What it shows
Sprinkling salt on ice makes it melt faster than plain ice, sugar helps a little, and sand hardly at all. Dissolved substances lower the temperature at which water freezes, so salted ice melts even when the room is cold.
The science
Pure water freezes and melts at 0 degrees Celsius. When salt dissolves in the thin film of water on the ice, the mixture freezes at a lower temperature, so at 0 degrees the ice next to the salt keeps melting instead of refreezing. Sugar dissolves too but gives fewer particles per spoon, so its effect is smaller. Sand does not dissolve, so it barely changes the melting. This is why salt is spread on icy roads in cold countries.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Four ice cubes of the same size
- Four saucers
- A teaspoon each of salt, sugar and sand
- A timer
- A ruler or a syringe (no needle) to measure melt water
- Notebook
Safety
For this project: Ice is slippery; wipe up drips so nobody slips. Do not eat the salted 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.
Put one ice cube on each saucer at the same time and label the saucers salt, sugar, sand and plain.
Sprinkle the spoon of salt, sugar and sand on their cubes; leave the fourth one plain.
Start the timer and look at the cubes every two minutes, writing down what you see.
Touch the salted cube carefully after five minutes; it feels colder than the others, which is a clue.
Note the time when each cube is half gone and when it has fully melted.
Measure the melt water in each saucer after fifteen minutes with the syringe.
Make a bar chart of melting time for the four cubes.
Write which helper melted the ice fastest and which did almost nothing.
What you should see
The salted cube melts first and feels coldest, the sugared cube is second, and the sand and plain cubes melt at nearly the same slow rate.
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 salt melt ice faster?
Salt water freezes below 0 degrees, so the ice next to the salt keeps melting.
2Why did sand not help?
Sand does not dissolve in water, so it cannot change the freezing temperature.
3Why did the salted ice feel colder?
Melting takes in heat, and the faster melting pulls more heat out of the ice and the saucer.
4Where is this idea used?
Salt is spread on roads in snowy places and used in old-style ice-cream makers.
Take it further
Make a cold bath of ice and salt in a bowl and see how quickly it freezes a small packet of juice.
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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