You and Your Students!
Vicki Cobb, Education World Science Editor
Turn up the heat and things melt, but -- amazingly -- the temperature doesn’t change!
Measurement, Matter (States and Physical Properties)
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.
Watch the weather reports and you know that water freezes at 32F (or 0C). 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 32F 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.
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 70F is lower than the temperature of the human body, chocolate melts in your mouth. Put a Hershey Kiss 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.
How does the latent heat of fusion explain why snow and ice hang around long after the weather gets warmer? (It takes a lot of heat to melt all that ice and snow, days of it.) After ice has formed, do you think its temperature can drop lower than 32F? Freeze a thermometer (for the freezer) in an ice cube and find out? (If you have a sub-zero freezer, you can guess the answer. Once ice has formed, the temperature can drop sharply.)