Acids, Bases, and Salts

The governing concept is the characteristic response of acid–base indicators. A basic solution changes red litmus to blue, and turmeric develops a reddish-brown colour in the presence of a base. Since both independent observations point to alkalinity, option B is correct. An acid would produce the opposite litmus change, neutrality would not give these characteristic responses, and “gaseous” describes a state rather than the solution’s chemical nature.

An active metal reacts with an acid to form a salt and release hydrogen gas, for example: zinc + hydrochloric acid → zinc chloride + hydrogen. Hydrogen is tested by bringing a burning splint near the collected gas; it burns rapidly with a characteristic ‘pop’ sound. Therefore option B is correct. Limewater milkiness tests carbon dioxide, while turmeric and phenolphthalein indicate alkaline conditions rather than hydrogen.

When a carbonate reacts with an acid, the products are a salt, water, and carbon dioxide; in ionic form, carbonate ions react with hydrogen ions to produce CO₂ and water. Passing this gas through limewater forms insoluble calcium carbonate, which makes the solution milky. Thus option B is correct. Hydrogen gives a pop test, whereas oxygen and nitrogen do not produce the characteristic limewater reaction.

Limewater is a dilute solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is passed through it, the reaction is Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The calcium carbonate formed is insoluble and appears as a white suspension, producing the milky appearance. Therefore option A is correct. The other substances listed do not form the characteristic precipitate responsible for this observation.

Initially, carbon dioxide reacts with calcium hydroxide in limewater to form insoluble calcium carbonate, causing milkiness. When excess CO₂ is passed, the precipitate reacts further: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Calcium hydrogen carbonate is soluble in water, so the solid precipitate disappears and the milkiness is removed. Hence option A is correct; the other choices do not explain this reaction.

Neutralisation occurs when an acid reacts with a base. The acid supplies hydrogen ions, H⁺, while the base supplies hydroxide ions, OH⁻. These ions combine in the net ionic reaction H⁺ + OH⁻ → H₂O, producing water. Sodium and chloride ions may remain as spectator ions, while the other pairs do not directly form water in this process. Therefore, option A is correct.

Sodium hydroxide, NaOH, is a strong base and an ionic compound. In water it dissociates almost completely according to NaOH → Na⁺ + OH⁻. The hydroxide ions increase the concentration of OH⁻ in the solution and give it basic properties. Hydrogen ions are associated with acids, whereas carbonate and nitrate ions are not present in sodium hydroxide. Thus option B is correct.

Hydrochloric acid, HCl, is a strong acid. When it dissolves in water, it ionises almost completely, forming hydronium ions and chloride ions; this is often represented at school level as HCl → H⁺ + Cl⁻. The H⁺ ions account for acidic properties. Hydroxide ions are produced by bases, and sodium or carbonate ions are not constituents of HCl. Therefore option A is correct.

The extent of ionisation distinguishes strong and weak acids. Hydrochloric acid is a strong acid, so its molecules ionise almost completely in water, producing a high concentration of H₃O⁺ ions. Ethanoic acid and carbonic acid are weak acids and ionise only partially; vinegar contains dilute ethanoic acid. Hence hydrochloric acid ionises more completely, making option A correct.

Electrical conductivity in an aqueous solution depends largely on the concentration and mobility of charged particles. At the same concentration, hydrochloric acid, a strong acid, ionises almost completely and produces more ions. Ethanoic acid is weak and ionises only partially, so it supplies fewer charge carriers and conducts less electricity. It is not a base and does contain water when tested as a solution. Option A is correct.

A metal oxide that reacts with an acid to produce salt and water is undergoing a neutralisation reaction. For example, CuO + 2HCl → CuCl₂ + H₂O. In this reaction, the metal oxide behaves like a base because it neutralises the acid. Therefore metal oxides are generally basic in nature, although a few oxides may be amphoteric. Option B is the appropriate answer.

Many non-metal oxides are acidic oxides. When they dissolve in or react with water, they can form acids; for example, CO₂ + H₂O ⇌ H₂CO₃. The resulting solution may show acidic behaviour, such as turning blue litmus red. The word ‘many’ is important because not every non-metal oxide behaves identically. Thus option A is correct, while ‘always neutral’ is too absolute.

Carbon dioxide is an oxide of a non-metal and shows acidic behaviour. With water it forms carbonic acid: CO₂ + H₂O ⇌ H₂CO₃. It also reacts with bases, such as Ca(OH)₂, to form a salt and water: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. These reactions justify its classification as an acidic oxide. It does not turn red litmus blue, so option A is correct.

Turmeric contains a natural indicator substance called curcumin. In an acidic or neutral medium, turmeric generally remains yellow, but in contact with a basic substance it changes to a reddish-brown colour. This colour change occurs because the indicator responds to the alkaline medium. Blue and green are not the characteristic turmeric response, and remaining yellow describes non-basic conditions. Therefore option B is correct.

Phenolphthalein is an acid-base indicator whose colour depends on the pH of the solution. It is colourless in acidic and approximately neutral solutions, but it turns pink or magenta in a sufficiently basic solution. Therefore an acidic solution does not produce the pink colour associated with a base. Since blue and green are not its characteristic responses, option B, colourless, is correct.

Methyl orange is an acid–base indicator whose colour depends on the acidity of the solution. It is red in an acidic medium and changes through an intermediate shade near its transition range, but it appears yellow in a basic or alkaline medium. Therefore, option B is correct. Blue and violet are not the standard alkaline colour of methyl orange, while red indicates an acidic medium.

Baking soda is sodium hydrogen carbonate, NaHCO₃. When it reacts with an acid, the reaction produces carbon dioxide gas, along with a salt and water; for example, an acid plus hydrogencarbonate releases CO₂. The gas gets trapped as small bubbles in the batter or dough, making it expand, rise, and become porous. Thus option A is correct; the other choices describe substances or changes not produced in this reaction.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Carbon dioxide escapes as a gas and water leaves as vapour, so sodium carbonate remains as the principal solid residue. Therefore option A is correct. Sodium chloride, calcium carbonate, and ammonium chloride are not the solid products of this decomposition.

Hard water mainly contains dissolved calcium ions, Ca²⁺, and magnesium ions, Mg²⁺, which react with soap to form scum. Washing soda, sodium carbonate, supplies carbonate ions that precipitate these ions as insoluble carbonates, such as calcium carbonate and magnesium carbonate. Removing them reduces water hardness, so option A is correct. Sodium and chloride ions generally remain soluble and do not cause ordinary hardness.

Washing soda is the hydrated salt sodium carbonate decahydrate, written as Na₂CO₃·10H₂O. The ten water molecules are called water of crystallisation and are part of the crystal structure. Therefore option A is correct. Sodium hydrogen carbonate is baking soda, calcium oxychloride is bleaching powder, and calcium sulphate hemihydrate is plaster of Paris, so those alternatives name different compounds.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. When it is heated carefully at about 373 K, part of its water of crystallisation is removed and calcium sulphate hemihydrate, CaSO₄·½H₂O, is formed; this is plaster of Paris. Controlled heating is essential because excessive heating can remove too much water and produce a different form. Therefore option A is correct, while mixing salt, cooling, or converting it to sugar is irrelevant.

Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. When water is added, it combines with water and re-forms gypsum, calcium sulphate dihydrate, CaSO₄·2H₂O. The newly formed interlocking gypsum crystals set into a hard mass, which is why the material is useful for casts and moulds. Thus option A is correct; no sodium chloride, carbon dioxide, or sugar is formed in this setting reaction.

Blue copper sulphate crystals are hydrated copper sulphate, CuSO₄·5H₂O. On heating, they lose their water of crystallisation and become anhydrous copper sulphate, CuSO₄, which is white or nearly white. Adding water can restore the blue hydrated form. Therefore option A is correct. Chlorine is not added, sodium is not produced, and carbon dioxide is not responsible for this colour change.

Water of crystallisation is the fixed number of water molecules chemically associated with the formula unit of certain crystalline salts. It is not merely moisture on the surface or any water used to dissolve a substance. For example, washing soda contains ten water molecules, Na₂CO₃·10H₂O, and blue copper sulphate contains five, CuSO₄·5H₂O. Therefore option A gives the correct definition; the other choices describe unrelated forms of water.

The governing concept is the preparation of bleaching powder, also called calcium oxychloride. Chlorine gas is passed over dry slaked lime, Ca(OH)₂, to form bleaching powder, commonly represented as CaOCl₂. Thus option A is correct. Sodium chloride is used to make brine, while gypsum and washing soda are not the reactants used in this preparation.