A MOLECULAR DESCRIPTION OF THE PROPERTIES OF MATTER
Matter has certain qualities. It is hard or soft, fluid or solid, hot or cold, light or heavy. In attempting to understand the processes that shape the earth's environment, we will be concerned mostly with only of few of these properties, temperature, heat content, pressure, and density. These variables (All can change and all are measurable as discrete values.) can be described as a reflection of the actions or qualities of the molecules that comprise matter.
Take, for example, temperature. We all know what temperature is in some way. We can feel hot and cold, but what are we really sensing? Or if we describe heat as moving from hot to cold, why does that happen? By considering the meaning of temperature on a molecular level, we can answer these and other questions.
One way to approach the problem is to imagine a meteor (or the space shuttle) entering the atmosphere. You have learned that the atmosphere is composed of gases, meaning the molecules are moving freely, not much affected by any of the other molecules in the gas except for an occasional collision. When the meteor "hits" the atmosphere, it begins to slow down -- it loses kinetic energy. According to the First Law, that energy must be transformed. We see part of this transformation as the glowing gases of a shooting star. A more significant transformation is the heating of the surrounding atmosphere. Consider what happens when the meteor, even a tiny one collides with a molecule of gas. The meteor slows a little. And the gas molecule? What would happen if you were hit by a runaway locomotive? Of course, you would be sent flying. Some of the energy of the train would be transferred to you as your motion. The same thing happens to the molecule of gas; it picks up speed as a result of the collision. But has this really happened or is it just an accounting trick based on some assumed First Law?
We have a common name for this process that slows the meteor, friction. If you rub your hands together, you can feel an effect of friction -- it tends to heat the matter involved. So friction not only slows the meteor it heats the meteor and the gas. And, it heat the gas by transferring its energy of motion to increasing the speed of molecules. In other words, heat and temperature are measures of the kinetic energy of the molecules in matter. When we measure temperature, we are in effect measuring the velocity of molecules or more accurately the average kinetic energy of molecules in the matter. So the transformation of kinetic energy to heat is really a shift of the external kinetic energy of moving matter, in this case the meteor, to internal kinetic energy of molecules in the meteor and in the gases of the atmosphere.
If you have read about the origins of the earth, you already know this because that assignment draws a similar conclusion -- that gas molecules must be in constant motion and the speed of motion is controlled by temperature. In that reading, the argument was made that if gas molecules in our atmosphere stopped moving they would fall to the earth's surface because of gravity's pull and become liquid. This was used to explain how the earth lost its primordial atmosphere of hydrogen and helium. However, the reading neglected to mention that hydrogen and helium still escape earth's gravitational attraction. If that's the case, why don't the other gases?
Kinetic energy is a measure of velocity and mass of an object.
EK = 1/2 (mass X velocity2)
Therefore, if two gases have the same temperature (the same average kinetic energy), a gas with light molecules will be moving, on average, much faster than a gas with heavy molecules. In terms of molecular weight (the relative weights of molecules compared to Carbon-12), hydrogen has a weight of 2; helium is 4; water is 18; nitrogen, 28; oxygen, 32; and carbon dioxide is 40. Because they are so much lighter, H2 and He are moving between 2 and 10 times faster than the other gases. (Use the formula to check my math.) At current Earth temperatures, only hydrogen and helium have enough speed to regularly escape the pull of gravity.
This description of temperature as kinetic energy of the molecules also lets us understand both heat content of matter and why heat moves from hot to cold. First, temperature alone does not determine heat content. Temperature is average kinetic energy. Heat content is total. So to have a lot of heat, you need more molecules. If you have 10 kg. of iron and 2 kg. of iron, both at 40° C, the larger body will have more heat content. It would have more heat even if it were somewhat cooler say 30° C to 40° C. Which way would heat flow in this case? As always from hot to cold.
And why is that? Well imagine a fast moving marble colliding with a slow moving marble. Some of the kinetic energy of the faster marble is transferred to the slower. The fast marble slows down and slow marble speeds up. This is an absolute requirement of the tendency to increase entropy. Entropy "wants" to spread the energy as evenly (as randomly) as possible. Local areas with high energy, a fast moving marble, represents a form of order. Because there is no potential energy change in the collision, entropy controls the nature of the exchange. Similarly when fast moving hot molecules collide with slower, colder molecules, there is only one possible outcome. Energy is transferred to the slower molecules making the hot matter cooler and the cold matter hotter.
A third property of matter that is derived from the movement of molecules and the fact that molecules occupy space is that they resist compression. Solids and liquids are especially difficult to compress because the molecules are close together. Nor do they expand dramatically if pressure on them is removed. Gases behave differently. They expand and contract relatively easily in response to changes in external pressure. For them to maintain a constant volume, the external pressure on the gas must equal the internal pressure of the gas. (Pressure is defined as the force exerted on a specific area. For example, pressure is commonly measured in pounds (force) per square inch (area). If a 1000 lbs. of force are applied to 100 sq in. Then the pressure is 10 lbs/sq.in.)
Our description of temperature as a measure of the kinetic energy of molecules can be use to infer the underlying cause of gas pressure. If you were to put a balloon over a bottle and heat the bottle, the balloon will increase in size as the air in the bottle expands. The expansion represents an increase in internal pressure of the gas pushing the wall of the balloon outwards. And, what happens when the air is heated? The molecules move faster, so there must be a connection between molecular motion and internal pressure. The faster moving molecules are now colliding more rapidly and with more force against each other and the wall of the balloon and so push each other farther apart and the balloon wall outwards. If the air is allowed to cool, then the molecules slow down and exert less internal pressure. The external pressure of the balloon pushes the molecules closer together causing the volume of the air to decrease.
Similar but much less dramatic changes occur in solids and liquids. When heated, the molecules move faster, but not as free particles. Instead they vibrate about a fixed position -- like weights on a spring. As they get more energy, the vibrations become stronger and each molecule is pushed a little farther apart and the volume of the matter increases slightly. You can see this change is you fill a coffee cup to the top with boiling water. Make a cup of instant coffee. As the coffee cools, you will notice a significant decrease in the volume of the liquid even before you drink it.
These changes in volume produce changes in another property of matter, density. The density of matter is its mass divided by its volume or the mass of specified volume of the material. In effect it allows you to compare the heaviness of different kinds of matter without worrying about their size. If you lift equal volumes of lead and water, you have no trouble recognizing that the lead is heavier. But its not so easy if you have a lot of water and little lead. But, the lead is still about 12 times denser than water and in a very basic way, lead is always heavier than water.
On a molecular level the density difference between the two is a result of lead having heavy, closely packed molecules while water's tend to be light and more loosely packed. Density is actually a measure of the mass or weight of the molecules in matter and their spacing. The biggest difference between lead and water is their molecular weights. But if you were to compare a liquid's density with a gas, it would be the spacing of the molecules that is important. When water boils, the vapor produced takes up approximately 1000 times the volume of the liquid. The density of water vapor, therefore, is 1000 times less than the density of liquid water because each vapor molecule occupies 1000 times more space than a liquid molecule.
Thinking back to the volume changes that occur in response to heating and cooling, it is clear that density is a variable property. If water is heated, it expands taking up more volume, but its weight doesn't change. Therefore, each milliliter, or cubic centimeter or liter ( measures of volume) weighs less. Density has decreased. Similarly hot rock is less dense than cold rock and hot air is less dense than cold air.
Understanding the molecular source of
these properties of matter will be
useful as a tool for better understanding the convection that occurs in
earth's mantle, oceans, and atmosphere. If you work to understand what
on the molecular level to cause changes in temperature, density,
or heat content, you will find it easier to understand why hurricanes
and why continents move. Even these events that occur on a scale of
and thousands of miles result directly from the actions of molecules.
Reading question: Use the molecular
descriptions of temperature and density to explain why hot air is less
dense than cold air.