After meal puzzle

Can your learners remove the ice by only touching it with the string?

This experiment should take 15 minutes.

Equipment

• Ice cubes
• Water
• Common salt (NaCl)
• Glass tumbler
• Plastic cup or beaker
• String, 10 cm

Health, safety and technical notes

• Read our standard health and safety guidance here.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete. 
• There are no particular safety issues associated with this experiment.

Commentary

This simple puzzle is based on an experiment in a book called After dinner science.1 It might be a memorable way of introducing the depression of freezing point.

Procedure

• The problem can be solved by first soaking the end of the string in the water.
• The end is then laid across the top of the ice cube and a little salt is sprinkled along each side of the string.
• The salt lowers the freezing point of the ice that it touches, causing it to melt.
• The heat needed to melt the ice is the water on the string.
• Within seconds, the string should have withdrawn from the adjacent ice and from become frozen to the ice cube so that it can be lifted.

Extension

Do other soluble substances - eg sugar and ‘low salt’ - work as well as common salt?

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.