Acids, Bases, and Salts

The governing relationship is pH = −log₁₀[H⁺]. If the hydrogen-ion concentration changes from [H⁺] to 10[H⁺], the new pH is −log₁₀(10[H⁺]) = −1 − log₁₀[H⁺], which is one unit lower. The solution therefore becomes more acidic. A numerical value cannot be fixed without the starting pH, but the general change is clear. Hence option A is correct.

Blue litmus turning red shows that the solution is acidic. An acidic solution contains hydrogen ions, \(\mathrm{H^+}\). Zinc reacts with these ions to release colourless hydrogen gas: \(\mathrm{Zn + 2H^+ \rightarrow Zn^{2+} + H_2}\). Hence, \(\mathrm{H^+}\) is the correct option. \(\mathrm{OH^-}\) is associated with basic solutions; \(\mathrm{Cl^-}\) can occur in HCl but does not produce the acidic behaviour. Exam tip: Blue litmus changing to red and \(\mathrm{H_2}\) evolution with a metal are key tests for an acid.

The governing reactions are carbonate–acid reaction and the limewater test for carbon dioxide. Acid plus sodium carbonate releases CO2. Initially, CO2 reacts with limewater: CO2 + Ca(OH)2 → CaCO3↓ + H2O, and insoluble CaCO3 causes milkiness. With excess CO2, CaCO3 + CO2 + H2O → Ca(HCO3)2, which is soluble, so the milkiness disappears. Hence option A is correct.

\(\text{HCl}\) is an acid because it releases \(\text{H}^+\) ions in water. \(\text{NaOH}\) is a base because it dissociates to give \(\text{OH}^-\) ions. Therefore the pair \((\text{HCl} \rightarrow \text{acid},\ \text{NaOH} \rightarrow \text{base})\) is correctly matched.

Washing soda has the formula \(\mathrm{Na_2CO_3\cdot10H_2O}\). The term \(10\mathrm{H_2O}\) shows that each formula unit of sodium carbonate is associated with 10 molecules of water of crystallisation. Therefore, option A is correct. Five water molecules occur in copper sulfate pentahydrate, \(\mathrm{CuSO_4\cdot5H_2O}\), not in washing soda. Exam tip: In a hydrated salt formula, the coefficient before \(\mathrm{H_2O}\) after the dot gives the number of water molecules of crystallisation.

A pH of 9 indicates that the salt solution is mildly basic. A salt made from a weak acid and a strong base, such as sodium acetate in the usual school-level example, undergoes hydrolysis: the anion reacts with water and produces OH⁻ ions. Therefore option A is the most probable condition. Strong acid–weak base salts are generally acidic, strong acid–strong base salts are approximately neutral, and acid alone does not define a salt.

The acidic effect of an ant sting can be reduced by neutralisation with a mild base. Baking soda, or sodium hydrogen carbonate (NaHCO₃), is mildly basic, so a paste of it can help counter the acid. Therefore, option A is correct. Lemon juice and vinegar are acidic, and dilute hydrochloric acid is also an acid, so none of them would neutralise the acidic effect. Exam tip: for an acidic sting or substance, choose a suitable mild base.

Acidic behaviour in the school-level aqueous model depends on the formation and movement of ions, especially H₃O⁺. Many substances that are acids do not release mobile hydrogen-containing ions in the dry state, so dry hydrogen chloride, for example, does not change dry litmus. In water, ionisation occurs and the solution conducts electricity and shows acidic properties. Thus option A is necessary; darkness, low temperature, or a metal container is not the governing condition.

Red litmus turning blue indicates that the solution is basic. Lime water is an aqueous solution of calcium hydroxide, \(\mathrm{Ca(OH)_2}\). When carbon dioxide is passed through it, a white precipitate of calcium carbonate forms: \(\mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O}\). Therefore, option A is correct. Sodium chloride solution is generally neutral, whereas vinegar and dilute sulphuric acid are acidic, so they do not turn red litmus blue. Exam tip: The milky appearance of lime water on passing \(\mathrm{CO_2}\) is a key test for calcium carbonate formation.

The pH scale is logarithmic. A decrease from pH 13 to pH 10 is a change of three units, meaning the hydrogen-ion concentration increases by 10³, or 1,000 times. Equivalently, the hydroxide-ion concentration and the corresponding basic character decrease by about 1,000 times, assuming comparable conditions. The solution is still basic at pH 10, so option A is correct; it has not become acidic.

Blue litmus turning red shows that the salt solution is acidic. A salt formed from a strong acid and a weak base, such as ammonium chloride from HCl and NH₄OH, undergoes hydrolysis in water and produces relatively more H₃O⁺ ions. Therefore option A is the suitable combination. A weak acid–strong base salt is generally basic, while a strong acid–strong base salt is approximately neutral.

Electrical conduction in an aqueous acid solution depends on the number and mobility of charged particles, not on colour. At equal concentration, the acid that ionises more completely produces more mobile H₃O⁺ and counter-ions, so it conducts better; this is the behaviour of a stronger acid relative to the other. A weak acid ionises only partially. Thus A gives the correct reason, while B, C, and D are irrelevant or false.

The governing relation is pH = −log₁₀[H⁺]. A rise of one pH unit means a tenfold decrease in hydrogen-ion concentration. The change from pH 2 to pH 5 is three units, so [H⁺] changes by 10³ = 1000 in the decreasing direction. For example, 10⁻² mol L⁻¹ becomes 10⁻⁵ mol L⁻¹. Therefore option A is correct; it is not merely a threefold change.

Turning red litmus blue shows that the sample is basic. Carbonates and hydrogen carbonates also behave as bases, but when an acid is added they release carbon dioxide, producing visible effervescence. Sodium hydroxide is a strong base and reacts with acid by neutralisation to form salt and water, without producing carbon dioxide. Thus option A fits both observations; options B-D contain carbonate or hydrogen carbonate and would give CO₂.

Plaster of Paris is calcium sulfate hemihydrate, CaSO₄·½H₂O. On adding water, it rehydrates and sets into gypsum, CaSO₄·2H₂O. A correct amount of water permits this reaction and gives a workable paste. Excess water makes the mixture too dilute; after setting, the extra water leaves pores and reduces strength, while the diluted paste may set less effectively. Therefore option A is correct.

Relief by neutralisation depends on the chemical nature of the injected substance. If a sting contains an acidic component, a suitable mild basic substance may reduce its effect; if it contains a basic component, a suitable mild acidic substance may be considered. The substances and medical risks are not necessarily identical for every sting, so one remedy cannot be assumed universally safe. Thus option A is correct.

Lemon juice contains citric acid, so it is acidic rather than basic. Acids can react with metals under suitable conditions; prolonged contact may allow copper to dissolve slowly and form copper-containing salts or other contaminants. This is why acidic foods are not generally stored in unlined copper vessels. The claim that copper never reacts with acids is false, and the other alternatives describe impossible or irrelevant changes. Therefore option A is correct.

The governing idea is salt hydrolysis. A salt produced from a strong base and a weak acid generally gives a basic aqueous solution because the anion of the weak acid reacts with water and generates some OH⁻ ions. Basic solutions turn red litmus blue. The exact strength depends on concentration and hydrolysis, but the expected school-level result is basicity. Therefore option A is correct; the other colour changes are not standard acid-base indicators.

Electrical conduction in an aqueous solution requires mobile charged particles. Sodium chloride dissociates in water into Na⁺ and Cl⁻ ions; when a potential difference is applied, these ions move toward opposite electrodes and carry current. Neutrality refers to the overall acid-base condition, approximately pH 7, not to an absence of ions. Thus a neutral NaCl solution can conduct electricity. Sugar does not provide ions, colour is irrelevant, and free electrons are not the main carriers in solution. Option A is correct.

The governing concept is that acids show their characteristic properties through ions in aqueous solution. Dry hydrogen chloride gas does not produce hydrogen ions without water, so dry litmus shows no acidic colour change. In moisture, HCl dissolves and ionises as HCl + H₂O → H₃O⁺ + Cl⁻. The hydronium ions turn blue litmus red, making option A correct; the other statements contradict acid behaviour.

The gas evolved is carbon dioxide. Initially, CO₂ reacts with lime water: CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O, and insoluble calcium carbonate makes the liquid milky. When excess CO₂ is passed, the precipitate reacts further: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂, which is soluble. Thus the milkiness disappears and option B is correct.

Salt hydrolysis determines the nature of a salt solution. Sodium carbonate, formed from a strong base and weak acid, gives a basic solution. Sodium chloride and potassium nitrate are approximately neutral because their ions hydrolyse negligibly. Ammonium chloride comes from the strong acid HCl and the weak base NH₄OH; NH₄⁺ hydrolyses to produce H₃O⁺, so its solution is acidic. Therefore D is correct.

A metal above hydrogen in the reactivity series can generally displace hydrogen from a dilute non-oxidising acid. Since zinc reacts with the same acid and releases hydrogen, while the unnamed metal does not under the stated conditions, the first metal may be below zinc in reactivity. It cannot be identified as sodium, and the acid must contain replaceable hydrogen. Hence A is the valid conclusion.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, so its crystals contain water of crystallisation. On exposure to comparatively dry air, some of this water is lost from the crystal lattice. The crystals become dull and powdery; this process is called efflorescence. Deliquescence involves absorbing moisture and dissolving, while neutralisation and displacement are chemical reaction types. Therefore B is correct.

Plaster of Paris is calcium sulfate hemihydrate, CaSO₄·½H₂O. It is intended to react with added water and set into gypsum, CaSO₄·2H₂O. If exposed to humid air before use, it can slowly absorb moisture and become gypsum prematurely. The partly hydrated material loses its ability to set properly when later mixed for application. Thus option C gives the correct reason; the other products are unrelated.