Energy changes, but bitesize.

Part 1Heat it steadily and watch the temperature

Take some ice straight from the freezer and heat it gently and steadily — say, with a Bunsen burner turned to a constant flame. If you measure the temperature every minute, you might expect it to climb in a straight line all the way up. It doesn't.

At first the temperature rises. Then, while the ice is melting into water, the temperature stops climbing and stays flat for a while — even though you're still heating it. Once it's all melted, the temperature rises again. Then, while the water is boiling into steam, it goes flat again.

Those flat bits are called plateaus. A graph of temperature against time, with these two flat sections, is a heating curve.

Part 2Why does the temperature go flat?

This is where the particle model explains everything. Temperature is a measure of how fast the particles are moving — how much kinetic energy they have. When you heat a solid, you're feeding energy in, the particles vibrate faster, and the temperature climbs.

But to melt, the particles need to break free from their fixed positions. That takes energy — and while the substance is melting, all the energy you put in goes into breaking those bonds between particles, not into making them move faster. So the temperature doesn't change. The same thing happens at the boiling plateau: the energy goes into pulling the particles right apart into a gas.

In short: during a change of state, the energy goes into changing state, not raising temperature. Once the change of state is finished, the temperature starts rising again.

Part 1Reactions that give out heat

Some chemical reactions give out energy. The energy is transferred from the reaction to the surroundings — that means everything around the reaction: the test tube, the water it's in, the air, your hand. Because energy is being given to them, the temperature of the surroundings rises.

A reaction that transfers energy to the surroundings is called exothermic. An easy way to remember it: exo sounds like "exit", and the energy exits the reaction. We can measure that an exothermic reaction has happened with a simple thermometer — the temperature goes up.

Part 2Everyday examples

Exothermic reactions are all around you. The big ones to remember:

Combustion (burning). Burning a fuel like methane, wood or a candle gives out a lot of heat and light. This is why a fire warms a room.
Neutralisation. When an acid reacts with an alkali, the mixture gets warmer — you can feel it on the side of the beaker.
Oxidation. When a substance reacts with oxygen (like iron slowly rusting, or hand-warmer iron filings reacting fast), energy is released.
Respiration. The reaction in your cells that releases energy from glucose is exothermic — it's part of why your body stays warm.

Part 1Reactions that take heat in

Some reactions do the opposite of exothermic ones: they take in energy from the surroundings. Because energy is being pulled out of the surroundings, the temperature of the surroundings falls — things around the reaction get colder.

A reaction that takes in energy from the surroundings is called endothermic. The clue is in the name: endo means "into", and the energy goes into the reaction. Measure it with a thermometer and you'll see the temperature go down.

Part 2Everyday examples

Endothermic changes are less obvious than exothermic ones, but here are the ones to know:

Thermal decomposition. Heating a compound to break it apart — like heating copper carbonate or calcium carbonate. It only keeps going while you keep heating it.
Citric acid + sodium bicarbonate. This is the fizzy reaction in sherbet. Mix the two and the temperature drops — that's why sherbet can feel cool on your tongue.
Some dissolving. Dissolving certain salts (like ammonium nitrate) in water takes in energy and makes the water colder.

This is exactly how an instant cold pack works. Snap it and a salt mixes with water; the endothermic process pulls heat in, so the pack turns cold and you can use it on a sports injury.

Part 1The practical: before and after

To find out whether a reaction gives out heat or takes it in, you do something beautifully simple: measure the temperature before and after the reaction.

Measure the starting temperature of the chemicals with a thermometer. Mix them, stir, and read the highest (or lowest) temperature you reach. Then work out the temperature change:

temperature change = final temperature − start temperature

The sign of the answer tells you everything:

· A rise (the change is positive, the temperature went up) means the reaction is exothermic — it gave out heat.
· A fall (the change is negative, the temperature went down) means the reaction is endothermic — it took heat in.

Part 2Making it a fair test

To compare reactions fairly, you need to think about your variables:

The variable you change (independent variable) — for example, which acid you use, or its concentration.
The variable you measure (dependent variable) — the temperature change.
The variables you keep the same (control variables) — the volume of liquid, the starting temperature, the same thermometer, the same type of cup or beaker. Keeping these the same is what makes it a fair test.

It helps to write your readings in a results table so you can compare them at a glance: