Carbon and its Compounds

The governing test is the limewater reaction with carbon dioxide. First, CO2 reacts with calcium hydroxide: Ca(OH)2 + CO2 → CaCO3 + H2O. Insoluble calcium carbonate forms the milky appearance. When excess CO2 is passed, it reacts further: CaCO3 + CO2 + H2O → Ca(HCO3)2. Calcium hydrogen carbonate is soluble, so the milkiness disappears. Thus option A gives the compounds in the correct order.

Carbon has four valence electrons and needs four more to complete its outer shell. Losing four electrons would require very high energy, while gaining four would produce an unstable, highly charged ion. Carbon therefore usually completes its valence shell by sharing electrons with other atoms, forming covalent bonds. Hence option A is correct; the other choices contradict carbon’s electronic structure or bonding behaviour.

Tetravalency means that an atom has valency four. Carbon has four valence electrons and can share these electrons to complete its outer shell. As a result, it can form four covalent bonds, either with hydrogen, other carbon atoms, or suitable atoms of other elements. The term does not mean four nuclei, four metals, or four exclusively ionic bonds, so option A is correct.

Catenation is the ability of an element to form covalent bonds with atoms of the same element, producing chains, branches, or rings. Carbon atoms are small, so their orbitals overlap effectively and carbon–carbon bonds are strong and stable. Combined with carbon’s tetravalency, this allows extensive structures. Insolubility, gas formation, or absence of valency does not cause catenation; therefore A is correct.

The enormous variety of carbon compounds is chiefly explained by two linked properties: tetravalency and catenation. Tetravalency lets each carbon form four covalent bonds, while catenation lets carbon atoms join into long, branched, or cyclic chains. These structures can also carry different functional groups, creating many substances. Carbon is not responsible because of only ionic or metallic bonding, nor because of insolubility; hence A is correct.

Hydrogenation is an addition reaction in which hydrogen is added to carbon-carbon double bonds of unsaturated vegetable oil, increasing saturation and often making the fat more solid. Nickel provides a surface that helps the reaction proceed faster by lowering the effective activation barrier, while it is regenerated rather than used up overall. Thus option A is correct. Nickel is not the solvent, an acid-base indicator, or the fuel in this process.

Hydrogenation is an addition reaction in which hydrogen is added across carbon–carbon double bonds of an unsaturated oil, usually with a suitable catalyst such as nickel. This decreases unsaturation and raises the melting point, so the liquid oil becomes a more saturated, semisolid or solid fat. Option A is correct; the other options describe unrelated changes involving acids, salts, gases, or metals.

Ethanol, with the formula CH3CH2OH, is a primary alcohol. On oxidation, it first forms ethanal and can then undergo further oxidation to ethanoic acid, CH3COOH, when the oxidising conditions are sufficient. Therefore option A is correct. Methanoic and propanoic acids contain different carbon skeletons, while carbonic acid is not the normal oxidation product of ethanol.

Alkaline potassium permanganate is commonly used as an oxidising agent in organic chemistry. It can promote oxidation by adding oxygen to an organic molecule or by removing hydrogen; for example, it oxidises ethanol through ethanal to ethanoic acid under suitable conditions. Thus option A is correct. It is not a reducing agent, lubricant, or fuel in this reaction.

When ethanol is heated with concentrated sulphuric acid at about 443 K, the acid acts mainly as a dehydrating agent. It removes a molecule of water from ethanol, producing ethene: CH3CH2OH → CH2=CH2 + H2O. Therefore option A is correct. Ethane would require reduction, ethanoic acid requires oxidation, and sodium ethoxide would require sodium rather than sulphuric acid.

Ethanol is an alcohol and ethanoic acid is a carboxylic acid. They react in the presence of concentrated sulphuric acid to produce an ester and water; specifically, ethyl ethanoate is formed: CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O. This is esterification, so option A is correct. Carbonate, base, and graphite are not products of this reaction.

Ethanoic acid reacts with sodium carbonate in an acid–carbonate reaction. The products are sodium ethanoate, water, and carbon dioxide: 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2. Effervescence occurs because of the escaping carbon dioxide, which also turns limewater milky. Therefore option A is correct; the other gases are not released in this reaction.

Acid strength depends mainly on the extent to which an acid ionises in water, not simply on whether it dissolves. Ethanoic acid establishes an equilibrium and releases only a fraction of its molecules as hydrogen ions and ethanoate ions: CH3COOH ⇌ H+ + CH3COO−. Thus it is a weak acid. Option A is correct; it does dissolve, is not a metal, and definitely contains carbon.

The sodium carbonate test distinguishes these substances because ethanoic acid reacts with sodium carbonate and releases carbon dioxide, producing visible effervescence. Ethanol does not normally give this carbonate reaction under the same conditions. The gas can be confirmed by passing it through limewater, which turns milky. Therefore option A is correct; bromine water is not the standard distinction here, and magnet or bulb tests are inappropriate.

Complete combustion means that the fuel reacts with an adequate supply of oxygen. Ethanol, C₂H₅OH, contains carbon and hydrogen; carbon is oxidised to carbon dioxide and hydrogen is oxidised to water. The balanced equation is C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Carbon monoxide and soot indicate incomplete combustion, while sodium carbonate and hydrogen are not combustion products of ethanol.

Incomplete combustion occurs when a carbon compound burns with insufficient oxygen. Instead of oxidising all carbon to carbon dioxide, part of the carbon forms carbon monoxide, CO. This gas is especially dangerous because it is colourless and odourless and reduces the blood’s oxygen-carrying capacity by binding strongly with haemoglobin. Oxygen and nitrogen are not the characteristic gaseous products, and hydrogen is not formed in this process.

Isomerism is the existence of different compounds that have the same molecular formula but different structural arrangements of atoms. Because carbon is tetravalent and can form chains, branches, and different bonding patterns, the same numbers of carbon and hydrogen atoms may be connected in more than one way. These structures have different properties. The other choices deny bonding or incorrectly claim that all formulae must differ.

The governing concept is esterification, in which ethanoic acid reacts with ethanol in the presence of concentrated sulfuric acid to form an ester, ethyl ethanoate, and water: CH3COOH + C2H5OH → CH3COOC2H5 + H2O. Esters commonly have pleasant, sweet or fruity odours, and ethyl ethanoate is associated with a fruity smell. Thus option A is correct; the other smells are not characteristic of this ester.

Ethanol, C₂H₅OH, is converted into ethanoic acid, CH₃COOH, when it reacts with an oxidising agent such as alkaline potassium permanganate or acidified potassium dichromate. This involves addition of oxygen and removal of hydrogen, which is the governing idea of oxidation. Hydrogenation adds hydrogen, saponification makes soap, and esterification forms an ester, so option A is correct.

When ethanol is heated with concentrated sulphuric acid at about 443 K, it undergoes dehydration: C₂H₅OH → C₂H₄ + H₂O. The hydroxyl group and a hydrogen atom are removed together as water, producing the C=C bond of ethene. This is not hydrogen addition, carbon removal, or salt formation. Therefore, option A correctly identifies the change.