KS3 · Chemistry

Neutralisation reactions

Drip alkali into an acid and the colour creeps from red to green. So where did the acid go? It hasn't vanished or been watered down. It's become something new.

Drop by drop to pH 7

1Acid + indicator23456

Stage 1 of 6: Acid + indicator. paused

Acid + indicator · 1/6drag to add drops

Add 2–3 drops of universal indicator to the acid. It goes red: low pH, so this is an acid.

Drag through the experiment and watch the indicator colour follow the pH.

Read the result

What is the colour telling you to do?

Choose the indicator, then the result you see.

Indicator used → What you see

5 test results.

On You test a solution. 2 branches to choose from.

You've just tested a solution. Pick the indicator you used, then the colour you see.

Watch out: Adding acid to an alkali makes the pH go DOWN to 7. Adding alkali to an acid makes it go UP to 7. Both roads end at green.

What really happened?

Where did the acid go?

Drops of sodium hydroxide solution (an alkali) were added to hydrochloric acid with universal indicator in it. The tube has just turned green.

Which is closest to what you think has happened to the acid?
How sure are you?

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

Hydrochloric acid (HCl)Sulfuric acid (H₂SO₄)Nitric acid (HNO₃)
Sodium hydroxidemetal: sodium
Potassium hydroxidemetal: potassium
Magnesium oxidemetal: magnesium
Zinc oxidemetal: zinc
Copper oxidemetal: copper

Each cell hides a short answer and the reason behind it. Predict before you tap.

Your turn

Build the equation, one decision at a time

Sulfuric acid reacts with potassium hydroxide solution. The salt that forms stays dissolved. Write the word equation, with state symbols.

  1. Here's the pattern, done in full: hydrochloric acid (aq) + sodium hydroxide (aq) → sodium chloride (aq) + water (l). In symbols: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l).Every neutralisation follows acid + base → salt + water.
  2. missing step
Which line is step 2?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • Adding an acid lowers the pH of a solution; adding a base or an alkali raises it. An alkali is a soluble base.
  • On universal indicator (pH 1 to 14), acids are red, orange or yellow, neutral is green at pH 7, and alkalis are blue or purple. Blue litmus turns red in an acid; red litmus turns blue in an alkali.
  • acid + base → salt + water. The solution is only neutral (pH 7) when the acid and base are mixed in the correct quantities.
  • Common bases are metal oxides and metal hydroxides, such as sodium hydroxide, potassium hydroxide, calcium oxide, copper oxide and magnesium oxide.
  • Salt name = the metal from the base + an ending from the acid: hydrochloric acid → chloride, sulfuric acid → sulfate, nitric acid → nitrate.
  • State symbols: (s) solid, (l) liquid, (g) gas, (aq) in solution. They are lowercase, in brackets and written as subscripts.

The big picture

Neutralisation is the reaction between an acid and a base. Adding alkali to an acid raises its pH; adding acid to an alkali lowers it. Mixed in the correct quantities they react to make a neutral (pH 7) solution of a salt and water, which universal indicator shows as green. The salt's name comes from the metal in the base and the acid used, and the reaction can be written as a word or symbol equation with state symbols.

Key points

1Neutralisation is a reaction: an acid and a base make new substances, a salt and water.
2To neutralise an acid, add alkali (the pH rises to 7). To neutralise an alkali, add acid (the pH falls to 7).
3Green on universal indicator means pH 7, neutral. Overshoot into blue or purple and drops of acid bring it back.
4The water made contains hydrogen and oxygen atoms from the acid and the base.
5In neutralisation equations, lab acids are (aq) and water is (l); the base and the salt can be (s) or (aq), so use the information given.
6HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Worked example

Problem

Zinc oxide powder, a solid, is added to nitric acid. The salt that forms stays dissolved. Write the word equation, with state symbols.

⚠ Watch out

Thinking the acid just disappears or gets watered down. It reacts: the acid and base turn into new substances, a salt and water. And watch the state symbol on water: at room temperature it's (l), never (aq).

🧠

Memory hook

Think of a salt's name like a person's name: the base gives it its first name (the metal) and the acid gives its surname — chloride, sulfate or nitrate.

✓

Check yourself

Potassium hydroxide solution is dripped into nitric acid with universal indicator in it. What colour do you start with, what colour means stop, and which salt is in the tube then?

Flashcards

(15)
What is neutralisation, as a word equation?
acid + base → salt + water
What happens to the pH when you add alkali to an acid?
It rises (increases) towards 7.
What happens to the pH when you add acid to an alkali?
It falls (decreases) towards 7.
What is an alkali?
A base that dissolves: a soluble base.
Universal indicator colour at pH 7?
Green: neutral.
Universal indicator colours for an acid?
Red, orange or yellow (low pH).
Universal indicator colours for an alkali?
Blue or purple (high pH).
Litmus: what shows an acid and what shows an alkali?
Blue litmus turns red in an acid; red litmus turns blue in an alkali.
When is the solution neutral after mixing an acid and a base?
Only when they are mixed in the correct quantities.
Hydrochloric acid makes salts ending in…?
…chloride.
Sulfuric acid makes salts ending in…?
…sulfate.
Nitric acid makes salts ending in…?
…nitrate.
Where does the first part of a salt's name come from?
The metal in the base.
Two families of common bases?
Metal oxides (e.g. copper oxide) and metal hydroxides (e.g. sodium hydroxide).
What do (s), (l), (g) and (aq) mean?
Solid, liquid, gas, and in solution (aqueous).

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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How this lesson was checked. This KS3 Chemistrylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 30 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.