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States and Matter and Chemical Cycles Index

A Chalk Cycle
A Sulfur Cycle

The Chapters Index

 

Water Vapour and Steam

Molecules at the surface of a volume of water will gradually evaporate as they gain enough energy to "jump" into the air. This can happen at an air temperature of around 15°C and above. Evaporation takes place slowly at these temperatures, although it is would be faster on a hot, dry summer's day.

When water is heated to 100°C then all of the molecules quickly gain energy. At boiling point the molecules turn into steam and move into the air. It does not take long for all of the water to change into steam.

 

 

THE LANGUAGE OF CHEMISTRY

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States of Matter

The three states of matter are solid, liquid and gas.

  • Solids have a fixed volume and usually have a fixed shape.

  • Liquids have a fixed volume but no fixed shape. They will take the shape of their container.

  • Gases have no fixed volume or shape. A gas will expand to fill any container it is placed into.

 

States of Matter

 

The state of a pure substance at any given time will depend on the temperature.

If we take water as an example:

Water example

Ice

The molecules of water are arranged close together in the ice crystal structure. Each molecule vibrates slightly but is held in place by the molecules around it.

Ice

 

Liquid Water

As ice receives thermal energy the molecules vibrate more. Eventually they vibrate enough to break free of the ice structure. This is melting and results in liquid water.

Liquid water

 

The water molecules are held close to each by attraction. The forces of attraction between the molecules are called "hydrogen bonds".

 

Water Vapour

If more thermal energy is introduced the molecules gradually gain enough energy to break away from the "hydrogen bonds" holding them together. The molecules at the surface of the water move into the air as water vapour. The molecules move freely in the air.

Steam

 

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