E. Earth and Space Systems · Heat in the Environment

Heat and Volume: Expansion and Contraction

E2.3 A1.2 A1.4 A3.1About 45 minutes

Learning goals

I can use the particle theory to explain why most solids, liquids and gases expand when heated and contract when cooled.
I can compare how much solids, liquids and gases expand.
I can explain why water is unusual when it cools and freezes.
I can describe how engineers and builders plan for expansion and contraction.

Words to know

volume
The amount of space something takes up.

expansion
An increase in volume. Most matter expands when it is heated.

contraction
A decrease in volume. Most matter contracts when it is cooled.

expansion joint
A gap built into a bridge, sidewalk or road so that the material can expand and contract without cracking.

density
How much mass is packed into a certain volume. Matter that expands becomes less dense.

freeze-thaw cycle
Water freezing and melting again and again, which can crack rock and roads.

Why do sidewalks have cracks on purpose?

Next time you walk on a concrete sidewalk, look at the straight lines cut across it every metre or two. Big bridges have metal "teeth" at their ends that fit together like fingers. These gaps are not mistakes. They are there because of heat.

A straight concrete sidewalk in a neighbourhood with lines cut across it at regular spaces. — Notice the lines cut across the sidewalk. These joints give the concrete room to expand on hot days and shrink on cold days without cracking at random. AI-generated image

Particles spread out when heated

Remember the particle theory: when particles gain energy, they move faster. Faster particles push each other farther apart, so the matter takes up more space.

Expansion: heating → particles move faster and farther apart → volume increases.
Contraction: cooling → particles slow down and come closer together → volume decreases.

The number of particles does not change, and the particles themselves do not get bigger. So the mass stays the same. Only the spacing changes. Because the same mass now takes up more space, the warmer matter is less dense.

The same 16 particles take up more space when they are warmer, because they move faster and push farther apart. The particles themselves stay the same size.

State	How much it expands when heated	Why
Solid	a little	Strong attraction holds the particles close together in their places.
Liquid	more than a solid	The attraction is weaker, so particles can spread out more easily.
Gas	the most	The attraction is very weak; faster particles spread out much more.

For the same temperature change, gases usually expand the most and solids the least.

Expansion and contraction around us

Sidewalks, roads and bridges have expansion joints. On a hot summer day the concrete or steel expands into the gaps instead of cracking or buckling.
Power lines sag lower on hot summer days and pull tighter in winter. Line workers leave enough slack for the cold.
Railway tracks can bend out of shape on very hot days if they cannot expand freely, so rail crews check them in heat waves.
Liquid thermometers work because the liquid inside expands and rises up the thin tube as it warms.
A tight metal lid on a glass jar often loosens under hot tap water, because the metal lid expands more than the glass.
Car and bike tires look flatter on a cold winter morning: the air inside contracts and its pressure drops.
A hot-air balloon rises because heated air expands, becomes less dense, and floats on the cooler, denser air around it.

The same sidewalk in summer and winter (gap sizes exaggerated so you can see them). The expansion joints give each slab room to grow in the heat, so the concrete does not crack or buckle.

Water breaks the rule

Water behaves strangely. As liquid water cools, it contracts, like most liquids, but only until about 4 °C. Below 4 °C it starts to expand again. When it freezes at 0 °C, its particles lock into an open pattern with lots of space, and the ice takes up about 9% more volume than the water did.

That is why:

Ice floats (it is less dense than liquid water), so lakes freeze from the top down and fish survive in the water below.
Water pipes can burst in an unheated cottage in winter.
Potholes form: water seeps into cracks in the road, freezes, expands and pushes the cracks wider. Ontario's many freeze-thaw days in late winter make this worse.
Rocks split in nature when water freezes in their cracks (frost wedging). This slowly breaks down cliffs and mountains.

Worked example

Let's explain the sidewalk gaps

Use the particle theory to explain why concrete sidewalks are built with gaps.

Observe: sidewalks get hot in summer sun and very cold in winter.
Particle theory: in summer, the particles in the concrete gain energy, vibrate faster and push slightly farther apart. The concrete expands.
Problem: a long slab with no gaps would push against itself as it expanded and could crack or buckle.
Solution: gaps (expansion joints) give each section room to expand. In winter, the concrete contracts and the gaps get a little wider.

The gaps give the concrete room to expand in heat and contract in cold without cracking.

Worked example

Let's calculate: freezing water

Water expands by about 9% when it freezes. About how much ice do you get from 500 mL of water?

9% of 500 mL = 0.09 × 500 = 45 mL.
500 mL + 45 mL = 545 mL.

About 545 mL of ice. That is why you should never freeze a full, sealed glass bottle of water.

Bar heights drawn to scale: 500 mL of water becomes about 545 mL of ice. That extra 9% is what bursts pipes and widens potholes.

Just for fun

Funny but true: the Eiffel Tower in Paris can grow about 15 cm taller on a hot summer day, because its iron expands. It is one of the few towers that gets a growth spurt every summer.

Hands-on activity

Balloon on a bottle (A1.2)

You need: an empty plastic bottle (500 mL or 1 L), a balloon, two large bowls, hot tap water, cold water with ice cubes, a clock, and a grown-up.

Stretch the balloon a few times. Then fit its opening over the mouth of the empty bottle (the bottle holds only air).
Predict: what will happen to the balloon when the bottle is warmed? When it is cooled?
A grown-up fills one bowl with hot tap water. Stand the bottle in it for 3 minutes. Observe the balloon.
Move the bottle to the bowl of ice water for 3 minutes. Observe again.
Draw the balloon at each step. Under each drawing, add a particle diagram of the air inside the bottle.

What to look for: in hot water the balloon stands up and partly inflates, because the air inside expands. In ice water it shrinks and may even be pulled into the bottle, because the air contracts.

Extra: water expands when it freezes. Fill a small plastic container (yogurt cup) half full of water, mark the water level with a marker, and freeze it overnight. Compare the ice level with your mark.

Safety (A1.4): a grown-up handles the hot water; use hot tap water only, never boiling water. A thin plastic bottle may soften slightly in hot water; that is okay. Balloons are a choking hazard: keep balloons and broken pieces away from young children, and check for latex allergies. Never freeze glass containers of water.

Your turn

What happens to the particles of a substance when it expands?

Does the mass of a metal rod change when it expands? Explain.

Which usually expands the most when heated: a solid, a liquid or a gas?

Why do bridges have expansion joints?

How does a liquid thermometer work?

Why does ice float on water?

Why can water pipes burst in winter?

More practice: warm-up

Heating makes most matter ______. Cooling makes most matter ______.

Do power lines sag more in summer or in winter?

Below what temperature does water start expanding again as it cools?

Why might your bike tires look flatter on a cold morning?

What do we call water freezing and melting again and again?

More practice: on your own

A jar's metal lid is stuck. Why does running hot tap water over the lid help?

Explain, using particles, why a hot-air balloon rises.

Explain how potholes form in Ontario roads in late winter.

How much ice do you get from 1 L (1000 mL) of water, if water expands by about 9% when it freezes?

In the balloon experiment, why does the balloon shrink in ice water?

Challenge

A lake freezes in December. Explain why the ice forms on top and why the water at the bottom of a deep lake stays at about 4 °C all winter.

A builder lays a 10 m long steel beam in winter when it is −20 °C. Should she leave a small gap at its end or fit it tightly between two concrete walls? Explain.

Rock climbers notice more fallen rocks at the base of cliffs in spring. Use what you learned to explain why.

Check your understanding

Quiz

Why does a substance expand when heated?

For the same temperature change, which usually expands the most?

What happens to water when it freezes?

Why do sidewalks have gaps cut across them?

When a metal rod expands, its mass...

A balloon on a bottle inflates when the bottle sits in hot water because...

For parents and teachers

Big idea (E2.3): heating makes particles move faster and spread out, so most matter expands; cooling makes it contract. Mass stays the same. Water is an exception between 4 °C and 0 °C and expands about 9% when it freezes. The balloon test is a simple investigation (A1.2) with safety steps (A1.4); the examples of bridges, rails and power lines connect to engineering and trades (A3.1).

How to help: look for expansion joints on sidewalks and bridges, and ask why they are there. In winter, notice tire pressure, frost heaves and potholes.

Common difficulties:

Thinking particles grow. Only the spacing changes.
Thinking ice is heavier than water. The same mass of ice takes more space, so it is less dense and floats.

Safety: hot tap water only, handled by an adult. Balloons are a choking hazard; check for latex allergy. Never freeze water in glass.