Mixing hot and cold water

Demonstration

Mixing two masses of water at different temperatures to discuss 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

Read our standard health & safety guidance

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.

Procedure

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

Teaching Notes

• 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 energy stored thermally.
• 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 energy dissipated in these ways are minimized.
• Therefore energy stored thermally in the warm water is transferred to the cold water, until they arrive at a common temperature.

This experiment was safety-tested in November 2005