Acids, Bases, and Salts

The governing concept is the definite crystal structure of hydrated salts. Some salts incorporate a fixed number of water molecules into their crystal lattice; this chemically associated water is called water of crystallisation. For example, CuSO₄·5H₂O contains five water molecules per formula unit. Therefore option A is correct. Surface moisture, water in every acid, and dissolved gas are not water of crystallisation.

The governing concept is dehydration of a hydrated salt. Blue copper sulphate is CuSO₄·5H₂O, whose blue colour is associated with its water of crystallisation. On heating, this water is removed and anhydrous CuSO₄ forms, which is white: CuSO₄·5H₂O → CuSO₄ + 5H₂O. Hence option A is correct; copper does not become gold and sulphate does not turn into oxygen.

Hard water contains dissolved calcium and magnesium ions. Washing soda supplies carbonate ions, which react with them to form sparingly soluble calcium carbonate and magnesium carbonate precipitates. Removing these ions reduces hardness and allows soap to lather more readily. Therefore option A is correct; the other choices describe unrelated changes that do not soften water.

Baking soda is sodium hydrogen carbonate, NaHCO3. On heating, it undergoes thermal decomposition: 2NaHCO3 → Na2CO3 + CO2 + H2O. The products are therefore sodium carbonate, carbon dioxide and water, exactly as stated in option A. The other options contain substances that are not formed in this decomposition and do not balance the reaction.

Baking powder contains baking soda and a mild edible acid. When the mixture becomes moist and is heated, the acid reacts with sodium hydrogen carbonate and releases carbon dioxide. The gas forms bubbles that expand during baking, making the dough or batter rise. Hence option A is correct; the reaction does not produce sodium metal or chlorine gas.

Bleaching powder is used for its oxidising and bleaching action. The active chlorine released from it can oxidise coloured substances, converting them into less-coloured or colourless products. The same action that removes stains can also fade dyes in coloured fabric, so careful use is necessary. Thus option A is correct; the other options do not describe bleaching powder’s chemical action.

Bleaching powder is not perfectly stable during storage. Contact with atmospheric moisture and carbon dioxide can cause gradual decomposition and reduce the amount of available chlorine, which is responsible for much of its bleaching and disinfecting action. Keeping it in a tightly closed, dry container slows this deterioration. Therefore option A is correct; the other statements are chemically irrelevant.

Bleaching powder releases available chlorine when used appropriately in water. Chlorine and its active species damage essential components of many disease-causing microorganisms, thereby reducing contamination and helping disinfect the water. Thus option A correctly explains its use. It does not sweeten water, convert it into oil, or produce sodium metal; those alternatives contradict its known chemical action.

Brine is concentrated aqueous sodium chloride. During electrolysis, chloride ions are oxidised at the anode to chlorine gas, water is reduced at the cathode to hydrogen gas, and sodium hydroxide remains in solution. The overall reaction is 2NaCl + 2H2O → 2NaOH + Cl2 + H2. Therefore option A is the correct product set.

In the chlor-alkali process, concentrated brine is electrolysed. Chloride ions, Cl⁻, move towards the positively charged anode and lose electrons there: 2Cl⁻ → Cl₂ + 2e⁻. Thus chlorine gas is liberated at the anode. Hydrogen is produced at the cathode and sodium hydroxide remains in solution, so the other options do not describe chlorine formation.

During electrolysis of brine, water is reduced at the cathode, where reduction means gain of electrons. The cathode reaction may be written as 2H₂O + 2e⁻ → H₂ + 2OH⁻. Therefore hydrogen gas is collected at the cathode. Chlorine forms at the anode, while sodium hydroxide remains in the solution, so the other choices are incorrect.

Plaster of Paris is calcium sulfate hemihydrate, CaSO₄·½H₂O. Moisture supplies water that converts it into gypsum, calcium sulfate dihydrate, according to CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. This hydration causes the material to set and become hard. Consequently, exposure to moisture can spoil its usefulness during storage, so it is kept in moisture-proof packaging. Option A is correct.

Gypsum is calcium sulfate dihydrate, CaSO₄·2H₂O. On controlled heating, it loses one and a half molecules of water of crystallisation and forms calcium sulfate hemihydrate, CaSO₄·½H₂O, known as plaster of Paris: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Thus A is correct. The calcium remains in the compound, sulphur does not become a gas, and water is removed rather than increased.

Dilution of a concentrated acid is highly exothermic: heat is released when the acid mixes with water. If water is poured quickly into acid, the small amount of water can heat suddenly, boil, and throw corrosive acid outward. Adding acid slowly to a larger volume of water allows heat to disperse and reduces splashing. Hence option A is the safe laboratory rule.

Dilution increases the total volume without removing all the acid particles. The hydrogen or hydronium ions become distributed through a larger volume, so their concentration per unit volume decreases. Consequently, the acidic effect becomes weaker, although the solution may still be acidic. The ions do not disappear, and dilution alone does not necessarily make the solution basic.

On the pH scale, values below 7 indicate acidity, while a higher pH within that range indicates lower hydrogen-ion concentration and therefore weaker acidity. For example, a change from pH 3 to pH 5 means the solution is less acidic, but pH 5 is still below 7. It has not become basic, and ions have not vanished.

When an acid reacts with a base, a neutralisation reaction occurs, producing salt and usually water. The formation of water from hydrogen ions and hydroxide ions is commonly exothermic: H⁺ + OH⁻ → H₂O, with heat released. Therefore the solution temperature can rise. Equal amounts do not change the governing concept; the temperature depends on concentrations and reaction conditions.

Phenolphthalein is colourless in acidic or neutral conditions and pink in a sufficiently basic medium. Adding acid consumes hydroxide ions and lowers the pH, so the basic character gradually decreases. When the indicator becomes colourless, the solution has reached the indicator’s colour-change range; it may be neutral or may have become acidic if more acid is added. Thus A is the precise choice.

Methyl orange is an acid-base indicator. Its molecular structure changes as the hydrogen-ion concentration of the medium changes, producing different colours: it is generally red in an acidic solution and yellow in a basic or sufficiently neutral solution, with an orange transition range. Therefore its colour provides evidence about the nature of the medium; it is not a gas-forming or metal-producing substance.

Electrical conduction in an electrolyte occurs through the movement of charged particles. When many acids, bases, or salts dissolve in water, they separate into mobile positive and negative ions, which carry current through the solution. In the dry solid, these ions are generally fixed in a lattice and cannot move freely. Therefore, option A is correct; the other choices confuse conductivity with electron speed, colour, or metallic conversion.

A soda-acid extinguisher uses sodium hydrogen carbonate, also called sodium bicarbonate, and an acid such as sulphuric acid. Their reaction produces sodium salt, water, and carbon dioxide: the carbonate or hydrogen carbonate part releases CO₂ when treated with acid. The gas is non-supportive of combustion and helps smother the fire. Thus option A is correct; the other reactions do not produce the required gas.

Marble is mainly calcium carbonate, CaCO₃. Acids react with carbonates to form a salt, water, and carbon dioxide: acid + CaCO₃ → salt + H₂O + CO₂. The escaping carbon dioxide appears as bubbles on the marble surface. Therefore option A correctly identifies both the substance and the gas-forming reaction. The other options either describe nonexistent materials or an incorrect physical process.

Marble is largely calcium carbonate, a carbonate compound that reacts with acidic substances. Pollutants such as sulphur dioxide and nitrogen oxides can form acids in rainwater. Repeated contact converts some calcium carbonate into soluble salts, water, and carbon dioxide, gradually eroding the surface and its carvings. Hence option A is correct; sunlight alone does not explain this characteristic chemical damage.

The relevant governing concept is neutralisation: an acid reacts with a base and the acidic effect is reduced. Baking soda, sodium hydrogen carbonate, is a mild base. When applied appropriately, it can react with the acidic material associated with an ant sting and lessen irritation. Thus option A is correct. A strong acid would worsen acidity, while the remaining choices describe impossible effects or the wrong gas-producing process.

Nettle stings are commonly associated with an acidic irritant. The acid-base principle predicts that a mild base can partially neutralise an acid and reduce its irritating effect. Therefore a mild basic substance is the most suitable answer, making option A correct. A strong acid would add to the acidity, while neutral sugar and dry salt do not provide the required neutralisation reaction. The treatment should still be used safely and appropriately.