What it shows
Crushed ice mixed with salt gets colder than ice alone, below the freezing point of water. The outside of a tin filled with the mix becomes so cold that water vapour from the air freezes on it as a white frost, which never happens with plain ice.
The science
Ice melts when it takes in heat. Salt lowers the temperature at which water can stay frozen, so the ice next to the salt melts even though it is already at zero degrees, and the melting pulls heat from the ice and the tin. The mixture can drop to about minus 10 to minus 15 degrees Celsius. The outside of the tin is then colder than the freezing point, so water vapour in the air that touches it turns straight to ice, forming frost. The plain ice tin only reaches zero degrees, so vapour condenses to liquid dew instead.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two clean, dry steel or aluminium tins (cold drink cans or food tins, edges smoothed by an adult)
- Crushed ice, about 2 cups
- Table salt, 4 to 6 tablespoons
- A spoon
- A thermometer that reads below zero, optional
- A tray to catch drips
Safety
For this project: An adult must smooth any sharp tin edges. Do not hold the salted tin for long; it can get cold enough to hurt skin. Do not eat the salty ice.
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
8 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.
Half fill both tins with crushed ice and put them on the tray side by side.
Add the salt to one tin only and stir it into the ice for a minute with the spoon.
Put the thermometer in each tin if you have one and read the temperature after two minutes.
Watch the outside of both tins; the plain one becomes wet with dew.
After five to ten minutes, the salted tin turns white with frost that you can scratch with a fingernail.
Breathe gently on the frosted tin and see the frost thicken where your warm, moist breath touched.
Record the lowest temperature reached in each tin and the time the frost first appeared.
Explain on a card why one tin got dew and the other got frost.
What you should see
The salt and ice tin reads well below zero, often around minus 10 degrees, and grows a white frost coat; the plain ice tin stays at about zero and only collects water drops.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is the salted ice colder than plain ice?
Salt makes the ice melt below zero, and melting takes heat out of the mixture.
2Where does the frost come from?
Water vapour in the air freezes onto the very cold tin.
3Why did the plain tin get wet instead of frosty?
It was at zero degrees, cold enough for dew but not cold enough to freeze the vapour.
4What is this trick used for?
Old-style ice-cream churns use ice and salt to freeze the cream.
Take it further
Put a small sealed packet of fruit juice into the salted ice and time how long it takes to freeze solid.
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
Science projects
Easy Science Projects for Class 1 to 5
Simple, safe projects young children can make with a little help
Science projects
Safe Chemistry Projects for School
Experiments and models with kitchen materials and proper safety