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Faster Freezing

  • Key Stage 3
  • Popular Activity
  • Topical

Type: Activity
Learning Strategy: Planning
Topic: Heat

Hot liquids sometimes freeze faster than cold. This counter-intuitive effect first gained publicity in 1969, when a 15 year old Tanzanian student, Erasto B. Mpemba, spotted it while making ice-cream. He had ignored instructions to cool his ice cream before putting it in the freezer. Surprisingly, it set before the pre-cooled mixtures his classmates had prepared.He asked for an explanation, but didn't get one. Even now, scientists are not completely sure why it happens; but they have plenty of ideas. In this planning activity students revise what they know about heat transfer and plan experiments to test one of these ideas.

Published: 15th June 2006
Reviews & Comments: 2

Learning objectives

Pupils will reinforce their understanding of heat transfer and changing state and identify ways of investigating questions.

Try the activity

You will need Acrobat Reader installed to open the activity sheets.

8I Heating and Cooling
Apply knowledge of heat transfer..
Learn about changes in state.
Draw and interpret graphs of data collected.

9M Investigating Scientific Questions
Identify questions that are suitable for scientific enquiry.
Plan and set targets for a piece of work.
Work together in a group.

Running the activity

Page 1 introduces the topic and page 2 presents the scientific background knowledge. Page 3 has an account of the findings that provoked scientific interest in hot liquids that freeze faster than cold ones. It also sets the task. Students have to plan an experiment to test one of the explanations from page 4 that scientists have proposed to explain the effect.

The students will need ice/salt freezing mixtures to test their ideas. 1part salt to 20 parts crushed ice works well. Very low temperatures can be achieved. Students must not touch the mixtures, and should not touch the glassware they are in (their skin could freeze to the glass).

Please e-mail a summary of your most interesting findings to [email protected]

Background Notes
At first sight, the Mpemba effect seems contrary to thermodynamics. However, most thermodynamicians believe that each observation can be explained with standard physical theory. Many variables can contribute to the observations, depending on the experimental set-up:

•Different definition of freezing (Is it the physical definition of the point at which water forms a visible surface layer of ice, or the point at which the entire volume of water becomes a solid block of ice?)
•Evaporation, reducing the volume to be frozen.
•Convection, accelerating heat transfers.
•The insulating effects of frost.
•The effect of boiling on dissolved gases.
•Supercooling. It is hypothesized that cold water, when placed in a freezing environment, supercools more than hot water in the same environment, thus solidifying the hot water faster.
•The effect of solutes such as calcium hydrogen carbonate being precipitated.

In a freezer, most heat is transferred by conduction and hardly any by radiation.

News links

Strange but true...Why can hot water freeze faster than cold?
A useful summary. The second page has a video of hotwater freezing as it is thrown into the air.
The Mpemba Effect: Hot Water Freezes before Cold
Interesting background information and ideas for practical investigations.
Does hot water freeze first?
A good summary.
Supercooling and the Mpemba effect: when hot water freezes quicker than cold,
Teacher background. The author attributes the Mpemba effect to differences in the behaviour of supercooled formerly hot water and formerly cold water.
Can hot water freeze faster than cold water?
Background information for teachers.
HyperPhysics: The Mpemba Effect
Interesting background informationfor teachers or students.
Why water freezes faster after heating.
The most recent ideas about the Mpenda effect.
Ice cream Recipes.
Ice cream recipes for those who want to repeat Mpemba's findings.

Reviews & Comments

Write your online review to share your feedback and classroom tips with other teachers. How well does it work, how engaging is it, how did you use it, and how could it be improved?

faster freezing

Jun 2nd, 2008

5 Star

the information is made my pupils do the activity with hot water at were surprised by their findings.

Reviewer: sampada bilgikar

Mpemba Effect

Jul 9th, 2006

5 Star

Pupils really enjoyed this one - particularly as it was a reaaly hot day!

They came up with some really well thought out ideas on why hot might cool faster - some a little off the mark but nevertheless interesting.

I think the whole idea of these activities is to get the children talking (and arguing) between themselves. This activity certainly did that.

Reviewer: Margaret Cooper