Gases: Properties and Behaviour (11.1)

Attractive forces between the molecules are great enough to overcome the movement of molecules due to kinetic energy.

Molecules are generally packed very closely together in a regular geometric pattern.

Molecules vibrate in their place but cannot move from place to place.

This makes solids hard and rigid.

This is why you need force, such as cutting or hammering, to change a solid’s shape.

This is why a solid keeps its shape and volume regardless of where it happens to be.

Cooling slows the molecular vibration but usually does not change the shape or nature of the solid.

Attractive forces between molecules are weaker than in solids, so molecules are able to move around within the liquid.

Molecules in a liquid are packed closely together, but often not as closely together as in solids, making the physical structure of liquids easily changeable and not rigid.

In addition to vibrating, molecules in a liquid can move around from place to place, meaning that liquids take the shape of whatever container they are in.

Since the particles attract each other they maintain a constant volume.

Pressure has very little effect on the molecules of a liquid.

Temperature changes liquids.

• Cooling makes the molecules move more slowly, allowing the attractive forces to catch and hold molecules together and form a solid.

• Heating makes the molecules move even faster; with more energy, molecules near the surface can escape from the liquid and leave as gas

The attractive forces between gas molecules are quite weak.

Molecules in a gas are very far apart.

Molecules can move about very freely; this is why gases not only take the shape of their container but they completely fill the container (have no fixed volume).

The large spaces between molecules make it possible to force them closer together under pressure (gases said to be “compressible”).

If cooled and pressurized, a gas molecule’s attractive forces for each other sometimes overcome the kinetic energy and the gas becomes a liquid.

Temperature has an effect on gases.

If heated, the molecules move faster and push harder on the container, but still remain a gas.

If cooled, the molecules move more slowly and eventually attractive forces begin to hold them together.