MYP Chemistry Characteristics of Gases Notes

Characteristics of Gases Explained by the Particle Model

1. According to the particle model of matter, gases are made up of tiny particles (atoms or molecules) that are in constant, random motion.
2. The distance between gas particles is much greater than in solids or liquids.
3. Intermolecular forces between gas particles are very weak (often treated as negligible in simple models).
4. Gas particles move in straight lines until they collide with one another or with the walls of the container.

Why Gases Can Be Compressed and Expand

Compression

Because gas particles are far apart, there is a lot of empty space between them.

1. When pressure is applied, the particles can be pushed closer together, reducing the volume of the gas.
2. The particles themselves do not get smaller; instead, the spaces between them decrease.

Expansion

1. Gases naturally spread out to fill any container.
2. When a gas is allowed to expand, its particles move apart and spread out to occupy the entire volume available.
3. If gas is transferred from one container to a larger one (at the same temperature and pressure), the particles simply distribute themselves throughout the new volume.
4. Unlike solids, gases have no fixed volume and no fixed shape – they always take the shape and volume of their container.

For a fixed mass of gas at constant temperature, the relationship between pressure and volume is inversely proportional:

$$P \propto \frac{1}{V}$$

This is Boyle’s Law, which you will study in more detail later.

Low Density

1. Gases generally have a much lower density than liquids and solids.
2. Because gas particles are so far apart, there are fewer particles per unit volume, so the mass per unit volume (density) is low.
3. When a gas is cooled, its particles move more slowly and can come closer together.
4. As the particles lose enough energy, intermolecular forces become more significant and the gas can condense into a liquid.
5. If the liquid is cooled further, the particles lose more energy, move even less, and may form a solid as the forces of attraction fully organise them into a more fixed structure.

Fluidity

1. Gases, like liquids, are fluids.
2. Gas particles are not fixed in place.
3. They move freely and can flow around obstacles.
4. Gas particles can flow and spread out, so gases have no fixed shape.
5. They will “pool” in a container, filling all available space, much like liquids flow to fill the bottom of a container (but gases fill the entire volume, not just the bottom).
6. This property of fluidity is why we can pump air through pipes, use gases in spray cans, and breathe air as it flows in and out of our lungs.

Understanding Gas Pressure

Each collision exerts a tiny force; together, these collisions create a measurable pressure on the container’s surface.

How Pressure Relates to Temperature

Temperature is directly linked to the average kinetic energy of gas particles. When the temperature of a gas increases:

1. The average kinetic energy of the particles increases.
2. The particles move faster.
3. Collisions with the container walls become more frequent and more forceful → the pressure increases (if volume is constant).

Boyle's Law

Boyle’s Law describes how the pressure and volume of a fixed mass of gas are related at constant temperature.

$$P \propto \frac{1}{V}$$

$$P V=\text { constant }$$

In practical form:

$$P_1 V_1=P_2 V_2$$