Acids, Bases, and Salts

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. At the cathode, water is reduced and hydrogen gas forms: 2H₂O + 2e⁻ → H₂ + 2OH⁻. At the anode, chloride ions are oxidised to chlorine: 2Cl⁻ → Cl₂ + 2e⁻. The remaining Na⁺ combines with OH⁻ to give sodium hydroxide. Therefore A is correct.

Acid strength depends on the extent of ionisation in water, not simply on concentration. Hydrochloric acid ionises almost completely and is classified as a strong acid. Ethanoic acid ionises only partially and is therefore a weak acid. Option A contains an acid and a base, while C and D do not present a strong-acid/weak-acid pair. Hence hydrochloric acid followed by ethanoic acid in option B is correct.

Dilution decreases the concentration of hydronium ions in an acidic solution. Since pH = −log[H₃O⁺], a decrease in hydronium concentration produces an increase in pH, although the solution may remain acidic for some time. Repeated equal additions of water continue to dilute it, so the pH moves upward toward 7 rather than downward. Therefore option C is correct; the other choices violate the pH concept.

Bleaching powder is commonly represented as calcium oxychloride, CaOCl₂, and is prepared by passing chlorine over dry slaked lime, Ca(OH)₂. The process can be summarised as 2Cl₂ + 2Ca(OH)₂ → Ca(OCl)₂ + CaCl₂ + 2H₂O, with the product mixture called bleaching powder. The other options describe unrelated substances or processes. Therefore option A is correct.

The governing concept is controlled dehydration of a hydrated salt. Gypsum is calcium sulphate dihydrate, CaSO4·2H2O. Gentle heating removes part of its water of crystallisation and forms plaster of Paris, CaSO4·½H2O. If heating is excessive, more water may be driven off and the desired setting properties can be lost. Therefore option B is correct; calcium, chlorine, and sodium are not being removed or formed.

The pH scale is logarithmic: pH = −log[H⁺]. A tenfold decrease in hydrogen-ion concentration raises pH by one unit. Making the solution 100 times, or 10² times, less acidic means [H⁺] becomes 100 times smaller, so the pH rises by log₁₀(100) = 2 units. Starting from pH 3, the approximate new pH is 3 + 2 = 5. Therefore option C is correct.

The key qualitative test is that carbon dioxide turns limewater milky by forming insoluble calcium carbonate: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. An acid reacts with a carbonate ion to release carbon dioxide, for example CO₃²⁻ + 2H⁺ → CO₂ + H₂O. Therefore the unknown solid may contain carbonate ions. Chloride and sulphate do not normally produce this gas in the stated test, and hydroxide neutralises acid without releasing CO₂, so option C is correct.

The governing safety principle is that dilution of concentrated acid is exothermic. If a small amount of water is poured into a larger volume of acid, the water can heat suddenly, boil locally, and eject corrosive acid droplets. The safe laboratory practice is to add acid slowly to a larger quantity of water while stirring. Therefore option A explains the increased risk; the other choices describe impossible or chemically unrelated effects.

The governing concept is the logarithmic definition of pH: pH = −log₁₀[H⁺]. If the hydrogen-ion concentration changes from [H⁺] to 10[H⁺], the new value is −log₁₀(10[H⁺]) = −1 − log₁₀[H⁺], so the pH falls by exactly one unit. Acidity therefore increases. Option A is correct; a tenfold concentration change does not produce a ten-unit or positive pH change.

The governing concept is electrolysis of brine in the chlor-alkali process. Common salt is sodium chloride, NaCl. When its aqueous solution is electrolysed, chlorine gas forms at the anode, hydrogen gas forms at the cathode, and sodium hydroxide remains in solution: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Hence option A is correct. The other statements confuse salt chemistry or contradict its known solubility.

The governing concept is the acid–metal displacement reaction, in which a suitable metal can displace hydrogen from an acid: metal + acid → salt + hydrogen. The amount of gas depends on the moles and concentration of acid, the amount and reactivity of the metal, and whether a reactant becomes limiting. Therefore equal amounts cannot be assumed. The distractors incorrectly claim oxygen formation, no reaction, or water as the only source.

The governing concept is the characteristic reaction of an acid with a metal and with a carbonate. Zinc displaces hydrogen from an acid: Zn + 2HCl → ZnCl₂ + H₂. A carbonate reacts with acid to form salt, water, and carbon dioxide: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Thus the first gas is hydrogen and the second is carbon dioxide. The reverse identification and oxygen or chlorine choices do not match these reactions.

The governing relation is pH = −log₁₀[H⁺]. A decrease of one pH unit means a tenfold increase in hydrogen-ion concentration. The difference between pH 2 and pH 6 is 6 − 2 = 4 units, so the ratio is 10⁴ = 10,000. Equivalently, [H⁺] at pH 2 is 10⁻² mol/L and at pH 6 is 10⁻⁶ mol/L; their ratio is 10⁴. Therefore option A is correct.

The governing concept is that acids show their characteristic behaviour through ions in aqueous medium. Dry hydrogen chloride gas has no water available to ionise into H₃O⁺ and Cl⁻, so dry blue litmus does not change colour. Moisture dissolves HCl and permits ionisation, producing hydronium ions that turn blue litmus red. Dry litmus is not necessarily defective, HCl is not basic when dry, and the acid comes from dissolved HCl rather than from the paper itself.

The pH scale is logarithmic, so a change of three pH units represents a thousand-fold change in hydrogen-ion concentration. Moving from pH 12 to pH 9 therefore greatly reduces the relative hydroxide-ion effect, although the solution remains basic because pH 9 is still above 7. Thus option A is the best statement; the solution is not acidic and hydroxide ions have not vanished.

In the chlor-alkali process, electrolysis of concentrated brine produces chlorine at the anode, hydrogen at the cathode, and sodium hydroxide in solution. A membrane separates the electrode compartments and limits contact between chlorine and sodium hydroxide, which could otherwise react to form unwanted products. It does not remove all water or deposit sodium metal. Therefore option A correctly states its purpose.

The governing concept is that ions must become mobile for the acid-carbonate reaction to proceed efficiently. In dry baking powder, the solid particles have limited contact and the ions are not freely solvated, so vigorous reaction and carbon dioxide release are suppressed. When water is added, the substances dissolve and react; heating can then accelerate gas formation. The ingredients are not identical, and carbon dioxide does not form continuously without reaction. Therefore option A is correct.

Use $\text{pH}=-\log_{10}[H^+]$. So
$[H^+] \propto 10^{-\text{pH}}$.
When pH changes from $3$ to $5$:
\[
\frac{[H^+]_{\text{initial}}}{[H^+]_{\text{final}}}=10^{(5-3)}=10^2=100.
\]
So the hydrogen ion concentration decreases by a factor of $100$.

Gypsum is calcium sulfate dihydrate, CaSO₄·2H₂O, so it contains water of crystallisation. Controlled heating removes part of this water and produces plaster of Paris, CaSO₄·½H₂O. When water is added, plaster of Paris rehydrates to gypsum and hardens: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. Hence option A correctly describes the reversible relationship.

Zinc can react with an acidic solution and displace hydrogen, producing hydrogen gas; in a simplified equation, zinc plus acid forms a salt and H2. Carbon dioxide, however, appears when an acid reacts with carbonate or hydrogen carbonate. Since acid addition produced no CO2, carbonate is not indicated, while hydrogen from zinc supports an acidic nature. Therefore option A is the best conclusion among the choices.

Since $\text{pH}=-\log_{10}[H^+]$, if $[H^+]$ becomes $\frac{1}{1000}$, then pH increases by $\log_{10}(1000)=3$.
So new pH $\approx 4+3=7$.

The governing concept is the reaction of carbon dioxide with limewater. The gas turns limewater milky because insoluble calcium carbonate forms: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. When CO₂ is passed in excess, the precipitate reacts further: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Calcium hydrogen carbonate is soluble, so the milkiness disappears. Therefore option C is correct; calcium carbonate is only the intermediate product.

Electrical conduction in an acid solution is caused mainly by mobile ions. At the same concentration, an acid that produces fewer ions through ionisation generally conducts electricity less effectively. Thus the most appropriate conclusion is that it ionises less and is likely to be a weak acid. Option B is opposite to the evidence, while C and D do not follow from conductivity. Strictly, ion mobility and experimental conditions can also affect conductivity, so option A is the best school-level conclusion.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Controlled heating removes part of its water of crystallisation and produces plaster of Paris, calcium sulphate hemihydrate, CaSO₄·½H₂O. If heating is excessive, still more water is driven off and anhydrous calcium sulphate may form. This material does not rehydrate and set in the desired way, so the useful plaster quality is reduced. Hence option A is correct; the other products are chemically unrelated.

Sodium hydroxide is deliquescent, so its pellets absorb water vapour from humid air and may become wet or dissolve. It also reacts with carbon dioxide in the atmosphere: 2NaOH + CO₂ → Na₂CO₃ + H₂O. The formation of sodium carbonate changes the composition of the material and lowers its purity for uses requiring NaOH. Therefore option A correctly includes both moisture absorption and chemical reaction with CO₂; the other statements have no chemical basis.