Acidic vs Basic Salts: What's the Difference?
Dissolve table salt in water and the pH stays at 7. Dissolve the same amount of ammonium chloride and the pH drops to about 5. Both are salts, both came from a neutralisation, and neither contains an obvious acid. The explanation is that "neutralisation" does not always produce something neutral.
The short answer: a salt's solution is acidic if it came from a strong acid and a weak base, basic if it came from a weak acid and a strong base, and neutral if both parents were strong. The reason is hydrolysis — ions left over from a weak parent react with water and shift the pH.
Quick comparison at a glance
| Parent acid | Parent base | Resulting solution | Example | Approx. pH of 0.10 M |
|---|---|---|---|---|
| Strong | Strong | Neutral | NaCl, KNO₃ | 7.0 |
| Strong | Weak | Acidic | NH₄Cl, NH₄NO₃ | 5.1 |
| Weak | Strong | Basic | CH₃COONa (8.9), NaF (8.1) | 8–9 |
| Weak | Weak | Depends on Ka vs Kb | CH₃COONH₄ | ≈ 7.0 |
Why any salt changes the pH
A salt dissolves into two ions, and the question is whether either of them reacts with water.
An ion left over from a strong parent does nothing. Cl⁻ is the conjugate base of HCl, an acid so strong its conjugate base has no measurable tendency to take a proton back. Na⁺ and K⁺ likewise just float around hydrated. Chemists call these spectator ions.
An ion left over from a weak parent is a different matter, because the conjugate of a weak acid or base is itself a weak base or acid — strong enough to react with water. That reaction is hydrolysis.
Acetate, from weak acetic acid, makes the solution basic:
CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻
Ammonium, from weak ammonia, makes the solution acidic:
NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺
How to work out which is which
- Split the salt into its two ions.
- Trace each back to the acid or base it came from.
- Ions from strong parents are spectators; ions from weak parents hydrolyse.
- Whichever hydrolysis happens decides the pH.
For NaNO₃: Na⁺ comes from NaOH (strong), NO₃⁻ from HNO₃ (strong). Neither reacts — neutral. For KF: K⁺ from KOH (strong), F⁻ from HF (weak). Fluoride hydrolyses — basic. For NH₄Br: NH₄⁺ from NH₃ (weak), Br⁻ from HBr (strong). Ammonium hydrolyses — acidic.
One extra family worth knowing: small, highly charged metal cations such as Al³⁺ and Fe³⁺ make solutions acidic even though the salt itself contains no hydrogen. In water the ion is really [Al(H₂O)₆]³⁺, and it pulls so hard on those bound water molecules that one of them releases a proton:
[Al(H₂O)₆]³⁺ + H₂O ⇌ [Al(H₂O)₅(OH)]²⁺ + H₃O⁺
so aluminium chloride and iron(III) nitrate solutions both test acidic.
Worked examples
1. Find the pH of 0.10 M sodium acetate. Acetate is the conjugate base of acetic acid, Ka = 1.8 × 10⁻⁵, so Kb = Kw ÷ Ka = 1.0 × 10⁻¹⁴ ÷ 1.8 × 10⁻⁵ = 5.6 × 10⁻¹⁰ [OH⁻] = √(5.6 × 10⁻¹⁰ × 0.10) = 7.5 × 10⁻⁶ M pOH = 5.13, so pH = 8.87 — mildly basic, as predicted. (The approximation check: 7.5 × 10⁻⁶ ÷ 0.10 = 0.0075%, far under the 5% rule.)
2. Find the pH of 0.10 M ammonium chloride. Ammonium is the conjugate acid of ammonia, and ammonia's Kb = 1.8 × 10⁻⁵, so Ka(NH₄⁺) = Kw ÷ Kb = 1.0 × 10⁻¹⁴ ÷ 1.8 × 10⁻⁵ = 5.6 × 10⁻¹⁰ [H₃O⁺] = √(5.6 × 10⁻¹⁰ × 0.10) = 7.5 × 10⁻⁶ M pH = 5.13 — mildly acidic, and the same 0.0075% check applies. Note the pleasing symmetry with example 1: the two pH values sit the same distance either side of 7.
3. Is sodium hydrogencarbonate acidic or basic? NaHCO₃ gives HCO₃⁻, which is amphiprotic. Its base character wins, and the solution is mildly basic at about pH 8.3 — and the same proton-accepting ability is why baking soda works as an antacid, mopping up stomach acid as HCO₃⁻ + H₃O⁺ → CO₂ + 2 H₂O.
4. Is ammonium acetate acidic or basic? Both ions hydrolyse. Because ammonia's Kb and acetic acid's Ka happen to be equal (both 1.8 × 10⁻⁵), the two effects cancel and the solution is very close to neutral.
Common mistakes to avoid
- Assuming "salt" means "neutral". Only salts of two strong parents are neutral. Most salts you meet in a lab shift the pH one way or the other.
- Letting spectator ions hydrolyse. Na⁺, K⁺, Cl⁻, Br⁻ and NO₃⁻ do not react with water. Writing "Cl⁻ + H₂O → HCl + OH⁻" is a standard lost mark: HCl is strong, so that reaction does not go.
- Reading the pH off the salt's own name. Ammonium chloride sounds like it should be basic because of the ammonia in its name. It is acidic, because it is ammonia's conjugate acid that is present.
FAQ
What is salt hydrolysis?
The reaction of a salt's ion with water to produce H₃O⁺ or OH⁻. It happens only for ions derived from a weak acid or a weak base.
Why is NaCl neutral?
Both of its ions come from strong parents — HCl and NaOH — so neither has any tendency to react with water.
Can a salt solution be strongly acidic?
An ammonium salt only gets mildly acidic — even a concentrated solution rarely falls much below pH 4. But salts of small, highly charged cations go further: 0.10 M AlCl₃ sits near pH 3 and 0.10 M FeCl₃ near pH 2.
How do I predict a weak-acid, weak-base salt?
Compare the acid's Ka with the base's Kb. Larger Ka gives an acidic solution, larger Kb a basic one, and equal values give roughly neutral.
The takeaway
A salt's pH is inherited from its parents. Ions that came from strong acids and strong bases are spectators; ions that came from weak ones hydrolyse and push the pH up or down. Identify the parent acid and base, decide which ion is a leftover from the weak side, and you can predict the solution's character before you touch an indicator.
Come from [What Is an Amphoteric Substance?] and [Conjugate Acid vs Conjugate Base], and revisit where the salts came from in What Is Neutralization? Acids Meet Bases and What Is pH? The pH Scale Explained Simply.
You just learned one topic the five-minute way. The series does it for your entire course — one printable page per topic: understand it, memorize it, test yourself. Five minutes. Next topic.
- Vol 1 · Semester 1 — atoms, moles, stoichiometry, bonding & gases (22 sheets)
- Vol 2 · Semester 2 — kinetics, equilibrium, acids & bases, electrochem (19 sheets)
- Vol 3 · The Hard Stuff — cram charts & decision trees for the units worth the most points (15 sheets)
Built for advanced-level high school and first-year college chem. 56 sheets, printable, Letter + A4.
See what's inside →
Comments
Post a Comment