Hot and Cold: Molecules in Motion

Activity Time: 10 minutes
Recommended Grades: Pre-K - 4 with parent participation; grade 5 and up with parent permission
Objective: In this experiment, explore how hotter things have faster moving particles, while particles in colder things move slower.

  • 2 clear glasses or jars- at least 6 oz
  • Food coloring (two colors, ideally)
  • Water
  • Microwave or kettle
  1. Fill one glass with cold water, chilled in the refrigerator or freezer.
  2. Fill the other glass with hot water. Heat in the microwave or a kettle, but be careful that it doesn’t get too hot!
  3. Place the two glasses side by side.
  4. At the same time, add three drops of one color to the hot glass, and three drops of another color to the cold glass.
  5. Do not stir! Watch the color as it moves and is distributed in each glass. What do you see?

You should notice the food coloring in the warm water spreading out faster than the food coloring in the cold water. If you didn’t observe this, try making your cold water a little colder and your warm water a little warmer. Also make sure you add the food coloring to each glass at the same time.

Water is made of molecules (two hydrogen atoms and one oxygen atom stuck together). Molecules in a liquid have enough energy to move around and pass each other. This is why water can flow and take the shape of the glass you pour it into. The molecules in solids, like ice, don’t have enough energy to move around very much so the solid keeps its shape. Molecules in a gas have lots of energy and spread out even more than molecules in a liquid.

Warm water has more energy than cold water, which means that molecules in warm water move faster than molecules in cold water. The food coloring you add to the water is pushed around by the water molecules. Since the molecules in warm water move around faster, the food coloring spreads out quicker in the warm water than in the cold water.

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