Freezing and Melting Water

Adding or removing heat

Water, like all other types of matter, requires the addition or removal of energy to change states. 

A block of ice is solid water. When heat (a form of energy) is added, the ice melts into liquid water. It has reached its melting point at 0°C. If you were to keep applying heat, then the water will turn into water vapour (gas). The water has reached its boiling point at 100°C.

If heat is removed from water vapour, the gas cools down and it condenses back into liquid water. Continue to cool the water (by removing heat), and it becomes solid ice. This is its freezing point.

• The process of changing water from a liquid to a gas is called evaporation.
• The process of changing water from a gas to a liquid is called condensation.

Pressure

Pressure is the force (e.g. your weight) applied to the surface of an object (e.g. the floor). When you stand up you apply pressure to the floor. 

Air pressure

Air pressure is the weight of the atmosphere pushing down on the earth. At sea level this pressure is called 1 Atmosphere. The air pressure at the top of Mt Everest is half of this (0.5 Atmosphere). 

If you were to boil water high up in the mountains, thousands of metres above sea level, you would not need to heat it as much because there is less air pressure. The air pressure is lower because there is less weight of air above – the air is not as dense.

Less pressure means it is easier for the molecules to move and less heat needs to be added to cause a change in state.

Watch this video to see different temperatures for boiling water at different altitudes.

Water pressure

Water pressure is the weight of the ocean pushing down on you. The deeper you go under the sea, the greater the pressure of the water pushing down on you. For every 10 meters you go down, the pressure increases by 1 Atmosphere. In the deepest ocean, the pressure is over 1,000 atmospheres, or the same as the weight of an elephant balanced on a postage stamp, or the equivalent of one person trying to support 50 jumbo jets! 

The Ross Ice Shelf is the same size as France. The sea under the Ross Ice Shelf is 200m deep.

What is the water pressure (in atmospheres) at the bottom of the Ross Sea? (remember each 10 metres of depth increases pressure by 1 atmosphere).

Pressure and freezing

Water under pressure is harder to freeze! Water below freezing point can still be liquid under pressure! Watch this video where a bottle of cola put in the freezer only turns to ice when the cap is removed releasing the pressure. 

• As water pressure increases the freezing point of water goes down.

This is very important in Antarctica where very deep water may be below freezing temperature. It is called supercooled water. Supercooled water is water that is still liquid even though it is below the freezing point. 

When does supercooling occur?

Supercooling is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid (as in the coke trick shown in the video).

When a liquid reaches its standard freezing point (usually 0 degrees Celsius for water) it will crystalize and become a solid. For the water to crystallize and form ice, there must be a seed crystal around which a crystal structure can form to create a solid. You can see this in this video.

Salinity

The oceans are salty so they don’t freeze at the same temperature as fresh water. Salt lowers the freezing point so sea water freezes at about -2 degrees Celsius. This is why salt is used on some roads in winter to stop ice from forming.