Heat Transfer ICSE Class 8: Evaporation, Boiling and Thermal Expansion

What Heat Transfer is about

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Physics: your school sets and marks the exam. Heat Transfer sits in the Theme 6: Heat Transfer theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Evaporation and boiling

Evaporation	Boiling
takes place at all temperatures	takes place at a fixed temperature (boiling point)
only from the surface	throughout the liquid (bubbles form inside)
slow and silent	rapid, with bubbles
causes cooling	needs a continuous supply of heat

Evaporation is faster when:

• the temperature is higher,
• the exposed surface area is larger,
• the air is moving (wind),
• the air is dry (low humidity).

Evaporation cools the surface: the faster-moving particles escape and take heat with them. Sweating cools our body; water in an earthen pot stays cool.

Worked example

Why do wet clothes dry faster on a sunny, windy day?

Try it first, then show the solutionSolution

1. Sunshine raises the temperature, so the water evaporates faster.
2. Wind carries away the water vapour near the clothes, so more water can evaporate.

Answer: Higher temperature and wind both speed up evaporation.

Why substances expand on heating

When a substance is heated, its particles vibrate or move more vigorously and push one another farther apart. So the substance takes up more space: it expands. On cooling, the particles slow down and come closer: it contracts.

The mass does not change on expansion; only the size (and so the density) changes. Since the volume increases while the mass stays the same, the density decreases on heating.

Worked example

A metal ball just passes through a metal ring. After the ball is heated, it does not pass through. Explain. What happens when it cools?

Try it first, then show the solutionSolution

1. Heating makes the ball's particles move more and spread out, so the ball expands.
2. On cooling it contracts back to its original size and passes through again.

Answer: The ball expands on heating and contracts on cooling.

Linear expansion

Linear expansion is the increase in length of a solid on heating. It is most noticeable in long objects: railway lines, bridges, overhead wires, pipes.

For a given material, the increase in length is (approximately) proportional to:

• the original length (a rail twice as long expands twice as much),
• the rise in temperature (twice the rise, twice the expansion).

Different materials expand by different amounts: for example a 10 m steel rail grows by about 1.2 mm for a 10 °C rise, and a 10 m copper wire by about 1.7 mm.

Worked example

A 10 m steel rail expands by 1.2 mm when its temperature rises by 10 °C. How much will a 20 m rail expand for a 30 °C rise?

Try it first, then show the solutionSolution

1. Twice the length: $2\times 1.2=2.4$ mm for a 10 °C rise.
2. Three times the rise: $3\times 2.4=7.2$ mm.

Answer: 7.2 mm

Superficial and volume expansion

• Superficial expansion: increase in area of a sheet or plate on heating. A hole in a metal plate also gets larger on heating, as if the hole were made of the metal.
• Volume (cubical) expansion: increase in volume on heating. Solids, liquids and gases all show it.

Liquids have no fixed shape, so for them only volume expansion is meaningful. A liquid in a thermometer rises in the narrow tube because it expands more than the glass bulb.

Worked example

A metal plate has a circular hole. When the plate is heated, does the hole get bigger or smaller?

Try it first, then show the solutionSolution

1. Every part of the plate expands, including the metal around the hole.
2. The plate behaves as if the hole were filled with the same metal, so the hole gets bigger.

Answer: The hole gets bigger.

Comparing solids, liquids and gases

For the same rise in temperature:
$\text{gases expand most}>\text{liquids}>\text{solids expand least}$.

In solids the particles are held most strongly; in gases the forces between particles are very weak.

Simple demonstrations:

• Solid: the ball-and-ring experiment.
• Liquid: a flask of coloured water with a narrow tube through its cork; warm the flask and the level in the tube rises.
• Gas: a balloon stretched over the mouth of an empty bottle swells when the bottle is placed in hot water.

Worked example

A flask full of coloured water has a narrow glass tube through its cork. When the flask is put in hot water, the level in the tube first falls slightly and then rises. Explain.

Try it first, then show the solutionSolution

1. The glass flask is heated first and expands a little, so the level dips.
2. Then the water warms up; liquids expand more than solids, so the water level rises well above its starting point.

Answer: Glass expands first (small dip), then the water expands more (rise).

Applications of expansion

Expansion can be a problem to allow for, or a useful effect:

• Gaps are left between railway rails and concrete slabs, and rollers are put under bridge ends, so that expansion in summer does not bend or crack them.
• Overhead wires are left slightly loose (sagging) in summer so that they do not snap when they contract in winter.
• A tight metal lid on a glass jar can be loosened by pouring hot water over it: the metal expands more than the glass.
• A bimetallic strip (two metals, such as brass and iron, joined together) bends on heating because brass expands more than iron; it is used in thermostats and fire alarms.
• Thermometers use the expansion of a liquid in a narrow tube.
• Hot-air balloons rise because heated air expands and becomes less dense.

Worked example

A straight brass–iron bimetallic strip is heated. Which way does it bend, and why?

Try it first, then show the solutionSolution

1. Brass expands more than iron for the same rise in temperature.
2. The brass side becomes longer, so the strip curves with brass on the outside (convex side) and iron on the inside.

Answer: It bends with brass on the outside.

Physics in real life: Heat Transfer

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

• Evaporation and Boiling
• Thermal Expansion of Solids, Liquids and Gases

Heat Transfer formula sheet

Order of expansion

$\text{gas}>\text{liquid}>\text{solid}$

For the same temperature rise.

Linear expansion (proportion)

$\Delta L\propto L\times \Delta T$

Same material: double the length or the rise, double the expansion.

Density on heating

$\rho =\frac{m}{V}$

Mass fixed, volume up, density down.

Heat Transfer: diagrams to practise

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

How the school exam asks Heat Transfer

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

Tricks to remember Heat Transfer

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

• Surface vs everywhere

  Evaporation = surface, any temperature. Boiling = everywhere, one temperature.

  Use it when: Boiling versus evaporation.

• L-A-V

  Linear (length), Area (superficial), Volume: one, two, three dimensions.

  Use it when: Types of expansion.

• Gas goes most

  Gas > liquid > solid: the freer the particles, the more they spread on heating.

  Use it when: Comparing expansivity.

• Brass bulges

  In a brass–iron strip, brass is on the bulging (convex) side when hot.

  Use it when: Bimetallic strip questions.

Heat Transfer: 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 Heat Transfer

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

• ✗Mistake: Saying evaporation needs 100 °C

  Evaporation happens at all temperatures; 100 °C is the boiling point of water.

  ✓Do this: Link evaporation to the surface, not to a fixed temperature.

  Usually costs: 1 mark.

• ✗Mistake: Saying mass increases on expansion

  Only the size changes; mass stays the same and density decreases.

  ✓Do this: Write 'volume increases, mass unchanged'.

  Usually costs: 1 mark.

• ✗Mistake: Wrong side in a bimetallic strip

  The metal that expands more ends up on the outside of the curve.

  Wrong working

  Heated brass–iron strip bends with iron outside

  Right working

  Brass outside

  ✓Do this: More expansion = longer = outside.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Thinking a hole shrinks on heating

  All parts of the plate expand, so the hole grows.

  ✓Do this: Imagine the hole filled with the same metal.

  Usually costs: 1 mark.

• ✗Mistake: Reversing the order of expansivity

  Gases expand most, solids least.

  ✓Do this: Remember 'the freer the particles, the more they spread'.

  Usually costs: 1 mark.

Practice questions on Heat Transfer

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

   For the same rise in temperature, which expands the most?
   (a) iron
   (b) water
   (c) air
   (d) glass

   Hint

   Gases expand most.

   Answer

   (c)

2. Q2 · 1 mark · Very short answerExam level

   What is the increase in length of a solid on heating called?

   Hint

   One dimension.

   Answer

   Linear expansion.

3. Q3 · 2 marks · Short answerExam level

   State two factors that increase the rate of evaporation.

   Hint

   Weather and container.

   Answer

   Higher temperature and larger surface area (also wind and low humidity).

4. Q4 · 2 marks · Short answerHarder exam level

   Why do wet clothes dry faster on a windy day than on a still day at the same temperature?

   Hint

   Vapour near the clothes.

   Answer

   Wind removes the water vapour near the clothes, so the air stays drier there and more water can evaporate.

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

   Assertion (A): Sweating helps to cool the body.
   Reason (R): Evaporation takes heat from the surface from which it occurs.
   (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

   Evaporation cools.

   Answer

   (c)

6. Q6 · 3 marks · Short answerHarder exam level

   A 10 m steel rail expands 1.2 mm for a 10 °C rise. How much does a 15 m rail expand for a 20 °C rise?

   Hint

   Scale by length and by temperature rise.

   Answer

   $1.2\times 1.5\times 2=3.6$ mm.

7. Q7 · 2 marks · Short answerHarder exam level

   Why is a liquid such as alcohol or mercury used in a thermometer?

   Hint

   Expansion in a narrow tube.

   Answer

   Liquids expand noticeably and evenly on heating, so the level in the narrow tube rises steadily and can be read on a scale.

8. Q8 · 2 marks · Short answerHarder exam level

   A metal plate with a hole in it is heated. What happens to (i) the plate (ii) the hole? Name the type of expansion.

   Hint

   Area.

   Answer

   (i) The plate's area increases (ii) the hole also gets bigger. This is superficial expansion.

9. Q9 · 3 marks · Short answerHarder exam level

   Describe an experiment to show that air expands on heating.

   Hint

   Balloon and bottle.

   Answer

   Stretch a deflated balloon over the mouth of an empty bottle. Put the bottle in a bowl of hot water: the balloon swells as the air inside expands. In cold water it shrinks back.

10. Q10 · 3 marks · Short answerHarder exam level

    Why does a thick glass tumbler sometimes crack when boiling water is poured into it?

    Hint

    Inner and outer layers.

    Answer

    Glass conducts heat poorly, so the inner layer heats and expands while the outer layer is still cool. The unequal expansion strains the glass and it may crack.

11. Q11 · 3 marks · Short answerHOTS / competency

    In a fire alarm, a brass–iron bimetallic strip must touch a contact when heated. On which side of the strip (brass side or iron side) should the contact be placed? Explain.

    Hint

    Which way does it bend?

    Answer

    On the iron side. When heated, brass expands more, so the strip curves with brass outside and bends towards the iron side, touching a contact there.

12. Q12 · 3 marks · Short answerHOTS / competency

    A 5 m concrete slab expands by about 0.6 mm for every 10 °C rise. What is the least gap to leave between slabs if the temperature may rise by 30 °C? What could happen with no gap?

    Hint

    Three times the rise.

    Answer

    $3\times 0.6=1.8$ mm. Without a gap, the slabs would push against each other on hot days and could crack or buckle.

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.Where does evaporation take place?Show answer

   At the surface of the liquid.

2. 2.Does evaporation cause heating or cooling?Show answer

   Cooling.

3. 3.Define linear expansion.Show answer

   Increase in length on heating.

4. 4.Define superficial expansion.Show answer

   Increase in area on heating.

5. 5.Which expands most on heating: solid, liquid or gas?Show answer

   Gas.

6. 6.Why are gaps left between rails?Show answer

   To allow for expansion in hot weather.

7. 7.Which metal expands more in a brass–iron strip?Show answer

   Brass.

8. 8.What happens to density on heating?Show answer

   It decreases.

Quick revision checklist

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

• ☐Can you give three differences between boiling and evaporation?
• ☐Can you name four factors that affect evaporation?
• ☐Can you define the three kinds of expansion?
• ☐Can you arrange gas, liquid and solid by expansion?
• ☐Can you explain gaps in rails and sagging wires?
• ☐Can you explain which way a bimetallic strip bends?

Heat Transfer: frequently asked questions