States of matter and the particle model

Ice keeps its shape, water takes the shape of its glass, and water vapour spreads through the available space. One model explains all three: matter is made of tiny particles that are always moving. Every explanation uses four features of those particles: how they are arranged, how they move, how far apart they are and how strongly they attract one another.

Solids, liquids and gases behave differently

Start with what you can observe. “Fixed” means the sample keeps its own shape or volume wherever you put it, under ordinary conditions.

Everyday evidence for each column:

• Solid. A steel key keeps the same shape and size in your pocket, in a drawer or in your hand.
• Liquid. Pour 200 cm³ of water from a tall measuring cylinder into a wide dish. Its shape changes to match the dish, but there are still 200 cm³ of water.
• Gas. Air let out of a small balloon into a large empty box spreads through the whole box. Air trapped in a bicycle pump can be pushed into a smaller space.

Check your understanding 1

Two syringes have their outlets sealed. One is full of water and the other is full of air. You push each plunger with the same force. Predict what happens in each.

Your two predictions using the observed compressibilities

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The plunger on the air syringe moves in a long way: the air is compressed into a smaller volume. The plunger on the water syringe hardly moves at all: liquid water is very difficult to compress.

Check your understanding 2

A sealed sample is very difficult to compress. A student concludes that it must be a solid. Is this enough evidence? What further observation would distinguish a solid from a liquid?

Your explanation and further observation

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No. Solids and liquids are both very difficult to compress, so the sample could be either. Put it in a differently shaped container: a solid keeps its own shape, while a liquid flows and takes the shape of the container while keeping its volume.

The particle model explains each property

All matter is made of tiny particles, such as atoms or molecules, that are always moving. The particles attract one another when they are a little distance apart and repel strongly if they are pushed into one another. The three states differ in the arrangement, motion, separation and forces of the same particles.

How far apart are gas particles? In the air around you, neighbouring particles are on average about ten particle widths apart. When 1 g of liquid water (about 1 cm³) boils into steam at 100 °C and atmospheric pressure, the steam fills about 1700 cm³. The particles do not grow; they spread out to about 12 times their distance apart in each direction.

Shape

• A solid has a fixed shape. Strong forces hold each particle in its place. The particles vibrate but stay in their positions, so the pattern, and the shape of the solid, stay the same.
• A liquid takes the shape of its container. The particles are not held in fixed positions, so they slide past one another. The liquid flows down into the lowest part of the container and spreads across it.
• A gas fills its container. The particles move quickly in all directions and the forces between them are too weak to hold them together, so they spread out until they fill the whole space.

Check your understanding 3

Dry sand can be poured from a bucket and takes the shape of the pile it forms. A student says this shows that sand is a liquid. Use the particle model to explain why sand is still a solid.

Your grain-versus-particle explanation

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Each grain of sand is a small piece of solid. Inside a grain, strong forces hold the particles in fixed positions, so every grain keeps its own shape. The pouring is whole grains sliding past one another, not particles sliding past one another as in a liquid.

Volume

• Solids and liquids have a fixed volume. Their particles stay very close together because the attractive forces keep them from spreading apart. The same number of particles takes up the same space.
• A gas has no fixed volume. Its particles are free to spread into any space that is available, so a gas takes the volume of whatever container holds it.

Worked example 1

Pouring a liquid into a different container

Problem

250 cm³ of water is poured from a tall measuring cylinder into a wide, flat dish. Explain why its shape changes but its volume does not.

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1. Pouring a liquid into a different container

   Method

   Explain shape and volume separately.

   Reason

   A liquid can rearrange its particles while their separation remains almost unchanged.

   Working

   Answer

   1. Shape: water particles are not held in fixed positions. They slide past one another, so the water flows and spreads across the bottom of the dish, taking its shape.
   2. Volume: the attractive forces keep the particles very close together, so they do not spread apart as they move.
   3. Answer: the same number of particles stays the same distance apart, so the water still occupies 250 cm³. Only the arrangement of the particles, and so the shape, has changed.

Compressibility

• Solids and liquids are very difficult to compress. Their particles are already very close together, so there is almost no space between them to remove. Pushing the particles closer makes them repel strongly.
• A gas is easily compressed. There are large spaces between its particles. Compressing a gas pushes the particles closer together and makes the spaces smaller. The particles themselves do not get smaller.

Worked example 2

Comparing air and water in syringes

Problem

A sealed syringe of air can be pushed in a long way; a sealed syringe of water hardly moves. Explain the difference using the particle model.

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1. Comparing air and water in syringes

   Method

   Compare particle separation in the two states.

   Reason

   Compression reduces the space between particles, rather than shrinking the particles.

   Working

   Answer

   1. Name the property: compressibility. Air is easily compressed; water is very difficult to compress.
   2. Separation in the gas: air particles are far apart, with large empty spaces between them.
   3. Separation in the liquid: water particles are already very close together, with almost no space between them.
   4. What compression does: pushing the plunger moves particles closer together. In air there is plenty of space to remove, so the volume decreases a lot. In water the particles are already close, and pushing them closer makes them repel strongly, so the volume hardly changes.
   5. Check: the particles themselves are not squashed in either syringe. Only the spaces between them change.

Flow and density

• Liquids and gases flow because their particles can move past one another. A solid cannot flow because its particles stay in fixed positions.
• A gas has a much lower density than a solid or a liquid. Its particles are far apart, so each cubic centimetre holds far fewer particles and so far less mass.

Particles in a solid do move: they vibrate about fixed positions all the time. What they cannot do is move out of their places.

Draw particle diagrams

Draw three equal-volume boxes and label them solid, liquid and gas of a model substance. Use the simple crystalline-solid model in the figure. These are equal spaces for comparison, not three stages of a sealed sample with the same mass. Then check your diagram against each point:

1. Draw every particle as a circle of the same size in all three boxes.
2. Solid: particles very close together in regular rows and columns, forming a block.
3. Liquid: particles still close, most of them touching, but in no regular pattern. They fill the bottom of the box up to a flat top surface.
4. Gas: far fewer particles in the same volume, far apart and spread through the whole box, from top to bottom.
5. Motion: small two-way arrows on some solid particles, short arrows on some liquid particles and a longer arrow on every gas particle, each pointing in a different direction.

Check your understanding 4

A student’s liquid box shows particles spread evenly through the whole box with gaps as wide as a particle, and their steam particles are drawn twice as big as the ice particles. Draw corrected liquid and steam boxes on paper, then describe the errors and corrections below. Check particle size, spacing and where particles are drawn.

Your drawing corrections and their physical reasons

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Liquid: the particles should be close together, mostly touching, and collected in the bottom of the box, not spread through the whole box. A liquid has a fixed volume and does not fill its container.

Steam: the particles should be the same size as in the ice and the water. Heating and changing state do not make particles bigger. Steam takes up more space because the same particles are much further apart.

Check your understanding 5 Compress a sample without losing particles

A sealed syringe contains 60 cm³ of air, then its plunger is pushed in until the air occupies 30 cm³. No air leaks out. Sketch particle diagrams before and after on paper, and explain which features must stay the same and which change. A learner draws half as many particles after compression. Correct that choice and compare the air’s density before and after.

Your sketch criteria, conserved particle count and density comparison

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Self-review the sketches: use the same number of same-sized particles before and after, because the syringe is sealed and compression does not shrink particles. Distribute them throughout the available gas space in both drawings, with smaller average gaps in the smaller volume. Keep the diagrams as a random gas arrangement, not fixed rows. Mass stays constant while volume halves, so density = mass/volume doubles. Drawing half as many particles would describe loss of air, which the prompt excludes; these diagrams follow one sealed sample, unlike the earlier equal-volume state comparison.

Common mistakes

• Saying the particles in a solid do not move. They vibrate about fixed positions all the time.
• Saying gas particles are bigger than liquid particles. The particles are the same; gas particles are much further apart.
• Saying particles expand when a substance is heated. They move faster, and their average separation can increase, but they do not change size.
• Saying the spaces between gas particles are filled with air. Air is itself made of particles; the spaces are empty.
• Saying there are no forces between liquid particles. Attractive forces keep them close, but not in fixed positions.
• Saying a gas has no volume. It has no fixed volume; it fills the volume of its container.
• Saying a liquid flows, so it must be easy to compress. Its particles can slide past one another but are already very close together.