An ice cube floats on water in a glass tumbler or a cup. Apart from a piece of string, available are those that you would expect to find set out on a meal table
Can your learners remove the ice by only touching it with the string?
This experiment should take 15 minutes.
- Ice cubes
- 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.
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.
- 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- Within seconds, the string should have withdrawn from the adjacent ice and from become frozen to the ice cube so that it can be lifted.
Do other soluble substances - eg sugar and ‘low salt’ - work as well as common salt?
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.
After meal puzzle - teacherPDF, Size 0.26 mb
After meal puzzle - studentPDF, Size 0.24 mb
The resources were originally published in the book In Search of More Solutions.
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