Acids, Bases, and Salts

An acidic solution contains hydrogen ions, H⁺, while sodium hydroxide dissociates in water to provide hydroxide ions, OH⁻. The central ionic neutralisation reaction is H⁺ + OH⁻ → H₂O. Sodium ions and any accompanying anions generally remain as spectator ions and may form a dissolved salt. Therefore option A correctly states the main change; the other options involve chemically unsupported transformations.

The governing concept is dilution and hydrogen-ion concentration. Adding water increases the volume, so the concentration of hydrogen ions decreases and the pH rises toward 7. However, dilution alone does not automatically remove every hydrogen ion or create an excess of hydroxide ions. Therefore, while the acid becomes weaker, it remains acidic as long as hydrogen ions still determine the solution’s behaviour. Option B is false because water does not convert every acid into a base, and D is false because pH does change.

Neutralisation depends on the amounts of reacting hydrogen and hydroxide ions, not merely on the volumes of the two solutions. Equal volumes can contain very different numbers of particles when their concentrations differ; their acids or bases may also differ in strength. If the acid supplies a greater effective amount of hydrogen ions than the base can neutralise, some acidity remains and the final pH stays below 7. Thus A is correct. Equal volume does not guarantee neutrality, and salt formation does not produce hydrogen gas here.

The governing idea is that neutralisation compares reacting amounts, not just liquid volumes. Equal volumes of an acid and a base may contain unequal concentrations of ions. A more concentrated or stronger base can provide more hydroxide ions than the acid can neutralise. After the reaction, excess hydroxide ions remain, so the mixture has a pH above 7 and is basic. Option C is wrong because equal volume has no fixed pH, while D is wrong because acid-base neutralisation commonly forms water along with salt.

The governing property of a base is its ability to neutralise an acid. Many metal oxides show this behaviour: they react with acids to produce a salt and water, for example, copper oxide reacts with hydrochloric acid to form copper chloride and water. This neutralisation reaction supports their generally basic classification. Option B describes acidic behaviour and is not universally true; metal oxides are not always gases, and option D has no chemical basis. The word generally also allows recognized exceptions such as amphoteric oxides.

An acidic oxide generally forms an acidic solution with water or reacts with a base to form salt and water. Many non-metal oxides show one or both of these behaviours; for example, carbon dioxide dissolves in water to form carbonic acid and reacts with bases to produce carbonates. Hence they are generally classified as acidic oxides. Option C describes a basic effect, while B and D make absolute claims that are not chemically valid for all non-metal oxides.

Copper oxide is a metal oxide, and its reaction with hydrochloric acid demonstrates the basic nature of many metal oxides. The reaction is CuO + 2HCl → CuCl₂ + H₂O. Copper(II) chloride dissolves in the solution and gives it a blue-green colour, while water is also formed. Thus the observation supports option A: copper oxide reacts with an acid to produce a salt and water. The other options contradict the reaction or the observed chemistry.

Sodium carbonate reacts with an acid to release carbon dioxide: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Carbon dioxide is identified by passing it through limewater, where it reacts with calcium hydroxide to form insoluble calcium carbonate, making the limewater milky. A pop sound is the test for hydrogen, not carbon dioxide. Litmus and turmeric describe general acid-base behaviour and are not the specific gas test required here.

Hard water contains dissolved calcium and magnesium ions, which interfere with soap and form scum. Washing soda, sodium carbonate (Na₂CO₃), supplies carbonate ions. These combine with calcium and magnesium ions to form insoluble calcium carbonate and magnesium carbonate precipitates, removing the troublesome ions from the water. Consequently, soap can lather more readily. It does not produce hydrogen, increase acidity, or destroy sodium ions, so option A correctly explains the softening process.

The governing concept is that pH measures the acidic or basic nature of water. Acid rain adds acidic substances and may lower the lake’s pH. Aquatic organisms often need a limited pH range for enzyme activity, respiration, reproduction, and maintaining body chemistry. Therefore, very low pH can disrupt life processes and reduce survival. The other options are scientifically unrelated or use absolute claims.

Universal indicator gives different colours over a pH range: blue generally indicates a basic solution, while green indicates a value close to neutral. When acid is added, hydrogen ions react with hydroxide ions in the basic solution, reducing its alkalinity. If the colour becomes green, the mixture has reached approximately neutral pH. It has not become more basic, and the observation does not prove that it is a strong acid.

The governing principle is neutralisation. A red universal-indicator colour represents a strongly acidic region, with a relatively high hydrogen-ion concentration. Adding a base supplies hydroxide ions, which react with hydrogen ions to form water. This decreases acidity and moves the pH toward 7. Green indicates a nearly neutral mixture, so option A is correct. The result does not show that the solution became strongly basic or that the base had no effect.

pH is related inversely to hydrogen-ion concentration: when hydrogen-ion concentration decreases, pH rises. An acidic solution contains excess H⁺ ions. Added hydroxide ions, OH⁻, react with them according to H⁺ + OH⁻ → H₂O. This neutralisation removes free hydrogen ions from the solution, so its acidity falls and its pH increases. The other choices describe neither the relevant reaction nor the reason for the pH change.

Electrical conductivity in an aqueous solution is caused by mobile charged particles, especially hydrogen ions and the corresponding negative ions. A highly ionised acid produces a larger concentration of such mobile ions in water, so it carries electric current more effectively. A dry gas, a solid, or a substance producing no ions cannot provide comparable ionic conduction. Therefore, option A is correct.

A basic oxide is generally identified by its reaction with acids, but aluminium oxide also reacts with strong bases. With an acid it forms an aluminium salt and water; with a strong base it forms a soluble aluminate. A substance showing both kinds of reaction is called amphoteric. Therefore, describing aluminium oxide as only basic is incomplete, and option A gives the correct reason.

Calcium oxide is a metal oxide and behaves as a basic oxide. Carbon dioxide is a non-metal oxide and behaves as an acidic oxide. They undergo a combination reaction: CaO + CO₂ → CaCO₃. Calcium carbonate is a salt formed when the basic oxide neutralises the acidic oxide. Hence option A correctly describes the reaction; the other choices do not match the reactants or reaction type.

Hydrochloric acid and sodium hydroxide undergo neutralisation: HCl + NaOH → NaCl + H₂O. Since sodium hydroxide is a base rather than a carbonate or reactive metal, the reaction normally produces no gas. Carbonates and hydrogencarbonates release carbon dioxide with acids, while zinc can release hydrogen. Therefore option A is the only suitable pair.

A metal carbonate reacts with an acid in three stages conceptually: the carbonate produces carbonic acid, which decomposes into water and carbon dioxide, while the metal ion combines with the acid radical to form a salt. The general equation is acid + metal carbonate → salt + water + CO₂. Active metals usually release hydrogen, whereas oxides and bases generally form salt and water only. Hence option A is correct.

The governing concept is the pH scale: at 25°C, pH = 7 is neutral, values below 7 are acidic, and values above 7 are basic. A higher pH corresponds to a lower hydrogen-ion concentration and, in a basic solution, a higher hydroxide-ion concentration. Therefore, among pH 12, 7, 5, and 2, the pH 12 solution has the greatest hydroxide concentration. pH 7 is neutral, while pH 5 and pH 2 are acidic.

The pH scale measures acidity through hydrogen-ion concentration: pH = −log[H⁺]. When pH rises from 3 to 6, the hydrogen-ion concentration decreases greatly, so the solution becomes less acidic. However, pH 6 is still below the neutral value of 7; it therefore remains acidic rather than becoming basic. Option B reverses the pH relationship, option C ignores the value 7, and option D is chemically impossible for an aqueous solution.

A basic solution has pH above 7 because its hydroxide-ion concentration is relatively high. Adding a suitable amount of acid supplies hydrogen ions, which react with hydroxide ions in the neutralisation reaction H⁺ + OH⁻ → H₂O. This reduces the basic effect and can bring the pH close to 7, provided the amount is controlled. More base would raise pH, while removing water or changing salt into a gas does not describe the required neutralisation.

Dilution increases the volume of an acid solution without removing its acidic substance completely. Consequently, the concentration of hydrogen ions decreases and the pH rises. The solution therefore becomes less acidic and moves toward neutrality, but it does not automatically cross pH 7 to become basic. Water does not convert every acid into a base, the ions are not destroyed, and pH 14 is not the inevitable result of dilution. The final pH depends on the acid and the amount of water added.

Dilution of a concentrated acid is highly exothermic, meaning that considerable heat is released when water and acid mix. If a small amount of water is poured into the acid, the water may heat rapidly, boil locally, and eject acid droplets. These droplets can cause serious chemical burns. The safe laboratory rule is to add acid slowly to a larger volume of water while stirring and wearing protection. The other choices contradict the actual thermal behaviour.

Sodium chloride is formed from hydrochloric acid, a strong acid, and sodium hydroxide, a strong base. In water, NaCl dissociates mainly into Na⁺ and Cl⁻ ions, which do not produce a significant excess of H⁺ or OH⁻. Its aqueous solution is therefore approximately neutral and does not change the colours of red or blue litmus paper. It is neither a strong acid nor a strong base, and it does not destroy the indicator under ordinary conditions.

Sodium carbonate is considered a salt of the strong base sodium hydroxide and the weak acid carbonic acid. In water, carbonate ions undergo hydrolysis and produce hydroxide ions: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻. The formation of OH⁻ makes the solution basic, so it can turn red litmus blue. It is not strongly acidic, it does dissociate and react in water, and it is a compound rather than merely a metal.