Matter & Its Properties
States of Matter
1st Year · 2nd Year · 3rd Year
- ✓By the end of this lesson students will be able to describe the properties of solids, liquids, and gases.
- ✓By the end of this lesson students will be able to explain the particle model of matter.
- ✓By the end of this lesson students will be able to identify and describe the changes of state (melting, freezing, boiling, condensation, sublimation).
- ✓By the end of this lesson students will be able to relate changes of state to energy changes and particle movement.
Key concepts
In a solid, particles are tightly packed in a fixed, regular arrangement. They vibrate about fixed positions. Solids have a definite shape and a definite volume. They cannot be compressed easily.
In a liquid, particles are close together but are not in fixed positions. They can slide past one another. Liquids have a definite volume but take the shape of their container. They cannot be compressed easily.
In a gas, particles are far apart and move randomly and rapidly in all directions. Gases have no definite shape and no definite volume; they fill their container. They can be compressed easily.
The particle model of matter states that all matter is made up of tiny particles (atoms, molecules, or ions) that are constantly in motion. The arrangement and movement of these particles, and the forces between them, determine the state of matter.
Melting is the change of state from a solid to a liquid. This occurs when a solid gains enough thermal energy for its particles to overcome the forces holding them in fixed positions and start to slide past each other.
Freezing is the change of state from a liquid to a solid. This occurs when a liquid loses enough thermal energy for its particles to slow down and settle into fixed positions.
Boiling and evaporation are both changes of state from a liquid to a gas. Evaporation occurs at the surface of a liquid at any temperature below the boiling point. Boiling occurs throughout the liquid at a specific temperature (the boiling point) when the liquid gains enough thermal energy for particles to escape as a gas.
Condensation is the change of state from a gas to a liquid. This occurs when a gas loses enough thermal energy for its particles to slow down and come closer together, forming a liquid.
Sublimation is the change of state directly from a solid to a gas, without passing through the liquid state. An example is solid carbon dioxide ('dry ice') turning directly into gaseous carbon dioxide.
Key facts to remember
- 1Matter exists in three main states: solid, liquid, and gas.
- 2The state of matter depends on the arrangement and movement of its particles and the forces between them.
- 3Adding thermal energy usually causes a substance to change from solid to liquid to gas.
- 4Removing thermal energy usually causes a substance to change from gas to liquid to solid.
- 5Melting, boiling, and sublimation involve the substance gaining energy.
- 6Freezing and condensation involve the substance losing energy.
- 7Solids have a definite shape and volume.
- 8Liquids have a definite volume but take the shape of their container.
- 9Gases have no definite shape or volume and fill their container.
Worked examples
Example 1
Describe the arrangement and movement of particles in a solid, a liquid, and a gas.
Answer
In a solid, particles are tightly packed in a fixed, regular arrangement and vibrate about fixed positions. In a liquid, particles are close together but randomly arranged and can slide past one another. In a gas, particles are far apart and move randomly and rapidly in all directions.
This question assesses your understanding of the particle model for each state of matter.
Example 2
Explain what happens to the particles of water when ice melts and then boils.
Answer
When ice melts, it gains thermal energy, causing its particles to vibrate more vigorously and overcome fixed positions, allowing them to slide past each other as a liquid. When liquid water boils, it gains further thermal energy, causing its particles to move faster and further apart, overcoming all forces of attraction and escaping as a gas.
Remember to link energy changes to the behaviour of particles during phase changes.
Example 3
Identify the change of state that occurs in each scenario: (a) Water vapour turns into liquid water droplets on a cold window. (b) A block of ice left out on a warm day turns into a puddle. (c) Solid carbon dioxide ('dry ice') produces a fog without forming a liquid.
Answer
(a) Condensation, (b) Melting, (c) Sublimation.
This example tests your ability to apply the definitions of changes of state to real-world situations.
Common mistakes
- ✗Confusing evaporation with boiling; evaporation occurs at the surface, while boiling occurs throughout the liquid at a specific temperature.
- ✗Not understanding that particles are always moving, even in solids (they vibrate about fixed positions).
- ✗Thinking that particles themselves change size or type during a change of state; only their arrangement, energy, and spacing change.
- ✗Incorrectly stating that particles in a liquid are fixed in position, similar to a solid.
- ✗Forgetting about sublimation as a direct change from solid to gas, bypassing the liquid state.
Exam tips
- ★Use clear diagrams to help visualise particle arrangements for each state of matter.
- ★Always remember to link changes of state to the gain or loss of thermal energy.
- ★Practise using the correct terminology for each change of state (e.g., 'melting' for ice, not 'dissolving').
- ★When describing particle movement, use precise terms like 'vibrate', 'slide past', and 'move randomly and rapidly'.
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