Making ice

The practical investigation is part of the In search of more solutions collection. Use these problem-solving activities to contextualise chemistry and engage learners in small group work. 

This resource accompanies the article Supercool water in Education in Chemistry which delves into the weird and wonderful nature of the world’s most familiar liquid.

Learning objectives

1. Plan a method to investigate how quickly ice forms.
2. Make careful observations and accurately record them in a table.
3. Use experimental data to draw conclusions.
4. Write an investigation report.

Introduction

This activity is based on an article by Martin Sherwood that appeared in Physics Education. 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 challenges the perception of scientific ‘facts’.

Erasto Mpemba was a student at Magamba Secondary School in Tanzania when he discovered the phenomenon, now known as the Mpemba effect, while making ice cream.

One day, to get a free space in the refrigerator, Erasto 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, Erasto’s ice cream froze first after about one hour, while his friend’s remained liquid for longer.

Learners will investigate: ‘Which makes ice faster, hot or cold water?’. This is an open-ended problem-solving activity, so the guidance given is necessarily incomplete. Task your class to use their method writing and planning skills to answer the question using the equipment listed. For more advice on teaching practical planning, read the Teaching science skills article: A practical solution. Learners must decide how to heat their water so we have not included any apparatus to do so in the equipment list. 

Planning

Time

Lesson 1

• 10–20 minutes to set the scene. You can use the student worksheet questions as part of this.
• 30 minutes to plan the experiment in groups.
• If appropriate, you can ask learners to plan the investigation on their own as a homework task.

Lesson 2

• 50 minutes to carry out the experiment.
• Learners will need access to a fridge/freezer to monitor their results. If they use small volumes of water, their ice will form within a lesson.
• Set completion of the report as homework.

Group size

2–3

Curriculum links

• States of matter and their properties.
• Changes of state.
• Energy – exothermic and endothermic reactions.

Scaffolding

There are two versions of the student sheet. The scaffolded student sheet (one star in the header) provides support with the planning process. Get learners to work through the questions together to write a method. The sheet also includes a sample table and spaces for learners to record their results and make their conclusion.

Give the unscaffolded student sheet (two stars in the header) to groups to work through planning the investigation on their own without prompts.

Refer to the equipment available during your introduction to the investigation and display it using the PowerPoint slide while the class plan. You can also encourage learners to try out their method before writing it down. Check all methods and correct them where needed before learners do the practical.

Use the questions in both versions of the student sheet to get learners thinking about changes of state and particle theory before the investigation or as a follow-up activity. There are two challenge questions at the end of the unscaffolded student sheet (two stars).

Technical notes

Safety and hazards

Read our standard health and safety guidance and carry out a risk assessment before running any live practical.

Remind learners to wear safety glasses and to take care when dealing with hot or boiling water.

Equipment

Materials (per group)

• Deionised water

Equipment (per group)

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

Explanation

Learners 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 phenomenon is still puzzling scientists 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. This is due to the hotter molecules having more energy to overcome the intermolecular forces. Therefore, the rapid cooling of the hot liquid may be due to the evaporation from this ‘hot top’.

Answers

Guidance for the planning activity and answers to the follow-up questions can be found in the Teacher notes.