Heat Loss

There are many timesthat the temperature of an object plays an important role in solving a crime. Heat from the engine of an automobile may indicate that it has just been driven. A warm cup of coffee on a kitchen table may mean that a suspect has just left. The temperature of a body can indicate to detectives when a death has occurred. Try the following activity to see how temperature can be used by forensic scientists.

Materials

- 2 cup (500 ml) of hot coffee
- coffee cup
- thermometer (measures up to 212oF [100oC])
- stopwatch or watch with second hand
- pen or pencil
- notebook
- adult helper

Procedure

NOTE: This activity requires adult help. Have your adult helper keep the hot coffee at the same temperature throughout the experiment by placing it on a stove or hot plate.

1. Have your adult helper pour 1 cup (250 ml) of hot coffee into the coffee cup.
2. Put the thermometer in the coffee, start your stopwatch, and record the temperature of the coffee in your notebook.
3. Continue to record the temperature of the coffee at 1-minute intervals until the temperature no longer changes. Empty the cup of coffee.
4. Leave the room. Have the adult helper pour another cup of hot coffee into the coffee cup and note the time that he or she pours the coffee.
5. Have the adult helper wait several minutes before inviting you back into the room.
6. Take the temperature of the coffee. Compare the temperature to your records. How many minutes has it been since your helper poured the coffee into the cup?

Explanation

When the object is removed from a source of heat, its temperature begins to drop. It cools rather rapidly until it reaches the temperature of the surrounding air, at which point its temperature stabilizes. The heat loss of the same size of object at the same initial temperature occurs at the same rate each time. This allows one to determine, with some degree of accuracy, the time that the object was removed from the heat source. In this activity, as long as the same amount of coffee at the same initial temperature was poured the second time, and as long as the room temperature did not change, you should have been able to estimate the time the coffee was poured.

At death, a body stops generating heat and its temperature drops gradually. At first the cooling is relatively rapid –approximately 1.5oF (0.8oC) per hour—but this rate slows after a few hours. In addition to size, initial temperature, and the temperature of the surroundings, body type will also affect the cooling rate. A thin body will cool more rapidly than a heavier one. By taking a body’s temp[erature, a forensic scientist can estimate the time of death.