C. Matter and Energy · Fluids

Squeeze It: Compressibility of Liquids and Gases

C2.4 A1.2 A1.4 A3.1About 45 minutes

Learning goals

I can explain why gases are compressible and liquids are nearly incompressible, using the particle theory.
I can describe technologies that use the compressibility of gases.
I can describe technologies that depend on liquids being nearly incompressible.
I can explain why air in a car's brake lines is a safety problem.

Words to know

compress
To squeeze matter into a smaller volume.

compressibility
How easily a substance can be squeezed into a smaller volume.

incompressible
Not able to be squeezed into a smaller volume (or only by a tiny amount).

compressed air
Air that has been squeezed into a smaller volume than normal, so it is under higher pressure.

pressure
How much force pushes on each unit of area.

The bike pump puzzle

Put your finger over the end of a bike pump and push the handle. It moves in partway, then pushes back like a spring. Now imagine the pump full of water instead. You'd push and... the handle would barely move at all.

Why can air be squeezed, but water can't? And why do engineers care so much about the difference?

A metal hand bicycle pump with a T-shaped handle and black hose lying next to a bicycle wheel. — Pushing the handle of a bike pump squeezes the air inside into a smaller volume. Gases can be compressed because their particles have lots of space between them. AI-generated image

Compressibility and the particle theory

To compress something means to squeeze it into a smaller volume.

Gases are very compressible. Gas particles have large spaces between them. Pushing on a gas forces the particles closer together, into those spaces.
Liquids are nearly incompressible. Liquid particles are already close together, touching their neighbours. There is almost no empty space to squeeze them into. Even a huge push changes a liquid's volume only by a tiny amount.
Solids are also nearly incompressible, for the same reason.

When you compress a gas, its particles are crowded together, so they hit the walls of the container more often. The pressure goes up. That's why the bike pump pushes back.

	Gases	Liquids
Spaces between particles	large	very small
Compressibility	very compressible	nearly incompressible
What happens when pushed	volume shrinks and pressure rises	volume hardly changes; the push is passed on through the liquid
Useful because	it can store lots of gas in a small tank, and act as a cushion or spring	it passes forces along instantly and firmly

Comparing the compressibility of gases and liquids.

Worked example

Let's compare air and water in a syringe

Two sealed medicine syringes (no needles) each hold 20 mL. One holds air, the other holds water. Jasmine presses each plunger as hard as she can with the tip blocked. The air syringe goes down to about 10 mL. The water syringe stays at 20 mL. Explain the results.

Step 1. Observe. Air: the volume roughly halved (20 mL → 10 mL). Water: no change she could see.
Step 2. Air, using the particle theory. Air particles have large spaces between them. Pushing forced them closer together, so the air took up less space.
Step 3. What happened to the pressure? The crowded air particles hit the plunger more often, so the pressure inside rose. If the temperature stays the same, halving a gas's volume about doubles its pressure. Jasmine felt this as the plunger pushing back harder and harder.
Step 4. Water. Water particles are already close together, so there is almost no space to squeeze into. The volume can't noticeably shrink.

Air is compressible because its particles have large spaces between them; squeezing it raised its pressure. Water is nearly incompressible because its particles are already close together.

Same 8 air particles, half the space: the air is compressible. The water particles are already touching, so the plunger can't move in.

Technologies that use compressible gases

Because gases can be squeezed and spring back, they make great cushions, springs and storage.

Tires and air mattresses: the compressed air inside cushions bumps.
Scuba tanks: air is squeezed into a small, strong tank, so a diver can carry enough to breathe underwater for a while.
Propane tanks for barbecues: propane gas is squeezed so much that it becomes a liquid, so a lot fits in a small tank.
Air suspension and air brakes on buses and transport trucks: compressed air acts as a spring and powers the brakes. Many city buses "kneel" (lower one side) by letting air out of their suspension, making it easier for people who use wheelchairs or strollers to board.
Pneumatic tools like nail guns and jackhammers run on compressed air.
Sports balls and packaging pillows use trapped air to bounce or protect.

Technologies that need incompressible liquids

Because a liquid can't be squeezed, a push on one end of a pipe full of liquid is passed straight through to the other end. That makes liquids perfect for hydraulic systems.

Car brakes: when the driver presses the brake pedal, a piston pushes on brake fluid. The fluid passes the push through thin tubes to all four wheels at once, squeezing the brakes.
Excavators, backhoes, dump trucks and car lifts use hydraulic oil to lift huge loads.

You'll learn more in the lesson Hydraulic and Pneumatic Systems.

Why air in the brake lines is dangerous

If air bubbles get into a car's brake lines, the pedal feels spongy. When the driver presses, part of the push is wasted squeezing the air bubbles instead of being passed to the brakes. The car may take longer to stop. Automotive service technicians (a skilled trade) "bleed" the brakes to remove the air and refill the lines with brake fluid.

The liquid can't be squeezed, so it passes the push straight to the brake. An air bubble compresses first, so the brake gets less push: a spongy pedal.

Just for fun

What did the air in the bike pump say to the plunger?

"Hey, give me some space!" (The water in the syringe didn't say anything. It just refused to budge.)

Hands-on activity

Syringe squeeze test

You need: two or three plastic medicine syringes with no needle (from a pharmacy, 10 mL to 60 mL), water, a mini marshmallow, and paper for a results table.

Air: pull the plunger back to the top mark to fill the syringe with air. Block the tip firmly with your fingertip. Push the plunger as hard as is comfortable. Record the smallest volume you reach. Let go. What happens?
Water: fill a syringe with water (no air bubbles: point the tip up and push out any bubbles over the sink). Block the tip and push again. Record the volume.
Marshmallow model: remove the plunger, drop in one mini marshmallow, and put the plunger back in until it's just above the marshmallow. Block the tip and pull the plunger out (lower pressure). Then push it in (higher pressure). Watch the marshmallow.
Explain each result using the particle theory and the word compressible.

What to look for: the air compresses a lot and the plunger springs back when you let go. The water barely moves. The marshmallow (which is full of tiny air bubbles) puffs up when you pull and shrinks when you push, because the air inside it expands and compresses.

Safety: use only needle-free syringes. Don't point a syringe at anyone's face. Don't eat the marshmallow after the test (it has been handled). Throw it out and wash the syringes.

Your turn

What does compressible mean?

Use the particle theory to explain why gases are compressible.

Why are liquids nearly incompressible?

When you compress a gas, what happens to its pressure? Why?

Name two technologies that use the compressibility of gases.

Why do car brakes use a liquid instead of air?

More practice: warm-up

Which is more compressible: air or water?

True or false: liquid particles have large spaces between them.

What does a scuba tank store?

What does "bleeding the brakes" remove?

Name a tool that runs on compressed air.

More practice: on your own

Why do the tires on a bike make the ride smoother than solid rubber wheels would?

A driver says the brake pedal feels "spongy" and goes down too far. What is the likely problem, and why is it dangerous?

A 30 mL syringe of air is squeezed to 15 mL at the same temperature. About how does the pressure change?

Why does a propane tank for a barbecue hold so much fuel for its size?

Explain the marshmallow-in-a-syringe result.

Challenge

An engineer is designing a seat for a snowmobile that bounces over rough trails. Should the seat's shock absorber use only a liquid or should it include a gas? Explain using compressibility.

A city bus "kneels" at a stop. Explain how its air suspension does this and who it helps.

Water is described as "nearly" incompressible, not completely incompressible. What do you think this means, and why doesn't it matter for car brakes?

Check your understanding

Quiz

Gases are compressible because...

When a gas is compressed at the same temperature, its pressure...

Which technology depends on a liquid being nearly incompressible?

Why does a car's brake pedal feel spongy when there is air in the brake lines?

Which uses compressed air?

A syringe of air is pushed from 40 mL to 20 mL at the same temperature. The pressure becomes about...

For parents and teachers

Big idea (C2.4): gases are compressible because their particles have large spaces between them; liquids are nearly incompressible because their particles are already close together. This difference decides how they're used: gases for cushioning and storage (tires, scuba tanks, air suspensions), liquids for passing forces along (hydraulic brakes and machines). The brake-bleeding example connects to a skilled trade (A3.1).

How to help: pick up needle-free medicine syringes at a pharmacy; they're useful for this lesson and the next three. Point out kneeling buses, air brakes hissing on transport trucks, and the warning on propane tanks.

Common difficulties:

Thinking compressed gas particles get smaller. They stay the same size; the spaces between them shrink.
Thinking liquids can be squeezed a lot. They can't, and that's exactly what makes hydraulic systems work.

Safety: use needle-free syringes only. Don't eat food used in experiments.