Making ice

This is best suited to an extended study.

Equipment

Materials per group

• Deionised water

Equipment per group

• Beakers, 100 and 250 cm³
• Insulating material
• Thermometers, –5 to +100 °C
• Access to a refrigerator and freezer
• Safety glasses

Health, safety and technical notes

• Read our standard health and safety guidance here.
• Wear eye protection. 
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• Take care when dealing with hot, or boiling water.

Commentary

Students will find that hot water freezes more quickly than cold water – more precisely, water freezes more slowly if the initial temperature is below room temperature. The explanation is not entirely clear, but may be because a hot liquid has a ‘hot top’ of mobile molecules with high kinetic energy.

These molecules can escape from the liquid phase more easily than colder molecules with lower kinetic energy in a cooler liquid. The rapid cooling of the hot liquid is due to the evaporation from this ‘hot top’.

This activity is based on an article that appeared in Physics Education. Erasto Mpemba was a student at Magamba Secondary School in Tanzania, and he discovered the phenomenon while making ice cream.

One day, in order to be sure of space in the refrigerator, Mpemba put his ice cream mixture into the fridge without letting it cool first. At the same time, one of his friends, who had let his mixture cool, also put his mixture into the fridge. To everyone’s surprise, Mpemba’s ice cream froze first after about 1 h, while his friend’s remained liquid for longer

Francis Bacon reported in 1620 that ‘Water slightly warm is more easily frozen than quite cold’, and some people may have come across the folklore ‘never pour hot water down a frozen drain because the water will only freeze faster’. The problem may challenge the perception of scientific ‘facts’.

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.