Enforcement Officer / Accounts Officer (EO/AO) 2020 recruitment test, advertisement 421. Transcribed from the Commission's own booklet (series A, NMJX-U-GTA); answer as per the official UPSC final answer key. This question is in Part B. Answer last reviewed 24 August 2026.

Which one of the following substances, when added to water, will not change the pH ?

(a)NaHCO3
(b)NH4Cl
(c)Na2CO3
(d)NaCl

Answer

Why

Correct — D, (d) NaCl. Whether a salt changes the pH of water depends on the acid and the base it came from. Sodium chloride is the salt of a STRONG acid, hydrochloric acid, and a STRONG base, sodium hydroxide. Dissolve it and it dissociates completely into Na+ and Cl- ions, but neither of those ions reacts with water: Na+ is the conjugate of a strong base and Cl- the conjugate of a strong acid, so both are far too weak to accept or donate a proton. There is no hydrolysis, the balance between H+ and OH- in the water is untouched, and the solution stays neutral at a pH of about 7. That is exactly what the emphasised 'not' in the stem is asking for. Each of the other three salts has at least one ion that does react with water and therefore does move the pH — two of them upwards and one downwards. The rule to carry away is short: a salt of a strong acid and a strong base is the only combination that leaves water's pH alone.

Why the others are wrong

(a)NaHCO3 — Sodium bicarbonate — baking soda — is the salt of a strong base, sodium hydroxide, and a WEAK acid, carbonic acid, so it does change the pH. The bicarbonate ion HCO3- takes a proton from water, releasing OH- and making the solution mildly BASIC, at a pH of roughly 8 for a typical solution. This is not a laboratory curiosity: it is why baking soda is used as an antacid to neutralise excess stomach acid, why it is a mild cleaning agent, and why it works in baking, where it releases carbon dioxide. Being mild is not the same as being neutral, and a candidate who reads 'baking soda is gentle' as 'baking soda does not change pH' has been caught by exactly the association this option is built on.
(b)NH4Cl — Ammonium chloride is the salt of a WEAK base, ammonia, and a strong acid, hydrochloric acid, so it changes the pH in the opposite direction from the other two wrong options. Here it is the CATION that hydrolyses: the ammonium ion NH4+ gives up a proton to water, producing NH3 and H3O+, so the solution turns mildly ACIDIC, at a pH of around 5. It is the only acidic salt in this set, which makes it useful for checking that you have the rule the right way round — if your reasoning makes ammonium chloride basic, the rule has been inverted somewhere.
(c)Na2CO3 — Sodium carbonate — washing soda — is the salt of a strong base and a weak acid, and it is the strongest pH shifter in this set. The carbonate ion CO3 2- is a considerably better proton acceptor than bicarbonate, so it pulls protons from water and leaves a large excess of OH-, giving a distinctly ALKALINE solution with a pH above 11 in ordinary concentrations. That alkalinity is precisely what it is used for: washing soda softens hard water and cuts grease in laundry. Anyone who has felt a washing-soda solution's soapy slipperiness has felt a solution well away from neutral.

Concept

Salt hydrolysis is the whole content of this question, and it reduces to a four-way classification. A salt is formed by an acid and a base, and the pH of its solution is set by whichever parent was WEAKER, because only the conjugate of a weak parent is a strong enough proton donor or acceptor to disturb water. Strong acid with strong base — NaCl, KNO3, Na2SO4 — gives a NEUTRAL solution; neither ion hydrolyses. Strong base with weak acid — Na2CO3, NaHCO3, CH3COONa — gives a BASIC solution, because the anion accepts protons from water and leaves excess OH-. Strong acid with weak base — NH4Cl, (NH4)2SO4, CuSO4 — gives an ACIDIC solution, because the cation donates protons and leaves excess H3O+. Weak acid with weak base — ammonium acetate, for instance — depends on which is weaker, comparing the ionisation constants Ka and Kb. Behind all of it is the self-ionisation of water: in pure water at 25 degrees Celsius, [H+] equals [OH-] equals 10^-7 mol per litre, giving pH 7, and Kw = [H+][OH-] = 10^-14. The pH scale runs from 0 to 14 with 7 as neutral, and it is logarithmic, so a solution of pH 5 has a hundred times the hydrogen ion concentration of one at pH 7. Two of these salts are among the handful every Indian syllabus names by their common names as well as their formulae — sodium bicarbonate as baking soda, sodium carbonate as washing soda — and their uses follow directly from their alkalinity.

This opens the chemistry block. It is a rule-application item rather than a recall item: one rule, four salts, and the work is to classify each parent acid and base as strong or weak. That is why it is worth learning the short list of strong acids (hydrochloric, sulphuric, nitric, and the other hydrohalic acids apart from hydrofluoric) and strong bases (the hydroxides of the alkali metals and the heavier alkaline earths) — everything not on those lists is weak, and the classification then follows without further thought. The emphasised 'not' means the answer is the one salt that does nothing, which is easy to lose if you are hunting for the salt with the most dramatic effect.

Key facts

A salt of a strong acid and a strong base does not hydrolyse and leaves water neutral at pH about 7 — NaCl is the standard example.
A salt of a strong base and a weak acid gives a basic solution: NaHCO3 and Na2CO3 both do.
A salt of a strong acid and a weak base gives an acidic solution: NH4Cl does.
Sodium bicarbonate is baking soda; its solution is mildly basic, around pH 8, and it is used as an antacid and in baking.
Sodium carbonate is washing soda; its solution is strongly alkaline, above pH 11, and it is used to soften hard water.
Ammonium chloride gives a mildly acidic solution, around pH 5, by hydrolysis of the ammonium ion.
Pure water at 25 degrees Celsius has [H+] = [OH-] = 10^-7 mol per litre, giving pH 7, and Kw = 10^-14.
The pH scale is logarithmic: a change of one pH unit is a tenfold change in hydrogen ion concentration.
The common strong acids are HCl, H2SO4, HNO3, HBr, HI and HClO4; the common strong bases are the alkali metal hydroxides.

The pH is set by whichever parent was the WEAKER

NaCl — strong acid with strong base→Hydrochloric acid and sodium hydroxide. It dissociates completely into Na+ and Cl-, but neither ion reacts with water: Na+ is the conjugate of a strong base and Cl- the conjugate of a strong acid, and both are far too weak to accept or donate a proton. There is no hydrolysis, the balance between H+ and OH- is untouched, and the solution stays neutral at about pH 7.

NaHCO3 — strong base with weak acid→Sodium hydroxide and carbonic acid. The bicarbonate ion takes a proton from water, releasing OH- and making the solution mildly BASIC, around pH 8. This is baking soda: an antacid for excess stomach acid, and a leavening agent.

Na2CO3 — strong base with weak acid→The strongest pH shifter in the set. The carbonate ion is a considerably better proton acceptor than bicarbonate, so it pulls protons from water and leaves a large excess of OH-, giving a distinctly ALKALINE solution above pH 11 at ordinary concentrations. This is washing soda, and that alkalinity is exactly what it is used for.

NH4Cl — strong acid with weak base→Hydrochloric acid and ammonia, and it moves the pH the other way from the two above. Here it is the CATION that hydrolyses: the ammonium ion gives up a proton to water, producing NH3 and H3O+, so the solution turns mildly ACIDIC, around pH 5. It is the only acidic salt in this set.

The fourth case, not on this page→Weak acid with weak base — ammonium acetate, for instance — depends on which parent is the weaker, compared through the ionisation constants Ka and Kb.

What sits underneath all of it→The self-ionisation of water. In pure water at 25 degrees Celsius [H+] = [OH-] = 10^-7 mol per litre, giving pH 7, and Kw = [H+][OH-] = 10^-14. The scale is logarithmic, so a solution at pH 5 carries a hundred times the hydrogen ion concentration of one at pH 7. The common strong acids are HCl, H2SO4, HNO3, HBr, HI and HClO4; the common strong bases are the alkali metal hydroxides.

Study next

Acids, bases and salts: Arrhenius, Bronsted-Lowry and Lewis definitionsSalt hydrolysis and the four combinations of strong and weak parentsThe pH scale, pOH, and the ionic product of waterBuffer solutions and how a mixture of a weak acid and its salt resists pH changeCommon salts and their uses — baking soda, washing soda, bleaching powder, plaster of ParisNeutralisation, titration and indicatorsHard water, its causes and methods of softening

Common traps

Missing the emphasised 'not' and picking the salt with the largest pH change instead of the one with none.
Assuming every dissolved salt must change the pH somehow. A salt of a strong acid and a strong base does not.
Reading 'mild' or 'edible' as 'neutral'. Baking soda is safe to swallow and is still clearly basic.
Confusing sodium carbonate with sodium bicarbonate. Both are basic, but the carbonate is far more strongly so.
Getting the direction wrong for ammonium chloride. A weak-base parent makes the solution acidic, not basic.
Forgetting that pH is logarithmic and treating a shift of two units as a small one.

Chemistry on EPFO EO/AO papers stays with school-level physical and inorganic chemistry, and acids, bases and salts is among the most reliable corners of it. The recurring shapes are: which salt gives a neutral, acidic or basic solution; what is the common name and use of a given compound; and simple pH comparisons. Formulae are usually printed with true subscripts, so read NaHCO3 and Na2CO3 carefully — the difference of one sodium is the difference between mildly and strongly basic.

Related PYQs

EPFO_EOAO_2020_Q52Open & attempt →

Which one of the following, on adding to water, will not scatter a beam of light ?

(a) Copper sulphate
(b) Chalk powder
(c) Milk
(d) Ink

Answer(a) Copper sulphate

The next item in this chemistry block, and the second of three consecutive negatively worded questions about what a substance does when it is added to water. Reading each of them to the end before choosing is the single habit this stretch of the paper rewards.

Practice

practice — not a real PYQ
An aqueous solution of ammonium chloride is
- (a)strongly basic
- (b)mildly basic
- (c)neutral
- (d)acidic
Answer(d) acidic
practice — not a real PYQ
Washing soda, used to soften hard water, is the common name of
- (a)sodium bicarbonate
- (b)sodium carbonate
- (c)sodium chloride
- (d)calcium sulphate hemihydrate
Answer(b) sodium carbonate

Why