Sunday, April 3, 2011


We started exploring density with our Science Fridays group. For this demonstration, we filled one glass with hot water and dyed it red (with food coloring), then put cool water into another glass and dyed it blue.

An index card was placed over the hot water glass...

...and then the glass was inverted. (Be sure to overfill the glass with the water in order for this to work properly and avoid spilling dyed water all over your table and floor!)

We placed the glass on top of the glass with the cool blue water, then removed the index card so that the waters could mix. The kids expected a whole lot of purple water would be the result.

But look! The red and blue waters stayed separate. But why?

When you heat water (or really, any liquid), you cause the molecules to move further apart from each other, making the water less dense. So the hot water floats on top of the cool water.

To prove that, we set up a control, using hot water for both the glass with the blue dye and the glass with the red dye.

Voila -- when the index card separating the two sides was removed, the water instantly all mixed to turn purple.

We'll be doing more experiments with density in the coming weeks.

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