☯ Concept Cartoons ☯
1 Chemistry Solids, Liquids and Gases 3 - What happens when water freezes?
Have you talked about your ideas? Do you agree with any of the characters or do you think something different? Do you all have the same ideas? Here are some ways of finding out more. Talk together about freezing and melting and how you can test your ideas. You could weigh some ice cubes in a beaker using an accurate balance, let the ice cubes melt in a cool place, then weigh the water in the beaker. Has the mass changed? How can you check if the volume changes? Observe how ice cubes behave when you put them in water. Do they float or sink? What does that tell you about the density of ice compared to water? Observe how ice melting compares with other things that melt easily (e.g. margarine). Why is it important for plumbers to know what happens to water as it freezes?
Here’s what a scientist might say. Did you find any evidence to support or justify these ideas? Are there any questions that you still need to answer? Water is an unusual substance. Normally when a liquid solidifies, the particles get a little bit closer together and the volume gets less. The same mass is spread over a slightly smaller volume, so it is more dense and sinks if you put it in a liquid made of the same substance. Most substances do this but water doesn’t. When water freezes to make ice, the volume increases. This means that the same mass is spread over a slightly bigger volume, so the density decreases. This is why ice floats on water. When ice melts its volume decreases but the mass doesn’t change. When something melts it does not gain or lose particles so it has to be the same mass. Try to sketch an approximate graph of the volume of 100 g water from -5 to 95°C. Do some research to check your answer and find out what is special about 4°C.