JAMB Chemistry 2021 Past Questions & Answers

The compound has a total of four carbon atoms, so its parent name is related to butane. The prefix 'iso-' is used for a common name when a structure has a methyl group attached to the second-to-last carbon of the main chain, which is the case here with the (CH₃)₂CH- group. Therefore, the alkyl group is called 'isobutyl', and with the attached bromine atom, the full common name is Isobutyl bromide.

Common mistake: Students often choose Butyl bromide because they correctly count the four carbon atoms but fail to recognize the branched 'iso-' structure, confusing it with the straight-chain n-butyl group.

The conversion of glucose to ethyl alcohol (ethanol) and carbon dioxide is a biochemical process called fermentation. This reaction is catalyzed by a mixture of enzymes found in yeast. The specific enzyme complex responsible for breaking down glucose into ethanol is called zymase.

Common mistake: Students often confuse the roles of different enzymes in carbohydrate chemistry and might mistakenly choose Maltase, which is the enzyme that breaks down maltose into glucose, not glucose into alcohol.

Albert Einstein developed the theory of special relativity, which includes the famous equation E=mc². This equation shows that energy (E) and mass (m) are equivalent and can be converted into one another. This principle is fundamental to nuclear chemistry and explains the massive energy released in nuclear reactions, where a small amount of matter is converted into a large amount of energy.

Common mistake: Students might mistakenly choose Lavoisier because he is famous for the Law of Conservation of Mass, but this law applies to ordinary chemical reactions, not the conversion of mass into energy which occurs in nuclear reactions.

The compound HC≡CCH₃ has a chain of three carbon atoms with a triple bond, making it an alkyne. The correct IUPAC name is 'Propyne'. However, 'Methyl acetylene' is a common name for this compound, derived by viewing it as an acetylene (HC≡CH) molecule where one hydrogen is replaced by a methyl group (–CH₃), making it the intended answer among the choices.

Common mistake: Students might mistakenly choose Acetylene because they recognize the triple bond but forget to count all the carbon atoms in the chain; Acetylene only has two carbons, while the given compound has three.

The compound CuCO₃ is made of a copper ion (Cu) and a carbonate ion (CO₃). The carbonate ion always has a charge of 2- (CO₃²⁻). For the whole compound to be neutral with no overall charge, the copper ion must have a charge of 2+ to balance it. When naming compounds with transition metals like copper, we use a Roman numeral to show its charge, so Cu²⁺ is called 'Copper (II)'. Therefore, the full name is Copper (II) carbonate.

Common mistake: Students might mistakenly choose Copper (I) carbonate because they see only one copper atom in the formula, confusing the subscript of the atom with its oxidation state.

The breathalyzer test works by using a chemical reaction to detect alcohol (ethanol) in a person's breath. The correct chemical is acidified potassium dichromate(VI), which is a strong oxidizing agent with a bright orange colour. When ethanol from the breath reacts with it, the ethanol is oxidized, and the orange dichromate is reduced to chromium(III) ions, which are green. This visible colour change from orange to green is how the police confirm the presence of alcohol.

Common mistake: Students often mistakenly choose potassium permanganate because it is also a well-known strong oxidizing agent with a distinct colour change, but potassium dichromate is the specific reagent used in traditional breathalyzers.

A mixture consists of two or more substances that are physically combined, not chemically bonded, while a pure substance is made of only one type of element or compound. Mercury is a chemical element with the symbol Hg, which means it is a pure substance. In contrast, air is a mixture of gases, milk is a mixture (colloid) of fat and protein in water, and cement is a mixture of various compounds.

Common mistake: Students sometimes choose Air because it appears uniform and invisible, forgetting that it is a well-known mixture of different gases like nitrogen, oxygen, and argon.

At room temperature and pressure, elements exist in different physical states. Bromine is one of only two elements on the periodic table that is a liquid at these conditions, the other being mercury, which is a metal. Among the non-metal options provided, Bromine is the correct answer as it is a reddish-brown liquid. Chlorine and Helium are gases, while Phosphorus is a solid at room temperature.

Common mistake: Students might mistakenly choose Chlorine because it is also a halogen like Bromine, but they forget that Chlorine is a gas at room temperature.

A chemical change is a process that forms new substances with different chemical properties, and it is usually irreversible. Cooking and digestion of food involve complex chemical reactions that break down large molecules into new, simpler ones. In contrast, freezing and heating water are physical changes because the substance remains water (H₂O), only its state or temperature changes.

Common mistake: Students often mistakenly include freezing of water as a chemical change, but it is a physical change because the chemical identity of water (H₂O) does not change, only its state from liquid to solid.

The atomic weight, also known as the mass number, represents the total number of protons and neutrons in an atom's nucleus. A nitrogen atom has an atomic number of 7, which means it has 7 protons. The most common isotope of nitrogen also has 7 neutrons, so its atomic weight is the sum of protons and neutrons, which is 7 + 7 = 14.

Common mistake: Students might mistakenly choose 12 or 16, as these are the atomic weights of carbon and oxygen, which are common elements often studied alongside nitrogen.