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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. 1Properties of solids, liquids and gases
  2. 2Particles in solids, liquids and gases
  3. 3Changes of state
  4. 4Melting point, boiling point and state
  5. 5Temperature, pressure and the volume of a gas
  6. 6Changes of state and heating curves
  7. 7Gas pressure and volume in terms of particles
  8. 8What diffusion is
  9. 9Explaining diffusion with particles
  10. 10Diffusion in gases and liquids
  11. 11Relative molecular mass and rate of diffusion
  12. 12Choosing the fastest gas

1. Properties of solids, liquids and gases

A solid has a definite shape and a definite volume. It does not flow and it cannot be compressed.

A liquid has a definite volume but takes the shape of its container. It flows and it cannot be compressed.

A gas has no fixed shape or volume. It spreads everywhere and it can be compressed easily.

Examiner insight

“Many candidates did not answer this question at all, and those that did talked mainly about the particles in a gas and not in terms of a general property.”

Worked example 12 marks

Three states of matter are solid, liquid and gas.

State two general properties of a solid.

  1. 1

    has a definite shape / does not take the shape of the container [1]

  2. 2

    has a definite volume / fixed volume [1]

  3. 3

    Any two of four are credited. The others: does not flow, cannot be compressed.

2. Particles in solids, liquids and gases

Describe each state with three things: separation, arrangement and motion. Keep them apart. In a solid the particles are touching, in a regular fixed pattern, and vibrating.

In a liquid the particles are close together, in no particular arrangement, and sliding over each other. Do not write that they have large spaces between them.

In a gas the particles are far apart and have no particular arrangement.

Examiner insight

“A few candidates confused the arrangement and separation of a gas with other states of matter.” “Some candidates confused the arrangement of the particles in the gas with the separation of them.”

Worked example 14 marks

Complete Table 5.1 to show the motion and separation of particles in a solid and a liquid.

physical statemotion of particlesseparation of particles
solid
liquid
  1. 1

    solid, motion: vibrating [1]

  2. 2

    solid, separation: touching / (very) close together [1]

  3. 3

    liquid, motion: sliding / moving over each other [1]

  4. 4

    liquid, separation: touching / close together [1]

3. Changes of state

Melting is solid to liquid. Freezing is liquid to solid. Condensing is gas to liquid. Liquid to gas is boiling or evaporating. Read the question's own words. Liquid bromine left in a jar at room temperature has evaporated, not boiled.

Water shows these changes.

melting H₂O(s) → H₂O(l)

freezing H₂O(l) → H₂O(s)

condensing H₂O(g) → H₂O(l)

boiling H₂O(l) → H₂O(g)

Examiner insight

“The commonest error for the change of state from gas to liquid was ‘boiling’ and the commonest error for the change of state from solid sulfur to gaseous sulfur was ‘burning’.”

Worked example 12 marks

Oxygen can exist in different states of matter.

Name the change of state for each description.

solid to liquid

gas to liquid

  1. 1

    solid to liquid: melting [1]

  2. 2

    gas to liquid: condensing / condensation [1]

4. Melting point, boiling point and state

A substance is a solid below the melting point. It is a liquid between the melting point and boiling point. It is a gas above the boiling point.

To deduce a state, compare the temperature with the melting point and boiling point. Then say which one it is lower or higher than.

Examiner insight

“Some candidates could state the correct physical state. They got confused with the explanation and would benefit from more practice of this type of question.” “Candidates should recall that a substance will be a liquid at a temperature between the melting and boiling points.”

Worked example 12 marks

Substance X has a melting point of 113 °C and a boiling point of 445 °C.

Deduce the physical state of substance X at –5 °C.

Give a reason for your answer.

  1. 1

    physical state: solid [1]

  2. 2

    –5 °C is below the melting point of +113 °C [1]

5. Temperature, pressure and the volume of a gas

At constant temperature, increasing the pressure makes the volume decrease. Pushing in the plunger of a sealed gas syringe makes the pressure increase.

Heating a gas at constant pressure makes its volume increase. The particles move further apart.

Reducing the temperature moves the particles closer together. Reducing the pressure moves them further apart.

Examiner insight

“Other candidates wrote inexact answers such as “the volume goes down with pressure” (instead of the volume goes down as pressure increases).” “A considerable proportion of those candidates who suggested that the particles moved further apart thought that the volume decreased.”

Worked example 11 mark

A gas syringe contains 80 cm³ of nitrogen gas.

State how the volume of nitrogen gas changes, if at all, when the pressure is increased at a constant temperature.

  1. 1

    volume decreases [1]

6. Changes of state and heating curves

When a solid melts, the atoms move further apart. They gain thermal energy from the surroundings. Evaporating also takes in thermal energy: it is endothermic.

On a heating curve the temperature rises, goes flat while the substance melts, rises again, and goes flat while it boils. On a flat section the temperature remains constant.

The temperature at which a liquid boils is also the temperature at which its gas condenses. A cooling curve is flat at that same temperature.

A flat part of a cooling curve is a change of state, or the substance reaching room temperature.

Examiner insight

“It is expected that all candidates know that temperature remains constant during a physical change such as boiling for example.” “Candidates should recognise that the temperature at which the liquid boils is also the temperature at which its gas condenses, so the ‘temperature – time’ curve should be flat and equal during these two state changes.”

Worked example 11 mark

A solid lump of tin is heated until it melts.

Which statements describe what happens when solid tin melts?

1 The atoms of tin move further apart.

2 The atoms of tin lose thermal energy to the surroundings.

3 The atoms of tin gain thermal energy from the surroundings.

4 The temperature of the tin increases.

A 1 and 2 B 1 and 3 C 2 and 4 D 3 and 4

  1. 1

    B: statements 1 and 3. The atoms move further apart and gain thermal energy from the surroundings. [1]

  2. 2

    Statement 4 is false: the temperature stays constant while the tin melts.

7. Gas pressure and volume in terms of particles

Gas pressure is caused by collisions between the gas particles and the sides of the container. It is not caused by particles colliding with each other.

Heating a gas makes the average velocity of its particles increase. Cooling a gas lowers their average kinetic energy, so the pressure falls.

Gas particles collide least frequently at low pressure and low temperature. A change in pressure does not change the particle speed.

Examiner insight

“Most candidates did not recognise that an increased pressure would not change the particle speed.”

Worked example 11 mark

A gas is placed in a sealed container. The gas has a pressure of one atmosphere and a temperature of 50 °C. It is then heated to 100 °C.

Which row describes the cause of the pressure of the gas and the effect of increasing the temperature of the gas?

the cause of gas pressurethe effect of increased temperature on the gas
Acollisions between gas particles and other gas particlescollisions between gas particles and other gas particles become less frequent
Bcollisions between gas particles and other gas particlesthe average velocity of the gas particles increases
Ccollisions between gas particles and the sides of the containercollisions between gas particles and other gas particles become less frequent
Dcollisions between gas particles and the sides of the containerthe average velocity of the gas particles increases
  1. 1

    D: collisions with the sides of the container cause the pressure, and the average velocity of the particles increases. [1]

Recap

  • Solid: definite shape and volume. Liquid: definite volume, takes the shape of its container. Gas: fills its container and can be compressed.
  • Solid: touching, regular fixed pattern, vibrating. Liquid: close, irregular, sliding over each other. Gas: far apart, irregular.
  • Melting: solid to liquid. Freezing: liquid to solid. Condensing: gas to liquid. Boiling or evaporating: liquid to gas.
  • Below the melting point: solid. Between melting and boiling points: liquid. Compare the temperature and name the point.
  • More pressure: smaller volume. More heat at constant pressure: larger volume.
  • Extended: on a heating curve the temperature stays constant while a substance melts or boils.
  • Extended: gas pressure comes from collisions with the sides of the container. Heating raises the average velocity.

8. What diffusion is

Diffusion is the overall movement of particles from an area of higher concentration to an area of lower concentration.

It carries on until the particles are spread evenly. Always name the process: diffusion.

Examiner insight

“A significant number of candidates suggested that the bulk movement of particles was from low to high concentrations.”

Worked example 11 mark

A gas jar of air sits above a gas jar of dark brown bromine vapour, with a glass plate between them. The glass plate is removed and the jars allowed to stand for 3 hours. Both jars then contain light brown bromine vapour.

Which statement explains the change in the gas jars after 3 hours? A Bromine has a greater density than air. B Bromine particles move from an area of high concentration to an area of low concentration. C Bromine particles move faster than the particles in air. D The particles in air are heavier than those in bromine.

  1. 1

    B: the particles move from high concentration to low concentration. [1]

9. Explaining diffusion with particles

In kinetic particle theory, the particles of a gas or a liquid move randomly.

This random movement spreads the particles out. The overall movement is from high to low concentration.

Give both ideas and name the particles. 'Particles move' is not enough: say the movement is random.

Examiner insight

“Most candidates did not refer to particles in their answers, which is key to describing the process of diffusion.” “Some candidates were able to either describe the movement of particles down a concentration gradient or the random movement of particles, but very few candidates were able to do both.”

Worked example 13 marks

A bottle of concentrated ammonia solution is opened at the front of a room. Soon a student at the back can smell ammonia. Describe and explain how the smell reaches the student.

  1. 1

    The process is diffusion. [1]

  2. 2

    Particles of ammonia move randomly. [1]

  3. 3

    They move from higher concentration at the bottle to lower concentration. [1]

10. Diffusion in gases and liquids

A liquid that evaporates forms a vapour: particles escape from the liquid. The particles of the vapour then spread out by diffusion.

Particles of a dissolved substance also diffuse through a liquid. The particles of a dye placed in water spread throughout the liquid.

In each case the particles move randomly, from higher concentration to lower concentration.

Examiner insight

“Very few candidates wrote about the particles escaping from the liquid to form a vapour.”

Worked example 11 mark

Liquid bromine is placed in a sealed gas jar.

After one hour at room temperature, the liquid bromine has ...1... .

The gas jar becomes completely full of bromine vapour by a process called ...2... .

Which row shows the correct words to complete gaps 1 and 2?

12
Aevaporateddiffusion
Bevaporateddistillation
Cboileddiffusion
Dboileddistillation
  1. 1

    A: the liquid bromine has evaporated, and the vapour fills the jar by diffusion. [1]

11. Relative molecular mass and rate of diffusion

A gas with a lower relative molecular mass (Mr) diffuses faster.

Its molecules move more quickly than the molecules of a gas with a higher Mr. Give the reason as Mr, not as 'lighter' or 'less dense'.

Examiner insight

“Better performing candidates were able to relate the higher rate of diffusion of but-1-ene to its lower Mr.” “Weaker responses simply referred to but-1-ene being ‘lighter’ or ‘less dense’ or having a ‘lower Ar’.”

Worked example 11 mark

Cotton wool soaked in concentrated hydrochloric acid is at one end of a tube. Cotton wool soaked in concentrated ammonia is at the other end. Damp universal indicator paper is midway between them.

The concentrated hydrochloric acid gives off hydrogen chloride gas, HCl, which dissolves in water to form an acid.

The concentrated ammonia gives off ammonia gas which dissolves in water to form an alkali.

Which row describes and explains the first change in the universal indicator paper?

colour of universal indicator paperreason
Abluemolecules of ammonia move more quickly than molecules of hydrogen chloride
Bredmolecules of ammonia move more quickly than molecules of hydrogen chloride
Cbluemolecules of hydrogen chloride move more quickly than molecules of ammonia
Dredmolecules of hydrogen chloride move more quickly than molecules of ammonia
  1. 1

    Mr: NH₃ = 14 + 3 × 1 = 17, HCl = 1 + 35.5 = 36.5. Ammonia has the lower Mr.

  2. 2

    A: blue, because molecules of ammonia move more quickly than molecules of hydrogen chloride. [1]

12. Choosing the fastest gas

To find the gas that diffuses fastest, add up the relative atomic masses in each formula to get Mr.

The gas with the lowest Mr diffuses fastest. The number of atoms in a molecule does not decide it.

Nitrogen, oxygen and fluorine exist as diatomic molecules: N₂, O₂ and F₂. Neon exists as single atoms.

Examiner insight

“Most candidates opted for CH4 as the gas which diffuses quickest. Many thought it was CO, presumably because it has the fewest atoms in a molecule.” “Candidates found this question challenging with the majority choosing fluorine, oxygen or nitrogen. Few candidates realised that these gases exist as diatomic molecules and thus chose neon as the gas with the highest rate of diffusion.”

Worked example 12 marks

Relative atomic masses: H = 1, C = 12, O = 16. Which of CH₄, CO and CO₂ diffuses fastest? Explain your answer.

  1. 1

    Mr: CH₄ = 12 + 4 × 1 = 16, CO = 12 + 16 = 28, CO₂ = 12 + 2 × 16 = 44. [1]

  2. 2

    CH₄ diffuses fastest because it has the lowest relative molecular mass. [1]

Recap

  • Diffusion is the overall movement of particles from higher to lower concentration.
  • The particles move randomly. Name the process and refer to particles.
  • A liquid evaporates to form a vapour, and the vapour then diffuses.
  • Extended: a gas with a lower Mr diffuses faster.
  • Extended: work out Mr from the formula of each gas as it exists, such as N₂. The lowest is fastest, whatever the number of atoms.

End-of-chapter exercise

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

  1. A substance has a definite volume but takes the shape of its container. State whether it is a solid, a liquid or a gas.1 mark
  2. Describe the arrangement and the motion of the particles in a solid.2 marks
  3. State the separation of the particles in a liquid and in a gas.2 marks
  4. Name the change of state when (a) ice turns into water and (b) steam turns into liquid water.2 marks
  5. Substance Y has a melting point of –39 °C and a boiling point of 357 °C. Deduce its physical state at 20 °C and at –50 °C. Give a reason for each answer.4 marks
  6. The plunger of a sealed gas syringe is pushed in. The temperature stays constant. State what happens to the volume and to the pressure of the gas.2 marks
  7. A balloon of helium is warmed. The pressure on it does not change. State what happens to the volume of the helium and to the separation of the helium atoms.2 marks
  8. A lump of solid lead is heated steadily. It starts to melt and the heating continues until all of it is liquid. State what happens to the temperature of the lead while it melts, and state two things that happen to the lead atoms while it melts.3 marks
  9. Substance Z is a gas at 140 °C. A sample is left to cool in a room at 22 °C. Table 9.1 gives its temperature each minute. (a) Name the change of state between 1 and 3 minutes. [1] (b) State the boiling point of Z. [1] (c) Deduce the physical state of Z at 4 minutes. Give a reason for your answer. [2] (d) Solid Z is later heated from 30 °C to 140 °C. State the two temperatures at which its heating curve is flat, and name the change of state at each one. [2] (e) A student says, ‘The curve is flat three times, so Z changes state three times.’ Explain why the student is wrong. [1] | time / min | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | | temperature / °C | 140 | 118 | 118 | 118 | 90 | 52 | 52 | 52 | 34 | 22 | 22 |7 marks
  10. A sealed steel cylinder and a balloon both contain air at 15 °C. The cylinder cannot change size. The balloon stretches freely, so the pressure of the air inside it does not change. Both are left in the sun and warm up to 40 °C. (a) State what happens, if anything, to the volume and to the pressure of the air in the cylinder. [2] (b) Explain your answer for the pressure in terms of particles. [2] (c) The volume of the balloon increases. State two things that happen to the particles of air in the balloon as it warms. [2] (d) The warm balloon is then squeezed, so the pressure of the air inside it increases. Its temperature stays at 40 °C. A student writes, ‘The pressure increases because the particles move faster and collide with each other more often.’ Identify the two mistakes in this sentence. [2]8 marks
  11. The smell of cooking spreads through a house. Name the process.1 mark
  12. State the direction of the overall movement of particles in diffusion.1 mark
  13. A drop of blue dye is placed in a beaker of still water. After an hour the water is evenly blue. Name the process and say what the dye particles do.2 marks
  14. Perfume is sprayed in one corner of a room. After a minute it is smelt across the room. Describe and explain this in terms of particles.3 marks
  15. Liquid bromine is placed in a sealed gas jar at room temperature. After some hours the jar is full of brown vapour. Name the two processes, in order, that took place.2 marks
  16. A gas jar of air sits above a gas jar of brown bromine vapour. A glass plate between them is removed. After 3 hours both jars are the same colour, although bromine has a greater density than air. Explain why.3 marks
  17. Relative atomic masses: H = 1, C = 12, N = 14, O = 16. Which of NH₃, N₂ and CO₂ diffuses fastest? Calculate the Mr of each and give a reason.3 marks
  18. Relative atomic masses: H = 1, C = 12. Propene, C₃H₆, and propane, C₃H₈, are both gases. Which diffuses faster? Calculate both Mr values and give a reason.3 marks
  19. Relative atomic masses: N = 14, O = 16, F = 19, Ne = 20. Nitrogen, oxygen, fluorine and neon are released together at one end of a long tube. A student uses the relative atomic masses to predict that nitrogen reaches the other end first and neon reaches it last. (a) Explain the student's mistake. (b) Give the correct order in which the gases reach the other end. Show your working.5 marks
  20. Relative atomic masses: H = 1, C = 12, N = 14, Cl = 35.5. Cotton wool soaked in concentrated hydrochloric acid is put in the left end of a long glass tube. At the same time, cotton wool soaked in a concentrated solution of methylamine is put in the right end. A strip of damp universal indicator paper lies along the whole tube. Hydrogen chloride gas, HCl, turns the paper red. Methylamine gas, CH₃NH₂, turns the paper blue. The red part and the blue part of the paper meet at a point P. (a) Calculate the Mr of each gas. (b) State which end of the tube P is nearer to. Explain your answer. (c) The experiment is repeated with ammonia, NH₃, in place of methylamine. Ammonia also turns the paper blue. State and explain how the position of P changes.6 marks

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