Acids, Bases, and Salts

The governing concept is the difference between electrical neutrality and the absence of ions. Sodium chloride dissolves in water and dissociates into Na⁺ and Cl⁻ ions. Their positive and negative charges are equal, so the solution is electrically neutral, but the mobile ions can still conduct electricity. Therefore option A is correct; neutrality does not mean that ions are absent, while the other options contradict the solubility and composition of sodium chloride.

The governing reaction is the reaction of a carbonate with an acid. Marble is composed mainly of calcium carbonate, CaCO₃. Acidic rain supplies hydrogen ions, and carbonate reacts to produce a calcium salt, water, and carbon dioxide; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. This gradually erodes the surface, so option A is correct. The other choices misidentify marble or reverse the effect of acids.

The governing concept is thermal decomposition of sodium hydrogen carbonate, commonly called baking soda. On heating, 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The released carbon dioxide accounts for the gas, while the remaining sodium carbonate dissolves to give a basic solution because carbonate ions hydrolyse water and produce hydroxide ions. Thus option B is correct; the other substances are not the solid product of this decomposition.

The governing concept is the removal of hardness-producing ions by precipitation. Hard water mainly contains dissolved Ca²⁺ and Mg²⁺ ions. Washing soda, Na₂CO₃, supplies CO₃²⁻ ions, which form sparingly soluble calcium carbonate and magnesium carbonate: Ca²⁺ + CO₃²⁻ → CaCO₃ and Mg²⁺ + CO₃²⁻ → MgCO₃. These precipitates can be separated, so option B is correct; sodium and chloride ions do not cause hardness.

The governing concept is neutralisation and correction of soil pH. Highly acidic soil has an excess of acidic substances and a low pH. Slaked lime, Ca(OH)₂, is a base; when applied in a carefully measured amount, it reacts with acidic components and raises the soil pH toward a more suitable range. Therefore option C is correct. Lemon juice, vinegar, and dilute acid are acidic and would generally intensify the problem rather than correct it.

The governing concept is the stability of bleaching powder and its available chlorine. Bleaching powder is used because it can release chlorine for bleaching and disinfection. Exposure to moisture and air promotes slow decomposition and loss of available chlorine, reducing its effectiveness. Thus option A is correct, and it should be stored in a dry, tightly closed container. The other options describe impossible or chemically unrelated changes, such as formation of gold or gypsum.

The governing concept is acid–base indicators and neutralisation. A red litmus paper turning blue initially shows that the solution is basic. As acid is added, it consumes hydroxide ions and neutralises the base. When neither red nor blue litmus changes colour, the solution is at the neutral point under the stated school-level conditions. Therefore option C is correct; highly acidic or basic solutions would affect at least one indicator.

The governing concept is the basic nature of most metal oxides. A basic metal oxide reacts with an acid in a neutralisation reaction, producing a salt and water; for example, CuO + 2HCl → CuCl₂ + H₂O. Thus option A correctly describes their usual behaviour. Option B is wrong because metal oxides are not always acidic, while C and D have no chemical basis. Some metal oxides can be amphoteric, but that does not change the general rule.

The governing indicator principle is that an acidic solution changes blue litmus to red. Many non-metal oxides, such as carbon dioxide and sulphur dioxide, dissolve or react with water to form acidic solutions. Therefore the oxide described is acidic, so option B is correct. Option A would be supported by a red-to-blue change, whereas metallic and neutral-solid descriptions do not explain the observed litmus result. The test concerns the solution formed in water.

The governing concept is an acid–base reaction that releases a gas. Baking soda, sodium hydrogen carbonate (NaHCO₃), reacts with the edible acid when moisture is present, and heating helps the reaction proceed: acid + NaHCO₃ → salt + water + CO₂. The carbon dioxide bubbles expand in the batter and make baked food rise and become porous. Therefore A is correct; chlorine, metal, and permanent acidity are not products or purposes of this mixture.

The governing concept is water of crystallisation. Hydrated copper sulphate, CuSO₄·5H₂O, is blue because water molecules are incorporated in its crystal structure. On heating, it loses this water and becomes white anhydrous CuSO₄. Adding water restores the hydrated form and the blue colour. Therefore option B is correct. Sodium chloride and sodium carbonate are not identified by this reversible blue–white change, while calcium carbonate is not a blue hydrated salt.

The governing concepts are salt hydrolysis and ionic conduction. A salt formed from a strong acid and a strong base, such as sodium chloride, generally gives a neutral solution with pH close to 7 because its ions do not significantly hydrolyse water. It conducts electricity because the dissolved salt separates into mobile Na⁺ and Cl⁻ ions. Therefore A is correct. Strong-acid/weak-base salts are acidic, weak-acid/strong-base salts are basic, and an ion-free salt could not conduct.

The governing process is the chlor-alkali process, in which concentrated aqueous sodium chloride (brine) is electrolysed. Chloride ions move to the anode and are oxidised: 2Cl⁻ → Cl₂ + 2e⁻. Thus chlorine gas is formed at the anode and is used for disinfecting or purifying water, making option B correct. Hydrogen is produced at the cathode, not the anode; sodium metal is not deposited from aqueous brine, and carbon dioxide is unrelated.

The governing concept is stoichiometric neutralisation, which depends on the number of reactive acid and base equivalents, not merely on equal volumes. For example, equal volumes of 0.1 M HCl and 0.1 M NaOH can approach pH 7, but unequal concentrations or a weak acid/base can leave excess H⁺ or OH⁻ and produce an acidic or basic pH. Therefore A is correct. Acids and bases do react, pH is not determined only by colour, and salts are not always acidic.

The governing concept is the reactivity series of metals. Metals above hydrogen can generally displace hydrogen from dilute acids. Magnesium is above hydrogen and reacts to form a magnesium salt and hydrogen gas, producing brisk bubbles: Mg + 2HCl → MgCl₂ + H₂. Copper is below hydrogen, so it does not normally liberate hydrogen from dilute non-oxidising acids. Therefore option A is correct; the other statements are chemically false.

The relevant concept is the preparation and uses of bleaching powder. Bleaching powder, commonly represented as CaOCl₂ at school level, releases available chlorine. This chlorine oxidises many coloured substances, so the material is used for bleaching textiles and paper-related materials. It also kills many disease-causing microorganisms in water, making it a disinfectant. Washing soda, gypsum, and baking soda do not provide this combined bleaching and disinfecting action. Hence B is correct.

The governing concept is pH and dilution. A basic solution has a hydroxide-ion concentration greater than that of neutral water. Adding water increases volume without adding an equivalent amount of base, so the hydroxide-ion concentration decreases. Consequently, pOH decreases and pH moves downward toward 7, although the solution remains basic if it is not diluted infinitely. It does not become acidic immediately, so option A is correct.

The governing process is the chlor-alkali process. When an aqueous solution of common salt, sodium chloride, is electrolysed, sodium hydroxide is formed along with chlorine and hydrogen: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Sodium hydroxide is a strong alkali used to make soap through saponification and is also important in paper manufacture. The other listed substances are not the relevant product of this process, so A is correct.

This is the lime-water test for carbon dioxide. Initially, carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate, which appears as a white precipitate and makes the solution milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. If excess carbon dioxide is passed, the precipitate reacts further to form soluble calcium hydrogen carbonate, Ca(HCO₃)₂, so the milkiness disappears. Thus option A explains the initial change.

The governing concept is that the pH of a salt solution depends on the acid and base from which the salt is formed. Sodium chloride, from a strong acid and strong base, gives an approximately neutral solution. Ammonium chloride is formed from a strong acid, hydrochloric acid, and a weak base, ammonia; its aqueous solution is acidic because ammonium ions hydrolyse and produce hydronium ions. Therefore B disproves the claim, while the other choices do not provide an acidic salt solution.

The governing concept is neutralisation and the definition of pH. An acidic solution contains an excess of hydrogen or hydronium ions. Added hydroxide ions react with them to form water: H₃O⁺ + OH⁻ → 2H₂O, or equivalently H⁺ + OH⁻ → H₂O. This lowers hydrogen-ion concentration, and because pH = −log[H⁺], the pH rises. The other options do not describe neutralisation, so A is correct.

The relevant concept is the reaction of food acids with reactive metals. Curd contains lactic acid and is therefore acidic. During prolonged contact, acids can react with copper, allowing copper compounds or salts to form and possibly contaminate the food. Excess copper intake can be harmful, so acidic foods are not safely stored in copper or brass vessels unless properly lined. The claims that curd becomes sweet, becomes water, or cannot react are false; hence A is correct.

The governing concept is the identification and composition of common salts. Baking soda is sodium hydrogen carbonate, NaHCO₃. Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. Bleaching powder is a chlorine-containing bleaching compound commonly represented at school level as CaOCl₂; it is not sodium chloride, NaCl, which is common salt. Therefore option D is the incorrect pair.

The governing concept is the characteristic reaction of acids. An acid turns blue litmus red, showing that the solution is acidic. Acids also react with carbonates to form a salt, water, and carbon dioxide; the carbon dioxide escapes as visible bubbles. Therefore option A is correct. A basic or neutral solution would not give both of these observations under the stated conditions.

The pH scale expresses hydrogen-ion activity: values well above 7 indicate a strongly basic solution. A pH of 13 is therefore consistent with a highly alkaline, potentially caustic substance that can damage skin and eyes. Option A is correct, so the solution should be handled with gloves and eye protection. Pure water is near pH 7, while acids have pH below 7.