Heat Makes Things Grow

Why it matters

On a hot day, tight metal lids come loose more easily and telephone wires sag lower. Almost everything grows a little bigger when it gets hot.

Predict first

You heat the same amount of a solid, a liquid and a gas by the same number of degrees. Which one grows in size the MOST?

Heating makes particles move faster and spread apart, so matter expands. Gases expand the most, liquids less, and solids the least, because gas particles are freest to move apart.

• The solid, because it is the heaviest
• The gas, because its particles are far apart and move apart the most on heating — correct
• They all expand by exactly the same amount
• The liquid, more than the gas

What you do

Order these by how much they expand when heated, from the most to the least.

Check yourself

In the ball-and-ring experiment, a cold iron ball just passes through the ring. After the ball is heated strongly, what happens?

Heating makes the iron ball expand slightly, so it becomes too big to pass through the ring. When it cools it contracts and fits again. This shows solids expand on heating.

Why does the liquid in a thermometer rise when it gets warmer?

When the temperature rises, the liquid in the thermometer expands and moves up the thin tube. The height it reaches tells us the temperature, because liquids expand on heating.

Why are small gaps left between the rails of a railway track?

Metal rails expand in hot weather. The gaps give the rails room to grow so they do not push against each other and bend. Bridges are built with similar expansion gaps.

• The ball expands and no longer fits through the ring — correct
• The ball shrinks and slips through more easily
• The ball turns into a liquid at once
• Nothing at all changes
• The liquid expands on heating, so it rises up the narrow tube — correct
• More liquid is magically created by the heat
• The glass shrinks and pushes the liquid up
• The liquid turns into a gas and floats
• So the rails have room to expand in hot weather without buckling — correct
• To save metal and make the track cheaper
• Because the rails shrink in summer and need extra space
• So rainwater can drain away