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States of matter

Particles are in constant motion. This animation shows the movement of gas molecules.
Particles are in constant motion. This animation shows the movement of gas molecules.

The particles that make up everyday objects are in constant movement.

These particles can be atoms (the basic units of matter) but are more commonly molecules (two or more atoms chemically bonded together).

The movement of particles is experienced in everyday life as temperature.

At low temperatures, the particles move slowly and tend to vibrate around specific positions. They are held in place by intermolecular forces.

At higher temperatures, particles move faster. They break free from their positions and move in straight lines. They change direction (and may lose speed) when they collide with other molecules.

Absolute zero is the lowest temperature theoretically possible. This happens at $$-273.15^{\circ}\text{C}$$. At absolute zero, particle movement is at a minimum.

There are three basic states of matter: solid, liquid and gas.

The state of a given material depends on two factors:

  • The strength of attractive intermolecular forces between the particles. These forces depend on the type of the material.
  • The kinetic energy (in the form of particle movements) of the material. If a particle is moving quickly (at high temperature), the kinetic energy of the particle is high.

Recall that the kinetic energy of an object is the energy it has due to its motion.

In a solid, the energy is too weak to overcome the intermolecular forces. The particles are tightly packed and vibrate about fixed points.

In a liquid, the speed of the particles is higher. The particles have enough energy to break away from fixed positions but are held closely together by intermolecular forces. The particles can move past one another.

In a gas, the particles have enough energy to overcome the intermolecular forces. The particles are spaced far apart and can move independently.

State Solid Liquid Gas
Packing of molecules Tightly packed Closely packed Loosely packed
Movement of molecules Can only vibrate and rotate in fixed positions Move randomly but not as mobile as gases Move randomly in all directions (faster than in liquids)
Physical properties Fixed shape and volume No definite shape but fixed volume No definite shape or volume.
Compressibility Hard to compress Hard to compress Compressible
Examples Tables, chairs, ice Milk, water, petrol Steam, air, helium