Acids, Bases, and Salts

Common salt, the crystalline substance commonly used in food, is chemically called sodium chloride and has the formula NaCl. It consists of sodium ions and chloride ions in an ionic lattice, generally obtained from sea water or rock salt. Therefore option B is correct. Sodium hydroxide is caustic soda, sodium carbonate is washing soda, and sodium hydrogen carbonate is baking soda, so those names refer to different compounds.

Brine is a concentrated aqueous solution of sodium chloride. During electrolysis, chloride ions produce chlorine at the anode, water produces hydrogen at the cathode, and sodium hydroxide remains in solution. Because the products include chlorine and an alkali, this industrial method is called the chlor-alkali process. Therefore option B is correct; acidification, simple water decomposition, and crystallisation do not describe this complete electrolytic process.

Sodium chloride is formed from the strong acid hydrochloric acid, HCl, and the strong base sodium hydroxide, NaOH. Its Na⁺ and Cl⁻ ions undergo negligible hydrolysis in water, so the solution is approximately neutral, with pH near 7. Hence option A is correct. Sodium carbonate and sodium acetate are salts of weak acids with a strong base and give basic solutions, whereas ammonium chloride gives an acidic solution because NH₄⁺ hydrolyses.

Sodium hydroxide has the formula NaOH and dissociates in water to produce sodium ions and hydroxide ions, OH⁻. A substance that supplies hydroxide ions in aqueous solution is classified as a base; NaOH is specifically a strong base because it dissociates extensively. Therefore option B is correct. It is not an acid, salt, or neutral oxide. Sodium chloride is the salt involved as the brine starting material, not the NaOH product.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, whose chemical name is calcium hydroxide, Ca(OH)₂. The reaction produces bleaching powder, commonly represented as calcium oxychloride, along with water. Therefore, option A is correct. Sodium chloride and water, sodium carbonate with acid, and gypsum with water are not the standard reactants for this preparation.

Bleaching powder releases chlorine or chlorine-containing species that destroy many harmful microorganisms. For this reason, it is used to disinfect drinking water and is also employed as a bleaching agent in textile and paper industries. Option A is correct. It is not used to sweeten food, melt metals, or harden soil; those choices do not describe its standard chemical applications.

Ammonium chloride, NH₄Cl, is formed by neutralisation of the strong acid HCl and the weak base NH₄OH. In water, NH₄⁺ undergoes hydrolysis and produces H₃O⁺ ions, so the solution is acidic. NaCl is essentially neutral, while CH₃COONa and Na₂CO₃ are salts of weak acids with strong bases and generally give basic solutions. Thus option A is correct.

Baking powder is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid. In the presence of moisture or heat, the acid reacts with baking soda to release carbon dioxide, which makes cakes and bread rise. Slaked lime is alkaline and is not used for this purpose. Exam tip: Remember: baking soda + mild edible acid = baking powder.

Baking powder contains sodium hydrogen carbonate along with an edible weak acid. On heating or on contact with moisture, they react and release carbon dioxide, CO₂. The gas forms bubbles within the dough or batter, and these bubbles expand during heating, making bread and cakes rise and become porous. Hydrogen, chlorine, and oxygen are not the leavening gas here, so option C is correct.

Sodium carbonate, Na₂CO₃, is formed from the strong base NaOH and the weak acid H₂CO₃. In water, the carbonate ion undergoes hydrolysis and generates OH⁻ ions, making the solution basic. A basic solution turns red litmus blue, so option C is correct. NaCl is nearly neutral, NH₄Cl is acidic, and CuSO₄ is not the expected basic salt in this comparison.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. Carbonate ions react with dissolved calcium and magnesium ions in hard water to form insoluble carbonates, removing them from solution. Thus washing soda softens water and can remove permanent hardness; option A is correct. The other choices describe neither its standard composition nor its principal household use.

Water of crystallisation is the definite number of water molecules chemically associated with the formula unit of certain crystalline salts. It is not merely moisture on the outside. For example, CuSO₄·5H₂O contains five water molecules per formula unit. This water can influence crystal colour, shape, and stability, so option A gives the correct meaning.

Hydrated copper sulphate crystals, CuSO₄·5H₂O, are blue because of their water of crystallisation. On heating, this water is removed and anhydrous copper sulphate, CuSO₄, is formed, which is white. The change is therefore due to loss of bound water, not addition of oxygen or conversion into common salt or an acid. Adding water can restore the blue hydrated form, so option B is correct.

Ammonium chloride, NH₄Cl, is produced by the neutralisation reaction HCl + NH₄OH → NH₄Cl + H₂O. Hydrochloric acid is strong, whereas ammonium hydroxide is a weak base, so option D matches the stated condition. NaCl comes from a strong acid and strong base; CH₃COONa comes from a weak acid and strong base; Na₂CO₃ is also associated with a strong base and weak acid.

Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. When mixed with water, it rehydrates and changes into gypsum, CaSO₄·2H₂O, forming a hard solid: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. Its rapid setting and rigid support help immobilise a fractured bone, so option A is correct.

The governing concept is the extent of ionisation in water. A strong acid ionises almost completely and produces a high concentration of hydrogen or hydronium ions. Hydrochloric acid, HCl, is therefore a strong acid, so option A is correct. Acetic acid in vinegar, carbonic acid, and citric acid ionise only partially and are commonly classified as weak acids. Strength refers to ionisation, not merely to the amount of acid present.

A weak acid ionises only partially in water, so it produces fewer hydrogen ions than a strong acid of comparable concentration. Acetic acid, the principal acid in vinegar, is a weak acid; therefore option D is correct. Hydrochloric acid and nitric acid are strong acids, and sulphuric acid is also treated as a strong acid at the school level. The distinction is based on ionisation, not on whether the substance tastes sour.

The governing concept is concentration: it describes the amount of solute present in a given amount of solution. A concentrated acid solution contains a relatively larger amount of acid, whereas a dilute solution contains a smaller amount, so option A is correct. Concentration must not be confused with strength. Strength concerns the extent to which an acid ionises in water; a weak acid may be concentrated, and a strong acid may be dilute.

The governing concept is exothermic dilution. When an acid dissolves and its particles become hydrated in water, heat is released; consequently, the temperature of the solution generally increases. Thus option B is correct. The temperature does not necessarily fall to zero or remain unchanged. Because rapid heat production can cause splashing, the safe laboratory rule is to add acid slowly to water with stirring, never to pour water quickly into concentrated acid.

The governing concept is that acids show their characteristic behaviour in aqueous solution. Dry hydrogen chloride gas does not dissociate to produce hydrogen ions without water, so dry litmus has no acidic ions with which to react. Therefore option A is correct. Litmus can identify acids, and hydrogen chloride itself is not basic. When the litmus is moist, HCl dissolves and ionises, allowing the expected colour change to occur.