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Cold water sinks because the water molecules are packed closer together (it's more dense). Warm water rises because the molecules are a bit farther apart and is less dense. In this video I placed an ice pack on one side of a glass vase (14 inches high, 10 inches wide and 1 inch deep) and let it sit for about a minute. I dropped blue food coloring on the cold side and red food coloring on the warmer side. The taping time was about 1.5 hours and time lapsed to about 30 seconds. 

What happens when cold water comes in contact with warm water?