Matter ICSE Class 8 Chemistry: Kinetic Molecular Theory and Conservation of Mass

What Matter is about

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Chemistry: your school sets and marks the exam. Matter sits in the Theme 1: Matter theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Postulates of the kinetic molecular theory

1. Matter is made of very small particles (molecules or atoms).
2. There are spaces between the particles (inter-particle space).
3. The particles attract each other (inter-particle force of attraction). The attraction is strongest when they are close.
4. The particles are in constant random motion; they have kinetic energy.
5. The kinetic energy increases with temperature: heated particles move faster.
6. Moving particles collide with each other and with the walls of their container.

These ideas explain diffusion (the smell of perfume spreading), and the properties of solids, liquids and gases.

Worked example

Use the kinetic theory to explain why the smell of incense spreads through a room.

Try it first, then show the solutionSolution

1. The particles of the scented vapour and of air are in constant random motion.
2. There are large spaces between gas particles, so the scent particles move into the spaces between air particles and spread out (diffusion).

Answer: Gas particles move randomly and spread into the spaces between air particles.

Solids, liquids and gases on the particle model

Feature	Solid	Liquid	Gas
Inter-particle space	least	more	very large
Inter-particle attraction	strongest	weaker	weakest (almost none)
Motion	vibrate about fixed positions	move about, slide past each other	move freely and fast in all directions
Collisions	—	frequent	very frequent, also with walls
Shape and volume	fixed shape and volume	fixed volume, no fixed shape	neither fixed

Gases can be compressed easily because of the large spaces between their particles; solids hardly at all.

Worked example

Why can a gas be compressed into a cylinder, but a solid cannot be squeezed noticeably?

Try it first, then show the solutionSolution

1. Gas particles have very large spaces between them, so they can be pushed closer.
2. Solid particles are already packed closely with almost no space between them.

Answer: Because of the large inter-particle spaces in a gas.

Change of state on the particle model

Heating (melting, boiling):

• particles gain kinetic energy and move faster;
• they overcome the forces of attraction, and the inter-particle space increases;
• solid → liquid → gas.

Cooling (condensation, freezing):

• particles lose kinetic energy and move slower;
• attraction pulls them closer; the inter-particle space decreases;
• gas → liquid → solid.

In a gas, faster particles collide more often and harder with the container walls; this is why heating a sealed gas raises its pressure.

Worked example

Explain, in terms of particles, what happens when ice melts.

Try it first, then show the solutionSolution

1. Heat gives the water particles in ice more kinetic energy, so they vibrate more strongly.
2. They partly overcome the forces of attraction holding them in fixed positions.
3. The particles can now move past one another with a little more space between them: the ice becomes water.

Answer: Particles gain energy, overcome attraction and become free to move: solid to liquid.

Law of conservation of mass

Statement: In a chemical reaction, mass is neither created nor destroyed. The total mass of the reactants equals the total mass of the products.

Explanation: in a reaction, atoms are only rearranged to form new substances; no atom is lost or gained, so the total mass stays the same.

Experiment: dissolve barium chloride and sodium sulphate in water in two separate tubes, place both in a flask, and weigh the whole set-up. Tip the tubes so the solutions mix: a white precipitate of barium sulphate forms. Weigh again: the mass is unchanged.
${\text{BaCl}}_2+{\text{Na}}_2{\text{SO}}_4\to {\text{BaSO}}_4+2\text{NaCl}$

Worked example

20.8 g of barium chloride reacts completely with 14.2 g of sodium sulphate, forming 23.3 g of barium sulphate. What mass of sodium chloride is formed?

Try it first, then show the solutionSolution

1. Total mass of reactants $=20.8+14.2=35.0$ g.
2. Mass of products must also be 35.0 g.
3. Sodium chloride $=35.0-23.3=11.7$ g.

Answer: 11.7 g

Reactions with gases from the air

Some reactions seem to break the law until the gases are counted.

• Burning magnesium: the ash (magnesium oxide) is heavier than the magnesium, because oxygen from the air has combined with it: $2\text{Mg}+{\text{O}}_2\to 2\text{MgO}$.
• Burning a candle or wood: the remains are lighter, because carbon dioxide and water vapour escape into the air.

In a closed container (no gas enters or leaves), the total mass stays the same. That is why the law applies to the whole system, including any atmospheric gases used or produced.

Worked example

2.4 g of magnesium burns in air and forms 4.0 g of magnesium oxide. Does this break the law of conservation of mass? What mass of oxygen was used?

Try it first, then show the solutionSolution

1. The product is heavier because oxygen from the air combined with the magnesium.
2. Mass of oxygen $=4.0-2.4=1.6$ g.
3. Reactants $2.4+1.6=4.0$ g $=$ products, so the law holds.

Answer: No; 1.6 g of oxygen was used.

Chemistry in real life: Matter

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Kinetic Molecular Theory and Change of State
• Law of Conservation of Mass

Matter formula sheet

Conservation of mass

$\text{mass of reactants}=\text{mass of products}$

Include gases from or to the air.

Barium chloride and sodium sulphate

$BaC{l}_2+N{a}_{2}S{O}_4\to BaS{O}_4+2NaCl$

White precipitate of barium sulphate.

Burning magnesium

$2Mg+O_2\to 2MgO$

Product heavier: oxygen added.

Matter: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Matter

The question patterns to expect, and the method that works for each.

Tricks to remember Matter

Short memory aids. Say each one aloud once, then use it on a question the same day.

• SAC

  Space, Attraction, Collisions: the three ideas used to explain every change of state.

  Use it when: Particle explanations.

• Atoms just swap partners

  In a reaction, atoms rearrange like dancers changing partners; nobody leaves the hall, so the mass stays the same.

  Use it when: Conservation of mass.

• Heavier ash? Air got in. Lighter? Gas got out.

  Always ask what gas joined or escaped.

  Use it when: Burning questions.

• Hot means fast

  Higher temperature means higher kinetic energy and faster particles.

  Use it when: Postulate questions.

Matter: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 3 + 3 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Matter

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying particles stop moving in solids

  Particles in a solid still vibrate about fixed positions.

  ✓Do this: Write 'vibrate about fixed positions'.

  Usually costs: 1 mark.

• ✗Mistake: Saying burning magnesium creates mass

  The extra mass is oxygen from the air.

  ✓Do this: Name the gas that joined or escaped.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Doing the conservation experiment in an open container with a gas formed

  Escaping gas makes the mass appear to fall.

  ✓Do this: Use a closed flask, or a reaction with no gas, such as BaCl₂ + Na₂SO₄.

  Usually costs: 1 mark.

• ✗Mistake: Unbalanced equation

  The law of conservation of mass requires equal numbers of each atom on both sides.

  Wrong working

  $BaC{l}_2+N{a}_{2}S{O}_4\to BaS{O}_4+NaCl$

  Right working

  $BaC{l}_2+N{a}_{2}S{O}_4\to BaS{O}_4+2NaCl$

  ✓Do this: Count atoms on both sides.

  Usually costs: 1 mark.

• ✗Mistake: Missing one of the three ideas in particle explanations

  The syllabus expects space, attraction and collisions.

  ✓Do this: Use all three words in your answer.

  Usually costs: 1 mark.

Practice questions on Matter

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Which state of matter has the largest inter-particle spaces?
   (a) solid
   (b) liquid
   (c) gas
   (d) all equal

   Hint

   Compressible.

   Answer

   (c)

2. Q2 · 1 mark · Very short answerExam level

   What happens to the kinetic energy of particles when a substance is heated?

   Hint

   Temperature.

   Answer

   It increases.

3. Q3 · 2 marks · Short answerExam level

   Why do liquids have a fixed volume but no fixed shape?

   Hint

   Attraction and motion.

   Answer

   Attraction keeps the particles close (fixed volume), but they can slide past one another, so the liquid takes the shape of its container.

4. Q4 · 2 marks · Short answerHarder exam level

   3 g of carbon burns completely in oxygen to give 11 g of carbon dioxide. What mass of oxygen was used?

   Hint

   Conservation of mass.

   Answer

   $11-3=8$ g.

5. Q5 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): When magnesium burns in air, the mass of the product is more than the mass of the magnesium.
   Reason (R): Oxygen from the air combines with the magnesium.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   Where does the extra mass come from?

   Answer

   (c)

6. Q6 · 2 marks · Short answerHarder exam level

   5.6 g of iron combines with 3.2 g of sulphur to form iron sulphide. What mass of iron sulphide forms?

   Hint

   Add the reactants.

   Answer

   $5.6+3.2=8.8$ g.

7. Q7 · 3 marks · Short answerHarder exam level

   Explain freezing of water in terms of particles.

   Hint

   Cooling.

   Answer

   Cooling lowers the particles' kinetic energy, so they move more slowly. Attraction pulls them closer, and they settle into fixed positions where they only vibrate: water becomes ice.

8. Q8 · 2 marks · Short answerHarder exam level

   Why does the smell of hot food reach us faster than the smell of cold food?

   Hint

   Temperature and particle speed.

   Answer

   At a higher temperature, the particles have more kinetic energy and move faster, so they diffuse through the air more quickly.

9. Q9 · 3 marks · Short answerHarder exam level

   25 g of calcium carbonate is heated and gives 14 g of calcium oxide and some carbon dioxide. Find the mass of carbon dioxide and say why the solid left weighs less.

   Hint

   Gas escapes.

   Answer

   $25-14=11$ g of carbon dioxide. It is a gas and escapes into the air, so the solid left is lighter.

10. Q10 · 2 marks · Short answerHarder exam level

    A sealed bottle of soda water is weighed, shaken and weighed again. Will the mass change? Explain.

    Hint

    Closed system.

    Answer

    No. The bottle is sealed, so no gas can escape; the total mass stays the same.

11. Q11 · 3 marks · Short answerHOTS / competency

    A student says 'a gas has no attraction between its particles, so it can be squeezed endlessly.' Correct the statement using the kinetic theory.

    Hint

    Particles themselves take up space.

    Answer

    Gas particles have very weak (not zero) attraction and large spaces between them, so a gas can be compressed a lot. But the particles themselves take up some space, so it cannot be squeezed endlessly; under very high pressure it may even turn into a liquid.

12. Q12 · 3 marks · Short answerHOTS / competency

    10 g of a solid compound is heated in an open crucible and the solid left weighs 7.5 g. A fresh 10 g sample is then heated in a sealed tube, and the total mass of the tube and contents does not change. Explain both results.

    Hint

    Something left the open crucible.

    Answer

    In the open crucible, a gas formed in the reaction (2.5 g) escaped, so the solid weighed less. In the closed tube nothing can escape, so the total mass of the system stays the same, as the law of conservation of mass says.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What does 'kinetic' mean?Show answer

   Related to motion.

2. 2.In which state is the inter-particle space largest?Show answer

   Gas.

3. 3.What happens to particles' kinetic energy when heated?Show answer

   It increases.

4. 4.State the law of conservation of mass.Show answer

   In a chemical reaction, total mass of reactants = total mass of products.

5. 5.Name the white precipitate formed when barium chloride and sodium sulphate react.Show answer

   Barium sulphate.

6. 6.Why is magnesium oxide heavier than the magnesium burnt?Show answer

   Oxygen from the air has combined with it.

7. 7.Which state can be compressed most easily?Show answer

   Gas.

8. 8.What is diffusion?Show answer

   The spreading of particles of one substance into another due to their random motion.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you state five postulates of the kinetic theory?
• ☐Can you explain melting and boiling using SAC?
• ☐Can you state the law of conservation of mass?
• ☐Can you describe the BaCl₂ + Na₂SO₄ experiment?
• ☐Can you explain heavier ash and lighter candle remains?
• ☐Is your equation balanced?

Matter: frequently asked questions