Acids, Bases, and Salts

The governing concept is the identification and use of common salts. Baking soda is the common name for sodium hydrogen carbonate, whose chemical formula is NaHCO₃. When heated, it produces carbon dioxide, and this gas helps dough or batter rise. Sodium chloride is common salt, calcium carbonate is found in substances such as limestone, and calcium sulphate hemihydrate is plaster of Paris. Thus, option B is correct.

The governing concept is water of crystallisation in salts. Washing soda is sodium carbonate decahydrate, written as Na₂CO₃·10H₂O, so each formula unit contains ten molecules of crystallisation water. It is used for cleaning and for removing the permanent hardness of water. Sodium hydrogen carbonate is baking soda, calcium oxychloride is bleaching powder, and sodium chloride is common salt. Therefore, option A is correct.

The governing concept is the preparation and hydration of salts. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. It is prepared by carefully heating gypsum, CaSO₄·2H₂O, to remove part of its water of crystallisation. When water is added, plaster of Paris changes back into gypsum and sets into a hard mass. Washing soda, common salt, and calcium carbonate are different compounds, so option A is correct.

The governing concept is the preparation and use of bleaching powder. Chlorine gas is passed over dry slaked lime, Ca(OH)₂, to form bleaching powder, commonly represented at school level as calcium oxychloride, CaOCl₂. Chlorine is therefore the required gas. Hydrogen, carbon dioxide, and nitrogen do not provide the active bleaching constituent in this preparation. Hence option B is correct.

The governing concept is chemical disinfection by available chlorine. In water, bleaching powder releases chlorine-containing species that produce active chlorine, including hypochlorous acid under suitable conditions. These species oxidise and destroy many disease-causing microorganisms, making the water safer. The process does not sweeten water, convert it into a metal, or freeze it. Therefore, option A correctly explains why bleaching powder is used for disinfection.

The governing concept is the identification of an important everyday salt. Common salt is sodium chloride, with the chemical formula NaCl. It is obtained mainly from seawater or rock salt and is used in food as well as in the chlor-alkali process to produce sodium hydroxide, chlorine, and hydrogen. Sodium carbonate, calcium sulphate, and sodium hydroxide are different compounds with different uses. Thus, option A is correct.

The governing concept is electrolysis of brine, an aqueous solution of sodium chloride. At the anode, chloride ions are oxidised to chlorine gas; at the cathode, water is reduced to hydrogen gas and hydroxide ions. The remaining sodium ions combine with hydroxide ions to form sodium hydroxide. Thus the three main products are chlorine, hydrogen, and sodium hydroxide, making option A correct. The other combinations are unrelated products.

The governing concept is the electrode reaction during electrolysis of brine. Brine contains chloride ions, and at the anode oxidation occurs: chloride ions lose electrons and form chlorine gas, represented by 2Cl⁻ → Cl₂ + 2e⁻. Hydrogen is produced at the cathode, while oxygen and carbon dioxide are not the main anode products in this process. Therefore, option B is correct.

The governing concept is reduction at the cathode during brine electrolysis. At the cathode, water gains electrons and produces hydrogen gas and hydroxide ions: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Chlorine is formed at the anode by oxidation of chloride ions. Nitrogen and sulphur dioxide are not products of the chlor-alkali process. Hence, the gas formed at the cathode is hydrogen, so option A is correct.

The governing concept is the distinction between bases and alkalis. A base is a substance that can neutralise an acid, but some bases do not dissolve in water. An alkali is specifically a base that dissolves in water and produces hydroxide ions in aqueous solution. Therefore, the water-soluble base in the question is an alkali, so option B is correct. Acid, salt and indicator are different classes of substances.

The governing concept is the definition of an alkali. Every alkali is a base, but an alkali must also be soluble in water. Some bases, such as copper(II) oxide, do not dissolve appreciably in water; they can still neutralise acids but are not called alkalis. Thus option A correctly explains the one-way relationship. The other statements either confuse acids with bases or give chemically false claims.

The governing concept is water of crystallisation in hydrated salts. Blue copper sulphate crystals are represented as CuSO4·5H2O, meaning that a fixed number of water molecules are incorporated into the crystal structure. This hydrated form has its characteristic blue colour. On heating, the water is removed and anhydrous copper sulphate becomes white. Hence option A is correct; sodium ions, chlorine gas and carbon dioxide are not responsible for this colour.

The governing concept is dehydration of a hydrated salt. Blue copper sulphate crystals contain water of crystallisation, commonly written as CuSO4·5H2O. When the crystals are heated, this bound water is driven off, producing anhydrous copper sulphate, which is white. Therefore option A gives the correct cause of the colour change. Heating does not form sodium, add carbon dioxide, or convert the salt into sugar.

The governing concept is controlled dehydration of gypsum. Gypsum is calcium sulphate dihydrate, CaSO4·2H2O. On careful heating, it loses part of its water of crystallisation and forms calcium sulphate hemihydrate, CaSO4·1/2H2O, commonly called plaster of Paris. Thus option A is correct. Washing soda, baking soda and common salt are different salts and are not products of gypsum heating.

The governing concept is rehydration and setting of plaster of Paris. Plaster of Paris is calcium sulphate hemihydrate, CaSO4·1/2H2O. When water is added, it combines with the material and converts it back into gypsum, CaSO4·2H2O. The newly formed crystalline gypsum interlocks and makes the mixture hard. Therefore option A is correct; no sodium chloride, gas, or sugar is produced in this setting reaction.

The governing concept is precipitation of hardness-causing ions. Hard water contains dissolved calcium and magnesium ions, which interfere with soap and produce scum. Washing soda, sodium carbonate (Na2CO3), reacts with these ions to form insoluble carbonates such as calcium carbonate and magnesium carbonate. The precipitates can be removed, reducing hardness. Hence option A is correct; gas formation, acidification and chlorine removal do not explain softening.

The governing concept is an acid–carbonate reaction that releases a gas. Baking soda is sodium hydrogen carbonate, NaHCO3. With an acid, it produces a salt, water and carbon dioxide; for example, NaHCO3 + H+ forms CO2 and H2O along with the salt. Carbon dioxide bubbles become trapped in the mixture, expanding it and making the food rise and become porous. Therefore option A is correct.

The governing concept is acid–base neutralisation. An ant sting commonly introduces formic acid, which can cause irritation. A mild solution of baking soda, a weak base, can partially neutralise the acid and may reduce discomfort. Therefore option A is the suitable school-level answer. Lemon juice, vinegar and another dilute acid are acidic substances, so applying them would not provide the intended neutralising effect. Any serious reaction requires proper medical care.

The governing concept is the reaction of food acids with reactive metals. Curd contains lactic acid, and many pickles contain acids such as acetic acid. These acids can react with copper when food is stored in a copper vessel for a long time, forming copper salts that may contaminate the food and be harmful in excess. Thus option A is correct. The other choices contradict the presence of acids or make unsupported claims about copper.

The governing concept is the reaction between acids and carbonates. Marble is mainly calcium carbonate, CaCO₃. Acids in rain react with this carbonate, producing soluble calcium salts, water and carbon dioxide; the surface can therefore dissolve and lose detail gradually. Thus option A is correct. Iron, sugar and basic rain do not explain marble damage, so options B, C and D are incorrect.