States of Matter (AQA GCSE Chemistry: Combined Science)
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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 boiling 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
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