Experiment 13.3: Seeing the Effect of Changing Temperature

What did the experiment show? Many science books use this kind of experiment to show that things tend to expand as the temperature increases. While this is true, it also demonstrates what temperature really measures. You see, by sealing the bottle with the balloon when it was cold, you trapped a lot of cold air in the bottle. As the air's temperature was increased by the hot water through conduction and convection, the air expanded, making the balloon grow larger. To understand temperature, you need to understand why the air expanded.

Recall from module 2 the kinetic theory of matter. Remember that all molecules of solids, liquids, and gases are constantly moving. The molecules of gases in the air are moving around at very high speeds because they have a lot more energy than molecules of liquids or solids (molecules of solids only vibrate back and forth). What are you doing when you heat something up? You are transferring energy to it. So when you placed the bottle in the hot water, the energy in the hot water started flowing into the bottle by conduction. Believe it or not, that's actually what heat is.

Heat—Energy that is transferred as a consequence of temperature differences

So heat is really kinetic energy that flows from one object to another. As energy flowed from the hot water to the bottle, two things happened. The bottle's temperature increased, and the water's temperature decreased.

What happened to that energy? As it flowed into the bottle, it was picked up by the molecules that made up the bottle as well as the molecules that made up the air in the bottle. What happened when these molecules absorbed this energy? They began to move faster. Since the bottle is a solid object, the molecules that made up the bottle began vibrating faster. The molecules that made up the air in the bottle, however, started moving around faster.

Think about what happened to the air molecules on a molecular level. As the temperature rose, the average kinetic energy of the molecules in the air increased. With increased kinetic energy, the molecules move faster and collide more often with the inner walls of the bottle and balloon. The faster moving molecules also hit the walls of the bottle and balloon with greater force. The increase in the number of collisions along with the increase of the force of the collisions causes an increase in the pressure of the air in the balloon, bulging the balloon outward.

How does this apply to temperature? When I put a thermometer in a substance, the molecules of that substance begin striking the thermometer. At the same time, the molecules that make up the thermometer are moving as well. Thus, collisions will be occurring between the molecules in the thermometer and the molecules of the substance that the thermometer is in. If the molecules of the substance are moving faster than those of the thermometer, energy will be transferred from the molecules of the substance to those of the thermometer. As a result, the molecules in the liquid in the thermometer heat up, which means they speed up and spread slightly further apart so they occupy a larger volume. Thus, the liquid in the thermometer is less dense. This causes the column of liquid to rise, and we see a high temperature.

If, on the other hand, the molecules of the thermometer are moving faster than those of the substance, energy will go from the thermometer to the substance. As a result, the thermometer cools down, which means its molecules slow down and fit in closer together, contracting. Thus, the liquid in the thermometer becomes more dense and the liquid column shrinks, showing a low temperature. So, a thermometer really measures the average speed at which the molecules of a substance are moving.

Since the speed of the molecules is directly related to their energy, we say that a thermometer measures the energy of the molecules of a substance. That is what temperature is!

Temperature—A measure of the energy of random motion in a substance's molecules

Therefore, in the end, temperature does not measure heat (energy that is transferred); instead, it measures the kinetic energy of the molecules in a substance.

If you think about it, this explains why you get hot when the temperature is high and get cold when the temperature is low. When the temperature is high, the molecules in the air colliding with your skin have much more energy than the molecule in your skin. As a result, energy is transferred from the air to you. This makes you hotter. When the temperature is low, the molecules in your skin have more energy than the molecules in the air colliding with your skin. As a result, energy is transferred from you to the air, and you end up getting colder.