Cambridge IGCSE0620

Solids, liquids and gases

Chemistry 0620 Chapter Notes

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Solids, liquids and gasesDiffusion
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In this chapter

  1. 1The Three States of Matter
  2. 2The Kinetic Particle Theory
  3. 3Changes of State and Energy
  4. 4Interpreting Heating Curves
  5. 5Diffusion and Particle Motion

1. The Three States of Matter

Matter can exist in three main physical forms, or states: solid, liquid, and gas. The state of a substance depends on its temperature and pressure. Each state has distinct properties related to its shape, volume, and how easily it can be compressed.

Key term

State of Matter: The distinct physical form in which a substance can exist, such as solid, liquid, or gas.

Worked example 13 marks

A substance is stored in a sealed jar. It completely fills the jar, taking its shape. If the lid is removed, the substance quickly escapes into the room. Identify the state of the substance and explain your reasoning based on its properties.

  1. 1
    1. Identify the properties described: The substance has no fixed shape (it takes the shape of the jar) and no fixed volume (it escapes to fill the room).
  2. 2
    1. Compare these properties to the known properties of solids, liquids, and gases.
  3. 3
    1. A gas is the only state of matter that has neither a fixed shape nor a fixed volume.
  4. 4
    1. Conclusion: The substance is a gas.

Recap

  • Solids have a fixed shape and a fixed volume.
  • Liquids have a fixed volume but take the shape of their container.
  • Gases have no fixed shape and no fixed volume; they expand to fill any container.
  • Solids and liquids are considered virtually incompressible, while gases are easily compressed.

Quick check

  1. Which state of matter has a fixed volume but can be poured?1 mark
  2. Why can a gas be compressed easily but a solid cannot?2 marks

2. The Kinetic Particle Theory

The Kinetic Particle Theory explains the properties of solids, liquids, and gases by describing matter as being made of tiny, constantly moving particles (atoms, molecules, or ions). The energy and forces between these particles determine the state of the substance. In solids, particles are held in a fixed, regular pattern (a lattice) and vibrate in place. In liquids, particles are closely packed but can move and slide past one another. In gases, particles are far apart and move randomly and rapidly.

Key term

Kinetic Particle Theory: A model stating that all matter is composed of tiny particles which are in constant motion, and the energy of this motion determines the state of matter.

Examiner insight

Examiners award full marks for answers that clearly describe all three aspects for each state: particle arrangement, particle movement, and the forces between particles.

Common pitfall

Simply stating that particles 'move' is not specific enough. You must describe the type of movement: 'vibrate in fixed positions' for solids, 'slide past each other' for liquids, or 'move rapidly and randomly' for gases.

Worked example 14 marks

Use the kinetic particle theory to explain why a solid has a fixed shape, but a liquid does not.

  1. 1
    1. In a solid, the particles are held in fixed positions within a regular lattice structure.
  2. 2
    1. There are very strong forces of attraction between the particles, preventing them from moving out of their positions.
  3. 3
    1. They can only vibrate about their fixed positions, which is why the solid maintains a fixed shape.
  4. 4
    1. In a liquid, the forces between particles are weaker. The particles are still close together but are able to move and slide past one another.
  5. 5
    1. Because the particles can move, the liquid does not have a fixed shape and can flow to take the shape of its container.

Recap

  • All matter is made of tiny, moving particles.
  • Solid particles vibrate in fixed positions within a lattice.
  • Liquid particles are close together but can slide past each other.
  • Gas particles are far apart and move randomly and quickly.
  • The forces between particles are strongest in solids, weaker in liquids, and almost non-existent in gases.

Quick check

  1. Describe the arrangement and movement of particles in a gas.2 marks

3. Changes of State and Energy

Substances change state when they gain or lose thermal energy. This energy changes the motion of the particles and their ability to overcome the forces holding them together. Melting (solid to liquid) and boiling (liquid to gas) require energy input to overcome these forces. Freezing (liquid to solid) and condensing (gas to liquid) release energy as forces are re-established. Evaporation is the change from liquid to gas at the surface of a liquid at any temperature below the boiling point. Sublimation is the direct change from solid to gas.

Key term

Sublimation: The direct change of state from a solid to a gas, or gas to a solid, without passing through the liquid state.

Common pitfall

Confusing boiling with evaporation. Remember that boiling is a bulk phenomenon at a fixed temperature, while evaporation is a surface phenomenon at a range of temperatures.

Fun fact

Dry ice is solid carbon dioxide. At room temperature and pressure, it sublimes directly into CO₂ gas, creating a dense white fog without leaving any liquid residue. This makes it perfect for stage effects!

Worked example 13 marks

Explain, in terms of particles and energy, what happens when water boils to become steam.

  1. 1
    1. When water is heated to its boiling point (100°C), the particles gain kinetic energy and move faster.
  2. 2
    1. At the boiling point, the particles gain enough energy to overcome the forces of attraction holding them together in the liquid state.
  3. 3
    1. The particles break away from the liquid and escape to form a gas (steam).
  4. 4
    1. In the gaseous state, the particles are far apart and move randomly and rapidly.

Worked example 23 marks

Explain the difference between boiling and evaporation.

  1. 1
    1. Boiling occurs at a specific temperature, the boiling point, whereas evaporation can occur over a range of temperatures below the boiling point.
  2. 2
    1. Boiling occurs throughout the entire volume of the liquid, which is why bubbles form. Evaporation only occurs at the surface of the liquid.
  3. 3
    1. Boiling is a rapid process, while evaporation is a slower process.

Recap

  • Melting (S→L) and boiling (L→G) are processes that absorb energy.
  • Freezing (L→S) and condensing (G→L) are processes that release energy.
  • Sublimation is the direct conversion between solid and gas (e.g., solid CO₂ or iodine).
  • Boiling happens at a fixed temperature, but evaporation can happen at any temperature.
  • During a change of state, the temperature of the substance remains constant.

Quick check

  1. What is the name for the process when a gas turns directly into a solid?1 mark

4. Interpreting Heating Curves

A heating curve is a graph that plots the temperature of a substance against time as it is heated at a constant rate. The graph shows distinct sections. Sloping sections show the temperature of a single state (solid, liquid, or gas) increasing. Horizontal sections, or plateaus, show a change of state occurring (melting or boiling). During these plateaus, the temperature remains constant because the heat energy being supplied is used to break the forces between particles (potential energy), not to increase their speed (kinetic energy).

Key term

Melting Point: The specific temperature at which a solid turns into a liquid at a given pressure.

Examiner insight

Examiners frequently ask for an explanation of the flat regions on a heating curve. A good answer must state that the energy is used to overcome forces between particles, not to raise the temperature.

Common pitfall

Assuming that because heat is being supplied, the temperature must always be rising. During melting and boiling, the added energy changes the state, not the temperature.

Worked example 14 marks

The graph shows the heating curve for a substance X.(a) What is the melting point of substance X?(b) What is the physical state of X in the region BC?(c) Explain what is happening to the energy supplied in the region DE.

  1. 1

    (a) The first plateau, where the solid melts, occurs at 60°C. So, the melting point is 60°C.

  2. 2

    (b) In region AB, X is a solid. It melts in region BC. Therefore, in region BC, substance X exists as a mixture of solid and liquid.

  3. 3

    (c) In region DE, the substance is boiling (changing from liquid to gas). The temperature is constant. The energy supplied is being used to overcome the forces of attraction between the particles, allowing them to escape as a gas. This is often called latent heat of vaporisation.

Recap

  • A heating curve plots temperature against time for a substance being heated.
  • A rising slope on the curve indicates the temperature of a single state is increasing.
  • A flat plateau on the curve indicates a change of state is occurring at a constant temperature.
  • The first plateau corresponds to the melting point.
  • The second, higher temperature plateau corresponds to the boiling point.
  • Energy supplied during a change of state breaks intermolecular forces, not increase kinetic energy.

Quick check

  1. On a heating curve for water, at what temperature would the second plateau occur?1 mark
  2. What is happening to the particles in the first sloping section of a heating curve?2 marks

5. Diffusion and Particle Motion

Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration. It is a passive process that results from the random motion of particles. Diffusion occurs in liquids and gases because their particles are free to move. It does not occur in solids as the particles are fixed in a lattice. The rate of diffusion is affected by two main factors: temperature (higher temperature means more kinetic energy and faster diffusion) and particle mass (lighter particles move faster and diffuse more quickly than heavier particles at the same temperature).

Key term

Diffusion: The net movement of particles from an area of higher concentration to an area of lower concentration as a result of their random motion.

Examiner insight

When explaining diffusion experiments like the ammonia and HCl tube, marks are given for correctly identifying the lighter gas, linking lower mass to faster speed, and concluding where the product will form.

Common pitfall

Forgetting to compare particle masses when explaining different rates of diffusion. It's not enough to say one diffuses faster; you must state it's because its particles are lighter.

Worked example 14 marks

A crystal of purple potassium manganate(VII) is placed at the bottom of a beaker of water. After several hours, the entire beaker of water has turned a pale purple colour. Explain this observation using the kinetic particle theory.

  1. 1
    1. Both water and solid potassium manganate(VII) are made of particles.
  2. 2
    1. The potassium manganate(VII) particles dissolve and begin to move out from the crystal into the water.
  3. 3
    1. The water particles are also in constant, random motion.
  4. 4
    1. The purple potassium manganate(VII) particles and the colourless water particles collide and mix.
  5. 5
    1. This random movement and mixing causes the purple particles to spread out from an area of high concentration (around the crystal) to an area of low concentration (the rest of the water) until they are evenly distributed. This process is diffusion.

Worked example 23 marks

Ammonia gas (Mr = 17) and hydrogen chloride gas (Mr = 36.5) are released at opposite ends of a long glass tube. Where they meet, they form a white ring of solid ammonium chloride. Will the ring form closer to the ammonia end or the hydrogen chloride end? Explain your answer.

  1. 1
    1. The gases will diffuse along the tube due to the random motion of their particles.
  2. 2
    1. The rate of diffusion depends on the mass of the particles. Lighter particles diffuse faster.
  3. 3
    1. Ammonia has a lower relative molecular mass (Mr = 17) than hydrogen chloride (Mr = 36.5).
  4. 4
    1. Therefore, ammonia particles will move faster and travel further along the tube in the same amount of time.
  5. 5
    1. The white ring will form where the gases meet, which will be closer to the hydrogen chloride end.

Recap

  • Diffusion is the net movement of particles down a concentration gradient.
  • It is caused by the random motion of particles.
  • Diffusion happens in gases and liquids, but not in solids.
  • Diffusion is faster at higher temperatures.
  • Lighter particles diffuse faster than heavier particles.

Quick check

  1. Why does the smell of perfume spread across a room?2 marks
  2. Which gas will diffuse faster: carbon dioxide (Mr = 44) or methane (Mr = 16)?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Describe the arrangement, movement, and forces between particles in a liquid.3 marks
  2. Oxygen has a melting point of -219°C and a boiling point of -183°C. What is the physical state of oxygen at (i) -200°C and (ii) 25°C? Explain your reasoning.4 marks
  3. Use the kinetic particle theory to explain why you can smell a cake baking in the oven from another room.3 marks
  4. A student heats a solid substance from 20°C to 120°C. The substance has a melting point of 55°C and a boiling point of 105°C. Sketch a graph of temperature against time for this process, labelling the axes and the melting and boiling points.5 marks
  5. Name the change of state for each of the following transformations: (a) Solid to gas, (b) Gas to liquid, (c) Liquid to solid.3 marks
  6. Explain why a puddle of water disappears on a warm, sunny day, even if the temperature is well below 100°C.3 marks
  7. Two balloons are filled with gas at the same temperature and pressure. Balloon A is filled with helium (Mr = 4) and Balloon B is filled with carbon dioxide (Mr = 44). If tiny holes appear in both balloons, which balloon will deflate faster? Explain your answer in terms of diffusion.3 marks
  8. Look at the heating curve for substance P. Explain, in terms of energy and forces, what is happening in the section of the graph where the temperature remains constant at 80°C.3 marks
  9. Explain why gases are easily compressed whereas liquids are virtually incompressible.4 marks
  10. Iodine is a grey solid at room temperature. When gently heated, it turns into a purple vapour without melting. What is this process called? Describe what happens to the iodine particles during this change.3 marks

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