Acids, Bases, and Salts

The governing concept is the extent of ionisation. Ethanoic acid, CH₃COOH, establishes a reversible equilibrium in water and only a fraction of its molecules release H⁺ ions: CH₃COOH ⇌ H⁺ + CH₃COO⁻. Hence option A is correct. Complete ionisation characterises a strong acid, while the metal and strong-base descriptions are chemically incorrect.

The governing concept is electrical conduction in ionic solutions. At equal concentration, hydrochloric acid ionises almost completely, producing many mobile H₃O⁺ and Cl⁻ ions. Ethanoic acid ionises only partially, so it produces fewer charge carriers. Therefore option A is correct. Conductivity is not evidence that HCl is a base, and it is soluble in water.

The governing concept is the acidic behaviour of non-metal oxides. Carbon dioxide reacts with bases such as sodium hydroxide: CO₂ + 2NaOH → Na₂CO₃ + H₂O. It can also form carbonic acid with water. Thus option A correctly identifies its reaction with bases. It does not form metals, turn red litmus blue, or represent common salt, so the other options are unsuitable.

The governing concept is neutralisation between a basic oxide and an acid. Copper(II) oxide reacts as follows: CuO + 2HCl → CuCl₂ + H₂O. Formation of salt and water from an oxide and an acid indicates that the oxide has basic character. Therefore option B is correct. Acidic, neutral and volatile describe different properties and do not explain this neutralisation reaction.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. The principal solid product is sodium carbonate, while carbon dioxide and water are gaseous products under the usual conditions. Hence option A is correct; the other salts do not result from this decomposition.

Baking powder contains baking soda and a mild edible acid. When the mixture becomes wet and is heated, the acid reacts with sodium hydrogen carbonate and produces carbon dioxide: acid + NaHCO₃ → salt + CO₂ + water. The gas forms bubbles that expand and make dough rise. Therefore option A is correct; hydrogen, chlorine, and metal are not produced in this process.

Hard water contains dissolved calcium and magnesium ions, which can form deposits and react with soap. Washing soda supplies carbonate ions that precipitate these ions as insoluble carbonates, such as calcium carbonate and magnesium carbonate. Removing them reduces hardness, so option A is correct. The process does not depend on hydrogen or chlorine gas and does not make the water acidic.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Controlled heating removes part of its water of crystallisation and forms calcium sulphate hemihydrate, CaSO₄·½H₂O, known as plaster of Paris: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Thus option A is correct; the other substances do not undergo this preparation reaction.

Plaster of Paris is prepared by partially heating gypsum, whose formula is CaSO₄·2H₂O. Partial dehydration removes one and a half water molecules, producing CaSO₄·½H₂O, called calcium sulphate hemihydrate. Calcium carbonate, sodium carbonate decahydrate, and sodium chloride are different salts, so option C is correct.

The hardening of plaster of Paris is a hydration reaction. Calcium sulphate hemihydrate, CaSO₄·½H₂O, combines with water and changes back into gypsum, CaSO₄·2H₂O. Gypsum forms an interlocking solid mass, making the paste hard. No sodium chloride, gas, or sugar is produced, so option A is correct.

Hydrated copper sulphate crystals have the formula CuSO₄·5H₂O. The fixed water molecules incorporated into the crystal structure are called water of crystallisation and are associated with its characteristic blue appearance. Sodium ions, chlorine gas, and carbon dioxide are not responsible for this colour, so option A is correct.

Blue copper sulphate is hydrated CuSO₄·5H₂O. On heating, its water of crystallisation is driven off, producing anhydrous copper sulphate, which is white. The colour change is therefore due to loss of bound water, not addition of sodium or chlorine and not formation of carbon dioxide. Hence option A is correct.

Water of crystallisation is the definite number of water molecules chemically incorporated into the crystal structure of some salts. For example, washing soda is Na₂CO₃·10H₂O and copper sulphate crystals are CuSO₄·5H₂O. It is not ordinary river water, arbitrary water in an acid, or a gas, so option A is correct.

Bleaching powder is manufactured by passing chlorine gas over dry slaked lime, calcium hydroxide, Ca(OH)₂. The reaction produces bleaching powder, commonly represented as CaOCl₂ in school chemistry. The lime must be dry because excess moisture interferes with the process. Common salt, gypsum, and washing soda are not the starting solid, so option A is correct.

Bleaching powder is useful for water disinfection because it releases available chlorine when added to water. This chlorine forms active chlorine-containing species that oxidise and destroy many disease-causing microorganisms. Its purpose is not to sweeten, freeze, or convert water into metal. Therefore, the scientifically correct explanation is option A.

Brine is a concentrated aqueous solution of sodium chloride. During electrolysis, chlorine is produced at the anode, hydrogen at the cathode, and sodium hydroxide remains in the solution. Because the process produces chlorine and an alkali, sodium hydroxide, it is called the chlor-alkali process. Neutralisation, evaporation, and filtration are different processes, so option B is correct.

The chlor-alkali process electrolyses aqueous sodium chloride, or brine. Chloride ions are oxidised at the anode to form chlorine, water is reduced at the cathode to form hydrogen, and sodium hydroxide accumulates in the solution. Therefore the three principal products are chlorine, hydrogen, and sodium hydroxide. Option A is correct; the other combinations do not result from brine electrolysis.

The chlor-alkali process is the electrolysis of concentrated sodium chloride solution, or brine. At the anode, chloride ions lose electrons and are oxidised: 2Cl⁻ → Cl₂ + 2e⁻. Therefore, chlorine gas is released at the anode, so option B is correct. Hydrogen is produced at the cathode, while oxygen and carbon dioxide are not the expected products in this process.

In the chlor-alkali process, aqueous sodium chloride (brine) is electrolysed. At the cathode, water molecules gain electrons and are reduced, producing hydrogen gas: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Thus option A is correct. Chlorine is formed at the anode by oxidation of chloride ions; nitrogen and sulphur dioxide are not products of brine electrolysis. Hydrogen can be tested with a lighted splint, which gives a pop sound.

Tooth enamel is mainly a mineral containing calcium compounds, and it can be attacked when the mouth becomes sufficiently acidic. Oral bacteria convert sugars into acids, lowering the local pH. When the pH falls below about 5.5, demineralisation of enamel becomes more likely and the risk of tooth decay rises. Therefore option A is correct; pH 7 is neutral, while 10 and 14 are alkaline values.

After food is eaten, bacteria in the mouth may produce acids from sugars. These acids lower the pH and can gradually demineralise tooth enamel. Basic toothpaste contains mildly alkaline substances that react with and neutralise excess acid, helping to keep the mouth less acidic. Hence option A is correct. Dissolving enamel would worsen decay, pH zero is extremely acidic, and increasing acid cannot protect teeth.

Acidic soil has a pH lower than the range preferred by many crops. Slaked lime, calcium hydroxide [Ca(OH)₂], is a basic substance. It reacts with excess acids in the soil and neutralises them, thereby raising the pH toward a more suitable value. Thus option A is correct. Adding it does not increase acidity, convert soil into gas, or produce sugar; the effect is an acid-base neutralisation.

Curd contains lactic acid, so it is an acidic food. If it remains in a copper vessel, the acid may react with the metal and form copper salts. Depending on the amount and conditions, such contamination can make the food unsuitable or harmful to consume. Therefore option A is correct. Curd is not acid-free, and copper neither converts it into pure water nor invariably makes food sweet; these statements do not describe a chemical reaction.

Laboratory acids and bases may be corrosive, poisonous, or irritating even when they are dilute or appear harmless. Tasting an unknown chemical can injure the mouth and digestive system and is therefore prohibited by laboratory safety rules. Option A is correct. Colour or sweetness cannot reliably identify a substance, and the claim that acids and bases have no effect is false. Indicators, pH paper, or approved testing methods should be used instead.

The governing concept is the reaction of an active metal with an acid. Zinc reacts with a dilute acid to form a salt and hydrogen: Zn + 2HCl → ZnCl₂ + H₂. Hydrogen is identified because it burns rapidly and produces a characteristic ‘pop’ when a burning splint is brought near it. Carbon dioxide would turn limewater milky, ammonia would affect red litmus, and oxygen would rekindle a glowing splint; therefore option A is correct.