Did you know you can make frost appear on the surface of a metal can using just ice, water and salt?
What you need:
A clean and empty tin can
Timer – optional
Fill the tin can about half full with ice and add a little water and a couple of tablespoons of salt.
Wait and watch the frost form.
Try comparing how fast the frost forms on two cans using ice cubes and crushed ice. Which would you expect to be faster, and why?
Remember that to make this a fair test, you should only change one variable ( ice crushed/not crushed ) and keep everything else ( amount of ice, water, salt, size of can ) the same.
The Science Bit
The air around us holds a lot of water which is called water vapour. We can often see this water vapour when it condenses on windows, cars, grass and cobwebs. We call this dew. Cold surfaces generally make the water vapour condense because colder air can’t hold as much water so what it can’t hold turns into droplets on surfaces. If the surface is very cold (below the freezing point of water) the condensed water vapour freezes, this is frost!
In our experiment we filled a can with crushed ice and a bit of water. The temperature of the icy water should be around the freezing point of water (zero degrees Celsius). However, for frost to form we need to get it even colder which we do by adding salt. Salt lowers the melting point of ice, reducing the temperature on the surface of the can to below freezing point. This makes the water vapour in the air condense and freeze on the can.
More investigations you might like:
Cool ice experiments.