Acidic, Basic and Neutral Salts

Introduction

Neutralisation makes a salt and water, so it is tempting to think every salt solution must be neutral. Test a few with universal indicator and the idea falls apart. Sodium chloride solution is green, pH 7. Ammonium chloride solution is orange-yellow, about pH 5. Sodium carbonate solution is blue-purple, about pH 11. The pH of a salt solution depends on the "parents" of the salt: the acid and the base that could have made it.

Core explanation

Every salt has a parent acid and a parent base. Sodium chloride comes from hydrochloric acid and sodium hydroxide. Ammonium chloride comes from hydrochloric acid and ammonia. Sodium ethanoate comes from ethanoic acid and sodium hydroxide. Whether each parent is strong or weak decides the pH.

Rule 1 — strong acid + strong base → neutral salt. Examples: NaCl, KNO₃, Na₂SO₄. Neither ion reacts with water, so the solution stays at about pH 7.

Rule 2 — strong acid + weak base → acidic salt solution. Examples: NH₄Cl, (NH₄)₂SO₄, NH₄NO₃. The ammonium ion is linked to weak base ammonia. In water it gives away a little H⁺:

NH₄⁺(aq) ⇌ NH₃(aq) + H⁺(aq)

The extra H⁺ ions make the solution weakly acidic, typically around pH 5.

Rule 3 — weak acid + strong base → alkaline salt solution. Examples: Na₂CO₃, CH₃COONa (sodium ethanoate), NaHCO₃. The negative ion comes from a weak acid, so it tends to take H⁺ back from water, leaving OH⁻ ions behind:

CO₃²⁻(aq) + H₂O(l) ⇌ HCO₃⁻(aq) + OH⁻(aq)

The extra OH⁻ ions make the solution alkaline. Sodium carbonate solution is quite strongly alkaline (around pH 11); sodium hydrogencarbonate is only mildly alkaline (around pH 8).

Weak acid + weak base. For a salt such as ammonium ethanoate, both ions react with water to similar extents, and the solution ends up close to pH 7. The outcome depends on the relative strengths of the parents.

Why the ions behave this way. A strong acid such as HCl ionises completely, so chloride ions have essentially no tendency to recapture H⁺ — they are "content" as ions. A weak acid such as carbonic or ethanoic acid only partly ionises, which means its negative ion has a real tendency to grab H⁺ back. Similarly, ammonia is a weak base, so its partner ion NH₄⁺ readily releases H⁺. These reactions of ions with water are called hydrolysis .

Metal ions can be acidic too. Solutions of salts of some small, highly charged metal ions, such as iron(III) chloride and aluminium sulfate, are noticeably acidic. The metal ion pulls strongly on the water molecules around it and helps release H⁺.

Step-by-step reasoning

To predict the pH of a salt solution:

1. Split the salt into its positive and negative ions. 2. Identify the parent base of the positive ion and the parent acid of the negative ion. 3. Decide whether each parent is strong or weak. 4. The strong parent "wins": strong acid gives acidic, strong base gives alkaline, both strong gives neutral.

Visual explanation

Imagine a tug-of-war between the parent acid and the parent base. When both are equally strong, the rope stays in the middle at pH 7. When the acid team is strong and the base team weak, the rope is pulled towards the acidic side, and the reverse gives an alkaline solution.

Real-world analogy

Mixing paint colours works similarly. Equal strengths of two colours give a balanced shade, but if one pigment is much more intense than the other, the mixture takes on that colour. The salt's pH takes on the character of its stronger parent.

Real-world example

Gardeners who add ammonium sulfate fertiliser often find their soil becomes more acidic over time, partly because ammonium ions release H⁺. Meanwhile, washing soda (sodium carbonate) is used in cleaning because its alkaline solution helps break down grease.

Why?

Why does the titration of a weak acid with a strong alkali end above pH 7? At the equivalence point the flask contains only the salt, such as sodium ethanoate, and water. That salt solution is itself alkaline, so the equivalence point lies above pH 7, which affects the choice of indicator.

Common misconception

"A salt is always neutral because acid and base have cancelled out." The amounts are matched, but the ions left behind can still react with water. Only salts of a strong acid and a strong base give truly neutral solutions.

Worked example

Question: Predict whether potassium carbonate solution is acidic, neutral or alkaline.

Reasoning: K₂CO₃ contains K⁺ (parent: potassium hydroxide, a strong base) and CO₃²⁻ (parent: carbonic acid, a weak acid). The strong parent is the base.

Answer: Alkaline, because carbonate ions react with water to produce hydroxide ions.

Quick check

1. Is ammonium nitrate solution acidic, neutral or alkaline? Answer: Acidic, because it comes from a strong acid (nitric acid) and a weak base (ammonia).

Exam focus

Learn the three rules linking parent strength to salt pH, and quote an example of each: sodium chloride neutral, ammonium chloride acidic, sodium carbonate alkaline. Examiners often link this to choosing indicators for titrations.

Advanced insight

The extent of hydrolysis can be quantified using acid dissociation constants. The weaker the parent acid, the more strongly its negative ion accepts H⁺, so the more alkaline its salt solution. This is why sodium carbonate (from the very weak hydrogencarbonate ion) is far more alkaline than sodium ethanoate at the same concentration.

Summary

The pH of a salt solution depends on the strength of its parent acid and base. Strong acid with strong base gives a neutral salt; strong acid with weak base gives an acidic solution; weak acid with strong base gives an alkaline solution. The cause is hydrolysis: ions from weak parents react with water, releasing H⁺ or OH⁻.

Practice questions

1. Predict the pH character of sodium sulfate solution and explain. Answer: Neutral, because it comes from a strong acid (sulfuric acid) and a strong base (sodium hydroxide). 2. Write the equation showing why ammonium ions make a solution acidic. Answer: NH₄⁺(aq) ⇌ NH₃(aq) + H⁺(aq) 3. Why is sodium ethanoate solution alkaline? Answer: Ethanoate ions come from a weak acid, so they take H⁺ from water, leaving extra OH⁻ ions. 4. Which would have a higher pH at the same concentration: sodium carbonate or sodium hydrogencarbonate solution? Answer: Sodium carbonate, which is more strongly alkaline (about pH 11 compared with about pH 8).