3.1 States of Matter

A free GCSE Chemistry lesson for AQA (8462) from OBA School: the particle model of solids, liquids and gases, changes of state, state symbols and the limits of the model. 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 arrangement and movement of particles in solids, liquids and gases
• Name the changes of state and link melting and boiling points to the strength of forces between particles
• Use melting and boiling point data to predict the state of a substance at a given temperature
• Write state symbols in chemical equations
• Explain the limitations of the simple particle model (Higher tier)

Key words

• Particle model: A model in which every substance is made of tiny particles, drawn as small solid spheres.
• Melting point: The temperature at which a solid turns into a liquid. Freezing happens at the same temperature.
• Boiling point: The temperature at which a liquid turns into a gas throughout the liquid. Condensing happens at the same temperature.
• Change of state: A physical change between solid, liquid and gas. No new substance is made, so it can be reversed.
• State symbol: A letter or letters in brackets after a formula: (s), (l), (g) or (aq).
• Aqueous: Dissolved in water. An aqueous solution has the state symbol (aq).

The three states of matter

In the particle model, every substance is made of tiny particles. The particles might be atoms, ions or molecules, depending on the type of bonding. How the particles are arranged and how they move decides the state.

• Solid; Arrangement close together, regular pattern; Movement vibrate about fixed positions; Shape and volume fixed shape and volume
• Liquid; Arrangement close together, random; Movement move around each other; Shape and volume flows, takes the shape of its container; fixed volume
• Gas; Arrangement far apart, random; Movement move quickly in all directions; Shape and volume fills its container; no fixed shape or volume; easily squashed

Gas pressure. Gas particles are always hitting the walls of their container. Each collision pushes on the wall, and all these pushes together make the pressure. If a sealed container of gas is heated, the particles move faster. They hit the walls more often and harder, so the pressure goes up.

Big idea: One particle on its own does not have the properties of the bulk substance. A single copper atom is not shiny or bendy, and a single water molecule is not wet. These bulk properties come from huge numbers of particles and the forces between them.

Changes of state and energy

To melt a solid or boil a liquid, energy must be transferred to the substance. The energy is used to overcome the forces of attraction between the particles. When a gas condenses or a liquid freezes, energy is transferred from the substance to the surroundings.

While a substance is melting or boiling, its temperature stays the same, even though it is still being heated. This shows up as a flat line on a heating curve.

A cooling curve has the same shape upside down: the temperature stays constant at the boiling point while the gas condenses, and at the melting point while the liquid freezes. Energy is released to the surroundings as forces form between the particles, which stops the temperature falling.

• evaporation; What happens liquid → gas at any temperature, only from the surface of the liquid; Energy taken in
• boiling; What happens liquid → gas at the boiling point, all through the liquid (bubbles form); Energy taken in
• sublimation; What happens solid → gas directly, without melting, e.g. solid carbon dioxide (dry ice); Energy taken in

All changes of state are physical changes: no new substance is made, and they can be reversed. Water molecules are H₂O in ice, in liquid water and in steam.

Key fact: The stronger the forces between the particles, the more energy is needed to separate them, so the higher the melting point and boiling point.

Predicting the state from melting and boiling points

Compare the temperature you are given with the melting point (mp) and boiling point (bp):

• below the melting point → solid
• between the melting point and the boiling point → liquid
• above the boiling point → gas

Worked example: state at room temperature (25 °C)

Substance X has mp = −7 °C and bp = 59 °C.
25 °C is above −7 °C, so X has melted. 25 °C is below 59 °C, so X has not boiled.
X is a liquid over a range of 59 − (−7) = 66 °C.

Answer: X is a liquid at 25 °C

Exam tip: Take care with negative numbers. −120 °C is colder than −20 °C. Sketch a number line if it helps.

State symbols

• (s); Meaning solid; Example NaCl(s), Mg(s)
• (l); Meaning liquid; Example H₂O(l), Br₂(l)
• (g); Meaning gas; Example O₂(g), CO₂(g)
• (aq); Meaning aqueous: dissolved in water; Example NaCl(aq), HCl(aq)

Worked example: adding state symbols

Magnesium ribbon is added to hydrochloric acid. Bubbles of hydrogen form and magnesium chloride solution is left.
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Answer: solid metal, acid and salt dissolved in water, gas given off

Watch out: Water itself is H₂O(l), not H₂O(aq). Only substances dissolved in water are (aq).

Limitations of the particle model (Higher tier)

The particle model is useful, but it is only a model. Its main limitations are:

• it shows no forces between the particles, so it cannot explain why different substances melt at different temperatures
• all particles are drawn as spheres, but many particles, such as molecules, are not spherical
• the spheres are drawn as solid, but atoms are mostly empty space
• it does not show the true size of particles or the real distances between them, especially in a gas

Exam tip: In a 3-mark limitations question, give three separate points. “It is not accurate” scores nothing on its own: say what it leaves out.

Common mistakes

• Saying particles in a solid do not move. They vibrate about fixed positions.
• Saying the particles themselves melt, expand or get bigger. The particles stay the same; only their arrangement, movement and spacing change.
• Saying the bonds inside a molecule break when it boils. For simple molecules only the weak forces between molecules are overcome.
• Writing H₂O(aq) for water. Pure water is (l).
• Reading the order of negative temperatures wrongly, e.g. thinking −50 °C is hotter than −10 °C.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the name of the change of state when a gas turns into a liquid? (Choose from: Evaporating, Condensing, Melting, Freezing.)

Answer: Condensing

Working: A gas turning into a liquid is condensing. It happens at the same temperature as boiling.

Question 2. Write the state symbol for a substance dissolved in water, e.g. sodium chloride solution.

Answer: aq

Working: NaCl(aq): (aq) means aqueous, dissolved in water.

Question 3. A liquid melts at −23 °C and boils at 77 °C. Over how many degrees is it a liquid?

Answer: 100 °C

Working: 77 − (−23) = 77 + 23 = 100 °C.

Related lessons

• Previous lesson: 2.6 The Transition Elements
• Next lesson: 3.2 Atoms and Ions
• In this chapter: 3.3 Ionic Bonding
• In this chapter: 3.4 Giant Ionic Structures
• In this chapter: 3.5 Covalent Bonding

Continue in the full course

This lesson is section 3.1 (AGCCH0301) of our free GCSE Chemistry 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 Chemistry course.