diametlogoWater can exist in 3 different states (solid, liquid, gas) and can change from one state to another when heated or cooled. Water is constantly changing state within the atmosphere, and within global and local weather systems.

The following terms are used to describe different changes of state.

 Scientific Term   Meaning   Example
 Freezing  Liquid to Solid   Making ice cubes in a kitchen freezer
 Melting  Solid to Liquid   Artic ice melts during the summer
 Evaporation  Liquid to Gas   Wet clothes dry outside on a sunny day
 Condensation  Gas to Liquid   Water droplets appear on a cold window pane 

Occasionally a gas can change directly into a solid and miss out the liquid phase. This process is called Deposition.

Deposition : Gas to Solid (formation of hoar frost on leaves and grass)

The opposite can also occur. Sublimation is the term used when a solid turns directly into a gas.

Sublimation : Solid to Gas (Snow in the coldest parts of Antarctica does not melt, but much of it does disappear in the summer, by sublimation straight from solid to water vapour)

cumulonimbusChange of State and Weather

Clouds develop when water vapour condenses to form water droplets. This usually occurs at heights above the Earth's surface where temperatures are lower than at ground level.

If temperatures are below freezing, clouds can also contain frozen water droplets (ice).

The Water Cycle provides examples of evaporation and condensation in the real world.

  • The Sun warms water in the oceans and seas causing evaporation to form water vapour.
  • Water vapour rises, cools and gradually condenses to form water droplets and clouds.
  • Water droplets collide and grow within the cloud until they are too heavy to remain in the cloud, and fall to Earth as precipitation, which can be rain, hail, sleet or snow.
  • Water then flows back into the oceans and seas and the whole process starts again.

FAAM BAE newpaintAs part of the Diamet Project, scientists use a research aircraft to fly into clouds and make measurements.

The aircraft carries lots of different instruments which can measure temperature, humidity, wind speed and direction, cloud droplet size, the number of cloud droplets, whether they are liquid (water) or solid (ice) and even the shape of the ice crystals.

Use your knowledge of condensation and evaporation to make your own cloud in a jar. Carry out the ' document Cloud in a Jar Challenge '

The Particle Model and Kinetic Theory

GlacierEverything on our planet is made of tiny, constantly moving particles. Another term for constantly moving is 'kinetic'.
Particles behave differently in solids, liquids and gases.

A solid, like ice, contains particles that have fixed positions and are strongly attracted to one another. The particles are closely packed and can only vibrate.

When heat is continually applied to a solid, the vibrations increase until the attraction forces between the particles are weakened and the particles can roll over one another. When this happens, the solid has melted to become a liquid.

If heat continues to be applied, the particles increase their movement until some of those on the surface of the liquid break away into the air. The liquid has begun to change into a gas. In a gas, the particles are not held together and move randomly.

StatesIf the gas is then cooled, the particles slow down and become closer together until they start rolling over each other. The gas is condensing into a liquid. If cooled further, the particles become closely packed and stop moving freely. The liquid has frozen into a solid.

The particles in water vapour, liquid water and solid ice are the same but in the water vapour state they have more energy and are moving faster. Heating and cooling can change a particle's 'state'.

If weather forecasters want to correctly predict where and when water vapour will cool down, condense and form clouds, they must be aware of all the heating and cooling affects in the atmosphere.

Demonstrate your particle model knowledge by completing the  document The Particle Kinetics Challenge (62 kB) !