Lets investigate how we can quickly melt ice and then magically pick it up using just a piece of thread.
You will need
- Some ice cubes
- a plate
- Cotton or thread
- Food colouring (optional)
- Place 2 ice cubes on your plate and sprinkle a teaspoon of salt onto one of them.
- After a couple of minutes look at the surface of the both the ice cubes. What effect has the salt had on the ice. Look at the surface of the ice cube, the melt water underneath.
- Drop a small amount of food colouring onto the surface of each ice cube to help you notice these differences.
- Using a fresh ice cube lay a 20cm length of thread over the surface of the ice cube.
- Cover the thread with salt and wait for 5 minutes.
- When the salt has frozen solid onto the ice, see if you can pick up one end of the thread and lift the ice cube up.
- What happens if you lay the thread over two ice cubes and cover them with salt?
- Can you pick up more than one ice cube?
Salt lowers the freezing point of water to below 0oC, so when you add salt to ice, it melts a thin layer of the ice really quickly. After a while the melt water will refreeze again as the water is still extremely cold.
When we place a piece of thread on the ice and cover it with salt the thread eventually becomes frozen into the salt allowing us to pick up the ice cube without our hands.
We use this idea in the winter adding salt to roads and pavements to prevent them from getting icy overnight.