## States of matter

Matter is anything that has a mass and a volume. Everything that exists is made up of matter. Matter exists in basically 3 different states,

• Solid, e.g ice, rocks
• Liquid, e.g water, petrol
• Gas, e.g steam, oxygen

#### Solid:

• particles do not move but only vibrate about fixed position
• strong forces of attraction hold the particles in fixed positions
• have definite shape and volume
• solids are incompressible (i.e. cannot be squashed up into a smaller volume)
• the particles of a solid have very low kinetic energy.

#### Liquid:

• particles can only move by sliding over one another
• liquids have a definite volume
• liquids take the shape of a container
• liquids are incompressible
• the particles of a liquid have moderate kinetic energy.

#### Gas:

• particles widely separated
• little or no attraction between the particles
• particles move freely and randomly in all directions
• gases fill whatever container they are placed in
• diffuse into all the space that is available
• are compressible (i.e. can be squashed up into a smaller volume)
• the particles of a gas have very high kinetic energy.

## Change of State

When substances change state, energy is either absorbed or given out.

#### Melting

Solid to Liquid. Heat is taken in because energy is needed to break the bonds between the particles in the solid.

#### Vaporisation

Liquid to Gas. Heat is taken in because energy needed to break the forces between the particles in the liquid.

#### Solidifying

Liquid to Solid. Heat is given out because energy is given out as particles bond together.

#### Condensation

Gas to Liquid. Heat given out because energy is given out as particles bond together

#### Sublimation

Gas to Solid directcly. Heat is given out because energy is given out as particles bond together.

Solid to Gas directly. Heat is taken in because energy needed to break the forces between the particles in the solid.

## Kinetic Theory

The properties of solids, liquids and gases can be explained by kinetic theory.

Kinetic theory states that matter is made of tiny particles that move all the time.

The main points of the theory are;

• All matter is made of tiny, invisible, moving particles.

• The particles move all the time. The higher the temperature the faster they move.

• Heavier particles move more slowly than light ones at a given temperature.

## Diffusion

Diffusion is the mixing of atoms or molecules due to their continuous and random motion. e.g. Mixing of bromine vapour and air.

Experiment to show that Gases diffuse at different rates.

One piece of cotton wool soaked in concentrated ammonia solution and another soaked in concentrated hydrochloric acid are put at opposite ends of a dry glass tube. After a few minutes a white cloud of ammonium chloride appears which shows the position at which the two gases meet and react.

The white cloud forms in the position close to the cotton wool soaked in hydrochloric acid as shown above. That is because the ammonia particles are lighter and have a smaller relative molecular mass than the hydrogen chloride particles (released from the hydrochloric acid) and so the ammonia particles move faster.

## Brownian Movement

Brownian movement is the constant random movement of tiny particles (e.g. smoke particles, or pollen on a drop of water) caused by collision with (invisible) air or water molecules, which are themselves in continuous and random motion.

## Heating Curve

• At O – In a solid the particles are held in fixed positions by strong forces of attraction.
• From O to A – Heat is applied and the particles start to faster until at A they start to break the bonds holding them together. The temperature increases propotionally.
• From A to B – The applied heat is used to break the bonds holding the particles together so they is no net increase in temperature until the solid has fully melted at B.
• From B to C – The substance is now in liquid form and the temperature increases propotionally to the applied heat energy.
• From C to D – The applied heat is used to break the bonds holding the particles together so they is no net increase in temperature until the liquid has fully vaporised.

The main points to note from the heating curve are:

• at every change of state there is no temperature rise.
• the energy absorbed during change of state is called latent heat energy (hidden energy). Latent heat of vaporisation during vaporisation and latent heat of fusion during melting.

## Cooling Curve

• At O – The substance is in gaseous state.
• From O to A – Heat is lost to the surroundings and the particles start to slow down and bond together. There is no loss in temperature because the heat lost to the surroundings is balanced to the heat released by bond formation. That is until the gas is fully condensed at A.
• From A to B – The substance is now in liquid form and the temperature decreases propotionally to the heat energy being lost to the surroundings.
• From B to C – There is no net decrease in temperature because the heat lost to the surroundings is balanced to the heat released by bond formation as the liquid turns to solid.
• After C – the substance is now a solid and the temperature is dropping as the heat is lost to the surroundings.

The main points to note from the cooling curve are:

• at every change of state there is no temperature rise.
• the energy released during change of state is called latent heat energy (hidden energy). Latent heat of vaporisation during vaporisation and latent heat of fusion during melting.

## Atoms

The atomic theory assumes that all elements are made up of “atoms”.

An atom is the smallest particle of an element that can exist or take part in a chemical change.

## Molecules

A molecule is a simplest group of atoms joined together by a chemical bond.

All elements are made up of atoms. Some gaseous elements (e.g. noble gases) are composed of single atoms moving around freely. But in other gaseous elements (e.g halogens) single atoms cannot exist on their own at ordinary temperatures. In these elements the free-moving particles consist of pairs of atoms. The two atoms forming a pair ( a molecule ) are joined together by a chemical bond.

This is the case with hydrogen (H2), oxygen (O2 and nitrogen (N2). Such substances are said to be diatomic.