Heat and Energy ICSE Class 9: Anomalous Expansion, Greenhouse Effect and Energy Degradation

What Heat and Energy is about

Weightage: CISCE publishes no chapter weightage and no question-paper layout for ICSE Class 9 Physics. Class IX is examined by your school: one 80-mark paper of 2 hours plus 20 marks of internal assessment. Heat and Energy is in the 5. Heat and Energy (i)-(iii) part of the 2028 syllabus.Source: CISCE ICSE 2028 Physics syllabus, Class IX

Key concepts

Heat and temperature

Heat is the form of energy that flows from a hotter body to a colder one because of their temperature difference. It is the total kinetic energy of the molecules involved, so it depends on the mass of the body. SI unit: joule (J). Another unit is the calorie: $1\text{cal}=4.186\text{J}$ exactly (1 kcal = 1000 cal).

Temperature is the degree of hotness or coldness of a body. It decides the direction of heat flow (from higher to lower temperature) and is related to the average kinetic energy of the molecules. SI unit: kelvin (K); in daily use, degree Celsius (°C): $T(\text{K})=t(°\text{C})+273$.

A bucket of warm water has more heat than a cup of boiling water, but a lower temperature.

Worked example

Convert 500 cal into joules, and 8372 J into calories.

Try it first, then show the solutionSolution

1. $500\times 4.186=2093$ J.
2. $\frac{8372}{4.186}=2000$ cal.

Answer: 2093 J; 2000 cal

Anomalous expansion of water

Most liquids expand steadily when heated. Water is unusual (anomalous):

• when water is heated from 0 °C to 4 °C, it contracts (volume decreases);
• above 4 °C it expands like other liquids.

So water has its minimum volume and hence maximum density (about 1000 kg m⁻³, or 1 g cm⁻³) at 4 °C.

Also, water expands on freezing: ice at 0 °C (about 917 kg m⁻³) is less dense than water at 0 °C, so ice floats.

Worked example

1 kg of water (volume 1000 cm³ at 0 °C, nearly) freezes into ice of density 0.917 g cm⁻³. Find the volume of the ice and the percentage increase.

Try it first, then show the solutionSolution

1. Volume of ice $=\frac{1000}{0.917}\approx 1090.5$ cm³.
2. Increase $\approx 90.5$ cm³, about 9%.

Answer: ≈ 1090.5 cm³; about 9% larger

Graphs of volume and density (0 to 10 °C)

Volume–temperature graph (temperature on the x-axis, volume of a fixed mass of water on the y-axis): the curve falls from 0 °C to a minimum at 4 °C, then rises from 4 °C to 10 °C.

Density–temperature graph: the mirror image: density rises from 0 °C to a maximum at 4 °C, then falls.

Both curves are smooth (no sharp corner) at 4 °C. Since density $=\frac{\text{mass}}{\text{volume}}$ and the mass is fixed, minimum volume means maximum density.

Worked example

Water at 0 °C is heated slowly to 10 °C. Describe how its density changes.

Try it first, then show the solutionSolution

1. 0 °C to 4 °C: water contracts, so density increases.
2. At 4 °C: density is maximum.
3. 4 °C to 10 °C: water expands, so density decreases.

Answer: Rises to a maximum at 4 °C, then falls.

Hope's experiment

Hope's apparatus is a tall metal cylinder filled with water, with a trough (jacket) round its middle filled with a freezing mixture (ice and salt). Thermometers are fitted near the top ($T_1$) and the bottom ($T_2$).

Observations as the middle is cooled:
1. At first, the lower thermometer falls faster: water cooled in the middle becomes denser (above 4 °C) and sinks. The lower thermometer reaches 4 °C and stays there.
2. Then the upper thermometer starts to fall quickly: water cooled below 4 °C becomes less dense and rises. The upper thermometer falls to 0 °C, and ice may form at the top.
3. Finally: top at 0 °C, bottom steady at 4 °C.

Conclusion: water has maximum density at 4 °C.

Worked example

In Hope's experiment, why does the lower thermometer stop at 4 °C while the upper one falls to 0 °C?

Try it first, then show the solutionSolution

1. Water at 4 °C is densest, so it collects at the bottom.
2. Water cooled below 4 °C is lighter and rises to the top, where it cools to 0 °C.

Answer: The densest water (4 °C) sinks; colder, lighter water rises.

Consequences of anomalous expansion

• Aquatic life survives in frozen lakes: as the air cools, surface water cools and sinks until the whole lake is at 4 °C. Further cooling makes the surface water lighter, so it stays on top and freezes. Ice is a poor conductor, so the water below stays at about 4 °C and fish survive.
• Water pipes burst in cold countries: water expands on freezing, pushing on the pipe walls.
• Rocks crack (weathering): water in cracks expands when it freezes and splits the rock.
• Ice floats because it is less dense than water.

Worked example

Give a reason: in very cold places, water pipes are wrapped with straw or other insulating material.

Try it first, then show the solutionSolution

1. Water expands when it freezes.
2. Insulation slows cooling so the water does not freeze and burst the pipe.

Answer: To stop the water freezing and expanding.

Greenhouse effect, global warming and energy degradation

Greenhouse effect: the Sun's short-wavelength radiation passes through the atmosphere and warms the Earth's surface. The warm surface emits long-wavelength infrared radiation, much of which is absorbed and re-emitted back by greenhouse gases (carbon dioxide, methane, water vapour, nitrous oxide, CFCs). This keeps the Earth warm enough for life.

Global warming: the rise in the Earth's average temperature because human activity adds more greenhouse gases. Causes: burning fossil fuels, deforestation, agriculture (methane), industrial gases. Impact on life: melting glaciers and polar ice, rising sea levels and flooding of coasts, extreme weather (heatwaves, droughts, floods), damage to coral reefs and ecosystems, effects on crops and on health.

Energy degradation: in every energy change some energy turns into forms that are less useful (mostly low-temperature heat spread out in the surroundings). The total energy is conserved, but its usefulness decreases. Examples: a bulb's electrical energy becomes light plus much wasted heat; friction in machines; heat lost from a car engine.

Worked example

A filament bulb uses 60 J of electrical energy each second, and only 6 J of it comes out as light. How much energy is degraded each second, and what fraction is useful?

Try it first, then show the solutionSolution

1. Degraded (mostly heat) $=60-6=54$ J each second.
2. Useful fraction $=\frac{6}{60}=10\%$.

Answer: 54 J per second; 10% useful

Physics in real life: Heat and Energy

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

• Anomalous Expansion of Water, Hope's Experiment and Its Consequences
• Greenhouse Effect, Global Warming and Energy Degradation

Heat and Energy formula sheet

Calorie and joule

$1\text{cal}=4.186\text{J}$

Exact; 1 kcal = 1000 cal.

Celsius and kelvin

$T(\text{K})=t(°\text{C})+273$

Temperature scales.

Density

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

Fixed mass: least volume → greatest density.

Maximum density of water

${\rho }_{max}\approx 1000{\text{kg m}}^{-3}\text{at}4°\text{C}$

1 g cm⁻³.

Useful fraction

$\text{useful fraction}=\frac{\text{useful energy out}}{\text{energy in}}$

The rest is degraded.

Heat and Energy: diagrams to practise

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

How the school exam asks Heat and Energy

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

Tricks to remember Heat and Energy

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

• Four is the floor

  Water's volume hits the floor (minimum) at 4 °C; its density hits the roof.

  Use it when: Graphs and Hope's experiment.

• Cold rises below four

  Below 4 °C, colder water is lighter and floats up.

  Use it when: Hope's experiment and frozen lakes.

• 4.186, exactly

  One calorie is exactly 4.186 joules.

  Use it when: Unit conversion.

• Short in, long trapped

  Sunlight comes in as short waves; the Earth's heat goes out as long waves, which greenhouse gases trap.

  Use it when: Greenhouse effect.

Heat and Energy: 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 9-mark question split into 3 + 2 + 4 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 and Energy

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

• ✗Mistake: Treating heat and temperature as the same

  Heat is energy; temperature is the degree of hotness. A large lake at 20 °C holds more heat than a cup at 90 °C.

  ✓Do this: Use 'heat' for energy (J) and 'temperature' for hotness (°C, K).

  Usually costs: 1 mark.

• ✗Mistake: Sharp V-shaped graph

  The volume–temperature curve is smooth at 4 °C, not a sharp corner.

  ✓Do this: Draw a smooth U with its lowest point at 4 °C.

  Usually costs: 1 mark.

• ✗Mistake: Wrong order of thermometer changes in Hope's experiment

  Students say the upper thermometer falls first.

  ✓Do this: Above 4 °C cooled water sinks, so the lower one falls first.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Confusing greenhouse effect with global warming

  The natural greenhouse effect keeps the Earth habitable; global warming is the extra warming from human-made emissions.

  ✓Do this: Define both separately.

  Usually costs: 1 mark.

• ✗Mistake: Saying energy is destroyed in degradation

  Energy is conserved; it only becomes less useful.

  ✓Do this: Write 'converted into less useful forms, mostly heat'.

  Usually costs: 1 mark.

Practice questions on Heat and Energy

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

   The SI unit of temperature is:
   (a) degree Celsius
   (b) kelvin
   (c) joule
   (d) calorie

   Hint

   Base unit.

   Answer

   (b)

2. Q2 · 1 mark · MCQExam level

   When water is heated from 0 °C to 4 °C, its density:
   (a) decreases
   (b) increases
   (c) stays the same
   (d) first decreases then increases

   Hint

   Volume decreases.

   Answer

   (b)

3. Q3 · 1 mark · Short answerExam level

   Convert 2.5 kcal into joules.

   Hint

   1 kcal = 4186 J.

   Answer

   $2.5\times 4186=10465$ J.

4. Q4 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): In winter, the bottom of a deep lake stays at about 4 °C even when the surface is frozen.
   Reason (R): Water is densest at 4 °C.
   (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

   Densest water sinks.

   Answer

   (c)

5. Q5 · 2 marks · Short answerExam level

   Give two consequences of the anomalous expansion of water in nature.

   Hint

   Lakes and rocks.

   Answer

   Aquatic life survives under frozen lakes; rocks crack when water in their cracks freezes and expands (also: pipes burst, ice floats).

6. Q6 · 2 marks · Short answerHarder exam level

   Why are glass bottles of water not filled to the top before being put in a freezer?

   Hint

   Expansion on freezing.

   Answer

   Water expands by about 9% when it freezes; without space, the ice pushes on the glass and can crack it.

7. Q7 · 2 marks · Short answerHarder exam level

   Sketch the density–temperature graph of water from 0 °C to 10 °C and mark the maximum.

   Hint

   Upside-down U.

   Answer

   A smooth curve rising from 0 °C to a maximum (about 1 g cm⁻³) at 4 °C, then falling to 10 °C; axes labelled temperature (°C) and density.

8. Q8 · 3 marks · Short answerHarder exam level

   A water heater supplies 25,000 cal of heat to a bucket of water and a kettle supplies 167,440 J. Which supplies more heat, and by how many joules?

   Hint

   Convert both to joules.

   Answer

   Heater: $25000\times 4.186=104650$ J. Kettle: 167,440 J (that is 40,000 cal). The kettle supplies $167440-104650=62790$ J more.

9. Q9 · 3 marks · Short answerHarder exam level

   What are greenhouse gases? Explain why an increase in carbon dioxide raises the Earth's average temperature.

   Hint

   Infrared absorption.

   Answer

   Gases such as CO₂, methane, water vapour, N₂O and CFCs that absorb infrared radiation. More CO₂ absorbs and re-emits more of the infrared emitted by the Earth, so less heat escapes to space and the average temperature rises.

10. Q10 · 3 marks · Short answerHarder exam level

    In a power station, 1000 MJ of coal energy produces 350 MJ of electrical energy. How much is degraded, and give two forms it takes.

    Hint

    Difference.

    Answer

    650 MJ degraded, mainly as heat in the cooling water and the flue gases, and as sound and friction in machinery.

11. Q11 · 3 marks · Short answerHOTS / competency

    In Hope's experiment, if the cylinder were filled with a liquid that does not show anomalous expansion, predict how the two thermometers would behave.

    Hint

    Colder liquid always denser.

    Answer

    Cooled liquid would always be denser and sink, so the lower thermometer would keep falling and reach the lowest temperature first; the upper one would fall more slowly. There would be no steady 4 °C reading at the bottom.

12. Q12 · 4 marks · Long answerHOTS / competency

    A sealed glass jar is completely filled with 500 cm³ of water at 4 °C. Say what happens to the volume of the water if it is (a) warmed to 10 °C (b) cooled to 0 °C without freezing (c) then frozen (ice density 0.917 g cm⁻³). Which change is most likely to crack the jar? Support your answer with a calculation.

    Hint

    Water has its least volume at 4 °C; mass stays 500 g when it freezes.

    Answer

    (a) It expands slightly: above 4 °C water expands like other liquids. (b) It also expands slightly: below 4 °C water expands on cooling (anomalous expansion), since its volume is least at 4 °C. Both changes are very small (well under 1%). (c) Mass $=500$ g, so ice volume $=\frac{500}{0.917}\approx 545$ cm³: an increase of about 45 cm³, roughly 9%. Freezing (c) is by far the most likely to crack the jar.

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.SI unit of heat?Show answer

   Joule (J).

2. 2.1 calorie = ? joulesShow answer

   4.186 J (exactly).

3. 3.At what temperature is water densest?Show answer

   4 °C.

4. 4.What happens to water's volume from 0 °C to 4 °C?Show answer

   It decreases (water contracts).

5. 5.What does Hope's experiment show?Show answer

   Water has maximum density at 4 °C.

6. 6.Name two greenhouse gases.Show answer

   Carbon dioxide and methane (also water vapour, nitrous oxide, CFCs).

7. 7.What is energy degradation?Show answer

   The conversion of energy into less useful forms, mainly heat, in every energy change.

8. 8.Why does ice float?Show answer

   Ice is less dense than water.

Quick revision checklist

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

• ☐Can you give three differences between heat and temperature?
• ☐Can you convert between calories and joules?
• ☐Can you sketch volume and density against temperature for 0–10 °C?
• ☐Can you describe Hope's experiment in order?
• ☐Can you explain why fish survive in frozen lakes?
• ☐Can you explain the greenhouse effect with wavelengths?
• ☐Can you give examples of energy degradation?

Heat and Energy: frequently asked questions