Solids, liquids and gases

​​In a nutshell

The kinetic model describes all substances as being made of atoms and molecules with different arrangements depending on the phase of the substance. Brownian motion refers to the random movements of particles due to elastic collisions and momentum transfer.

The kinetic model

The kinetic model describes all substances as being made of atoms and molecules. The arrangement of these particles depends on the phase of the substance, that is if it's solid, liquid or gaseous:

Solids:

• Atoms or molecules are arranged regularly and closely packed.
  
• They are held in position by strong electrostatic forces.
• Their kinetic energy makes them vibrate.
• When more energy is added in the form of heat, the particles vibrate more and break away from other particles, meaning that the solid is melts.

Liquids:

• Atoms or molecules are close together but can change position and flow past each other.
  
• There still are forces of attraction between them that keep them close.
• They have more kinetic energy than solids.
• When energy is added to the system the particles break away from the liquid and become gas.

Gases:

• Atoms or molecules are far apart.
  
• Electrostatic forces between them are negligible.
• They have more kinetic energy than liquids.
• Gases occupy a much larger volume than liquids and solids of the same mass.

The closer the spacing between the particles, the denser the substance. In general, solids have greater density than liquids and gases.

Curiosity: Liquid water is more dense than solid water (ice). This is because when water freezes, the molecules in ice are further apart than when in liquid form, resulting in ice floating!

Brownian motion

When pollen is placed on water it will continue to move randomly without stopping. This movement was given the name Brownian motion after the physicist that discovered it and it took Einstein to solve the mystery.

The reason why the pollen moves is because of continuous elastic collisions with water molecules, which result in a transfer of momentum. This was the first concrete proof for the kinetic model.