States of Matter (Edexcel GCSE Chemistry)
Revision Note
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The Three States 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
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Interconversions
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
Interconversion between the three states of matter
Examiner Tips and Tricks
Solids, liquids and gases have different physical properties. The difference in these properties comes from differences in how the particles are arranged in each state.
Predicting Physical State
The physical state of a substance under certain conditions can be predicted from a given set of data.
Normally you are given melting and boiling point data for a substance and asked to predict its physical state in specified conditions.
At temperatures below the melting point:
The substance is will be in the solid state
At temperatures above the melting point but below the boiling point:
The substance will be in the liquid state
At temperatures above the boiling point:
The substance will be in the gaseous state.
Worked Example
Predicting the state
The table below indicates melting and boiling point data for four different substances named A, B, C and D. Predict the states of the following substances:
Substance A at -150 ºC
Substance B at 50 ºC
Substance C at 1400 ºC
Substance D at 400 ºC
Melting & Boiling Points Table
Answer
A boils at temperatures above -173 ºC so at -150 ºC A is a gas
B melts at 1736 ºC so at 50 ºC it is a solid
C melts at 1105 ºC and boils at 1450 ºC so at 1400 ºC it is a liquid
D melts at 650 ºC so at 400 ºC it is a solid
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