Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Expansion of water on freezing


The expansion of water when it freezes breaks an iron bottle.

Apparatus and materials

Iron flask for freezing

Freezing mixture of ice and salt (or solid carbon dioxide)


Cloth, piece of

Tongs or heat-proof gloves (if using solid carbon dioxide)


Health & Safety and Technical notes

While the flask is in the freezing mixture, cover the bucket with a cloth. This will ensure that no debris is ejected. 
If solid carbon dioxide is used, it must be handled with tongs or heat-proof gloves to prevent injury to skin.

To avoid a long wait and uncertain timing, cool the flask beforehand. 

Before the demonstration, boil the water that will be used in the flask to make it air-free.



a Fill the iron flask with water. 

b Remove all air bubbles and screw the plug in. 
c Bury the bottle in the freezing mixture and cover it with a cloth. (If solid carbon dioxide is available, the iron flask can be embedded in that.)

Teaching notes

1 The breaking of the flask will be clearly heard. Students are not surprised when glass breaks, but the strong iron flask is another matter. 

2 When water freezes it swells. The volume of the ice is about 10% greater than the volume of the water. Water is unusual in expanding when freezing, but it is not the only substance which does so. Iron also expands on freezing and this is why it can be cast. 
3 As the water becomes solid, the molecules take up fixed positions in relation to each other. Average spacings between molecules are larger than when the water is in a liquid state. 
This experiment was safety-checked in May 2006


Related guidance

Making dry ice


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