Melting Ice - the cool way!
It has to be said that in Australia we are less preoccupied with melting ice than in countries such as Canada where it causes serious issues for people every winter by making road slippery and dangerous.
Even still ice as the solid form of water is a pretty interesting substance to explore. So let's get started with this fun experiment to see if you can speed up the process of ice melting!
- 2 x ice-cubes that are the same size
- 2 x bowls, small dishes or glasses that are identical
- 1 tablespoon of salt
- Some small toys such as Lego people to freeze in the ice-cubes
- An extra ice-cube
- 1 tablespoon of sugar
- Other toys to make a scene with
- Place each ice-cube into its own dish or glass.
- Put the tablespoon of salt over one of the ice-cubes.
- If you are using sugar, put a tablespoon of sugar on the next ice-cube.
- Do not put anything on your final ice-cube.
- Place the ice-cubes close together away from direct sunlight so that they are all in the same conditions other than the salt/sugar variable.
Once you have set the experiment up make a note somewhere of which ice-cube you think will melt the fastest.
Set a timer and return to the ice-cube set up every 10 minutes. Make a note of what you are observing. Is one of the ice-cubes melting faster than the other(s). Are the salt or the sugar disappearing?
The ice-cube in the salt should melt the fastest followed by the one in sugar if you have used that. The last to melt should be the ordinary ice.
The reason is that both salt and sugar reduce the melting point of ice (this is called “freezing point depression). It means that instead of melting at 0°C the ice melts at less than 0°C.
How does it work? It is the contact between the ice and the molecules in the salt and sugar that cause the freezing point depression. The more molecules there are in a substance, the lower the melting point of the ice.
The reason that the salt helps the ice to melt faster than the sugar is that the molecules in salt are smaller than those in sugar, this means that where there are equal amounts of salt and sugar, there are more molecules in the salt!
For younger kids
Younger kids will be encouraged to learn and to put their own spin on things if you let them play. Instead of presenting this as a straight out science experiment, give it some context by creating a scene.
We made a scene from the North Pole by using water based paints on a couple of pieces of laminated boards and then using a Duplo Eskimo and a dog frozen in the ice. We pretended that we had to rescue them!
You could also freeze a tray of ice and pretend it was a road and that you need to melt the ice to make it safe for cars to drive on!
For older kids
Does it make a difference if the salt and sugar are in the ice-cubes when they freeze?
Boil some water and pour ½ a cup into a mug. Dissolve 1 tablespoon of salt in the water, stir until there is no residue at the bottom,
Pour the salt water into an ice-cube tray and then fill the remaining spaces with fresh water (add a touch of food colour to the fresh water so that you know which is which).
Once the water has frozen, remove one salt water cube and one fresh water cube and place them on identical surfaces. Observe the cubes every 10 minutes or so. The salt water cube should melt first.