Last month we talked about how snow begins to melt from a sidewalk before it melts on grass, and that this is due to the heat capacity of cement versus grass. We found that the lower the heat capacity of a material or substance, the faster heat will “move” into that material and warm the snow. In order to fully explain this observation, however, it is important to understand the difference between heat and temperature. 

Materials

Containers for ice water and for heating water
Food coloring
Glass tumblers, 5
Ice water and tap water
Optional: Thermometer

Be Safe!

Adult supervision is necessary when pouring hot liquids. Do NOT heat the water to boiling, and SLOWLY pour the hot water into glass tumblers to prevent cracking. Do not use thick-walled tumblers.

Experiment

Fill a container with ice water and heat water in another container using a microwave or on a stovetop. Pour ice water into the first tumbler and tap water into the second tumbler. Slowly pour hot water into the third tumbler. Allow the tumblers to sit for five minutes to allow eddy currents to settle. After five minutes, add a drop of food coloring to each tumbler and observe.

Fill a fourth tumbler with hot water, and to the fifth tumbler add only enough hot water so that the tumbler is less than one-quarter full. Set the tumblers aside for 10 minutes. After 10 minutes, add a drop of food coloring to each glass and watch what happens. 

What’s happening?

You probably noticed that the food coloring mixed much faster in hot water than in cold water. The reason for this is that when energy, or heat, is added to the water, the water molecules move faster and as they bump into the food coloring molecules they cause those molecules to also move faster and disperse more quickly throughout the water. We can measure how fast water molecules are moving with a thermometer, since temperature is a measure of the kinetic energy or speed of molecules. The food coloring is basically acting like a thermometer in that it allows you to see how fast the molecules are moving!

Water in the fourth and fifth tumblers should initially have been at the same temperature. However, the fourth tumbler had more heat in it because it contained more hot water. But, if we assume that the surrounding air removes heat from both vessels at the same rate, then the temperature of the hot water in the fifth tumbler, because it had less heat, should decrease faster. Adding food coloring to the fourth and fifth tumblers, you should observe the food coloring disperse more quickly in the fourth (full) tumbler than in the fifth tumbler that was only a quarter-full. 

In a thought experiment, imagine what would happen if you added a little water to a mug and heated it in the microwave for 60 seconds, and did the same thing with a full mug of water. You’re adding the same amount of heat energy to both mugs, but the temperature will be higher in the mug containing less water.

Explore more at

https://www.teachstarter.com/us/blog/science-experiments-for-kids-water-density-temperature-us/

To view more “ChemShorts for Kids” activities, go to: https://chicagoacs.org/ChemShorts

To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts

- PAUL BRANDT

These liquids are at the same temperature, but the liquid on the left contains more heat.