Core Concepts
In this article, we learn about what a gas is, diatomic gases, the three properties of gases, gas laws, and gas phase changes.
Topics covered in other articles
- What is Pressure?
- Dalton’s Law of Partial Pressure
- Avogadro’s Law
- Boyle’s Law
- Charles’ Law
- Van der Waal’s Equation
- Kinetic Gas Theory
- Gay Lussac’s Law
- Combined Gas Law
- Henry’s Law
What are gases?
Gases are the highest-energy state of matter. What makes them unique is that they have no fixed shape or volume. Unlike solids and liquids, gases take the shape of their container. They’re highly compressible and can change volume under different pressures and temperatures. Having an introductory understanding of gases is crucial as they play a key role in various chemical reactions and processes.
Gases made up of one element are called monoatomic. Common examples are neon, argon, or helium. However, there are some elements that exist as diatomic molecules, meaning that in their natural gaseous form, these elements exist as diatomic molecules with two atoms bonded together. A common example of this is oxygen! The chemical formula of oxygen in the air we breathe is not 
, but rather 
. Bromine, iodine, nitrogen, chlorine, hydrogen, oxygen, and fluorine are all diatomic elements.
Properties and Laws of Gases
There are three significant properties of gases — pressure, temperature, and volume.
Pressure refers to the force exerted by gas molecules on the walls of their container. It is a result of the collisions between gas particles and the container’s surface. When gas molecules move faster or there are more gas molecules in a given volume, the pressure increases. Pressure is measured in units like Pascals (Pa) or atmospheres (atm). To measure pressure, either a barometer or a manometer can be used.
Temperature is a measure of the average kinetic energy of gas molecules. In simpler terms, it indicates how fast the gas particles are moving. When a gas is heated, its molecules gain kinetic energy, move more vigorously, and its temperature rises. Conversely, cooling gas decreases the average kinetic energy of its particles and lowers the temperature. Celsius (°C), Kelvin (K), and Fahrenheit (°F) are most commonly used to measure temperature.
Gas Laws
Gas laws are laws that describe the relationships between these different properties of gases.
Boyle’s Law states that, at constant temperature, the volume of a given amount of gas is inversely proportional to its pressure. This means that if volume is decreased, pressure will increase, and if volume is increased, pressure will decrease. Mathematically, Boyle’s law can be expressed as 
.
Charles’s Law states that, at a constant pressure, the volume of a given amount of gas is directly proportional to its temperature. This means that when you increase the temperature of a gas while pressure remains constant, its volume will increase, and when you decrease the temperature, its volume will decrease. Charles’s law can be written as 
Gay-Lussac’s Law states that, at a constant volume, the pressure of a given amount of gas is directly proportional to its temperature. This means that if you heat up a gas while volume remains constant, its pressure will increase, and, if you cool it down, its pressure will decrease. Gay-Lussac’s Law is expressed as 
.
So far, all of these laws have discussed the relationships between two of the properties of gases while a third property remains constant. However, there is another law, called the ideal gas law, which incorporates all three of the properties of gases: pressure, volume, and temperature. The ideal gas law is a combination of Boyle’s Law, Charles’s Law, and Gay-Lussac’s Law. It can be expressed as 
. Learn more about the units of the ideal gas law and the differences between an ideal and real gas here.
Gas Phase Changes
- Gas to solid — Deposition
- Solid to gas — Sublimation
- Gas to liquid — Condensation
- Liquid to gas — Vaporization