Plastic water bottle
Clear plastic straw
Food colouring (any colour)
Two large bowls
Water – hot and cold
Put a few drops of food colouring in the plastic bottle. Fill the bottle till the top with lukewarm water.
Insert the straw halfway into the bottle.
Seal the bottle’s opening using the modelling clay. The seal should go around the straw and should be compact enough to allow the water to move through the straw.
Use the marker pen to mark the level of water in the straw.
Then, take one of the bowls and fill it with hot water. Place the bottle in the bowl and leave it aside for a few minutes. Mark the level of water in the straw again, by using the marker.
Next, fill the other bowl with cold water and place the bottle in it. Leave it undisturbed for a few minutes before marking the level of water in the straw.
The bottle can also be placed in a sunny spot outside and the markings can be made once every two hours to understand the temperature.
How does it work?
At the end of the experiment, the straw will contain three or more different markings, based on the different temperatures the bottle has been placed in– the normal temperature, in hot water and in cold water.
When water heats up, it becomes less dense and rises to the surface. When it cools down, the density increases, and it begins to sink. This cycle is called convection. When the bottle thermometer is placed in hot water, the heat makes the water expand. But since the bottle is sealed, the water has space but to move through the straw. This experiment helps to understand how thermometers function.