Can Water Float?

There are many things that will float on water: pieces of wood, wax, Styrofoam, and many other things. They float because they are less dense than the water. Now for the question. Can water float on water? Is there a way to make water less dense?

To try this, you will need:

  • two identical drinking glasses or jars
  • cold water
  • hot water
  • food coloring
  • an index card or stiff paper

Fill one glass to the brim with cold water and add a couple of drops of food coloring. Fill the other glass with hot water. Place the index card over the top of the cold water glass, turn it upside down and place it carefully on top of the hot water glass. (I recommend trying this in the sink or outside, in case there is a spill.)

Pull the card out from between the glasses. The water from the two cups will mix, and the food coloring spreads through both.

Refill the glasses and try it again, but this time put the hot water on top. When the card is pulled out the water does not mix! The food coloring stays on top.

Why? Hot materials are less dense than cold. Hot water rises just as hot air does. This sets up convection currents which mix the two solutions. When the hot water is already at the top there is no mixing. It floats on top of the cold water. You now have water floating on top of water.

Understanding this experiment will help you understand how to properly use air conditioners and heaters. Heaters should be run at floor level. The air will rise causing currents which move the hot air around the room. Air conditioners should be located up high, so the cool air drops down and mixes with the rest of the air. If you do things the other way, the heat stays at the top of the room and the cold air at the bottom!

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