CONCEPTS IN BIOLOGY

PART II. CORNERSTONES: CHEMISTRY, CELLS, AND METABOLISM

2. The Basics of Life

2.4. Molecules and Kinetic Energy

Common experience shows that all matter has a certain amount of kinetic energy. For instance, if you were to open a bottle of perfume in a closed room with no air movement, it wouldn’t take long for the aroma to move throughout the room. The kinetic molecular theory explains this by saying that the molecules diffuse, or spread, throughout the room because they are in constant, random motion. This theory also predicts that the rate at which they diffuse depends on the temperature of the room—the higher the air temperature, the greater the kinetic energy of the molecules and the more rapid the diffusion of the perfume.

Temperature is a measure of the average kinetic energy of the molecules making up a substance. The two most common numerical scales used to measure temperature are the Fahrenheit scale and the Celsius scale. When people comment on the temperature of something, they usually are making a comparison. For example, they may say that the air temperature today is

 “colder” or “hotter” than it was yesterday. They may also refer to a scale for comparison, such as “the temperature is 20°C [68°F].”

Heat is the total internal kinetic energy of molecules. Heat is measured in units called calories. A calorie is the amount of heat necessary to raise the temperature of 1 gram of water 1 degree Celsius (°C). The concept of heat is not the same as the concept of temperature. Heat is a quantity of energy. Temperature deals with the comparative hotness or coldness of things. The heat, or internal kinetic energy, of molecules can change as a result of interactions with the environment. This is what happens when you rub your hands together. Friction results in increased temperatures because molecules on one moving surface catch on another surface, stretching the molecular forces that are holding them.

They are pulled back to their original position with a “snap,” resulting in an increase of vibrational kinetic energy. Heat (measured in calories) and temperature (measured in Celsius or Fahrenheit) are not the same thing but are related to one another.

The heat that an object possesses cannot be measured with a thermometer. What a thermometer measures is the temperature of an object. The temperature is really a measure of how fast the molecules of the substance are moving and how often they bump into other molecules, a measure of their kinetic energy. If heat energy is added to an object, the molecules vibrate faster. Consequently, the temperature rises, because the added heat energy results in a speeding up of the movement of the molecules. Although there is a relationship between heat and temperature, the amount of heat, in calories, that an object has depends on the size of the object and its particular properties, such as its density, volume, and pressure.

Thermometer

Why do we take a person’s body temperature? The body’s size and composition usually do not change in a short time, so any change in temperature means that the body has either gained or lost heat. If the temperature is high, the body has usually gained heat as a result of increased metabolism. This increase in temperature is a symptom of abnormality, as is a low body temperature.

2.4. CONCEPT REVIEW

8. On what basis are solids, liquids, and gases differentiated?

9. What relationship does kinetic energy have to the three phases of matter?

10. What is the difference between temperature and heat?

11. What is a calorie?