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6.2–6.3 States of Matter and Changes of State

GCSE Physics · AQA · Chapter 6: Molecules and Matter

A free GCSE Physics lesson for AQA (8463) from OBA School: solids, liquids and gases, the particle model, changes of state, melting and boiling points, heating curves. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the properties of solids, liquids and gases and the arrangement and motion of their particles
  • Explain why gases are much less dense than solids and liquids
  • Name the changes of state and explain why mass is conserved when a substance changes state
  • Define melting point and boiling point and use a temperature–time graph to find them
  • Explain the differences between evaporation and boiling

Key words

  • Change of state: A change between solid, liquid and gas. It is a physical change: no new substance is made and it can be reversed.
  • Melting point: The temperature at which a pure solid turns into a liquid. It is the same as the freezing point of the liquid.
  • Boiling point: The temperature at which a pure liquid boils (turns to gas throughout). It is the same as the condensing temperature of the gas.
  • Sublimation: A solid turning straight into a gas without becoming a liquid first, e.g. solid carbon dioxide (dry ice).
  • Evaporation: Particles escaping from the surface of a liquid that is below its boiling point.
  • Latent heat: The energy transferred when a substance changes state without changing temperature.

Solids, liquids and gases

  • Shape; Solid fixed; Liquid takes the shape of its container; Gas fills its whole container
  • Volume; Solid fixed; Liquid fixed; Gas changes – it can be squashed
  • Can it flow?; Solid no; Liquid yes; Gas yes
  • Density; Solid high; Liquid high; Gas very low

The particle model (kinetic theory) explains these properties. Everything is made of tiny particles that are always moving. The hotter a substance is, the faster its particles move.

  • Solid – particles held next to each other in fixed positions by strong forces. They can only vibrate, so a solid keeps its shape.
  • Liquid – particles are still in contact, but they can slide past each other at random. So a liquid flows and takes the shape of its container.
  • Gas – particles are far apart (on average) and move much faster, at random. So a gas spreads out to fill its container and can be compressed.
  • For the same substance, the particles have least energy in the solid and most energy in the gas.

Why gases have a low density

In a solid or a liquid the particles are packed closely together, so a lot of mass fits in a small volume. In a gas the same particles are spread far apart, with mostly empty space between them, so the same mass takes up a much bigger volume. Density = mass ÷ volume, so the gas has a much lower density.

Worked example: boiling a little water

1 g of liquid water takes up about 1 cm³. When it boils, it becomes about 1700 cm³ of steam at 100 °C.
The mass is still 1 g, but the volume is about 1700 times bigger.

Answer: steam is about 1700 times less dense than liquid water

Key fact: Solids are usually a little denser than their liquids. Water is unusual: ice is less dense than water (about 920 kg/m³ compared with 1000 kg/m³), which is why ice floats.

Changes of state and conservation of mass

A change of state is a physical change: no new substance is made, and if the change is reversed the substance gets back its original properties. Ice, water and steam are all water molecules.

When a substance changes state, the number of particles stays the same, so its mass is conserved. If 250 g of ice melts, it makes 250 g of water. If a kettle loses 80 g of water when it boils, 80 g of steam has been made – it has just left the kettle.

Exam tip: The particles themselves do not change size, shape or mass when a substance changes state. Only their arrangement, motion and energy change.

Melting points, boiling points and heating curves

While a pure substance is changing state, its temperature stays the same. The melting point is the temperature at which the solid melts (the liquid freezes at the same temperature). The boiling point is the temperature at which the liquid boils (the gas condenses at the same temperature). For pure water at normal air pressure these are 0 °C and 100 °C.

On the flat sections, energy is still being supplied, but it is used to break the bonds between particles instead of making them move faster. This energy is called latent heat (“hidden” heat), because the thermometer reading does not change. Melting is also called fusion.

Impurities change these temperatures. Salt dissolved in water lowers its freezing point, so roads gritted with salt do not ice over until the temperature is a few degrees below 0 °C.

Finding a melting point in the lab

Plot a graph of temperature against time. The flat section gives the melting point. A cooling curve (letting the hot liquid cool) has a flat section at the same temperature, where the liquid is freezing. For a liquid’s boiling point, heat the tube directly without the water bath. Safety: wear eye protection and take care with hot water.

Evaporation and boiling

  • Where?; Evaporation only at the surface of the liquid; Boiling throughout the liquid – bubbles of vapour form inside it
  • When?; Evaporation at any temperature below the boiling point; Boiling only at the boiling point
  • Speed; Evaporation slow; Boiling fast, while energy is supplied
  • Temperature of liquid; Evaporation can fall (the fastest particles escape); Boiling stays at the boiling point

Wet washing dries on a line because water evaporates at air temperature. A cold window mists up because water vapour in the air condenses on it.

Common mistakes

  • Saying the particles themselves expand, melt or get hotter. The particles stay the same; their spacing, motion and energy change.
  • Thinking that no energy is supplied on the flat part of a heating curve. Energy is still supplied – it changes the state instead of the temperature.
  • Mixing up evaporation and boiling. Evaporation is from the surface at any temperature; boiling happens throughout the liquid at the boiling point.
  • Writing that mass is lost when water boils away. The steam has the same mass as the water that boiled; it has just left the container.

Revision questions

Try these, then check your answers below each one.

Question 1. Which statement describes the particles in a gas? (Choose from: Far apart, moving quickly in random directions, Touching, in a regular pattern, vibrating, Touching, moving around each other at random, Far apart, vibrating about fixed positions.)

Answer: Far apart, moving quickly in random directions

Working: Gas particles are far apart on average and move fast in random directions. That is why gases fill their container, can be compressed and have a low density.

Question 2. True or false? When 50 g of ice melts, it makes less than 50 g of water because the water takes up less space.

Answer: False

Working: False. The number of particles stays the same, so the mass is conserved: 50 g of ice makes 50 g of water. (The volume does change slightly.)

Question 3. A kettle full of water has a mass of 1.62 kg. After it has boiled for a while, its mass is 1.47 kg. What mass of steam was produced, in grams?

Answer: 150 g

Working: 1.62 − 1.47 = 0.15 kg = 150 g. This mass of water has turned to steam and left the kettle; mass is conserved.

Related lessons

Continue in the full course

This lesson is section 6.2–6.3 (AGCPH0602) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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