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How do we know a chemical reaction has happened, and why do some reactions give out heat while others take it in?

Recognise the signs of a chemical reaction, balance simple word and symbol equations, and classify reactions as exothermic or endothermic

A focused N(A)-Level answer on reactions. The signs of a chemical change, conservation of mass, balancing simple equations, and telling exothermic from endothermic reactions.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

SEAB wants you to recognise when a chemical reaction has happened, to balance simple word and symbol equations, and to decide whether a reaction is exothermic or endothermic. The central idea is that in a chemical reaction atoms are rearranged, never created or destroyed, and that energy is given out or taken in as bonds change.

The answer

Signs of a chemical reaction

A chemical reaction makes new substances. Tell-tale signs include:

a colour change,
a gas given off (bubbling or fizzing),
a solid forming when two solutions are mixed (a precipitate),
a temperature change (heating up or cooling down),
light given out (as in burning).

Conservation of mass

In a chemical reaction, atoms are only rearranged, so no atoms are gained or lost. This means the total mass of the products equals the total mass of the reactants. This is why equations must balance.

Balancing equations

A word equation names the reactants and products, for example:

\text{magnesium} + \text{oxygen} \rightarrow \text{magnesium oxide}

A symbol equation uses formulae and must be balanced, meaning the same number of each type of atom appears on both sides. You balance by placing big numbers in front of formulae, never by changing a formula:

2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}

Exothermic and endothermic reactions

Energy changes go in two directions:

an exothermic reaction gives out heat to the surroundings, so the temperature rises (for example burning fuel or neutralisation),
an endothermic reaction takes in heat from the surroundings, so the temperature falls (for example some dissolving and thermal decomposition reactions).

Balancing the reaction of hydrogen and oxygen

Hydrogen burns in oxygen to form water. Balance the equation $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$ .

Step 1: Count the atoms on each side

Left: $2$ hydrogen, $2$ oxygen. Right: $2$ hydrogen, $1$ oxygen. The oxygen does not match.

Step 2: Balance the oxygen

Put a $2$ in front of the water to give $2$ oxygen atoms on the right:

\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}

Step 3: Recount the hydrogen

The right now has $2 \times 2 = 4$ hydrogen atoms, but the left has only $2$ .

Step 4: Balance the hydrogen

Put a $2$ in front of the hydrogen on the left:

2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}

Now both sides have $4$ hydrogen and $2$ oxygen atoms, so it is balanced.

Examples in context

Example 1. A hand warmer. A reusable hand warmer uses an exothermic change that releases heat to your hands when activated. Because the reaction gives out energy to the surroundings, the pack feels warm for some time.

Example 2. An instant cold pack. A sports cold pack uses an endothermic change: when a chemical dissolves in water inside the pack, it takes in heat from the surroundings, so the pack feels cold and can be used to treat a sprain.

Try this

Cue. State two signs that a chemical reaction has happened. Any two of: colour change, gas given off, precipitate forms, temperature change, light given out.
Cue. Balance the equation $\text{Na} + \text{Cl}_2 \rightarrow \text{NaCl}$ : $2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}$ .
Cue. A reaction makes the beaker feel hot. State whether it is exothermic or endothermic. Exothermic, because it gives out heat.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksMagnesium burns in oxygen to form magnesium oxide. (a) Write the word equation. (b) Balance the symbol equation:

\text{Mg} + \text{O}_2 \rightarrow \text{MgO}

Show worked answer →

(a) Magnesium plus oxygen gives magnesium oxide.

(b) Balance so each side has equal atoms. Oxygen comes as $\text{O}_2$ , so:

$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$

Now there are $2$ magnesium and $2$ oxygen atoms on each side.

What markers reward: a correct word equation, and a balanced symbol equation with equal atoms of each element on both sides.

Original3 marks(a) State what is meant by an exothermic reaction. (b) A student adds a chemical to water and the beaker feels cold. State whether this change is exothermic or endothermic and explain your answer.