IMAT Chemistry · Acids, Bases and pH

Salt Solutions: Neutral, Acidic or Basic

A dissolved salt is neutral only if neither of its ions reacts with water; the ion from a weak acid makes the solution basic, and the ion from a weak base makes it acidic.

All Acids, Bases and pH notes

Why this matters

This is one of the two topics the ministry papers have asked most often in this chapter: three questions in 2025 and 2026 asked whether a dissolved salt gives a neutral, acidic or basic solution. Two older questions asked which salt solution is alkaline.

Concept 1 of 2: Predicting the pH of a salt solution from its parent acid and base

Think of every salt as made from an acid and a base. The ion that came from a strong acid or strong base is a spectator: it has no pull on protons. The ion that came from a weak partner does react with water. An anion of a weak acid takes

\mathrm{H^+}

from water and leaves

\mathrm{OH^-}

; a cation of a weak base gives

\mathrm{H^+}

to water.

Definition

Salt hydrolysis is the reaction of a salt's ions with water that shifts the pH away from 7.

Spectator ions (no effect): $\mathrm{Na^+}$ , $\mathrm{K^+}$ , $\mathrm{Li^+}$ , $\mathrm{Ca^{2+}}$ , $\mathrm{Ba^{2+}}$ , $\mathrm{Cl^-}$ , $\mathrm{Br^-}$ , $\mathrm{I^-}$ , $\mathrm{NO_3^-}$ , $\mathrm{ClO_4^-}$ .
Basic anion: $\mathrm{CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^-}$ .
Acidic cation: $\mathrm{NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+}$ .
The effect is weak: a 0.1 mol/L salt solution is usually only mildly acidic or basic, never as extreme as a strong acid or base of the same concentration.

Parent acid and base	Ion that reacts with water	Solution	Examples
Strong acid + strong base	Neither ion	Neutral, pH 7	$\mathrm{NaCl}$ , $\mathrm{KNO_3}$ , $\mathrm{KI}$ , $\mathrm{LiBr}$
Strong acid + weak base	The cation gives $\mathrm{H^+}$	Acidic, pH below 7	$\mathrm{NH_4Cl}$ , $\mathrm{NH_4NO_3}$ , $\mathrm{(NH_4)_2SO_4}$
Weak acid + strong base	The anion takes $\mathrm{H^+}$ , leaving $\mathrm{OH^-}$	Basic, pH above 7	$\mathrm{CH_3COONa}$ , $\mathrm{Na_2CO_3}$ , $\mathrm{KF}$ , $\mathrm{Na_3PO_4}$
Weak acid + weak base	Both ions	Depends on Ka against Kb	$\mathrm{CH_3COONH_4}$ is close to pH 7 Ka of the ammonium ion and Kb of the ethanoate ion are almost equal, so their effects cancel.

Practice this conceptself-check · 4 quick reps

Neutralisation does not always give a neutral solution

A salt is neutral only when both its acid and its base were strong. Sodium ethanoate, made from a weak acid and a strong base, gives a basic solution; ammonium chloride gives an acidic one. Do not choose pH 7 just because the substance is called a salt.

Concept 2 of 2: Acid salts and salts of small, highly charged metal ions

Some salts still carry a proton from their parent acid. Whether the solution ends up acidic or basic depends on which is stronger: that ion's urge to give the proton away or its urge to take another one. Small metal ions with a high charge, such as

\mathrm{Al^{3+}}

and

\mathrm{Fe^{3+}}

, pull so hard on the water molecules around them that those waters release protons.

Definition

An acid salt contains an anion with an acidic H: $\mathrm{NaHSO_4}$ , $\mathrm{NaHCO_3}$ , $\mathrm{NaH_2PO_4}$ , $\mathrm{Na_2HPO_4}$ . Despite the name, its solution can be acidic or basic.
$\mathrm{HSO_4^-}$ comes from a strong acid and is itself a fairly strong acid, so $\mathrm{NaHSO_4}$ solution is clearly acidic.
$\mathrm{HCO_3^-}$ is a better base than acid, so $\mathrm{NaHCO_3}$ solution is slightly basic (about pH 8.3).
Hydrated metal ions act as acids: $\mathrm{[Al(H_2O)_6]^{3+} \rightleftharpoons [Al(H_2O)_5(OH)]^{2+} + H^+}$ . Solutions of $\mathrm{AlCl_3}$ , $\mathrm{FeCl_3}$ and $\mathrm{AlBr_3}$ are acidic.

Salt	Ion that reacts	Solution	Why
$\mathrm{NaHSO_4}$	$\mathrm{HSO_4^-}$	Acidic (about pH 1.5 at 0.1 mol/L)	Gives $\mathrm{H^+}$ easily; almost no pull on protons
$\mathrm{NaHCO_3}$	$\mathrm{HCO_3^-}$	Slightly basic	Takes $\mathrm{H^+}$ more readily than it gives one
$\mathrm{NaH_2PO_4}$	$\mathrm{H_2PO_4^-}$	Slightly acidic	Gives $\mathrm{H^+}$ more readily than it takes one
$\mathrm{Na_2CO_3}$	$\mathrm{CO_3^{2-}}$	Clearly basic (about pH 11 to 12)	Anion of a weak acid, no H to give
$\mathrm{AlCl_3}$ , $\mathrm{FeCl_3}$	The hydrated metal ion	Acidic	The small 3+ ion makes its water ligands release $\mathrm{H^+}$

Practice this conceptself-check · 3 quick reps

A hydrogen salt is not automatically acidic

Having an H in the anion does not decide the pH.

\mathrm{NaHSO_4}

is acidic, but

\mathrm{NaHCO_3}

is slightly basic. Ask whether the anion gives up its proton more easily than it takes another one.

Some chlorides are acidic

A chloride ion is a spectator, but the metal ion may not be. Aluminium and iron(III) salts give acidic solutions, while sodium and potassium chlorides are neutral.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (2)

Predicting the pH of a salt solution from its parent acid and base4 rows

Parent acid and base	Ion that reacts with water	Solution	Examples
Strong acid + strong base	Neither ion	Neutral, pH 7	$\mathrm{NaCl}$ , $\mathrm{KNO_3}$ , $\mathrm{KI}$ , $\mathrm{LiBr}$
Strong acid + weak base	The cation gives $\mathrm{H^+}$	Acidic, pH below 7	$\mathrm{NH_4Cl}$ , $\mathrm{NH_4NO_3}$ , $\mathrm{(NH_4)_2SO_4}$
Weak acid + strong base	The anion takes $\mathrm{H^+}$ , leaving $\mathrm{OH^-}$	Basic, pH above 7	$\mathrm{CH_3COONa}$ , $\mathrm{Na_2CO_3}$ , $\mathrm{KF}$ , $\mathrm{Na_3PO_4}$
Weak acid + weak base	Both ions	Depends on Ka against Kb	$\mathrm{CH_3COONH_4}$ is close to pH 7 Ka of the ammonium ion and Kb of the ethanoate ion are almost equal, so their effects cancel.

Acid salts and salts of small, highly charged metal ions5 rows

Salt	Ion that reacts	Solution	Why
$\mathrm{NaHSO_4}$	$\mathrm{HSO_4^-}$	Acidic (about pH 1.5 at 0.1 mol/L)	Gives $\mathrm{H^+}$ easily; almost no pull on protons
$\mathrm{NaHCO_3}$	$\mathrm{HCO_3^-}$	Slightly basic	Takes $\mathrm{H^+}$ more readily than it gives one
$\mathrm{NaH_2PO_4}$	$\mathrm{H_2PO_4^-}$	Slightly acidic	Gives $\mathrm{H^+}$ more readily than it takes one
$\mathrm{Na_2CO_3}$	$\mathrm{CO_3^{2-}}$	Clearly basic (about pH 11 to 12)	Anion of a weak acid, no H to give
$\mathrm{AlCl_3}$ , $\mathrm{FeCl_3}$	The hydrated metal ion	Acidic	The small 3+ ion makes its water ligands release $\mathrm{H^+}$

Watch out for (3)

Neutralisation does not always give a neutral solution
→ Predicting the pH of a salt solution from its parent acid and base
A hydrogen salt is not automatically acidic
→ Acid salts and salts of small, highly charged metal ions
Some chlorides are acidic
→ Acid salts and salts of small, highly charged metal ions

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