Acids, Bases, and Salts

This is a neutralisation reaction: an acid reacts with a base to form salt and water. The balanced equation is HNO₃ + NaOH → NaNO₃ + H₂O. Nitric acid supplies the nitrate ion, NO₃⁻, and sodium hydroxide supplies Na⁺; these ions combine in a 1:1 ratio to form sodium nitrate. Chloride, sulphate and carbonate are absent, so the other options cannot be formed.

Potassium hydroxide and hydrochloric acid undergo neutralisation. The balanced equation is KOH + HCl → KCl + H₂O. Potassium hydroxide contributes K⁺, while hydrochloric acid contributes Cl⁻; the ions combine to form potassium chloride, and H⁺ combines with OH⁻ to form water. Sodium, sulphate and calcium ions are not present, so options A, B and D do not follow from the reactants.

Magnesium oxide is a basic metal oxide, so it reacts with hydrochloric acid in a neutralisation reaction. The balanced equation is MgO + 2HCl → MgCl₂ + H₂O. Magnesium combines with chloride ions to form magnesium chloride, while the oxide ion combines with hydrogen ions to form water. No sulphate, sodium or carbonate is present, so the other products are not possible.

An oxide that reacts with water to produce an acidic solution is classified as an acidic oxide. Sulphur dioxide is an oxide of the non-metal sulphur; it reacts with water to form sulphurous acid: SO₂ + H₂O → H₂SO₃. This acidic product explains the observation. A basic oxide would produce a basic solution, a neutral oxide would not show this behaviour, and “metallic oxide” is not the correct chemical classification here.

The relevant experiment compares dry hydrogen chloride with hydrogen chloride dissolved in water. Dry HCl gas does not change the colour of dry litmus because ions cannot move or form effectively without an aqueous medium. When HCl dissolves in water, it ionises to H₃O⁺ and Cl⁻, and the hydronium ions produce acidic behaviour. Thus the lack of effect on dry litmus demonstrates the need for water.

Indicator colours reveal the solution’s acid-base nature. Methyl orange is yellow in a basic or sufficiently less acidic medium, while phenolphthalein is pink in a basic medium, generally above its transition range. Since both observations are consistent with a basic solution, option C is correct. An acidic solution would make methyl orange red and phenolphthalein colourless; a neutral solution would not make phenolphthalein pink.

Both indicator tests identify an acidic medium. Methyl orange turns red in an acidic solution, and acids turn blue litmus red because hydrogen or hydronium ions are present. Therefore the observations independently support option A. A basic solution would turn methyl orange yellow and blue litmus would remain blue, while a neutral solution would not produce either red change. Being a salt solution alone does not guarantee basicity.

The governing concept is water of crystallisation, which is chemically bound water present in certain salt crystals. Blue copper sulphate is hydrated copper sulphate, CuSO4·5H2O. On heating, it loses this water and changes from blue to white anhydrous copper sulphate; adding water restores the blue colour. The other options do not show this characteristic reversible colour change in this test.

A hydrated salt contains a definite number of water molecules within its crystal structure. Washing soda is sodium carbonate decahydrate, Na2CO3·10H2O, so each formula unit is associated with ten molecules of water of crystallisation. Therefore, option A is correct. It is still an ionic salt; it is not merely water, ion-free, or a gas, as claimed by the other options.

Plaster of Paris is prepared by heating gypsum, CaSO4·2H2O, under controlled conditions so that part of its water of crystallisation is removed, producing CaSO4·½H2O. If gypsum is overheated, excessive water may be removed and anhydrous calcium sulphate can form. This material does not rehydrate and set in the desired way, so option C is correct; the other choices describe impossible changes.

The relevant property is the setting reaction of plaster of Paris. When water is added to calcium sulphate hemihydrate, it changes back into gypsum, CaSO4·2H2O, and forms a hard solid mass. It can therefore be shaped in a mould before setting. Option A is correct; immediate dissolution, acid formation, and hydrogen release are not properties responsible for making statues or moulds.

On heating, baking soda decomposes according to 2NaHCO3 → Na2CO3 + H2O + CO2. The sodium carbonate left in the food is alkaline and can impart an unpleasant bitter or soapy taste when present in excess. Thus option A correctly identifies its basic nature. It is not a strong acid, a gas, or sugar, so options B, C, and D are chemically unsuitable.

Baking powder contains sodium hydrogen carbonate and a mild edible acid. During mixing and heating, the acid reacts with sodium hydrogen carbonate and releases carbon dioxide: acid + NaHCO3 → salt + water + CO2. The gas expands and forms bubbles that remain in the batter, making the cake light and fluffy. Therefore option C is correct; the other gases are not produced.

Hard water contains Ca2+ and Mg2+ ions. These ions react with soap anions to form insoluble scum, so less soap remains available for lather. Washing soda, Na2CO3, supplies carbonate ions that precipitate calcium and magnesium as insoluble carbonates, softening the water. Hence option A is correct; the other choices do not describe softening or lather formation.

The aqueous solution of sodium chloride is called brine. During electrolysis, chloride ions are oxidised at the anode to form chlorine, while water is reduced at the cathode to form hydrogen and hydroxide ions. The sodium ions combine with hydroxide ions to produce sodium hydroxide in solution. Thus option A lists all three useful products; the other options are unrelated.

In electrolysis of brine, chloride ions lose electrons at the anode and form chlorine gas: 2Cl− → Cl2 + 2e−. Chlorine reacts with water to produce disinfecting species, so it is widely used in controlled water treatment. Therefore option A is correct. Making milk sour, setting plaster, and dissolving tooth enamel are not the intended uses of chlorine in this process.

At the cathode during brine electrolysis, water gains electrons and produces hydrogen gas: 2H2O + 2e− → H2 + 2OH−. Hydrogen is used to hydrogenate unsaturated vegetable oils, converting them into more saturated fats such as vanaspati. Hence option B is correct. Bleaching powder is associated with chlorine, while the remaining options do not describe hydrogen’s relevant industrial use.

The chlor-alkali process electrolyses brine, or concentrated sodium chloride solution, and produces chlorine, hydrogen, and sodium hydroxide (NaOH). The alkali left in solution is sodium hydroxide, which is used in making soaps and detergents, paper, and other chemicals. Therefore, option A is correct. The other choices describe no standard industrial use of sodium hydroxide in this context.

Bleaching powder, commonly represented as calcium oxychloride (CaOCl2) at school level, is prepared by passing chlorine gas over dry slaked lime, Ca(OH)2. The reaction produces the bleaching compound used for disinfecting water and bleaching materials. Thus option A gives the correct preparation method. The other mixtures do not provide the required chemical reaction or product.

Bleaching powder is useful because it contains available chlorine, which gives it bleaching and disinfecting action. Moisture and contact with air can gradually decompose or reduce the available chlorine, lowering the substance’s effectiveness. Keeping it in a dry, tightly closed container slows this deterioration. Therefore, option A is correct; the other options describe impossible changes unrelated to its storage.