How can water molecules in a gas state be moving slower than water molecules in a liquid state? Remember, the state that a molecule is in does not just depend on the speed of the molecule; it also depends on the strength of the attraction between molecules. If a molecule is by itself, it does not matter how slow it is moving… it will be in a gas state because the molecule will be moving freely.
When two water molecules come close together, they are attracted to each other just like magnets. The strength, or force, of the attraction depends on the distance between them. The two water molecules on the left in the simulation below are attracted to each other. Notice how they are constantly drawn toward each other when they are close together, but “ignore” each other when they are farther apart. On the other hand, the two water molecules on the right in the simulation are not attracted to each other at all. Notice how they move independently and do not interact, except when they bounce off of each other like hard spheres.
It is actually very difficult for two water molecules to enter a liquid state all by themselves. If the two water molecules on the left in the simulation above were not trapped in a small box together, chances are, they would eventually drift apart and never come close to each other again. The force of attraction between them would only be strong enough to hold them together if they were moving very, very, very, very slowly. Meanwhile, the two water molecules on the right will never enter a liquid state because they are not attracted to each other at all and will always move independently.
But if we pack more water molecules into the small box, we can see how the water molecules clump up together. The water molecules in the center of the clump are held together for a short period of time, while the water molecules on the edges of the clump are constantly breaking away and rejoining. These water molecules are not moving independently of each other. However, it is important to note that, even when molecules are strongly attracted to each other, this attraction does not slow them down… they continue moving and bouncing off of each other. This is because the water molecules still have kinetic energy, which means they are still moving. The only way for the water molecules to slow down is if they lose kinetic energy, which would mean cooling down the box and removing heat from the system.
Increasing the number of water molecules again and including the presence of gravity makes the simulation even more realistic. Now the water molecules are forming what almost looks like a water drop on top of a penny. Notice how the water molecules on the edge of the water drop are attracted by and pulled back toward the other water molecules in the center of the drop.
In order to simulate water molecules in a drop of water, we had to cheat a little bit by keeping the speed of the molecules extremely slow. Increasing the speed of the water molecules causes the water drop to break apart and the water molecules to enter something that looks much more like a gas state (the water molecules would certainly fly apart and enter a full gas state if they were not trapped in the box together).
So how does a drop of water stay together when the average speed of a water molecule at room temperature is approximately 590 m/s? Well, in real life, a drop of water does not consist of just 36 water molecules surrounded by a vacuum (empty space); it consists of over 1,000,000,000,000,000,000,000 water molecules surrounded by air.
The mass of a water molecule is 18 atomic mass units. 6.022 × 1023 water molecules would have a mass of 18 grams. 6.022 × 1023 is known as Avogadro’s number. It is used to find the number of molecules in a sample of a pure substance if you know the mass of the sample and the mass of the pure substance’s molecule. You will learn more about atomic mass units and pure substances later in this unit.
With over 1,000,000,000,000,000,000,000 molecules in a drop of water, most of the water molecules will be deep in the center of the drop, not close to the surface. This means that most of the water molecules will be held more tightly by molecular attraction and will not break away from the water drop so easily. And since the drop of water is surrounded by air, the molecules in the air will be exerting pressure on the surface of the drop equal to about one pound of force (or 4.5 newtons). This force will push the water molecules closer together, increasing the attraction between water molecules even more; and the molecules in the air will also physically knock many of the water molecules breaking away from the surface of the drop back into the drop, keeping more water molecules in the liquid state.
Unlike water, air is not a pure substance; there is no such thing as an “air” molecule. Air is actually a mixture of many molecules. Dry air contains roughly 78% nitrogen (N2), 21% oxygen (O2), argon (Ar), carbon dioxide (CO2), and small amounts of other gases. Air can also contain a variable amount of water vapor (H2O), on average around 1%.
Air pressure is created when the molecules in air bounce off of each other and other objects. At room temperature, the average speed of the nitrogen and oxygen molecules is approximately 450 m/s (1000 mph). While we generally do not notice it since air is all around us and pushing equally in all directions, air pressure can actually generate extremely strong forces. Standard atmospheric pressure (the average air pressure at sea level) is about 14.7 pounds per square inch (or 101,325 N/m2 [pascals]). This means that there are almost 1400 pounds of force pushing down on a sheet of paper lying flat on a table. Without this air pressure pushing down on it, a drop of liquid water at room temperature would boil off in a second, entering the gas state as water vapor.
A group of water molecules will be in a liquid state only if the attraction between those molecules is strong enough to hold them together, keeping the individual water molecules from moving freely. In a liquid state, the water molecules are held together in tiny clusters. Those clusters are “fluid,” which means that water molecules are constantly breaking away from one cluster and joining another. (A water drop is not a single cluster; it is a collection of many, many smaller clusters.) This fluidity is what distinguishes a liquid from a solid. In a solid state, the water molecules are held together so strongly that they form a single giant cluster (a crystal), and the attraction is enough to keep individual molecules from moving out of position or breaking away.