A reaction rearranges atoms

In a chemical reaction, the reactants react to make new substances called products. The atoms don't vanish or appear — they separate from the atoms they were joined to and join up in new ways.

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.

Nothing is lost

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 wax reacts with oxygen from the air, and the gases made (carbon dioxide and water vapour) float away. If the candle burned in a sealed jar on the balance, the reading would stay exactly the same. Mass is always conserved.

Where did the mass go?

Word equations

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":

• Use + to separate two or more substances on the same side.
• The arrow is not an equals sign — it means "reacts to make".
• Read it as: "magnesium reacts with oxygen to make magnesium oxide".

Tip: when a metal joins with oxygen the product is named "metal oxide". That's why magnesium + oxygen makes magnesium oxide.

Symbol equations

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.

Read the equation

Is mass destroyed when a candle burns?

What do a rusting bike and a bonfire have in common?

Combustion (burning)

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:

A metal burns to make its oxide: magnesium + oxygen → magnesium oxide. No carbon, so no carbon dioxide.

Watch out: combustion always needs oxygen. With too little oxygen you get incomplete combustion, which can produce poisonous carbon monoxide and sooty carbon instead.

Products of burning

Thermal decomposition

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). Heating limestone is also the first step in making 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

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:

• Rusting is slow oxidation of iron (needs oxygen and water).
• Combustion is fast oxidation that releases lots of energy.

Link: "burning a metal in oxygen" and "iron rusting" are both examples of the metal being oxidised — one is just much faster than the other.

Displacement reactions

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:

Name the reaction type

Read each reaction and tap the type that best describes it.

What do a rusting bike and a bonfire have in common?

How does an indigestion tablet ease the burn?

Acids and alkalis

An acid is a substance that is neutralised by an alkali. An alkali is a base that dissolves in water; it neutralises an acid. A substance that is neither is neutral.

• Acids — pH below 7, e.g. lemon juice, vinegar, hydrochloric acid.
• Alkalis — pH above 7, e.g. bleach, soap, sodium hydroxide solution.
• Neutral — pH 7, e.g. pure water.

Stay safe: never taste or touch a chemical to test it. Alkalis can damage skin and eyes just as acids can. Use an indicator instead (you'll see this soon).

The pH scale

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 extremely acidic. Pure water sits right in the middle at 7.

Indicators show pH by colour

An indicator is a substance that changes colour depending on the pH. Universal indicator gives a whole rainbow of colours across the scale:

• Red / orange / yellow → acid (below 7)
• Green → neutral (pH 7)
• Blue / purple → alkali (high pH)

Another common indicator is litmus: it turns red in acid and blue in alkali.

Tip: litmus only tells you acid or alkali. To find the pH number, match the universal indicator colour to a pH chart.

Acid or alkali?

Tap a substance (with its universal-indicator colour), then tap the box for its pH range.

Neutralisation

When an acid and an alkali react, they make new substances and the pH moves towards 7. This is called neutralisation. The products are a salt and water:

Remember: acid + alkali → salt + water. Indigestion can cause a burning feeling, often linked to stomach acid. Indigestion tablets contain a base that neutralises stomach acid, which eases the burning.

Products of neutralisation

How does an indigestion tablet ease the burn?

Reacting metals with acids

Many metals react with acids. The products are a salt and hydrogen gas — you often see bubbles fizzing off the metal:

• The fizzing is bubbles of hydrogen gas.
• Test for hydrogen: a lit splint gives a squeaky pop.
• More reactive metals fizz faster.

Watch out: metal + acid makes a salt and hydrogen — not water. That's different from neutralisation (acid + alkali → salt + water).

Fizzing metal

Catalysts

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.

What a catalyst does

The key ideas to know

You've covered the key ideas in the KS3 "Chemical reactions" topic. Press Finish to see your score.