What it shows
Salt dissolving in the thin layer of melt water on ice lowers its freezing point, so the ice bath drops below 0 C and freezes a sweet milk mixture into soft ice cream. A thermometer shows how much colder salted ice gets.
The science
Pure ice melts and freezes at 0 C. Salt dissolves in the melt water and makes it harder for water molecules to rejoin the ice crystal, so the ice keeps melting and takes heat from its surroundings. This brings the mixture down to about minus 10 C or lower, cold enough to freeze the milk and sugar mixture, which itself freezes slightly below 0 C.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 1 cup cold milk
- 2 spoons sugar and a few drops of vanilla, optional
- 2 strong zip bags, one small and one large
- About 3 cups of crushed ice
- 6 spoons of rock salt or common salt
- A thermometer if available
- A towel or old cloth for holding the bag
Safety
For this project: Use only food-grade ingredients, and do not eat if anyone is allergic to milk. Wear a cloth around the bag because it gets very cold, and an adult must crush the ice with a clean cloth and rolling pin.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Put the milk, sugar and vanilla in the small bag, press out the air and seal it very tightly.
Half fill the large bag with ice and measure its temperature if you have a thermometer.
Add the salt to the ice, mix it in and note the new, lower temperature after one minute.
Push the sealed small bag into the ice and salt, then seal the large bag.
Wrap the large bag in a towel and shake or roll it from side to side for about ten minutes.
Open the large bag, wipe the salt off the small bag so none gets into the milk, and then open it.
Repeat with a second bag of ice but no salt and compare how much of the milk freezes.
What you should see
The milk in the salted ice turns thick and creamy within ten minutes, while the milk in plain ice stays a liquid, and the thermometer reads several degrees below zero in the salted ice.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the ice get colder when salt is added?
Salt lowers the freezing point so the ice melts, and melting absorbs heat from the surroundings.
2Why shake the bag?
Moving the mixture brings warmer milk next to the cold wall so it freezes evenly in small crystals.
3Why seal the inner bag tightly?
So that salty water does not leak into the milk.
4Where else is this used?
Spreading salt on icy roads in cold countries so that ice melts below 0 C.
Take it further
Time how fast the ice cream sets using one, two and three spoons of salt and plot the results.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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