States of Matter and Changes of State

Study Notes

Solids have a fixed shape and volume.
Liquids have a fixed volume but take the shape of their container.
Gases have no fixed shape or volume.
Solids have high density, liquids have medium density, and gases have low density.
The particles in solids vibrate in fixed positions, in liquids they move and slide past each other, and in gases they move randomly and quickly in all directions.

Melting is when a solid changes into a liquid.
- It occurs at the melting point (m.p.).
Freezing is when a liquid changes into a solid.
- It occurs at the same temperature as the melting point.
Evaporation is when a liquid changes into a gas at the surface.
- It occurs over a range of temperatures below the boiling point.
Boiling is when a liquid changes into a gas.
- It occurs at the boiling point (b.p.).
- Bubbles form within the liquid.
Condensation is when a gas changes into a liquid.
- It occurs over a range of temperatures.
Sublimation is when a solid changes directly into a gas.
Deposition is when a gas changes directly into a solid.

Heating curves show how the temperature of a substance changes as it is heated.
Cooling curves show how the temperature of a substance changes as it is cooled.
The temperature remains constant during phase changes.
- This is because the heat energy is being used to overcome intermolecular forces, not to increase kinetic energy.
The temperature increases in the regions where the substance is not changing state.
- This is because the heat energy is being used to increase kinetic energy, not to overcome intermolecular forces.

Pure substances have fixed melting and boiling points.
- On heating curves, phase changes are represented by horizontal lines.
Impure substances melt and boil over a range of temperatures.
- On heating curves, phase changes are represented by sloping lines.

The volume of a gas can be changed by changing the temperature or pressure.
- Increasing the pressure decreases the volume (inversely proportional).
- Increasing the temperature increases the volume (directly proportional).
These changes are due to the large intermolecular spaces in gases.

Diffusion is the movement of particles from an area of high concentration to an area of low concentration.
It is the movement of particles down a concentration gradient.
It continues until equilibrium is reached.
It occurs in fluids (liquids and gases) but not in solids.
The rate of diffusion is faster at higher temperatures and for particles with lower relative molecular masses.

Lighter particles diffuse faster than heavier particles.
For instance, ammonia molecules (NH3) diffuse faster than hydrogen chloride molecules (HCl) because they are lighter.