Answer:
When you heat up water, the water molecules start moving around faster and faster. They bounce off each other and move farther apart. Because there's more space between the molecules, a volume of hot water has fewer molecules in it and weighs a little bit less than the same volume of cold water. So hot water is less dense than cold water. When you put the two together with the hot water on the bottom, the hot water rises to the top, mixing with the cold water along the way and creating purple water.
Explanation:
Why doesn't the water mix when the hot water is on top?
When the cold water is on the bottom, the hot water doesn't have to rise--it's already on top. The cold blue water stays on the bottom and the hot red water stays on top.
