Consider the following statements : While diluting concentrated nitric acid solution 1. the concentration of [H₃O⁺] ions/volume increases 2. water must be added slowly to concentrated acid 3. acid must be added slowly to water Which of the statements given above is/are correct?
- (a)1, 2 and 3
- (b)1 and 2 only
- (c)3 only
- (d)2 and 3 only
Correct — C, 3 only. Only the third line survives. Dissolving a strong acid in water gives out a great deal of heat, and NCERT's Class 10 chemistry chapter states the rule in so many words: 'The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns.' That kills statement 2, which is the dangerous inversion of the rule — pour water on to concentrated nitric acid and the first drops boil on contact and throw acid back at you. Statement 1 fails on the arithmetic of dilution. The same chapter says that mixing an acid with water 'results in decrease in the concentration of ions (H₃O⁺/OH⁻) per unit volume'. You are not adding any more acid, so the number of hydronium ions barely changes while the volume grows; ions per unit volume must therefore fall, which is why a diluted acid has a higher pH than the concentrate it came from.
- (a)1, 2 and 3 — Accepts everything, including a statement that contradicts statement 3 and one that gets the direction of dilution backwards. Statements 2 and 3 are opposite instructions, so an option holding both can never be right.
- (b)1 and 2 only — The worst of the four, because it keeps the unsafe instruction and drops the safe one. It also asserts that hydronium ions per unit volume rise on dilution, when the whole meaning of dilution is that they fall.
- (d)2 and 3 only — Tempting, because statement 3 really is correct. But statements 2 and 3 say the reverse of each other — water into acid, and acid into water — and only one of them can be the laboratory rule.
Hydration of a strong acid is strongly exothermic. When concentrated sulfuric or nitric acid meets water, the heat comes out at the point of mixing, and where it goes next decides whether the operation is safe. Add the acid to a large volume of water and the water absorbs that heat over its whole bulk, its high specific heat capacity keeping the temperature rise small. Add water to the acid and the heat is dumped into a thin surface layer, which can reach boiling point and spit acid out of the vessel, or crack the glass through uneven expansion.
The item is a straight lift from the school chemistry chapter on acids, bases and salts, down to the phrase 'concentrated nitric acid' and the odd wording 'ions/volume'. Statements 2 and 3 are mirror images, so a candidate who notices that at once knows the answer contains one of them and not both — which already eliminates the first and fourth options. The remaining work is statement 1, and the trap there is the word 'concentration' pulling against the word 'diluting'. Adding acid to water does raise the total quantity of hydronium ions in the beaker, but concentration is a quantity per unit volume, and the volume rises faster. The paper prints the ion as [H₃O⁺] in square brackets, the chemist's notation for concentration, which makes the statement's own claim self-defeating.
- NCERT Class 10 Science: 'The acid must always be added slowly to water with constant stirring.'
- The same chapter: mixing an acid or base with water 'results in decrease in the concentration of ions (H₃O⁺/OH⁻) per unit volume. Such a process is called dilution'.
- Dissolving an acid or a base in water is described there as 'a highly exothermic' process.
- Water's large specific heat capacity, about 4.18 kJ per kg per kelvin, is what lets it soak up that heat with only a small temperature rise.
- Because hydronium concentration falls on dilution and pH rises as that concentration falls, diluting an acid raises its pH towards 7.
Statements 2 and 3 are opposite instructions, so no correct option can contain both.
- Reading 'concentration of ions per volume' as though it meant the total number of ions in the beaker.
- Missing that two of the three statements are opposite instructions, which alone rules out two options.
- Assuming the hazard is a chemical reaction between acid and water. It is heat of hydration, a physical effect.
As a statements item on dilution, as an assertion-and-reason pair about adding acid to water, or as a one-line question on what happens to hydronium concentration or pH when an acid is diluted.
Assertion (A): To dilute sulphuric acid, acid is added to water and not water to acid. Reason (R): Specific heat of water is quite large.
- (a) Both A and R are true, and R is the correct explanation of A
- (b) Both A and R are true, but R is not a correct explanation of A
- (c) A is true, but R is false
- (d) A is false, but R is true
Answer(a) Both A and R are true, and R is the correct explanation of A
The same laboratory rule, asked as an assertion and reason. It supplies the physics this question leaves implicit — water's large specific heat capacity is why a big volume of it can absorb the heat of mixing without the temperature running away.
- practice — not a real PYQ
When a solution of an acid is diluted with water, the concentration of hydronium ions
- (a)increases
- (b)decreases
- (c)remains unchanged
- (d)first increases and then decreases
Answer(b) decreases — the number of hydronium ions stays about the same while the volume grows, so their concentration per unit volume falls and the pH rises.
- practice — not a real PYQ
Concentrated sulfuric acid must be added to water rather than the other way round mainly because
- (a)the reaction is endothermic
- (b)hydrogen gas is liberated
- (c)the dissolution is highly exothermic
- (d)sulfuric acid is denser than water
Answer(c) the dissolution is highly exothermic — the large volume of water absorbs the heat, whereas water poured on to the acid boils at the surface and can throw acid out.