Teacher Preparation

Teacher content information

Students can use the Predict, Observe, Explain table on the Student Worksheet to describe and reflect upon this experiment.

Convection is one of the three ways that heat can be transferred, along with conduction and radiation.

Convection is the transfer of heat energy through the movement of a fluid, such as air or water.

Teaching Sequence

Work through this resource material in the following sequence:

20 minutes - Tune in: What will change?
20 minutes - Find out: Experiment time!
20 minutes - Reflection: What did we learn?

Tune in: What will change?

Step 1.

Ask students which they think will freeze quicker: hot or cold water?

Students should be able to explain their reasoning: cold water, which is closer to the freezing point of water at 0°C, will freeze sooner than the hot water.

Step 2.

Students write their predictions in the first section of the Student Worksheet.

Find out: Experiment time!

Step 1.

Take your two foam cups and label one "hot" and the other "cold".

Step 2.

Fill one foam cup with cold water and one foam cup with the same amount of very hot tap water.

Step 3.

Place the cups in the freezer at the same time and start the timer.

Step 4.

Check the cups every fifteen minutes. To the surprise of most the hot water in fact freezes first!

How does it work?

This is known as the Mpemba effect after a Tanzanian student who noticed that hot ice cream mix freezes faster than cold. However, it was in fact first observed by Aristotle, then later Francis Bacon and René Descartes.

Each molecule of water is made up of two hydrogen atoms and a single atom of oxygen, which are bonded together with a covalent bond.

But the separate water molecules (in a cup, for example) are bound together, too, by weaker forces generated by hydrogen bonds. These occur when a hydrogen atom from one molecule of water sits close to an oxygen atom from another.

When you bring water molecules into close contact a natural repulsion between the molecules causes the covalent bonds to stretch and store energy.

As the liquid warms up, the hydrogen bonds between water molecules stretch as the water gets less dense and the molecules move further apart.

That extra stretch in hydrogen bonds allows the covalent bonds to relax and shrink a little, giving up their energy. The process of covalent bonds giving up energy is equivalent to cooling (as they're losing 'heat' energy), so warm water (in theory) cools faster than cold.

This is the Mpemba effect!

However, as the following video explains, the fact that some water samples freeze faster than other samples could be down to impurities in the water: each sample of water will have different purity levels. So what is the answer? Well, we don't really know! Watch this clip to find out more:

Does Hot Water Freeze Faster Than Cold Water? (https://youtu.be/UjIdzcxSe3g)

For more information on the Mpemba effect, watch the following clip:

Why Does Hot Water Freeze Faster Than Cold Water? (https://www.youtube.com/watch?v=8Io2hL7tmj0)

Which Freezes First: Hot or Cold Water?

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