We all scream for ice-cream.
There's nothing better on a warm day than a nice scoop of ice cream. A quick trip to the freezer, and you have a refreshing treat. You may even have a machine at home to make your own. Plug it in, and off it goes.
People have been enjoying ice cream long before freezers were invented. They used a simple bit of GCSE science to make their dessert though you still need ice to start the process.
Before freezers, ice was collected from frozen lakes in winter and stored underground in ice houses. These could be natural caves or cellars dug into the ground. Beneath the earth, it stays cooler than on the surface, so the ice stays frozen long into the summer.
The other thing you will need for your ice cream is salt. That's it; ice and salt. No electricity or fancy equipment. Just plain old ice and salt. Of course, you will also need your ice cream mixture ready to freeze.
You will need a large and small bowl to make the ice cream. The ice goes in the larger bowl, and the ice cream mixture in the smaller one. Set the smaller bowl on the ice and make sure the ice surrounds it as much as possible. Now pour some salt on the ice, spreading it evenly. Lastly, whisk the ice cream mixture until it freezes.
How does this work?
You should know that Ice melts at 0°C. To get the ice cream to freeze, we need a colder temperature. When you mix an impurity into a substance, you lower its freezing point. When the salt is added to the ice, its temperature can go down by as much as 20⁰C, enough to freeze the ice cream.
Why does the temperature drop?
The water molecules in ice are held together by weak hydrogen bonds. You need to break these, which takes energy to melt the ice, this is taken in from the environment, so melting is endothermic. Endothermic means a process that takes heat in.
Freezing involves making hydrogen bonds between water molecules. This releases energy, so this is exothermic, letting heat out.
At the melting point, the exothermic and endothermic changes are happening equally; the ice melts at the same rate as it freezes. We say the processes are in equilibrium, so the temperature remains constant.
When you add salt to the ice, you lower its melting point. The ice still melts but can no longer refreeze. The melting continues to take in energy from the surroundings, but heat is no longer released to the surroundings because there is no freezing. The equilibrium has been lost, so the temperature drops.
Why don't you try this at home? Find a recipe for your favourite ice cream and freeze it the old-fashioned way.
