LITo describe neutralisation as the reaction of an acid with a base producing a salt and water.
SC: I can:
Try these 2questions before today's new content. Click an answer for instant feedback — your teacher will walk through them with you.
When an acid is added to a base, both lose their dangerous properties. What two new substances do you think are formed?
In a neutralisation reaction, an acid and a base react to form a salt and water.
This happens because acids are a source of hydrogen ions (H⁺), while bases are a source of hydroxide ions (OH⁻). In an acid–base neutralisation reaction, the H⁺ from the acid and the OH⁻ from the base combine to form water, which is neutral, having a pH of 7 at 25 °C.
Word form: hydrogen ion + hydroxide → water.
COMMON ACIDS & BASES
ACIDS
Neutralisation in three tiers — general (acid + base → salt + water), molecular (HCl + NaOH → NaCl + H₂O), and net ionic (H⁺ + OH⁻ → H₂O). The boxed products are neutral with pH = 7.
The other parts of the acid and base combine to produce a salt. The reaction of a strong acid with a strong base results in a neutral solution with a pH of 7 and a neutral ionic salt.
For example, nitric acid reacts with lithium hydroxide to form the salt lithium nitrate (LiNO₃) and water.
Word form: nitric acid + lithium hydroxide → lithium nitrate + water.
An antacid tablet or liquid is a base used to neutralise excess hydrochloric acid in the stomach when you have indigestion or heartburn. The active ingredient in milk of magnesia is magnesium hydroxide, Mg(OH)₂. When you swallow it, it meets the stomach's HCl(aq) and the neutralisation runs:
Word form: magnesium hydroxide + hydrochloric acid → magnesium chloride + water.
The H⁺ ions that were causing the burn get mopped up into water, the pH of the stomach contents creeps back toward neutral, and you feel better. Note the diprotic balance — Mg(OH)₂ has two OH⁻ groups, so it neutralises two HCl molecules.
Other common antacid ingredients are calcium carbonate (CaCO₃, in chewable tablets like Tums) and sodium hydrogen carbonate (NaHCO₃, baking soda). Those are carbonates, not bases, and they react via a different mechanism (acid + carbonate → salt + water + CO₂ ↑) — that's why some antacids fizz and make you burp. We cover that pattern fully in the next lesson.
Watch the demo: a beaker of milk of magnesia is dosed with universal indicator, then vinegar is added a bit at a time. The colour walks from purple/blue (Mg(OH)₂ alkaline, pH ≈ 10) through green (neutralisation point, pH 7) and on to red as more acid is added. That's exactly the equivalence-point chemistry above, in a single beaker — the same Mg(OH)₂ + acid → salt + water reaction your stomach runs every time you take milk of magnesia.
When you've added exactly enough base to neutralise all of the acid, you reach the equivalence point. For a strong acid + strong base pair (e.g. HCl + NaOH), the equivalence point sits at pH = 7 — the salt that forms is neutral, and only water is left to set the pH. During a slow titration, a digital pH meter would draw a curve that rises smoothly from the starting acidic pH (≈ 1) up through 7 as more base is added.
Universal indicator is red in a solution of 0.1 M hydrochloric acid. If calcium hydroxide (Ca(OH)₂) is added to the solution, the acid and base neutralise each other and the universal indicator turns green (Figure 4.18).
Figure 4.18 — Universal indicator tracks the neutralisation: red in acid, through orange/yellow as base is added, to green at the neutral endpoint.
We do · whole class · 3 prompts
Sulfuric acid H2SO4(aq) is added to sodium hydroxide NaOH(aq). Predict the products and write the balanced equation. (This is the "1:2 ratio" gotcha — pay attention.)
Prompt 1 — name the salt and the water co-product.
Prompt 2 — why is the ratio 1:2 (not 1:1)?
Prompt 3 — write the balanced symbol equation with state symbols.
Wrong — in chemistry, "salt" = any ionic compound formed when an acid neutralises a base.
| Acid + base | Salt |
|---|---|
| HCl + NaOH | NaCl (table salt) |
| HNO₃ + KOH | KNO₃ (potassium nitrate) |
| H₂SO₄ + KOH | K₂SO₄ (potassium sulfate) |
| HCl + Mg(OH)₂ | MgCl₂ (magnesium chloride) |
Rule: the cation comes from the base; the anion comes from the acid. Whatever ionic compound that gives you — that is the salt.
Wrong — not balanced. Sulfuric acid is diprotic: it donates 2 H⁺ ions per molecule, so you need 2 NaOH to neutralise one H₂SO₄.
Atom check: H 4↔4, S 1↔1, O 6↔6, Na 2↔2. ✓
Rule: count the acidic H atoms first, then multiply the base accordingly. Same idea for triprotic acids (e.g. H₃PO₄ needs 3 NaOH).
Wrong — they tell you what's actually happening in the beaker.
For neutralisation, the convention is:
Rule: acid (aq) + base (aq) → salt (aq) + water (l) for every neutralisation in this topic.
Hydrochloric acid reacts with potassium hydroxide. Fill in the four blanks:
The balanced symbol equation:
One thing I now understand about neutralisation that I didn't understand before:
Next class (Fri 8 May, P3): acid + metal — the second of the four reactions of acids.