Acids, Bases, and Salts

The chlor-alkali process electrolyses concentrated aqueous sodium chloride, or brine. At the cathode, water gains electrons and produces hydrogen gas and hydroxide ions. At the anode, chloride ions produce chlorine gas. The sodium ions remaining in solution combine with hydroxide ions to form sodium hydroxide. Therefore option B is correct; calcium hydroxide, oxygen, carbon dioxide, and ammonium hydroxide are not the products in this process.

Dilution of concentrated acid with water is highly exothermic, so a considerable amount of heat can be released. Adding acid slowly to a larger quantity of water allows the heat to disperse through the water and prevents a sudden local temperature rise. This reduces violent boiling and dangerous splashing. Option A is correct. The safe laboratory rule is always ‘add acid to water’, never water to concentrated acid, with stirring and suitable protection.

A salt formed from a strong acid and a strong base, such as sodium chloride from hydrochloric acid and sodium hydroxide, generally gives a solution close to pH 7 because its ions do not significantly hydrolyse water. It still dissociates into mobile ions, so the solution conducts electricity. Therefore option A is correct. Salts in B and C generally make acidic or basic solutions, while D cannot conduct in solution.

Blue litmus turning red shows that the aqueous solution is acidic. Many non-metal oxides, such as carbon dioxide and sulphur dioxide, form acidic solutions in water and are called acidic oxides. An acidic oxide reacts with a base in a neutralisation reaction to produce salt and water; for example, CO₂ + 2NaOH → Na₂CO₃ + H₂O. Hence option A is correct.

The governing concept is neutralisation: an acid reacts with a base to produce salt and water. Many metal oxides, such as copper(II) oxide and magnesium oxide, react with acids in this way, so they show basic character. Therefore option A is correct. Option B reverses the usual rule, while options C and D describe properties that are chemically impossible or irrelevant to this reaction.

Sour foods commonly contain acids such as acetic, citric, or lactic acid. These acids may react slowly with the copper or zinc components of brass, especially during prolonged storage, producing dissolved metal salts. Therefore option A identifies the relevant food-safety risk. Options B and C have no chemical basis, and option D is unsafe because contact and storage time can permit reaction.

The pH relationship is pH = −log[H⁺], so every increase of one pH unit represents a tenfold decrease in hydrogen-ion concentration. The change from pH 4 to pH 7 is three units; therefore the concentration changes by 10³ = 1000 in the decreasing direction. Hence option A is correct, not B, which treats the pH difference as an ordinary multiplier.

The governing idea is salt hydrolysis and the relative strength of the parent acid and base. Ammonium chloride comes from a strong acid and weak base; the ammonium ion hydrolyses and makes the solution acidic. Sodium carbonate comes from a weak acid and strong base; carbonate hydrolysis produces hydroxide ions, making it basic. Thus A is correct.

When oral bacteria metabolise sugars, they produce acids that lower the pH near the teeth. Tooth enamel, which is mainly a mineral containing calcium compounds, begins to lose minerals when the surrounding pH falls below its critical value of about 5.5. Therefore option A is correct. The other choices do not describe acid attack or the chemistry of tooth decay.

A carbonate reacts with an acid to form a salt, water, and carbon dioxide: carbonate + acid → salt + H₂O + CO₂. Passing carbon dioxide through limewater produces insoluble calcium carbonate, which makes the liquid milky. Therefore option B is correct. Hydrogen, oxygen, and nitrogen do not give this characteristic limewater test under these conditions.

Phenolphthalein is colourless in acidic or neutral conditions but turns pink in a sufficiently basic solution. If adding acid makes the pink colour disappear, the solution must initially have contained enough base to produce that colour. The acid lowers the alkalinity through neutralisation. Thus option B is correct; the other choices cannot explain the initial pink indicator colour.

Acidic behaviour in this test requires the gas to dissolve in water and ionise, producing hydrogen ions or hydronium ions that interact with the indicator. Dry litmus provides no water film, so the necessary ions are not formed at the indicator surface and no colour change is observed. Therefore A is correct; the gas still has chemical properties.

A pH of 4.5 indicates that the soil is distinctly acidic. Slaked lime, calcium hydroxide, is a basic substance that can neutralise some of the excess acidity and raise the soil pH when applied in a suitable, tested amount. Therefore option A is correct. Vinegar, lemon juice, and dilute acid would add acidity and make the soil condition worse rather than correct it.

Bleaching powder is manufactured by passing chlorine gas over dry slaked lime, calcium hydroxide. In simplified form, the process is represented as calcium hydroxide reacting with chlorine to produce bleaching powder along with other products. Thus option C is correct. Hydrogen, carbon dioxide, and oxygen are not the characteristic reactant gases used for this preparation, so they cannot explain the manufacture of bleaching powder.

The governing concept is the distinction between a base and an alkali. A base is a substance that can neutralise an acid, but some bases are insoluble in water. Only a base that dissolves in water is called an alkali; for example, sodium hydroxide is an alkali. Therefore option B correctly corrects the student. The other statements confuse solubility, acidity, or neutralisation.

The governing concept is acid-base neutralisation. A basic solution initially has a pH above 7 because hydroxide ions are present in greater effective concentration. As acid is added, hydrogen ions react with hydroxide ions to form water, so the basicity decreases. Near the neutralisation point, the pH approaches 7, making option A correct. The other choices contradict the direction of the pH change or describe an impossible transformation.

The governing concept is the removal of hardness by precipitation. Hard water contains dissolved calcium and magnesium ions. Washing soda, sodium carbonate, supplies carbonate ions that react with these ions and form insoluble calcium carbonate and magnesium carbonate. The precipitates can then be separated, reducing hardness. Thus option A is correct. Sodium ions are not destroyed, and the process neither makes water acidic nor produces chlorine.

The governing concept is the relationship between acidity and pH. Acid rain forms when atmospheric acidic oxides produce acids in rainwater. The dissolved acids increase hydrogen-ion concentration and lower the pH. Such water can react more readily with metals, carbonates, and some building materials, increasing corrosion or damage. Therefore option A is correct. Sugar, absence of ions, and pH 14 do not describe acid rain.

The governing concept is the indicator test for an acidic solution. Excess hydrogen ions in water indicate that the solution is acidic, meaning its pH is below 7. Blue litmus changes to red in an acidic medium, so option A follows directly. Blue litmus does not normally become pink or white in this test, while remaining blue would indicate a neutral or basic condition rather than excess hydrogen ions.

The governing concept is neutralisation of a basic solution. A pH of 11 and pink phenolphthalein both indicate an alkaline medium. When acid is added, its hydrogen ions react with hydroxide ions to form water: H+ + OH− → H2O. Consequently, the effective hydroxide-ion concentration or basic effect decreases first. Option A is correct; chloride ions and sodium mass are not the primary quantity reduced, and water does not lose its chemical identity.