Help discover why warm water freezes quicker than cold water. The Royal Society of Chemistry offered £1000 for the best and most creative explanation of this phenomenon, known as the Mpemba effect. View the winning entry, try the experiment for yourself, or read the original paper.

The Mpemba effect

The Mpemba effect is the phenomenon where hot water freezes quicker than cold water. The Royal Society of Chemistry offered £1000 to the person or team producing the best and most creative explanation as to why this occurs.

We received 22 000 entries to the competition. These were whittled down to just 11 finalists by the panel of expert judges and the public vote.

The winner of the competition, Nikola Bregovic, research assistant in the field of physical chemistry at the Department of Chemistry, University of Zagreb, Croatia, was announced on the 10th of January 2013.

The Mpemba competition announcement

Announcement video of the Royal Society of Chemistry's competition to explain why hot water freezes quicker than cold water - also known as the Mpemba effect.

How the question came about

A discussion between Erasto Mpemba, Dr Dennis Osborne and Ray de Souza describing how the question, "why does hot water freeze quicker than cold water?" came about.

The Mpemba competition awards ceremony

On the 10th of January 2013, the winner of the Mpemba competition - Nikola Bregović, was announced at Burlington House, London.

Nikola Brekovic – competition winner

Click here to see full screen of video Click here to see full view of video The winner of the RSC- Hermes 2012 Mpemba competition is Nikola Bregović, research assistant in the field of physical chemistry at the Department of Chemistry, University of Zagreb, Croatia.

If you teach primary science, see the headings below to find out how to use this resource:

Skill development

Children will develop their working scientifically skills by:

  • Using a range of scientific equipment to take accurate and precise measurements or readings, with repeat readings where appropriate.
  • Drawing conclusions and raising further questions that could be investigated, based on their data and observations.
  • Using appropriate scientific language and ideas to explain, evaluate and communicate their methods and findings.
  • Selecting and planning the most appropriate ways to answer science questions, recognising and controlling variables where necessary, including:
    • Carrying out comparative and fair tests.
  • Recording data and results using scientific diagrams and labels, classification keys, tables, scatter graphs, bar and line graphs.
  • Asking their own questions about scientific phenomena.

Learning outcomes

Children will:

  • Observe that some materials change state when they are heated or cooled, and measure or research the temperature at which this happens in degrees Celsius.

Concepts supported

Children will learn:

  • That materials and substances can change from one state to another, and that these changes are often reversible.
  • That hot water freezes quicker than cold water.

Suggested activity use

This resource could be used with a whole class to carry out the investigation. Children could work in small groups to carry out a simple comparative test on whether hot or cold water freezes first.

This initial experiment may lead to children asking follow-up questions that they would like to investigate, e.g. does water with salt in freeze quicker than water without? Do all hot liquids freeze quicker than cold liquids?

Practical considerations

To carry out this activity, access to a freezer would be required, and if children are working in groups, quite a large amount of freezer space is needed.

Thermometers will need to be provided for children to measure the temperature of their water.

Please take into account health and safety considerations, and act accordingly. Extra care and adult supervision will be required when working with hot water.

Children will need to check the experiment at regular intervals to see which one freezes first.