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

Mixing hot and cold water


Mixing two masses of water at different temperatures to discuss thermal energy transfer.

Apparatus and materials

Plastic buckets, 2

Thermometer (demonstration or digital display)

Domestic balance

Supply of hot water and supply of cold water

Health & Safety and Technical notes

The temperatures could be read with a mercury thermometer, but this would not allow the class to see the reading. Digital thermometers with large displays are now reasonably priced.



Weigh 3 kg of hot water into one of the plastic buckets. Weigh 2 kg of cold water into the other. Note the temperature of each. Pour the cold water into the hot water and stir. Take the final temperature.


Teaching notes

1 Discuss with students what happens when hot water is mixed with cold water. Light containers are used so that the thermal capacity of the container itself can be ignored.
When the waters are mixed the temperature ends up somewhere between the two initial temperatures. You might ask:
'Has anything stayed the same during the mixing?'
Because the masses of water are different the temperature changes should not be equal.
'What if we multiply the temperature change by the mass of water?'
This product does stay the same (approximately), and does so in many exchanges. It is directly proportional to the change in thermal energy.
2 The energy transfer between the two lots of water is not 100% because some energy comes from the bucket and some is transferred to the environment. But in this experiment the thermal transfers are minimized.
3 Therefore all energy lost from the thermal store of warm water is gained by the thermal store of cold water, until they arrive at a common temperature.

This experiment was safety-checked in November 2005