Changes of state

In a nutshell

The arrangement of particles in solids, liquids and gases can be changed. This summary will cover how changes between these states can occur and allow you to predict the states of substances.

Particle model

Definition

The arrangement of particles in either solids, liquids or gases due to how much energy they hold.

The particle model explains how particles behave in each of the states.

STATE OF MATTER	PROPERTIES
Solid	Strong forces of attraction hold particles in a regular lattice Holds a definite shape and volume Particles vibrate about their position
Liquid	Weak forces of attraction allow particles to move freely Holds a definite volume but not a definite shape Particles move with random motion
Gas	Very weak forces of attraction allow very free movement Does not hold a definite shape or volume Particles move with random motion

However, this model is not perfect. In reality, particles are not perfect spheres - they are ions, atoms, or molecules of different shapes and sizes. In addition, the graphic representation does not show the strength of forces between particles.

State changes

Substances can change states and these changes are also reversible. This is shown below.

Chemistry; States of matter and mixtures; KS4 Year 10; Changes of state

	State change		Explanation	Example
1.	Deposition	Gas to solid	Particles change from a gas to a solid without passing through the intermediate liquid phase.	Frost formation
2.	Sublimation	Solid to gas	Particles change from a solid to a gas without passing through the intermediate liquid phase.	Dry ice
3.	Condensing	Gas to liquid	Particles no longer have enough energy to overcome attractive forces, so bonds form between them.	Water condenses on cold surfaces
4.	Boiling	Liquid to gas	Particles gain even more energy and vibrate more, breaking the forces holding particles together. This occurs at the boiling point and is known as evaporation.	Water (C) evaporates from a kettle as steam (A)
5.	Melting	Solid to liquid	Particles gain energy and vibrate more, weakening forces between particles. This occurs at the melting point.	A snowflake (B) melts
6.	Freezing	Liquid to solid	Particles have even less energy, so more bonds form between them.	Making ice cubes

Predicting states

You must be able to make predictions on the state of a substance given information such as melting or boiling point.

Rules for state changes:

If the temperature is below the melting point, the state is solid.
If the temperature is above the boiling point, the state is gas.
If the temperature is between melting and boiling point, the state is liquid.

Example

Methanol melts at $-98\ \degree C$ and boils at $65\ \degree C$ . Predict the state of methanol at:

a) $-100\ \degree C$ 

b) $0\ \degree C$ 

c) $173\ \degree C$ 

a) Solid; the temperature is below melting point.

b) Liquid; the temperature is between melting and boiling point.

c) Gas; the temperature is above boiling point.