Solids, liquids and gases
Looking at states of matter
Everything you can see and feel is called matter. Scientists sort matter into three groups or states called ‘solids’, ‘liquids’ and ‘gases’. These states of matter behave in different ways. The ways they behave are called their properties.
Solids
Solids keep the same shape. Solids take up the same amount of space. Solids keep the same volume. Solids cannot be compressed (squashed) or poured.
Liquids
Liquids take the shape of the container they are in. Liquids can be poured. Liquids cannot be compressed. Liquids take up the same amount of space, whatever shape their container.
The volume of a liquid does not change.
Gases
Gases flow like liquids. They will fill any closed container they are in.
Gases are very easy to compress. The volume of a gas can change. Gases weigh very little. Generally, you cannot see or feel gases, but you can sometimes smell them, and you can feel air moving on your face.
Questions
1. What are the three states of matter?
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Answer: Solids, liquids and gases
2. Which state of matter can be compressed (squashed) easily?
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Answer: Gas
3. Which state of matter cannot be poured?
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Answer: Solid
4. List the properties of solids.
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Answer: Hard, fixed shape, fixed volume, not pourable, not compressible
5. Name a property of liquids that they do not share with solids.
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Answer: Liquid particles can slide past each other
6. Name a property of gases that they share with liquids.
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Answer: They can flow
7. Name a property of gases that they do not share with solids or liquids.
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Answer: Gas particles are free to move in all directions and fill their container
Scientists look at what matter does
Scientists try to explain what they see. Here are some examples of how matter behaves that scientists have tried to explain.
- You can smell food cooking in another room.
- Some substances get bigger when you heat them.
- Liquids, such as water, change to a gas when you heat them.
- Substances change from liquid to solid if you cool them.
Scientists think about why these things happen and try to come up with ideas to explain it. They form an hypothesis, which is a suggestion for an explanation. This hypothesis can then be tested by carrying out more investigations.
When an hypothesis has been tested and widely accepted as valid by other scientists, it is called a theory.
The best theory to explain how matter behaves uses the idea of particles. Particles are tiny portions of matter. This theory says that all matter is made up of tiny particles arranged in different ways.
Particle theory
All matter is made up of tiny particles that are much too small to see.
The particles are arranged differently in solids, liquids and gases.
Solids
In solids the particles are arranged in a fixed pattern. The particles are held together strongly and are tightly packed together. This is why solids have a fixed shape.
The particles in a solid can vibrate (make small movements) but they stay in the same place.
They stay in the same place.
Liquids
In liquids the particles touch each other. The particles are held together weakly. The particles can move past one another but they still touch each other. Liquids can change shape.
Gases
In gases the particles do not touch each other. They are a long way apart. The particles spread out by themselves. The particles can spread out to fill up the space they are in. Gases can change shape.
Explaining the properties
Matter can only flow (be poured) if the particles can move past one another.
Matter can only change volume if the particles in it can spread out or move closer together.
Solids
The particles in a solid are very close together. This makes it difficult for the volume of a solid to be made smaller. Solids have a fixed shape because attractive forces hold the particles together. These forces stop the particles from moving around. The particles can only vibrate. This means that a solid cannot flow.
Liquids
The volume of a liquid cannot be changed. The particles are very close together and cannot be squashed. The particles touch each other but they can move past each other. The attractive forces between the particles are weak enough to allow them to move but strong enough to hold them together.
Gases
Particles in a gas are a long way apart so they can move quickly in all directions. The particles can move easily because there are no attractive forces between them. This means that gas has no fixed shape or volume.
When you squash a gas, the particles move closer together and the gas takes up less space.
No particles?
A space where there are no particles at all is called a vacuum. A vacuum contains nothing.
Think like a scientist
Particle theory
Scientists observe the world around them and think carefully about what they see. Development of the particle theory was based on the observations that scientists made about how solids, liquids and gases behave.
Scientists saw that most solids cannot be compressed. Can you think of any solids that do not fit the rules of particle theory? Think about the properties of a sponge or a marshmallow. Can a sponge be compressed?
Questions
1 Use particle theory to explain how a sponge can be a solid, but it can also be compressed.
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A sponge is made of solid material, so the particles are closely packed. However, the sponge contains many air gaps between the particles. These air spaces can be compressed, making the whole sponge compressible even though the solid material itself is not.
2 How well does particle theory explain the properties of solids, liquids and gases?
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Particle theory clearly explains why solids keep their shape, why liquids flow but don’t compress, and why gases spread out and can be compressed. It also explains the differences in particle movement and forces between states of matter.
3 What are the strengths of the particle theory?
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It helps explain a wide range of physical behaviors like compression, shape, flow, and diffusion by considering how particles are arranged and move in each state of matter.
4 What are the weaknesses of the particle theory?
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It does not explain the forces between particles in detail or how temperature and energy affect them at the atomic level. Also, it cannot describe complex material behavior like elasticity or plasticity.