Acids, Bases, and Salts

The governing concept is salt hydrolysis and the strength of the parent acid and base. NaCl is formed from the strong acid HCl and the strong base NaOH. Its Na⁺ and Cl⁻ ions undergo negligible hydrolysis in water, so the solution is approximately neutral; therefore A is correct. Sodium acetate is basic, ammonium chloride is acidic, and ammonium acetate is approximately neutral only under a more detailed comparison of weak-acid and weak-base strengths, making NaCl the clear school-level choice.

The governing concept is that HCl shows acidic behaviour in the presence of water. Dry hydrogen chloride molecules do not produce hydrogen or hydronium ions on dry litmus paper because water is absent. Without those ions, the acid–base indicator cannot undergo its characteristic colour change, so option A is correct. HCl is not basic; the composition of litmus and the colour of the gas do not explain the absence of the reaction.

Sodium carbonate solution is basic because carbonate ions hydrolyse in water and generate hydroxide ions: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻. It can therefore turn red litmus blue. Sodium chloride is approximately neutral, ammonium chloride is acidic because it comes from a weak base and strong acid, and sodium hydrogen sulfate is acidic. Hence option C is correct.

The pH scale indicates the acidic or basic nature of a solution: pH 7 is neutral, values below 7 are acidic, and values above 7 are basic. Since pH 3 is below 7, this solution is acidic. Acids change blue litmus to red, so option A is correct. Options B, C, and D do not describe the standard blue-litmus response to an acidic solution.

A pH greater than 7 indicates a basic or alkaline solution. Because the given pH is 10, the solution is basic. A base changes red litmus paper to blue, so option A is correct. Red litmus remaining red would suggest no basic change, while yellow or colourless are not the characteristic observations for this indicator test. The conclusion follows directly from the pH scale and litmus behaviour.

On the pH scale, acidity increases as the pH value decreases. Since 2 is lower than 5, the pH 2 solution is more acidic. In terms of hydrogen-ion concentration, a one-unit pH difference represents a tenfold difference; therefore pH 2 has a much greater hydrogen-ion concentration than pH 5. Thus option A is correct, not B, C, or D.

A solution with pH above 7 is basic, and basicity increases as the pH value rises. Both pH 8 and pH 12 are basic, but 12 is higher than 8, so the pH 12 solution is more basic. The difference also corresponds to a much lower hydrogen-ion concentration at pH 12. Therefore option A is correct; options C and D misclassify both solutions.

Phenolphthalein is an acid-base indicator. It is colourless in acidic and neutral conditions, but it develops a pink colour in a basic solution, especially when the pH is sufficiently alkaline. The observed pink colour therefore indicates that the solution is basic, making option A correct. Acidity would keep the indicator colourless, and being a salt alone does not determine the observed colour without knowing its solution properties.

Methyl orange is an acid-base indicator whose colour depends on the medium. It appears red in an acidic solution and changes through orange toward yellow as the medium becomes less acidic or basic. Since the observation is red, the solution is acidic, so option A is correct. A basic solution would produce yellow, while neutral and metallic are not supported by the indicator’s red colour.

Turmeric contains curcumin, which acts as a natural indicator. In an acidic or neutral medium it generally remains yellow, but a basic substance changes it to a reddish-brown colour. Therefore the observed reddish-brown turmeric paper shows that the solution is basic, so option A is correct. The other options do not explain the characteristic colour change produced by an alkaline medium.

Onion is an olfactory indicator because its characteristic smell changes in different chemical media. In a basic medium, the onion odour is reduced or becomes difficult to detect, whereas it generally persists in an acidic medium. Thus a decrease in smell indicates a basic solution, making option A correct. This observation is not a test for insolubility, and neutral or acidic descriptions do not match the usual indicator response.

Hydrogen chloride shows acidic behaviour when it dissolves in water and ionises, producing hydronium ions, H₃O⁺, which are responsible for the acidic reaction. Dry HCl gas in contact with dry blue litmus has no water available for this ionisation, so the ions are not produced and the litmus remains unchanged. Hence option A is correct; colourlessness or basicity is unrelated to the result.

Acids display their characteristic properties in aqueous solution because water allows their molecules to ionise and produce hydrogen ions, more accurately represented as hydronium ions, H₃O⁺. These ions cause acidic behaviour, including litmus change and reaction with suitable substances. Without water, the ions are not produced freely, so the acid does not show the usual aqueous properties. Therefore option A is correct.

The governing concept is the ionisation of bases in water. A soluble base such as sodium hydroxide dissociates to produce Na⁺ and OH⁻ ions, and the presence of OH⁻ gives the solution its basic properties, such as turning red litmus blue. Therefore option A is correct. Hydrogen ions are associated with acidity, while hydroxide ions do not form metals or carbon dioxide in this context.

The governing safety principle is that dilution of a concentrated acid is highly exothermic. If a small amount of water is poured into a large quantity of acid, the released heat can rapidly heat and boil the water at the contact surface. Steam and acid droplets may then be thrown out, causing burns. Hence option A is correct. The safe method is to add acid slowly to water with stirring, never water to acid.

The governing concept is an exothermic change during acid dilution. When acid particles become surrounded by water molecules, energy is released as heat. Adding the acid slowly to a larger volume of water allows this heat to spread safely and prevents local overheating or splashing. Therefore option A is correct. The process is not defined by absorption of light, formation of sound, or zero heat change.

The governing reaction is metal–acid displacement. Zinc, being above hydrogen in the reactivity series, displaces hydrogen from hydrochloric acid: Zn + 2HCl → ZnCl₂ + H₂. The gas released is hydrogen, and a sample gives a characteristic ‘pop’ when tested with a burning splint. Thus option A is correct. Oxygen is not released, chlorine is not produced, and carbon dioxide requires a carbonate or hydrogen carbonate reactant.

The governing rule is that a sufficiently reactive metal displaces hydrogen from an acid. In general form, metal + acid → salt + hydrogen gas. For example, Mg + 2HCl → MgCl₂ + H₂. Therefore option A is correct. Oxygen is not the usual product of this reaction, water is associated with acid–base neutralisation, and carbon dioxide is produced when an acid reacts with a carbonate or hydrogen carbonate.

The governing reaction is acid + metal carbonate → salt + water + carbon dioxide. Carbon dioxide is identified by passing it through limewater; it reacts with calcium hydroxide to form insoluble calcium carbonate, making the limewater milky. Hence option A is correct. A pop sound is the test for hydrogen, while litmus colour changes indicate acidic or basic solutions rather than specifically identifying this gas.

The governing pattern for a metal carbonate reacting with an acid is acid + carbonate → salt + water + carbon dioxide. For example, Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Thus option A lists all three products. Hydrogen belongs to the usual acid–metal reaction, whereas oxygen, a new metal, and a base are not expected products in this reaction.

The governing concept is gas evolution in an acid–hydrogen carbonate reaction. Sodium hydrogen carbonate reacts with an acid to form a salt, water, and carbon dioxide; for example, NaHCO₃ + HCl → NaCl + H₂O + CO₂. The rapid escape of CO₂ bubbles produces effervescence. Therefore option A is correct. No gas freezes, water does not become solid, and the salt is not burning.

The governing concept is neutralisation and ionic exchange. In water, HCl provides H⁺ and Cl⁻ ions, while NaOH provides Na⁺ and OH⁻ ions. H⁺ combines with OH⁻ to form water, and the remaining Na⁺ and Cl⁻ ions form sodium chloride: HCl + NaOH → NaCl + H₂O. Therefore option A is correct; the other pairings do not describe this reaction.

The governing concept is neutralisation, in which the characteristic effects of an acid and a base are reduced by reaction with each other. Hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions form a salt: HCl + NaOH → NaCl + H₂O. Hence option A is correct. The reaction produces definite products; it is not merely a colour change and does not generally form a metal and gas.

An acidic solution has a pH below 7 because it contains a relatively high concentration of hydrogen ions. A base reacts with and neutralises some of this acidity. As base is added gradually, the hydrogen-ion concentration generally falls, so the pH rises toward 7. It may rise above 7 if excess base is added. Therefore, option A is correct; option B gives the opposite trend, while C and D are scientifically incorrect.

A basic solution normally has a pH greater than 7 because its hydroxide-ion concentration is relatively high. When acid is added, hydrogen ions react with hydroxide ions and reduce the solution’s basicity. Consequently, the pH generally decreases, first moving toward 7; with enough acid it can become less than 7. Thus option A is correct. The other options either reverse the trend or make unsupported absolute claims.