Density and Kinetic Theory

Density

Density is a measure of mass per unit volume. This can be calculated by the formula: density = mass/volume.
Density is measured in kilograms per cubic metre (kg/m³).
If an object’s volume or mass increase while the other stays the same, its density will also increase. An object with more mass in the same volume is denser.
The density of an object determines whether it will float or sink in a liquid. A denser object will sink.
Different substances have different densities because the mass and volume of the atoms or molecules they contain differ.

Kinetic Theory

Kinetic Theory explains the different states of matter (solid, liquid, gas) and their behaviours.
All substances are composed of tiny particles known as atoms or molecules. The way these particles move and interact explains the properties of different states of matter.
In solids, particles are closely packed in a fixed, regular pattern. They vibrate around a fixed position, which explains why solids maintain a fixed shape and volume.
In liquids, particles are less closely packed. They move randomly and slide past each other, which is why liquids can flow and take the shape of their container but have a fixed volume.
In gases, particles are free to move in all directions. They move randomly and with more energy compared to in solids and liquids, explaining why gases can expand to fill their containers and change both shape and volume.
The temperature of a substance is a measure of the average kinetic energy of the particles within the substance. Higher temperatures mean higher kinetic energy and faster particle movement.
Substances can change state when heat is added or removed. For example, heating a solid provides more kinetic energy to the particles, helping them overcome the forces holding them in place and move more freely, leading to melting.
Pressure in gases can be explained by the kinetic theory. When gas particles collide with the walls of their container, they exert a force on the walls which is measured as pressure. More frequent collisions, faster moving particles, or a smaller container will increase gas pressure.