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We have been learning about the phases of matter. The phases of matter are solids, liquids, gasses, and plasmas. Each state has a different particle arrangement and movement. The particle arrangement and movement affect a substance's volume. Let's explore the volumes of solids, liquids, and gases!
Volume in Solids
Solids have set shapes and volumes. Often, volume is a physical property. For example, I have a young niece who loves to play with blocks. I can calculate the block's volume by multiplying its length, width, and height. If you don't have a perfect shape, you use water displacement. Archimedes famously ran down the street naked, shouting Eureka, after discovering water displacement.
In chemistry, a solid's volume is a chemical property. Does that surprise you? But, how often have you seen perfect cubes in your chemistry class? And if you drop a chunk of salt in water, it dissolves. A solids volume comes from its molecular geometry.
In crystalline solids, the volume depends on the unit cell. Many unit cells are cubic. It is easy to calculate their volume. However, there are non-cubic unit cells. Their volumes are more laborious to calculate. To experimentally find a crystal's density, chemists use x-ray crystallography.
For molecular solids, the volume depends on the spacing between the molecules. You can calculate the density with quantum mechanics! Physical and theoretical chemists use molecular orbital theory to calculate densities.
Practically, what if you need to find the volume of a solid? Use density! You can look up any molecule's density. To determine the mass, you only need a scale. Divide the mass by density, and you have volume!
Volume in Liquids:
A liquid's container determines its shape. However, all liquids have fixed volumes. We measure liquid volumes by the capacity of their container. For example, imagine pouring water into a graduated cylinder. The water now has a cylindrical shape. So, we use the cylindrical volume equation, . Thankfully, a graduated cylinder already has volume markings. We don't have to do any math! What if you only know a fluid's mass? Again, use density!
Volumes of Gases
A gas has neither a fixed shape nor a fixed volume. It will fill the space of its container. If the container expands, the volume increase. If the container shrinks, the volume decrease. If we know the volume of our container, we have the volume of our gas. For example, you buy a propane canister. The canister is sixteen ounces. So, you have sixteen ounces of propane gas.
Using container volume for gas volume is problematic. It does not tell us how much gas we have. In our propane canister, we could have 1 mole of gas or 10 moles of gas. So, we need a standard pressure. In collision theory, we learned that gases have random motion. They collide with the walls of their container. Pressure is the force of the collisions per unit area. For example, your car's tire pressure is pounds per square inch (psi). The tire pressure in my car is 30 psi. So, the air in my tire exerts thirty pounds of force on each square inch of the tire. In our propane canister example, they are all filled to the same pressure.
Sometimes, we do measure gas volumes. For example, we sell cubic feet of natural gas. But, the ideal gas law teaches us that volume depends on pressure and temperature. So, a company selling natural gas has to record the temperature and pressure. Before selling the natural gas, they correct the volume to standard temperature and pressure (STP). Remember, one mole of an ideal gas at STP is 22.4 liters.
Because gases cannot have a fixed shape, they are expandable and compressible. We use the compressibility of gasses to power our cars. A car's engine has pistons attached to a crankshaft. As gasoline combusts, it produces carbon dioxide. The pressure from the carbon dioxide moves the piston. The piston's movement rotates the crankshaft. The rotating crankshaft spins a series of gears that turn the tires. And off you go!
