Acids, Bases, and Salts

The governing concept is salt hydrolysis and the relative strengths of the reacting acid and base. A strong acid with a strong base generally gives a neutral salt, whereas a strong acid with a weak base tends to give an acidic salt, and a weak acid with a strong base tends to give a basic salt. Therefore the acid and base strengths determine the salt’s behaviour in water; container shape, colour, and zero temperature do not.

The governing concept is the pH scale and the colour range of a universal indicator. An excess of hydroxide ions over hydrogen ions means the solution is basic and has a pH above 7. Universal indicator usually changes through green near neutral to blue and then violet as basicity increases. Red, orange, and yellow represent acidic ranges, so they do not fit the stated ion balance. The exact blue or violet shade depends on the pH.

The governing concept is laboratory safety when handling a soluble, corrosive base. A strong base can damage skin and eyes, and dissolving some bases in water releases heat; therefore direct contact and careless mixing are unsafe. The substance should be handled with gloves, eye protection, suitable tools, clear labelling, and controlled procedures. Option B is correct. Eating it or storing it without a label creates additional hazards, while bare-hand use ignores its caustic nature.

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.

Red litmus turning blue indicates that the salt solution is basic. A salt formed from a weak acid and a strong base commonly gives a basic solution because the anion of the weak acid hydrolyses water and produces hydroxide ions; sodium carbonate is a familiar example. A strong-acid/weak-base salt is generally acidic, while a strong-acid/strong-base salt is approximately neutral. Hence A is correct.

Carbonates react with acids to release carbon dioxide, for example: sodium carbonate plus hydrochloric acid produces sodium chloride, water, and CO₂. The blue colour produced from red litmus also shows that the aqueous solution is basic. Sodium carbonate satisfies both tests. Sodium chloride is nearly neutral, ammonium chloride is acidic, and copper sulphate does not give carbon dioxide with an acid. Therefore A is correct.

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.

Diluting many concentrated acids is exothermic: hydration of ions releases heat. If water is poured rapidly into acid, the small amount of water can heat suddenly, boil locally, and eject acid droplets. Adding acid slowly to a larger quantity of water allows heat to spread more safely. The hazard is therefore rapid heat release and splashing, not gas formation, solidification, or colour change. Option A is correct.

Strength describes the extent to which a base ionises in water; a strong base, such as sodium hydroxide, ionises almost completely. Concentration describes how much dissolved base is present in a given volume of solution. Thus a strong base may be dilute, and a weak base may be concentrated. The two properties are independent, so option A is the accurate distinction.

Water of crystallisation is a definite number of water molecules incorporated into the crystal structure of some salts. Heating can remove this water, producing a colour change and sometimes visible moisture; adding water may restore the hydrated form and its original colour. Hydrated copper sulphate is a standard example. Common salt does not explain this reversible colour change, and oxygen or chlorine gas are not crystal water. Hence A is correct.

Gypsum has the formula CaSO₄·2H₂O, so it contains two water molecules per calcium sulphate unit. Controlled heating removes one and a half of these water molecules and forms plaster of Paris, CaSO₄·½H₂O. Therefore half a water molecule remains per calcium sulphate unit. The other values do not match the balanced formula or the known dehydration process, so A is correct.

When hydrogen-ion concentration is much greater than hydroxide-ion concentration, the solution is acidic and its pH is below 7. Universal indicator shows red at very low pH and orange in a moderately strong acidic range, so red or orange is the most likely colour. Blue and violet indicate alkaline conditions, while green is near neutral. The indicator is not necessarily colourless. Therefore A is correct.

Ammonium chloride is formed from hydrochloric acid, a strong acid, and ammonium hydroxide, a weak base. In water, the ammonium ion undergoes hydrolysis and produces hydronium ions, making the solution acidic; chloride ions do not significantly offset this effect. Consequently blue litmus turns red. A strong-acid/strong-base salt would be nearly neutral, and the other choices contradict its ionic nature. Thus A is correct.

The governing concept is the reactivity of metals with acids. A sufficiently reactive metal displaces hydrogen from a dilute acid, producing hydrogen gas, whereas a less reactive metal may show little or no reaction. If gas is evolved rapidly, that metal is reacting faster under the same conditions. Colour, container shape, and the acid’s name do not explain the difference. Therefore option A is correct.

Hard water contains dissolved calcium and magnesium ions, which interfere with soap and produce scum. Washing soda, sodium carbonate (Na2CO3), supplies carbonate ions. These ions precipitate calcium carbonate and magnesium carbonate, removing the hardness-causing ions from the water. No hydrogen gas is required, sodium ions are not destroyed, and the water does not become an acid. Thus option A is correct.

The governing idea is that mouth bacteria can metabolise food residues, especially sugars, and release acids. These acids lower the oral pH; when the pH remains low, tooth enamel can lose minerals and decay becomes more likely. Cleaning or rinsing removes residues and reduces acid production, helping the pH move away from the harmful acidic range. It does not dissolve enamel or increase acidity, so option A is correct.