Thermal Physics

What stays the same when a substance changes state?

The number of molecules and mass.

What changes when a substance changes state?

The energy of the particles.

Are changes of state physical or chemical?

Physical (reversible).

Name the four key changes of state.

Melting, freezing, boiling/evaporation, condensing.

Shape & volume of a solid?

Definite shape and definite volume.

Can solids flow or be compressed? Why?

No. Because particles are held in fixed positions by strong forces and are already packed very closely, so they can’t move past each other or be squashed closer.

Arrangement of particles in a solid?

Very close together, regular pattern.

Motion of particles in a solid?

Vibrate about fixed positions.

Forces & distance in a solid?

Strong forces, very small fixed distance.

Energy of particles in a solid?

Lowest.

Density of solids?

High.

Shape & volume of a liquid?

No fixed shape, definite volume.

Can liquids flow or be compressed? Why?

Liquids can flow but cannot be compressed. Because particles are close together but not in fixed positions, so they can slide past each other, and there’s little empty space between them, so not compressible.

Arrangement of particles in a liquid?

Close together, random arrangement.

Motion of particles in a liquid?

Slide past each other.

Forces & distance in a liquid?

Weaker than solids but still close.

Energy of particles in a liquid?

Greater than solids.

Density of liquids?

Medium.

Shape & volume of a gas?

No fixed shape, no fixed volume.

Can gases flow or be compressed? Why?

Yes, gases can flow and are highly compressible. Because particles move randomly with high energy and there are large spaces between them, so they can be squeezed closer or spread out.

Arrangement of particles in a gas?

Widely separated, random.

Motion of particles in a gas?

Random, high speeds.

Forces & distance in a gas?

Forces negligible, very large spacing.

Energy of particles in a gas?

Highest.

Density of gases?

Low.

How do molecules move in a gas according to the particle model?

They are in constant, random motion at high speeds.

What does random motion mean?

Gas molecules travel in no fixed path and change direction suddenly when they collide with container walls or other molecules.

Why do gas particles collide with container walls?

Because they are moving randomly and spread throughout the container.

What causes pressure in a gas?

Collisions of gas particles with the walls of the container.

Why do faster-moving particles cause higher pressure?

Because they collide with the container walls more frequently and with greater force, so the force per unit area (pressure) increases.

How does increasing temperature affect gas pressure (at constant volume)?

It increases particle speed, leading to more frequent and forceful collisions, so pressure rises.

How do gas molecules exert a force on container walls?

When molecules collide with the walls, they exert a tiny force at right angles to the surface. The combined effect of many collisions produces a net force.

Why does pressure increase if the force exerted per unit area is greater and more frequent?

Because pressure is force ÷ area, so stronger and more frequent collisions mean higher pressure.

What is Brownian motion?

The random movement of microscopic particles in a liquid or gas caused by collisions with smaller, fast-moving molecules.

Who first described Brownian motion and how?

Robert Brown, who observed pollen grains moving randomly in water under a microscope.

What causes Brownian motion?

Light, fast-moving atoms or molecules collide with larger visible particles (like pollen or smoke), nudging them in random directions.

Why do pollen or smoke particles move randomly in a fluid?

Because invisible atoms/molecules constantly collide with them, changing their speed and direction.

Why can’t we see the atoms or molecules causing Brownian motion?

They are too small to observe with a microscope, but their presence is inferred from the motion of larger particles.