Classroom Exercises


Starring...    You and Your Students!

Props Required

• ice cubes

• thermometers

• Styrofoam coffee cups

• water

• chocolate

Setting the Scene   (Background)
When you teach the properties of matter, you make distinctions betweens solids, liquids, and gases. Water, for instance, changes easily from one state to another depending on heat (or the lack of it). But heat and temperature are not the same thing, as the two acts of this little performance make abundantly clear.

The Plot
Act I
Watch the weather reports and you know that water freezes at 32°F (or 0°C). Snow and ice melt when the external temperature warms up. But did you know that while ice is melting, the temperature doesn't change? Ice water remains a constant 32°F until the ice has melted.

Put two ice cubes in a Styrofoam cup and add cold water. Stir with a thermometer. What is the temperature? Stir the ice water and check the temperature every 20 minutes. Does the temperature ever change? Keep checking even after all the ice has melted. Now do you get a temperature change?

Here's how to think about what's happening: Molecules are always in motion.

• At room temperature, liquid water molecules "slide" past one another.

• When you add heat, the water molecules speed up. (You can see this in a pot of boiling water.)

• Ice molecules don't move much; rather, they vibrate in place.

Temperature is a measure of this molecular motion. The faster the molecules move, the higher the temperature. But there are exceptions to this. When ice melts, heat energy goes into making the molecules vibrate faster; they are no longer held in the fixed position of a solid. Although you are adding heat, the temperature doesn't change in the ice-water system until all the ice has melted. When the ice is completely melted, the water temperature will rise slowly as the warmth of the room increases the motion of the liquid water molecules. This will continue until the water is the same temperature as the room. The heat needed to melt ice is called its latent heat of fusion.

Act II
Here's a sweet way to experience the latent heat of fusion of something else that melts -- namely, chocolate. Seventy percent of chocolate candy is a solid fat, called cocoa butter. One of the most important properties of cocoa butter is the temperature at which it melts. It has a sharp melting point; that means that it changes from a solid to a liquid very quickly. Since its melting point of 70°F is lower than the temperature of the human body, chocolate melts in your mouth. Put a piece of chocolate on your tongue and hold it in your mouth as it melts. Resist the temptation to chew. It may get stuck to the roof of your mouth. Rub your tongue back and forth on the melting candy. Notice the cool feeling on your tongue and roof of your mouth. The melting chocolate is using heat from your body to melt and, as a result, your skin feels cooler. Since the difference in temperature between the melting point of chocolate and your body's temperature is not very large (22.6 degrees), you don't notice the cooling effect of the candy unless you pay attention.