What You Need:
- Plastic water bottle
- Modelling clay
- Plastic straw
- Food colouring
- Put a few drops of food colouring into the water bottle and fill it to the top with lukewarm water.
- ćInsert a straw into the bottle and mould the clay around it to seal the straw in place.
- When you have a tight seal, the water should go up into the straw.
- Use a marker to mark the level of the water in the straw.
- ŹSet the bottle in a bowl of hot water with the help of an adult. Watch the water level for a while and then mark the level again.
- źSet the bottle in a bowl of ice and watch what happens. Then mark the water level.
As the water heats up, it expands and becomes less dense, rising to the surface. When it cools down, it contracts, becoming more dense and sinks down. This cycle is called convection. When the water in the bottle thermometer heated up, it expanded. But since the bottle was sealed, it had nowhere to go but up through the straw. Similarly, when the water became cold, it contracted and went down through the straw. What you have done is marked the two opposite ends of water temperature on your thermometer. Real thermometers don't use water inside because it doesn't respond to temperature change very quickly. Homemade thermometers aren't actually measuring temperature, they are just measuring temperature changes.