GCSE · Chemistry · Edexcel · Spec 1CH0

Neutralisation reactions and the H+ + OH- -> H2O ionic explanation

Different acids, different alkalis, different salts — yet in every acid–alkali neutralisation the same two ions do all the work while the rest just watch. Let's catch them.

Chemistry · Neutralisation

Which ions actually react?

Reaction A: hydrochloric acid + sodium hydroxide. Reaction B: sulfuric acid + potassium hydroxide. Reaction C: hydrochloric acid + calcium hydroxide. For each ion, decide: does it take part, or does it just watch?

Still to sort

Takes part — it changes (0)

By the end it is no longer a free ion in the solution.

Where the line is: If you can find the same ion, still dissolved, after the reaction, it does not belong here.

Spectator — dissolved and unchanged (0)

The same ion is in solution before and after the reaction.

Where the line is: Ending up in the salt's name is not the same as reacting: the salt here stays dissolved as its separate ions.

12 of 12 still to sort.

In water, an acid releases H⁺ ions and an alkali releases OH⁻ ions — but other ions come along for the ride. Sort all twelve and look at what ends up in each column.

Watch out: An ion that turns up in the salt's name hasn't necessarily reacted. The salts made here stay dissolved as separate ions — exactly as those ions started.

The ionic equation of neutralisation

Tap each part. Count the atoms on each side — then add up the charges.

Tap a molecule to see what it does in the reaction.

Bases and alkalis

Where did the OH⁻ come from?

A base is any substance that neutralises an acid, making a salt and water. An alkali is a base that dissolves in water. Tick every set each substance belongs to.

  • A Bases
  • B Alkalis
  1. sodium hydroxide
  2. potassium hydroxide
  3. copper oxide
  4. iron oxide
  5. hydrochloric acid

Predict, then check

Commit to an answer before you look.

Potassium hydroxide neutralises nitric acid. What are the products?

Your turn

Derive the ionic equation yourself

Write the ionic equation for the neutralisation of hydrochloric acid by calcium hydroxide solution.

  1. Write the balanced equation: 2HCl(aq) + Ca(OH)₂(aq) → CaCl₂(aq) + 2H₂O(l)Two HCl, because each Ca(OH)₂ brings two OH⁻ ions to neutralise.
  2. missing step
Which line is step 2?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • Neutralisation: acid + base → salt + water. Those are the only two products.
  • A base is any substance that neutralises an acid. Alkalis are the bases that dissolve in water, such as sodium hydroxide.
  • In water, an acid dissociates and releases H⁺ ions; an alkali releases OH⁻ ions.
  • For every acid–alkali neutralisation the ionic equation is H⁺(aq) + OH⁻(aq) → H₂O(l).

The big picture

When an acid and a base react, the only products are a salt and water. In water an acid releases H⁺ ions and an alkali releases OH⁻ ions. Those two ions join to make water; every other ion is a spectator that stays dissolved and unchanged. Strip the spectators out and every acid–alkali neutralisation boils down to the same ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l), balanced for atoms and for charge.

Key points

1Salt name = the metal from the base + an ending from the acid: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.
2Hydroxides contain OH⁻ ions. Metal oxides contain O²⁻ ions. Most bases, like copper oxide, don't dissolve in water.
3Spectator ions don't take part: they stay dissolved and appear unchanged on both sides, so they are left out of the ionic equation.
4Method: write the balanced equation, split the aqueous compounds into ions, cross out the spectators, and write what remains.
5The ionic equation balances charge as well as atoms: +1 and −1 make 0, and water is neutral.

Worked example

Problem

Sulfuric acid neutralises potassium hydroxide solution. Name the salt, identify the spectator ions and write the ionic equation.

⚠ Watch out

Thinking the salt's ions react to make the salt. In hydrochloric acid + sodium hydroxide, Na⁺ and Cl⁻ are dissolved before the reaction and still dissolved after it — sodium chloride solution is just those same ions. They're spectators; only H⁺ and OH⁻ change.

🧠

Memory hook

Spectator ions are like the crowd at a match: in the stadium the whole time, but they never touch the ball. Only H⁺ and OH⁻ play — and they always end up as H₂O.

✓

Check yourself

Potassium hydroxide neutralises hydrochloric acid. Name the salt, the two spectator ions and the ionic equation. (Answers: potassium chloride; K⁺ and Cl⁻; H⁺(aq) + OH⁻(aq) → H₂O(l).)

Flashcards

(14)
What are the only products when an acid neutralises a base?
A salt and water.
What is a base?
Any substance that neutralises an acid, making a salt and water.
What is an alkali?
A base that dissolves in water, giving an alkaline solution — for example sodium hydroxide.
Is copper oxide an alkali?
No. It's a base (it neutralises acids) but it doesn't dissolve in water, so it isn't an alkali.
Which ion do hydroxides contain, and which do metal oxides contain?
Hydroxides: OH⁻. Metal oxides: O²⁻.
How do you name the salt from a neutralisation?
Metal from the base first, then an ending from the acid.
Which salt endings come from hydrochloric, sulfuric and nitric acid?
Chloride, sulfate and nitrate.
What happens to an acid when it dissolves in water?
It dissociates (ionises), releasing H⁺ ions.
Which ion does an alkali release in water?
The hydroxide ion, OH⁻.
Write the ionic equation for any acid–alkali neutralisation.
H⁺(aq) + OH⁻(aq) → H₂O(l)
What is a spectator ion?
An ion that doesn't take part in the reaction: it stays dissolved and is unchanged on both sides of the equation.
In hydrochloric acid + sodium hydroxide, which ions are spectators?
Na⁺ and Cl⁻ — the ions that make up the dissolved salt.
Why is the ionic equation of neutralisation balanced for charge?
+1 (H⁺) and −1 (OH⁻) add to 0, and water is a neutral compound with no charge.
What are the four steps for writing an ionic equation?
Balanced equation → split aqueous compounds into ions → remove spectator ions → write what's left.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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