Acids, Bases, and Salts

Carbon is a non-metal, and its oxide carbon dioxide dissolves in water to form carbonic acid: CO2 + H2O ⇌ H2CO3. This makes the solution acidic, so option C is correct. Calcium oxide, magnesium oxide and sodium oxide are generally metal oxides and show basic behaviour, although some oxides can have special or amphoteric properties.

The governing concept is amphoterism. An amphoteric oxide reacts with an acid as a base and with a strong base as an acid, so it can show both types of chemical behaviour. Aluminium oxide and zinc oxide are common examples; ZnO + 2HCl → ZnCl2 + H2O, and ZnO reacts with NaOH in alkaline solution. Hence option C is correct.

An amphoteric substance can react with both an acid and a base. Zinc oxide reacts with hydrochloric acid to form zinc chloride and water: ZnO + 2HCl → ZnCl2 + H2O. It also reacts with a strong alkali such as sodium hydroxide to form a soluble zincate. Therefore, option A correctly explains its name; the other choices do not define amphoterism.

Blue copper sulphate crystals are hydrated copper sulphate, CuSO4·5H2O, which contain water of crystallisation. On heating, this bound water is removed and anhydrous copper sulphate, CuSO4, is formed; it is white. Therefore, option A is correct. No chlorine, hydrogen or sodium is released because none of those species is part of copper sulphate crystals.

Anhydrous copper sulphate is white because it lacks water of crystallisation. When water is present, it becomes hydrated copper sulphate, CuSO4·5H2O, which is blue. This clear colour change makes it a useful qualitative test for water or moisture. Thus option A is correct; oxygen, hydrogen and chlorine require different chemical tests and do not cause this characteristic change.

Plaster of Paris, or calcium sulphate hemihydrate, reacts with water and changes back into gypsum, which is hard. Moisture from air can therefore begin the setting process before the material is needed. Option A is correct because premature setting makes the powder unusable. The other options describe chemically impossible changes: it does not become sugar, gas, or metal. Hence it must be stored in a dry, airtight container.

Gypsum is hydrated calcium sulphate, represented as CaSO4·2H2O. On controlled heating, it loses part of its water of crystallisation and forms plaster of Paris, CaSO4·½H2O. Thus option A correctly identifies the main chemical change. The process does not increase the number of metals, convert a salt into sugar, or involve the complete disappearance of an acid. Excessive heating can remove more water, so control is important.

Acid strength describes the degree to which acid molecules ionise in water; a strong acid ionises almost completely under suitable conditions. Concentration describes the amount of dissolved acid present in a given volume of solution. Therefore, a solution may be concentrated but contain a weak acid, or dilute but contain a strong acid. Option B is correct; the other statements incorrectly treat strength and concentration as identical or opposites.

Electrical conduction in an aqueous solution depends mainly on the number and mobility of charged particles. Hydrochloric acid is a strong acid and ionises nearly completely, producing a greater concentration of H+ and Cl− ions at equal analytical concentration. Ethanoic acid is weak and ionises only partially, so it produces fewer ions. Therefore option A has higher conductivity; neither solution has zero conductivity and they are not generally equal.

An aqueous solution conducts electricity when it contains sufficient mobile ions. Sugar dissolves as neutral molecules and does not appreciably ionise, so the solution has very few charge carriers and does not light the bulb in the simple test. An acid ionises in water and supplies mobile H+ and corresponding negative ions, allowing current to pass. Thus option A is correct; B, C, and D conflict with these observations.

Marble is composed mainly of calcium carbonate, CaCO3. Acids in acid rain react with carbonate, producing soluble calcium salts, water, and carbon dioxide; a representative reaction is CaCO3 + 2H+ → Ca2+ + H2O + CO2. Repeated exposure gradually dissolves and roughens the surface of a statue. Option A is correct; ordinary water does not instantly dissolve marble, and the other claims contradict the reaction.

The pH scale indicates the acidity or basicity of water: lower pH means greater acidity. Aquatic organisms, eggs, and food webs generally function within particular pH ranges. If acid rain lowers the pH greatly, physiological processes can be disturbed and sensitive organisms may die or reproduce poorly. Option A is correct. A pH of 7 is neutral, visual clarity does not determine pH, and every body of water has a temperature.

Dissolving or diluting a concentrated acid in water is generally exothermic: hydration of ions and other interactions release energy as heat. That released heat raises the temperature of the solution and may warm the container. Option A is correct. Heat is not completely removed, water does not become a metal, and the effect is not caused by acid simply stopping gas formation. For safety, acid should be added slowly to water while stirring.

Dilution of concentrated acid releases heat. If a small amount of water is poured into a large volume of acid, the water can heat suddenly, boil, and scatter corrosive acid droplets. The safer method is to add acid slowly to a larger volume of water with continuous stirring, allowing heat to disperse. Therefore option B is correct. Heating on a flame or mixing without proper equipment is dangerous and unacceptable.

Olfactory indicators are substances whose characteristic smell changes, becomes weak, or may disappear in acidic or basic media. The indicator therefore provides information through the sense of smell rather than through a colour change. Onion and vanilla are common school-level examples. Option A is correct. An indicator does not lose all mass, form a metal, or make water disappear merely because it is placed in an acid or base.

Turmeric is a natural acid-base indicator. Its yellow colour changes to reddish brown in a basic or alkaline medium. Since soap solution generally contains alkaline substances formed from a fatty acid and a strong base, it produces this colour change. Therefore, option B is correct. An acidic or neutral solution would not normally turn turmeric reddish brown, while metallic nature is unrelated to indicator behaviour. The substance should be tested safely rather than tasted or rubbed directly.

A universal indicator is a mixture of indicators that gives different colours over the pH scale. Orange generally corresponds to an acidic pH, usually more acidic than yellow but less strongly acidic than red. Therefore, option A is the appropriate conclusion. Green is associated with a nearly neutral solution around pH 7, while violet or purple suggests a strongly basic solution. The colour indicates the solution’s acid-base nature, not whether it contains a metal.

Universal-indicator colours are related to pH. Violet or purple is found at the high-pH end of the indicator scale, so it indicates a strongly basic or alkaline solution. Hence option C is correct. Strong acids are represented by red shades, neutrality near pH 7 by green, and the colour change itself requires an aqueous ionic environment; it does not mean that no ions are present. The exact pH depends on the indicator chart and concentration.

pH is defined as the negative logarithm of hydrogen-ion concentration: pH = −log[H⁺]. Adding water dilutes the acid, so the hydrogen-ion concentration per unit volume decreases. A smaller [H⁺] gives a larger pH value, although the solution may still remain acidic if its pH is below 7. Therefore, option A is correct. Dilution does not automatically make the pH zero or leave it unchanged.

For a basic solution, dilution lowers the hydroxide-ion concentration and reduces its basic strength. Because the ionic product of water relates hydrogen and hydroxide ions, the pH moves downward toward 7 as the solution is diluted, though it generally remains above 7 unless sufficient dilution or other conditions make it neutral. Thus option B is correct. Dilution cannot make the pH exceed 14, become zero, or immediately produce an acidic solution.

An acid reacts with a metal carbonate to produce a salt, water and carbon dioxide. For the pair in option A, the balanced equation is 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂. This exactly gives all three products named in the question. Sodium hydroxide and water do not provide an acid-carbonate reaction, sodium chloride is already a salt, and sugar with water is not the required reaction. Therefore, option A is correct.

An acid-base reaction is called neutralisation. In a typical neutralisation, hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions form a salt; for example, HCl + NaOH → NaCl + H₂O. Thus option C is correct. Acid with a metal commonly releases hydrogen, and acid with a carbonate or hydrogencarbonate releases carbon dioxide, so those pairs can produce gas.

A sufficiently reactive metal can displace hydrogen from a dilute acid. The products are a salt and hydrogen gas; for example, Zn + 2HCl → ZnCl₂ + H₂. Therefore, option A is correct. Acid and base usually undergo neutralisation to form salt and water, while acid and metal carbonate form salt, water and carbon dioxide. Mixing water with common salt does not normally produce hydrogen gas.

The governing safety principle is that dilution of concentrated acid is highly exothermic: considerable heat is released when acid mixes with water. If a small amount of water is poured into acid, that water can heat and boil suddenly at the surface, throwing corrosive acid droplets outward. Therefore option A is correct. The safe procedure is to add acid slowly to a larger volume of water with stirring and protection; the other options describe no relevant chemical effect.

The governing concept is the logarithmic nature of pH: pH = −log10[H+]. An increase of one pH unit means a tenfold decrease in hydrogen-ion concentration. The change from pH 3 to pH 6 is three units, so [H+] becomes 10 × 10 × 10 = 1000 times smaller. Thus acidity decreases by a factor of 1000, making option A correct; it is not merely three times lower.