conditions. Why Other Options Are Wrong: - Platinum: It is a metallic element and a precious metal, but not a gemstone in the usual gemological sense used in such questions. - Emerald: It is a mineral compound of beryllium aluminium silicate with impurities and therefore contains several different elements. Why Other Options Are Wrong (continued): - Ruby: It is aluminium oxide with chromium impurities and is not a single element crystal. Common Pitfalls: Some learners may pick platinum simply because it is an element, forgetting that the question asks specifically for a gemstone. Others might assume that all sparkling stones are directly elements. Another pitfall is not recalling the mineral compositions of emerald and ruby, which are classic examples of compounds. Carefully distinguishing between metals and gemstones and between elemental solids and mineral compounds will help avoid these mistakes. Final Answer: Diamond is the only gemstone listed that is made from a single chemical element, carbon, throughout its crystal lattice.

Question 17

According to basic atomic theory in chemistry, which scientist's law or hypothesis most directly explains the law of conservation of mass in chemical reactions?

Accepted Answer

Introduction / Context: The purpose of this chemistry question is to connect well known scientific laws and hypotheses with the fundamental law of conservation of mass. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. Learners must know which theoretical framework provides a direct explanation of this law at the atomic level. Connecting named laws with core principles improves conceptual understanding and recall for examinations. Given Data / Assumptions: - Four options are given: Hund rule of maximum multiplicity, Dalton atomic theory, Avogadro law of gases, and Berzelius hypothesis of chemical combination. - The question asks which one directly explains why mass is conserved during chemical reactions. - We assume standard definitions taught in introductory chemistry courses. Concept / Approach: The law of conservation of mass can be explained by the idea that matter consists of small, indivisible particles called atoms that cannot be created or destroyed in ordinary chemical changes. Dalton atomic theory states that atoms are indivisible in chemical reactions and that a chemical reaction involves only rearrangement of atoms. If atoms are simply rearranged and not created or destroyed, the total mass remains constant. Hund rule deals with electron filling in orbitals, Avogadro law relates gas volume to number of molecules, and Berzelius hypothesis concerns electrochemical combinations. None of those is as directly tied to mass conservation as Dalton atomic theory. Step-by-Step Solution: Step 1: Recall the statement of the law of conservation of mass: in any ordinary chemical reaction, the total mass of reactants equals the total mass of products. Step 2: Review Dalton atomic theory. One of its key postulates is that atoms cannot be created or destroyed in chemical reactions, they are only rearranged to form new substances. Step 3: If atoms are never created or destroyed, then the total number of atoms of each element before and after a chemical reaction remains fixed, leading to no net gain or loss of mass. Step 4: Consider Hund rule, which describes how electrons occupy degenerate orbitals with parallel spins. This does not comment on macroscopic mass conservation. Step 5: Avogadro law states that equal volumes of gases under the same conditions of temperature and pressure contain equal numbers of molecules, which is important for gas stoichiometry but does not directly explain mass conservation. Step 6: Berzelius hypothesis addresses electrochemical aspects of compounds and valencies but again does not directly state why mass remains constant in reactions. Step 7: Therefore, Dalton atomic theory is the correct choice because it directly links the behaviour of atoms with conservation of mass. Verification / Alternative check: Textbooks often present the historical development of the law of conservation of mass together with Dalton atomic theory. The logic is that if atoms are hard, indivisible spheres in ordinary chemical changes, and reactions only rearrange which atoms are bonded together, then nothing truly disappears or appears from nothing. This naturally leads to balanced chemical equations that conserve both atoms and mass. This conceptual alignment confirms that Dalton atomic theory is the best theoretical explanation for the law of conservation of mass at the basic level. Why Other Options Are Wrong: - Hund rule of maximum multiplicity: This rule explains how electrons fill orbitals to maximise total spin, which is a quantum mechanical concept, not directly connected to overall mass conservation. Why Other Options Are Wrong (continued): - Avogadro law of gases: This law deals with gas volumes and numbers of molecules at given conditions, useful for gas calculations but not a direct statement about mass conservation. - Berzelius hypothesis of chemical combination: This hypothesis focuses on electrochemical ideas of how elements combine but does not fundamentally explain why mass is conserved in all chemical reactions. Common Pitfalls: Learners sometimes confuse any famous chemistry law with being the explanation for many different concepts. For example, Avogadro law is well known and might be chosen simply because it is familiar. Others might select Hund rule because they remember it from atomic structure lessons without carefully connecting it to the idea of mass conservation. To avoid such mistakes, always match the wording of the law to the concept in question and recall the exact statements of each law or theory. Final Answer: Dalton atomic theory most directly explains the law of conservation of mass in chemical reactions by stating that atoms are neither created nor destroyed but only rearranged.

Question 18

According to the modern periodic law, the chemical and physical properties of elements are periodic functions of which fundamental atomic property?

Accepted Answer

Introduction / Context: This question tests understanding of the modern periodic law, which underlies the arrangement of the periodic table. Earlier versions of the periodic law used atomic mass as the organising principle, but with the discovery of protons and atomic number, the modern version was adopted. Knowing exactly which property controls periodic trends is essential for reasoning about groups, periods, and periodic properties such as ionisation energy, electronegativity, and atomic radius. Given Data / Assumptions: - The question lists four possible controlling properties: valency, atomic mass, number of neutrons, and atomic number. - We are asked which one appears in the wording of the modern periodic law. - We assume standard definitions used in school level chemistry. Concept / Approach: The modern periodic law states that the physical and chemical properties of elements are periodic functions of their atomic numbers. Atomic number is defined as the number of protons in the nucleus, which also equals the number of electrons in a neutral atom. While Mendeleev originally arranged elements by atomic mass, anomalies such as the position of tellurium and iodine were resolved when atomic number was used instead. Valency and number of neutrons are related but do not define the primary sequence of the periodic table, and atomic mass is no longer the defining property in the modern law. Step-by-Step Solution: Step 1: Recall the exact wording of the modern periodic law: properties of elements are periodic functions of their atomic numbers. Step 2: Consider valency. Although valency shows periodicity and is influenced by position in the table, it is an outcome of periodic behaviour, not the cause used to arrange elements. Step 3: Atomic mass was used in the original Mendeleev periodic law, but this led to a few misplacements that were corrected when atomic number was adopted. Step 4: The number of neutrons can vary among isotopes of the same element and does not uniquely define the element, so it cannot serve as the ordering principle for the periodic table. Step 5: Atomic number, which is the count of protons in the nucleus, uniquely identifies each element and forms the basis of the modern periodic law and the present arrangement of the periodic table. Verification / Alternative check: If you open any modern chemistry textbook or periodic table chart, the elements are listed in increasing order of atomic number from hydrogen to oganesson. Group trends such as similar valence electron configurations and recurring properties appear at regular intervals when ordered this way. This visual pattern shows that atomic number is the fundamental ordering property, and all references to the modern periodic law explicitly mention atomic number rather than atomic mass or valency. Why Other Options Are Wrong: - Valency: It does show periodic behaviour but is a property that results from atomic structure and position in the periodic table, not the primary ordering principle in the modern law. - Atomic mass: It was used in the original periodic law but was replaced by atomic number in the modern formulation because atomic mass created some inconsistencies in element placement. - Number of neutrons: Different isotopes of the same element have different numbers of neutrons but share the same atomic number, so neutron count cannot define the periodic order of elements. Common Pitfalls: Students sometimes remember that Mendeleev used atomic mass and mistakenly apply that fact to the modern periodic law. Others may confuse atomic mass and atomic number because both are numbers printed in periodic table boxes. To avoid this, remember that atomic number is always the smaller whole number and defines the element, while atomic mass is usually a decimal reflecting a weighted average of isotopes. The wording modern periodic law is a clear signal that the correct property is atomic number. Final Answer: Atomic number is the fundamental property in the modern periodic law, and the properties of elements are periodic functions of their atomic numbers.

Question 19

In the context of textile and polymer chemistry, which one of the following materials is correctly classified as a regenerated cellulose fibre?

Accepted Answer

Introduction / Context: This chemistry question focuses on the classification of common textile fibres. Learners often encounter names like rayon, nylon, terylene, and silk in everyday life, but may not know which are natural, synthetic, or regenerated fibres. Regenerated fibres are produced by chemically processing natural polymers, particularly cellulose from wood pulp, into new fibre forms. Identifying rayon as a regenerated cellulose fibre is important for understanding material science and textile technology. Given Data / Assumptions: - Four options are given: Terylene, Rayon, Nylon, and Silk. - The question asks which is a regenerated fibre, specifically a regenerated cellulose type. - We assume standard industrial definitions of natural, synthetic, and regenerated fibres. Concept / Approach: Fibres can be grouped into natural fibres, synthetic fibres, and regenerated fibres. Natural fibres such as cotton and silk come directly from plants or animals. Synthetic fibres like nylon and terylene are fully man made polymers derived from petrochemicals. Regenerated cellulose fibres start from natural cellulose, which is chemically dissolved and then reformed into fibre. Rayon is the classic regenerated cellulose fibre produced from wood pulp or cotton linters through a viscose or other process. Therefore, the approach is to identify which option fits this regenerated cellulose description. Step-by-Step Solution: Step 1: Recall that rayon is produced from natural cellulose sources such as wood pulp. The cellulose is dissolved in chemicals and then regenerated as fibres through spinning processes. Step 2: Because the raw material is natural cellulose but the fibre is formed after chemical treatment and regeneration, rayon is described as a regenerated cellulose fibre. Step 3: Terylene is a trade name for a type of polyester. Polyesters are synthetic polymers made from petrochemical derived monomers like ethylene glycol and terephthalic acid, so terylene is a fully synthetic fibre. Step 4: Nylon is another synthetic polymer, a polyamide, made from chemicals such as adipic acid and hexamethylenediamine, and does not come from regenerated cellulose. Step 5: Silk is a natural protein fibre produced by silkworms, not a regenerated fibre. It is directly spun by the organism without chemical regeneration steps. Step 6: Comparing all four, rayon is the only fibre that fits the description of a regenerated cellulose fibre. Verification / Alternative check: Standard school textbooks and textile references classify rayon under man made fibres but often place it in a separate regenerated category because its base polymer is natural cellulose. In contrast, nylon and terylene are grouped strictly as synthetic fibres, while silk appears among natural animal fibres. Checking such tables or charts quickly confirms that rayon is the only regenerated fibre among the given options, validating the answer. Why Other Options Are Wrong: - Terylene: It is a synthetic polyester and does not originate from natural cellulose, so it is not a regenerated cellulose fibre. - Nylon: It is a synthetic polyamide produced entirely from petrochemical monomers and does not involve regeneration of natural cellulose. - Silk: It is a natural protein fibre made by silkworms and does not involve any regeneration process from cellulose or other polymers. Common Pitfalls: Learners may think that any man made fibre is synthetic and may not distinguish regenerated fibres from fully synthetic ones. Another mistake is to assume that because rayon behaves somewhat like natural fibres, it must be fully natural. Remember that regenerated means the starting material is natural but is dissolved and then regenerated in new form. Keeping this definition in mind helps correctly classify rayon as regenerated cellulose and distinguish it from purely synthetic fibres such as nylon and terylene. Final Answer: Rayon is the regenerated cellulose fibre among the options, produced by chemically processing and reforming natural cellulose.

Question 20

Assuming a zinc atom has mass number 64, how many neutrons are present in a dipositive zinc ion Zn2 plus, given that ionisation does not change the nucleus?

Accepted Answer

Introduction / Context: This chemistry question tests basic nuclear composition concepts, namely the relationship between atomic number, mass number, and number of neutrons. It also checks whether learners understand that forming an ion by losing or gaining electrons does not affect the protons or neutrons in the nucleus. Zinc is a common example used in such questions, and here we assume a specific mass number to make the calculation clear and unambiguous. Given Data / Assumptions: - The species considered is a dipositive zinc ion Zn2 plus. - The mass number of zinc is taken as 64 for this question. - The atomic number of zinc is 30, which means zinc has 30 protons. - Ion formation changes electrons, not the nucleus, so the number of neutrons stays the same as in the neutral atom. Concept / Approach: For any nuclide, mass number is the sum of protons and neutrons. Atomic number is the number of protons and defines the element. The number of neutrons can be calculated using the simple relation: number of neutrons = mass number minus atomic number. When an atom forms a positive ion, it loses one or more electrons, but the protons and neutrons in the nucleus are unchanged. Therefore, the neutron count in Zn2 plus is the same as in neutral zinc of mass number 64. Step-by-Step Solution: Step 1: Note that zinc has atomic number 30, so every zinc atom or ion has 30 protons in its nucleus. Step 2: The question specifies that the mass number of zinc to be used is 64. The mass number is the total of protons and neutrons. Step 3: Use the relation number of neutrons = mass number minus atomic number. Therefore, number of neutrons = 64 minus 30. Step 4: Perform the subtraction: 64 minus 30 gives 34 neutrons. Step 5: Recognise that forming a Zn2 plus ion means zinc has lost two electrons, but these electrons are not counted in mass number. Protons and neutrons remain unchanged, so the ion still has 34 neutrons. Verification / Alternative check: A quick check is to remember that typical isotopes of zinc include Zn 64, Zn 66, Zn 68, and so on. For Zn 64, the difference between 64 and the atomic number 30 must be the neutron count, which is 34. Some learners remember this from isotope tables, but even without memorising, the simple subtraction method always works. Checking that 34 plus 30 equals 64 confirms that the calculated neutron number is consistent with the mass number given. Why Other Options Are Wrong: - 36: This would imply a mass number of 66 for zinc if the atomic number is 30, which is not the isotope specified in the question. - 38: This would match an isotope with mass number 68, not the given mass number 64. - 40: This would correspond to a mass number of 70, again different from the assumed mass number 64. Common Pitfalls: A common error is to think that the charge on the ion somehow affects the number of neutrons. Learners might incorrectly add or subtract the charge value from the neutron count. Another mistake is to confuse mass number with atomic mass or to forget the atomic number of zinc. Finally, some students may subtract in the wrong order and compute 30 minus 64 instead of 64 minus 30. Carefully applying the relation mass number = protons plus neutrons and remembering that electrons are not included in mass number avoids these problems. Final Answer: 34 neutrons are present in a Zn2 plus ion when zinc has a mass number of 64.

Chemistry Questions