C. Matter and Energy · Fluids

Pressure, Volume and Temperature

C2.6 C2.4 A1.2 A1.4About 45 minutes

Learning goals

I can describe how a gas's pressure changes when it is compressed or heated.
I can describe how a gas's volume changes when it is heated in a flexible container.
I can explain why heating a liquid in a closed container is dangerous.
I can use the particle theory to explain everyday examples, like tires losing pressure in winter.

Words to know

pressure
The amount of force pushing on each unit of area. Gas pressure comes from particles hitting the walls of their container.

kilopascal (kPa)
A unit of pressure. The air around us at sea level pushes with a pressure of about 101 kPa.

rigid container
A container whose volume can't change, such as a metal can or a steel tank.

flexible container
A container whose volume can change, such as a balloon or a plastic bag.

pressure relief valve
A safety valve that opens to let fluid out if the pressure inside gets too high.

The winter tire light

On the first really cold morning of November, lots of drivers in Ontario see a warning light on the dashboard: low tire pressure. Nobody let air out of the tires overnight. So where did the pressure go?

Today you'll find out how pressure, volume and temperature are linked in fluids.

A car tire on a frosty morning, with frost on the tire and on the grass beside it. — On a cold morning the air particles inside a tire move more slowly and hit the walls less often and less hard, so the tire pressure drops. AI-generated image

What is gas pressure?

Gas particles are always zooming around. Each time a particle hits a wall of its container, it gives the wall a tiny push. Billions of these hits every second add up to a steady push on every part of the wall. That is gas pressure.

The pressure goes up when particles hit the walls more often or harder. It goes down when they hit less often or less hard.

Pressure is measured in pascals (Pa) or kilopascals (kPa). The air around you at sea level presses on you with about 101 kPa. Car tires are usually filled to about 200 to 250 kPa above that.

Three relationships for gases

1. Squeeze a gas (temperature stays the same): smaller volume → higher pressure. The particles are crowded into less space, so they hit the walls more often. (You saw this with the syringe.) The reverse is also true: give the gas more room and its pressure drops.

2. Heat a gas in a rigid container (volume stays the same): higher temperature → higher pressure. Heated particles move faster, so they hit the walls harder and more often. Cooling does the opposite.

3. Heat a gas in a flexible container (pressure stays about the same): higher temperature → bigger volume. The faster particles push the stretchy walls outward until the inside and outside pressures balance again. A balloon gets bigger in the sun and smaller in the cold.

Three gas relationships. The dashed line and dashed circle show where the piston and balloon started. Longer arrows on the particles mean faster particles.

What you do to the gas	What stays the same	What happens	Particle theory reason
squeeze it (decrease volume)	temperature	pressure increases	particles hit the walls more often
give it more room (increase volume)	temperature	pressure decreases	particles hit the walls less often
heat it	volume (rigid container)	pressure increases	faster particles hit harder and more often
cool it	volume (rigid container)	pressure decreases	slower particles hit more gently and less often
heat it	pressure (flexible container)	volume increases	faster particles push the walls outward

How pressure, volume and temperature are related for a gas.

What about liquids?

Liquids expand only a little when heated. But because liquids are nearly incompressible, that little bit of expansion matters a lot in a closed, full container: the liquid has nowhere to go, so the pressure rises very quickly.

A home's hot water tank has a temperature and pressure relief valve. If the water gets too hot and the pressure climbs too high, the valve opens and lets some water out, so the tank can't burst.
A car's cooling system has an overflow tank where expanding coolant can go.
That's why you should never heat a sealed container of food or drink, like an unopened can of soup, on the stove or in a microwave.

Two more effects: compressing a gas quickly warms it (a bike pump's barrel gets warm when you pump hard), and letting a compressed gas expand quickly cools it (a spray can feels cold after a long spray).

Worked example

Let's explain the winter tire light

A car's tires were filled to the right pressure in a warm garage. The next morning, the car is parked outside at −20 °C and the low-pressure light comes on. No air leaked out. Explain why.

Step 1. What changed? The temperature dropped a lot.
Step 2. What stayed about the same? The amount of air (no leak), and the tire's volume (a tire is fairly stiff, almost like a rigid container).
Step 3. Use the particle theory. Cold air particles move more slowly, so they hit the inside of the tire less often and less hard.
Step 4. Conclude. Fewer, gentler hits → lower pressure.
Step 5. What should the driver do? Check the pressure with a gauge and add air. Many drivers check their tires each fall as temperatures drop.

The tire pressure dropped because the cold made the air particles inside move more slowly, so they hit the inside of the tire less often and less hard.

Inside a tire. Both tires hold the same 8 air particles. In the cold, the particles move more slowly (shorter arrows), so they hit the walls less often and less hard: lower pressure.

Read the warning labels

Spray cans (aerosols) carry warnings like "Do not puncture or incinerate" and "Do not expose to heat". The can holds gas under pressure. Heating it raises the pressure even more, and the can could burst. Never leave spray cans in a hot car or near a heater or campfire.

Snack science

Every popcorn kernel is a tiny sealed container. When it's heated, the water inside turns to steam and the pressure builds until the hard shell bursts: pop! It's the one time a sealed container exploding is exactly what you want (but only popcorn, and only in a popper or microwave bag).

Hands-on activity

Balloon and bottle: heating and cooling a gas

A grown-up handles the hot water. Use hot tap water only, never boiling water.

You need: an empty plastic water or pop bottle (500 mL), a balloon, two large bowls, hot tap water, cold water with ice cubes, and an empty plastic bottle with its cap.

Part 1: flexible container

Stretch the balloon over the mouth of the empty bottle (the bottle is full of air).
A grown-up stands the bottle in a bowl of hot tap water for 2 minutes. Observe the balloon.
Move the bottle into the bowl of ice water for 2 minutes. Observe again.

Part 2: a closed container in the cold

Put the cap tightly on the second empty bottle at room temperature.
Put it in the freezer for 30 minutes. Observe it.
Take it out and let it warm up. Observe again.

Record and explain each observation using pressure, volume, temperature and particles.

What to look for: in hot water, the balloon inflates (the warm air expands into the balloon). In ice water, it goes limp and may be pulled into the bottle's mouth. The capped bottle crumples in the freezer: the cold air inside has lower pressure, so the higher outside air pressure pushes the sides in. It pops back out as it warms.

Safety: hot water can scald; a grown-up pours it. Balloons are a choking hazard for young children; throw away broken pieces right away. Check for latex allergies before using balloons.

Your turn

What causes gas pressure, according to the particle theory?

A gas is squeezed into a smaller volume at the same temperature. What happens to its pressure? Why?

A sealed metal can of air is heated. What happens to the pressure inside? Why?

A balloon is taken from a warm house out into a −15 °C winter day. What happens to its volume?

Why does a hot water tank have a pressure relief valve?

Why do spray cans say "Do not expose to heat"?

More practice: warm-up

What unit is often used for pressure?

Heating a gas in a balloon makes the balloon...

Cooling a gas in a rigid container makes the pressure...

About what is the air pressure around us at sea level?

Should you ever heat an unopened can of soup? Why?

More practice: on your own

Why does the barrel of a bike pump get warm when you pump hard?

A sealed bag of chips puffs up when it's taken up a mountain by car. The temperature didn't change much. Explain.

In the balloon-and-bottle activity, why did the balloon go limp in the ice water?

Explain why a capped empty plastic bottle crumples in a freezer.

Why do drivers in Canada need to check their tire pressure in the fall?

Challenge

A tightly sealed, completely full plastic bottle of water and a sealed bottle half-full of water are both left in a hot car. Which is more likely to bulge or leak? Explain using compressibility.

Describe one way to keep a gas's pressure constant while heating it, and predict what happens to its volume.

A scuba tank is filled quickly at a dive shop and feels warm. An hour later it has cooled and the gauge reads a lower pressure. Was air lost? Explain.

Check your understanding

Quiz

Gas pressure is caused by...

A gas in a rigid container is heated. Its pressure...

A balloon full of air is cooled. Its volume...

Squeezing a gas into a smaller volume at the same temperature makes its pressure...

Why is heating a sealed, full container of liquid dangerous?

Car tires often show low pressure on a cold winter morning because...

For parents and teachers

Big idea (C2.6): qualitatively, for a gas: squeezing it raises its pressure; heating it in a rigid container raises its pressure; heating it in a flexible container increases its volume. Liquids barely expand, but because they can't be compressed, heating a full, closed container of liquid raises the pressure quickly. No formulas are required here; the particle theory is the explanation.

How to help: check the car's tire pressure together in the fall, and look at the safety valve on your hot water tank (don't touch it). Read the warnings on a spray can together.

Common difficulties:

Not saying what is held constant. Ask: "Could the container change size?"
Thinking cold air "leaks" from tires. Pressure drops even with no leak.
Thinking particles grow when heated. They move faster; they don't get bigger.

Safety: you handle the hot tap water. Balloons are a choking hazard for young children and some people are allergic to latex. Never heat sealed containers or spray cans.