Sticky Cups

This video from the Royal Society of Chemistry (RSC), aimed at primary teachers, demonstrates an experiment which may be used when learning about the properties of gases.

Two plastic beakers are heated with hot water and a kitchen roll soaked in hot water placed between them and they are stuck together. After being left to cool down, the two cups stick together when lifted. This happens because the air in the beakers expands when heated, and the gas molecules move around quickly some moving out of the beakers.

When the beakers are joined at first the heated molecules move quickly and so they keep the pressure up inside the beakers. As the gases cool, the molecules slow down and the pressure inside the beakers is reduced so it is less than the air pressure acting on the outside of the beakers pushing them together.

This demonstration is designed to help children understand about the properties of gases around us and to stimulate discussion of what has been observed.

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This resource is part of Royal Society of Chemistry