Properties of gases

Gases are all around us. They fill the air we breathe and the spaces we move through. But what exactly are gases, and what properties do they have? In this lesson, we'll explore the basic properties of gases, why these properties are important, and how they relate to the gas laws.

What are gases?

Gas is one of the four fundamental states of matter, the others being solid, liquid, and plasma. The molecules in a gas are in constant motion and are spread far apart, which is different from the arrangement of molecules in solids and liquids.

Properties of gases

Understanding the properties of gases is important for many aspects of science and everyday life. Here are some of the main properties:

1. Compressibility

The most important property of gases is that they are compressible. This means you can squeeze them into smaller volumes:

P1V1 = P2V2
    

In this formula:

• P1 is the initial pressure.
• V1 is the initial volume.
• P2 is the final pressure.
• V2 is the last section.

According to Boyle's law, as pressure increases, volume decreases, keeping temperature constant. Think of compressing air in a syringe: As you press the plunger, the air molecules move closer together, increasing the pressure.

2. Expansion

Gases expand to fill the container they are in. Unlike solids that have a definite shape and volume, or liquids that have a definite volume, gases have neither of these. For example, if you release helium from a balloon into a room, the helium will expand to fill the entire room evenly.

3. Low density

Gases have a much lower density than solids or liquids. This means there are fewer molecules in a unit of space. Consider the difference: The same volume of water in a glass, when boiled into steam, occupies much more space because as a gas, the molecules are more spread out.

4. Diffusion

Diffusion is the property of gases to spread out and mix with other gases. This is how a drop of perfume can quickly spread across a room. The molecules move randomly, collide with each other and eventually mix evenly throughout the space.

5. Pressure

Gas pressure is the force exerted by gas molecules when they collide with the surfaces they collide with. Gas laws explain how variables such as volume, temperature, and the number of molecules can affect pressure.

6. Dependence on temperature

The properties of gases are highly dependent on temperature. Heating a gas increases the energy of the molecules, resulting in an increase in pressure if the volume is constant, or an increase in volume if the container is flexible.

Gas Laws

Several gas laws describe the behavior of gases with respect to their various properties:

Boyle's law

Boyle's law explains the relationship between the pressure and volume of a gas at a constant temperature. It states that the pressure of a given quantity of gas is inversely proportional to its volume:

P1V1 = P2V2
    

This means that if you decrease the volume, the pressure increases, and vice versa, provided the temperature remains the same.

Charles's law

Charles's law describes how gases expand when heated, and gives the relationship between the volume of a gas and its temperature at constant pressure:

V1/T1 = V2/T2
    

In this formula:

• V1 is the initial volume.
• T1 is the initial temperature.
• V2 is the last section.
• T2 is the final temperature.

This shows that the volume of a gas is directly proportional to its temperature. If you keep a balloon in the sun, it expands due to the increase in temperature.

Gay-Lussac's Law

This law relates the pressure of a gas to its temperature when its volume is held constant:

P1/T1 = P2/T2
    

If you increase the temperature, the pressure also increases, provided the volume remains unchanged.

Avogadro's law

Avogadro's law asserts that the volume of a gas is proportional to the number of moles, provided the temperature and pressure are constant:

v1/n1 = v2/n2
    

Where:

• V1 is the initial volume.
• n1 is the initial amount in moles.
• V2 is the last section.
• n2 is the final amount in moles.

This means that more gas particles occupy more space, which can be clearly seen when the balloon is inflated.

Ideal Gas Law

This general formula incorporates all preceding observations into a single ideal law:

PV = nRT
    

Here:

• P is the pressure.
• V is the volume.
• n is the moles of the gas.
• R is the ideal gas constant.
• T is the temperature.

According to the ideal gas law, the behaviour of a gas can be predicted if any three of its properties are known, while the fourth property is unknown.

Real-world applications

Understanding gases and gas laws has many practical applications in real life:

1. Weather balloons

Weather balloons use the laws of gas to rise into the low-density atmosphere, and provide important data about atmospheric conditions.

2. Car airbags

Airbags in cars rely on rapidly expanding gases, and follow appropriate gas regulations, to protect passengers during a collision.

3. Breathing

The process of breathing depends on changes in pressure and volume in our lungs, which are calculated through these principles.

Practice problems

Let's test your knowledge with some practical problems:

1. A gas is compressed from 10 liters to 2 liters, while the initial pressure was 1 atm. What is the final pressure?
2. The volume of a balloon containing 2 moles of gas at 300 K temperature is 6 L. If the moles are doubled, what is the new volume?

Try solving these yourself using the principles discussed!

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