Acids, Bases, and Salts

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.

Common salt is the familiar crystalline ionic compound sodium chloride, with the formula NaCl. It consists of sodium ions and chloride ions in a regular lattice and is widely used in food. Sodium carbonate, calcium sulphate, and sodium hydroxide are separate compounds with different formulas and properties. Thus option A gives the correct chemical name.

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.

Litmus is a natural acid-base indicator. In an acidic solution, the indicator changes from blue to red because the acidic medium affects the litmus dye. Therefore option A is correct. Blue litmus remains blue in a neutral solution and generally does not show this red change in a basic solution. Although other indicators may display different colours, the characteristic blue-litmus test for an acid is red.

Litmus helps distinguish acidic and basic solutions. A basic solution changes red litmus paper to blue because the alkaline medium produces the characteristic response of the litmus indicator. Hence option B is correct. Red litmus does not remain the expected test colour in a base, and the change is not to white or black. This test should be performed with a suitable indicator strip rather than by touching or tasting an unknown chemical.

A sour taste is a common property associated with acids because acidic substances release hydrogen ions in aqueous solution. For example, citric acid in lemon and tamarind acids produce a sour taste. Therefore option A is correct. Bases are often bitter and soapy, salts may have varied tastes, and pure water is generally tasteless. However, taste is only a general property and must never be used to identify an unknown laboratory chemical.

The governing concept is the characteristic nature of acids and bases. Aqueous bases generally feel slippery or soapy because they react with oils on the skin; soap solutions are commonly alkaline. Therefore, option B, base, is correct. Acids are usually associated with sourness and can be corrosive, salts do not generally have a soapy feel, and metals are solids with metallic properties. Chemicals must never be identified by touching them in a laboratory.

The governing concept is the relation between taste and chemical nature. Lemon juice has a sour taste because it contains natural acids, chiefly citric acid. Hence option A, acid, is correct. A base is commonly bitter and may feel slippery, while a salt is not identified here by the characteristic sourness of lemon juice. Lemon juice is not a metal; it is an aqueous mixture containing water, citric acid, and other dissolved substances.