# 2.2.1 States of Matter

### The Physical Forms of Matter

• The three states of matter are solids, liquids and gases
• A substance can usually exist in all three states, dependent on temperature (and pressure)
• State changes occur at the melting point (solid to liquid, liquid to solid) and at the freezing point (liquid to gas and gas to liquid)
• Melting and freezing occur at the melting point
• Boiling and condensing take place at the boiling point
• Individual atoms themselves do not share the same properties as bulk matter
• The three states of matter can be represented by a simple model
• In this model, the particles are represented by small solid spheres

Summary of the Properties of Solids, Liquids and Gases

### Energy & State Changes

• The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles
• The stronger the forces of attraction, the more energy that is needed to overcome them for a state change to occur
• Therefore, the stronger the forces between the particles the higher the melting point and boiling point of the substance
• When matter changes from one state to another due to changes in temperature or pressure, the change is called an interconversion of state
• It is a physical change involving changes in the forces between the particles of the substances, the particles themselves remain the same, as do the chemical properties of the substance
• Physical changes are relatively easy to reverse as no new substance is formed during interconversions of state
• The interconversions have specific terms to describe them:

A Summary of State Changes

#### Melting

• Melting is when a solid changes into a liquid
• The process requires heat energy which transforms into kinetic energy, allowing the particles to move
• It occurs at a specific temperature known as the melting point which is unique to each pure solid

#### Boiling

• Boiling is when a liquid changes into a gas
• This requires heat which causes bubbles of gas to form below the surface of a liquid, allowing for liquid particles to escape from the surface and from within the liquid
• It occurs at a specific temperature known as the boiling point which is unique to each pure liquid

#### Freezing

• Freezing is when a liquid changes into a solid
• This is the reverse of melting and occurs at exactly the same temperature as melting, hence the melting point and freezing point of a pure substance are the same
• Water for example freezes and melts at 0 ºC
• It requires a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature which is unique for each pure substance

#### Evaporation

• When a liquid changes into a gas
• Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquids surface at low temperatures, below the boiling point of the liquid
• The larger the surface area and the warmer the liquid/surface, the more quickly a liquid can evaporate
• Evaporation occurs over a range of temperatures, but heating will speed up the process as particles need energy to escape from the surface

#### Condensation

• When a gas changes into a liquid, usually on cooling
• When a gas is cooled its particles lose energy and when they bump into each other, they lack energy to bounce away again, instead grouping together to form a liquid

#### Sublimation

• When a solid changes directly into a gas
• This happens to only a few solids, such as iodine or solid carbon dioxide
• The reverse reaction also happens and is called desublimation or deposition

Changing State

### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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