In this mini-lesson you'll see how a chemical reaction is really just atoms being rearranged — and how we write reactions as word and symbol equations. You'll meet combustion, thermal decomposition, oxidation and displacement, then explore acids, alkalis, the pH scale and neutralisation.
Work through each screen, answer the questions as you go and collect ⭐ stars. Press Start when you're ready.
In a chemical reaction, the reactants turn into new substances called products. The atoms don't vanish or appear — the bonds between them are broken and remade, so the same atoms end up joined in a different way.
Watch out: a chemical reaction makes new substances. Melting ice or dissolving sugar are only physical changes — no new substance forms, so they are not reactions.
Because atoms are only rearranged, the total mass stays the same. The mass of the products always equals the mass of the reactants.
Common mistake: "the candle got lighter, so mass was destroyed." Not true — the gases (carbon dioxide and water vapour) floated away. If you trapped them, the mass would be exactly the same. Mass is always conserved.
A word equation is the simplest way to show a reaction. Reactants go on the left, products on the right, and an arrow (→) means "reacts to make":
Tip: when a metal joins with oxygen the product is named "metal oxide". That's why magnesium + oxygen makes magnesium oxide.
A symbol equation uses chemical formulae instead of names. It carries more information — including how many of each atom there are:
The big numbers in front (called balancing numbers) tell you how many molecules. Because atoms are conserved, the number of each type of atom must be the same on both sides — the equation is balanced.
Left: 2 × H₂ = 4 H, plus O₂ = 2 O.
Right: 2 × H₂O = 4 H and 2 O. Both sides match ✓
Combustion is a reaction with oxygen that releases energy as heat and light. When a fuel that contains carbon and hydrogen burns fully, it makes carbon dioxide and water:
Watch out: combustion always needs oxygen. With too little oxygen you get incomplete combustion, which can produce poisonous carbon monoxide and sooty carbon instead.
Thermal decomposition is when heat breaks one compound down into two or more simpler substances. The word "thermal" means heat, and "decomposition" means breaking apart.
This happens when limestone is heated strongly in a kiln to make quicklime (calcium oxide) for cement.
Spot it: in decomposition there is usually one substance on the left and more than one on the right — the opposite pattern to combining reactions.
Oxidation is a reaction in which a substance gains oxygen. Rusting and burning are both oxidation. When iron rusts it slowly gains oxygen from the air:
Link: "burning a metal in oxygen" and "a metal rusting" are both examples of the metal being oxidised — one is just much faster than the other.
In a displacement reaction a more reactive metal pushes out (displaces) a less reactive metal from its compound. It takes the less reactive metal's place:
Read each reaction and tap the type that best describes it.
Many everyday substances are either acidic or alkaline (their opposite). A substance that is neither is called neutral.
Watch out: "alkali" is the opposite of "acid" — not the same thing. Acids and alkalis react together and cancel each other out (you'll see this soon).
The pH scale runs from 0 to 14 and tells us how acidic or alkaline something is. Lower numbers are more acidic; higher numbers are more alkaline; pH 7 is neutral.
Common mistake: neutral is pH 7 — not pH 0. pH 0 is a very strong acid. Pure water sits right in the middle at 7.
An indicator is a substance that changes colour depending on the pH. Universal indicator gives a whole rainbow of colours across the scale:
Another common indicator is litmus: it turns red in acid and blue in alkali.
Tip: a colour change alone tells you acid vs alkali; to read the actual pH number you compare the colour to a pH chart.
Tap a substance (with its universal-indicator colour), then tap the box for its pH range.
When an acid and an alkali react, they cancel each other out and move the pH towards 7. This is called neutralisation. The products are a salt and water:
Remember: acid + alkali → salt + water. This is why indigestion tablets (an alkali) settle an acidic stomach.
Many metals react with acids. The products are a salt and hydrogen gas — you often see bubbles fizzing off the metal:
Watch out: metal + acid makes a salt and hydrogen — not water. That's different from neutralisation (acid + alkali → salt + water).
A catalyst is a substance that speeds up a reaction but is not used up itself — so you can use it again and again. It is not one of the products.
Key idea: a catalyst changes the speed of a reaction, not the products. Because it isn't used up, a small amount lasts a long time. Living things use special catalysts called enzymes.
A reaction: atoms are rearranged; mass is conserved.
Equations: reactants → products (word or balanced symbol).
Combustion: fuel + oxygen → carbon dioxide + water.
Thermal decomposition: heat breaks one compound into more.
Oxidation: a substance gains oxygen (e.g. rusting).
Displacement: a more reactive metal pushes out a less reactive one.
pH: 0–14; 7 is neutral; indicators show pH by colour.
Neutralisation: acid + alkali → salt + water.
Metal + acid: → salt + hydrogen. Catalyst: speeds up, not used up.
You've covered the whole KS3 "Chemical reactions" topic. Press Finish to see your score.
You've worked through Chemical reactions for KS3 Science. 🎉
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