Rising Heat

Hot and Cold

• blue or green colored ice
• red food coloring
• warm water
• large container

Make blue or green colored ice (see the beginning of Mixing Colors for an explanation). Fill the large container with warm water. Make sure the container is as still as possible (as in, not on a rickety folding table). Try to minimize it's movement throughout this activity. On one side drop in a few ice cubes, on the other side put in a few drops of red food coloring. Now observe.

The red is hot water and the green is cold.

You can talk about a few things with this activity. One is why the food coloring spreads out instead of just staying where you dropped it. This is because the molecules in water are always moving. Their movement knocks around the food coloring molecules and causes them to spread out. You may also notice that the red spreads out faster than the blue. This is partly because the blue starts out trapped in the ice, but also because the molecules in the hot water are moving faster. Heat is energy. If you have more energy, you move around more. The same is true for molecules. So the faster moving warm water molecules knock into the food coloring more than the slower moving cold water molecules, thus spreading the color around faster.

The main point of my explanation when I did this was about the layering of the colors (I had this as part of my weather themed activity set). If you don't disturb the water, eventually the red water will be in the top half of the container, and the blue water will be in the bottom half. This is because the blue food coloring is staying with the cool water, and the red with the warm water. The reason the different temperature waters separate is because of density. Density is mass/volume, or how much stuff there is in a certain amount of space. Hot water is less dense than cool water. My friend had a really good intuitive explanation as to why this is true:

Think of a bunch of kids sitting at a table. Right now that don't have much energy, so they stay where they are. Then you give them a bunch of energy. Now they are probably running all around the room, and only a few are still at the table. The kids represent the molecules in the fluid. The energy you give them is heat. The table is the space we're comparing. In the first case there are a lot of kids at the table, or a lot of stuff in the area, so it is pretty dense. In the second kids, only a few kids are at the table so there is less stuff in the area and the density is lower.

Learn More:
http://www.landa.com/docs/HotWatervsColdWater.pdf
http://www.mansfieldct.org/Schools/MMS/staff/hand/atomsheat.htm
http://phet.colorado.edu/en/simulation/density