Selina ICSE Class 9 Physics Solutions Chapter 6 Heat And Energy

Selina ICSE Solutions


Q1. What is heat? Write its S.I. unit.
The energy of the body’s molecular constituents’ haphazard motion is known as heat.
“Joule” is its SI unit.

Q2. Two bodies at different temperatures are placed in contact. State the direction in which the heat will flow.
A hot body (one with a greater temperature) will transfer heat to a cold one (body at a lower temperature).

Q3. Name the S.I. unit of heat and how is it related to the unit calorie?
S.I. unit of heat is ‘joule’.
Here, 1 joule = 0.24 cal

Q4. Define temperature and write its S.I. unit.
The parameter that reveals a body’s thermal condition is temperature (i.e. the degree of hotness or coldness). Kelvin is the S.I. unit for measuring temperature.

Q5. Why does a piece of ice when touched with a hand, appear cool? Explain.
A chunk of ice feels chilly when we contact it because heat is transferred from our hand (a hot body) to the ice (a cold body).

Q6. Distinguish between heat and temperature.
Heat is a type of energy created when molecules in a substance move randomly, but temperature determines which way the heat will flow when two bodies with differing temperatures come into contact. Even when two objects have the same quantity of heat, their temperatures might vary.

Q7. What do you understand by the thermal expansion of a substance?
The term “thermal expansion of the material” refers to the expansion of a substance upon heating.

Q8. Name two substances which expand on heating.
Iron and brass both expand when heated.

Q9. Name two substances that contract on heating.
Water shrinks when heated between 0°C and 4°C. Silver iodide shrinks when heated from 80 to 141 degrees Celsius.

Q10. What do you mean by the anomalous expansion of water?
The anomalous expansion of water is the expansion of water when it is cooled from 4°C to 0°C.

Q11. At what temperature is the density of water is maximum? State its value.
At 4 °C, water has its highest density. It is worth 1000 kgm-3.

Q12. State the volume changes that you will observe when a given mass of water is heated from 0°C to 10°C. Sketch a temperature-volume graph to show the behavior.
When a given mass of water is heated from 0°C to 4°C, it contracts, i.e. its volume decreases.
On heating from 4°C to 10°C, it expands, i.e. its volume increases.

Q13. Draw a graph to show the variation in density of water with temperature in the range from 0°C to 10°C.

Q14. A given mass of water is cooled from 10°C to 0°C. State the volume changes you will observe. Represent these changes on a temperature-volume graph.

Q15. Describe an experiment to show that water has maximum density at 4°C. What important consequences follow this peculiar property of water? Discuss the importance of this phenomenon in nature.
Hope’s experiment to demonstrate that water has maximum density at 4°C:

The device used by Hope consists of a tall metallic cylinder with two side holes, P at the top and Q nears the bottom, both of which are equipped with thermometers T1 and T2. A cylindrical trough carrying a freezing combination of ice and salt surrounds the middle portion of the cylinder. Pure water is installed in the cylinder and is kept at room temperature.
(i) The temperatures of thermometers T1 and T2 are initially equal.
(ii) The temperature measured by the top thermometer T1 initially decreases and then stabilizes at 4°C, whereas the temperature measured by the lower thermometer T2 initially starts to rise.
(iii) Next, the temperature reported by lower thermometer T2 stays constant at 4°C, whereas the temperature recorded by higher thermometer T1 continuously drops to 0°C before becoming stable.
As a result, the lower thermometer registered a temperature of 4°C, while the higher thermometer recorded a temperature of 0°C.
Water in the centre of the cylinder contracts and becomes denser as the frozen mixture cools it; as a result, it falls to the bottom, rapidly lowering the reading of the lower thermometer T2. Since the water temperature in the higher section does not vary, neither does the reading of the upper thermometer T1. This continues until the water temperature in the entire area below the centre reaches 4°C. The water in the centre area expands as the temperature drops below 4 owing to anomalous expansion, which causes its density to drop and cause it to rise. As a result, the upper thermometer T1 swiftly drops to 0°C, when water instantly freezes to create ice. This demonstrates that at 4°C, water reaches its maximum density.
This unusual water expansion aids in protecting aquatic life during extremely cold temperatures. When the temperature drops during the winter, the top layer of water in a pond contracts, solidifies, and descends to the bottom. Thus, a circulation is established until the pond’s full volume of water reaches its maximum density at 4°C. The top layer grows as the temperature drops more and stays on top until it freezes. As a result, although the top layers are frozen, the water towards the bottom is just 4°C, making it easy for fish to thrive there.

Q16. A deep pond of water has its top layer frozen during winter. State the expected temperature of water layer (i) Just in contact with ice and (ii) At the bottom of pond.

(i) Water that is directly in touch with ice is at 0°C
(ii) The pond’s bottom water temperature is 4°C.

Q17. Draw a diagram showing the temperature of various layers of water in an ice covered pond.

Q18. Explain the following
(i) Water pipes in colder countries often burst in winter.
(ii) In winter, water tanks (or ocean) start freezing from the surface and not from the bottom.
(iii) Fishes survive in ponds even when the atmospheric temperature is well below 0°C.
(iv) A hollow glass sphere which floats with its entire volume submerged in water at 4°C, sinks when water is heated above 4°C.
(v) A glass bottle completely filled with water and tightly closed at room temperature is likely to burst when kept in the freezer of a refrigerator.
(i) During cold winter nights, when the air pressure begins to drop below 4°C, water in the pipelines expands and puts a lot of pressure on the pipes, which leads to their breaking.
(ii) During the winter, as the temperature drops, the water’s surface in the tank shrinks, hardens, and sinks to the bottom. Thus, a circulation is established until all of the tank’s water achieves its maximum density at 4°C. The top layer grows as the temperature drops more and stays on top until it freezes. As a result, the top of a tank’s water begins to freeze, not the bottom.
(iii) In extremely cold conditions, the anomalous expansion of water protects aquatic life. A pond’s top layer of water constricts, grows denser, and descends to the bottom as the temperature drops. As a result, circulation is established until the pond’s water reaches its maximum density at 4°C. The top layer grows as the temperature drops more and stays on top until it freezes. As a result, although the top layers are frozen, the water towards the bottom is just 4°C, making it easy for fish to thrive there.
(iv) Water loses density when heated over 4 degrees Celsius. As a result, the hollow glass sphere experiences less upthrust from the water, which makes it, sink.
(v) The water in the bottle begins to expand in the freezer when the temperature drops below 4°C. The bottle may explode if it is totally filled and shut firmly since there is no room for the water to expand.



Q1. Calorie is the unit of:
a) Heat
b) Work
c) Temperature
d) food
Solution: a) Heat

Q2. 1 J equals to:
a) 0.24 cal
b) 4.18 cal
c) 1 cal
d) 1 kcal
Solution: a) 0.24 cal

Q3. S.I. unit of temperature is:
a) Cal
b) Joule
c) Celsius
d) kelvin
Solution: d) kelvin (K)

Q4. Water is cooled from 4 °C to 0 °C. It will:
a) contract
b) expand
c) first contract, then expand
d) first expand, then contract
Solution: b) Expand

Q5. Density of water is maximum at:
a) 0°C
b) 100°C
c) 4°C
d) 15°C
Solution: c) 4°C


Q1. What is an ecosystem? Name its two components.
A self-contained group of both living and non-living objects is referred to as an ecosystem. Sunlight was necessary for an ecosystem to continue functioning. The following are an ecosystem’s two primary elements:
1. Abiotic component: This category is made up of all the inanimate elements, such as rocks, soil, water, and air temperature.
2. The term “biotic component” refers to all living things. All plants and animals are considered to be biotic components.

Q2. What is the source of energy for all ecosystems?
The Sun is the main source of energy for all ecosystems.

Q3. State the importance of green plants in an ecosystem.
The majority of the energy that falls on green plants is absorbed, and via the process of photosynthesis, they create food for human use. Being primary producers, plants play a crucial role in the environment. They also keep the earth’s carbon dioxide and oxygen levels in balance.

Q4. Differentiate between the producers and consumers.
Consumers cannot produce their own food, unlike producers like plants and some microbes, who can do so using the energy of the sun. For food, they are dependent on producers.

Q5. State the functions of decomposers in an ecosystem.
A decomposer’s job is to disassemble dead creatures so that it can eat them later. The deceased organisms’ nutrients are returned to the soil for subsequent use by the farmers. When these dead creatures are reintroduced to the soil, bacteria and fungus utilise them as food by breaking down the complex organic molecules into simpler nutrients. Producers can then continue the cycle with the simpler items. Every ecosystem benefits from the presence of these decomposers.

Q6. What is a food chain?
A food chain illustrates the interdependence of several living entities in a given habitat or ecosystem. Because it can produce its own food using energy from the Sun, a plant frequently starts a food chain. The passage of food and energy from one creature in an ecosystem to another is represented by a series of events known as a food chain. Food chains demonstrate how energy is transferred from the sun to producers, consumers, and decomposers.

Q7. Draw a simple diagram showing a food chain.

Q8. Describe the energy flow in an ecosystem.

Ecosystems keep themselves going by recycling nutrients and energy from outside sources. Primary producers (plants, algae, and certain microbes) at the lowest trophic level utilise sun energy to create organic plant material through photosynthesis. The second trophic level is made up of herbivores, or animals that only consume plants. The third trophic level is made up of predators that prey on herbivores; if bigger predators are present, these reflect higher trophic levels. Decomposers, which include bacteria, fungus, and others, decompose wastes and dead creatures to replenish the soil’s nutrients.

On average, 10% of the net energy produced at one trophic level is transferred to the following one. Respiration, growth and reproduction, faeces, and non-predatory mortality are among the processes that lessen the energy transmitted between trophic levels.
In terms of energy flow, decomposers are typically more significant than producers due to the slow pace of energy transfer across trophic levels. Large volumes of organic matter are broken down by decomposers, who subsequently return nutrients to the environment in inorganic forms that primary producers may then utilise.

Q9. State the law which governs the energy flow in an ecosystem.

The ecosystem’s energy flow is governed by the principles of thermodynamics. The first law of thermodynamics states that although energy may be changed from one form to another, it cannot be produced or destroyed.
The second rule of thermodynamics states that whenever energy is put to use, a portion of it is always transformed into an inefficient form, mostly heat through friction and radiation.

Q10. Show that the energy flow in an ecosystem is linear.

Ecosystems have linear energy flow, or energy that travels in a single direction. Plants absorb sun energy, and some of it is transformed into food. These plants are subsequently devoured by primary consumers, who in turn consume secondary consumers, who in turn consume tertiary consumers. This cycle has only one direction. Decomposers feed on the dead and decayed matter and replenish the soil’s nutrients in the process. The energy eventually degrades to its final condition. Energy flow is linear since it does not cycle back to the sun to make the process cyclical.

Q11. Draw a simple diagram showing the energy flow in a food chain.

Q12. Draw a diagram to show that the energy flow in an ecosystem is governed by the law of conservation of energy.



Q1. Food chain begins with
a) Respiration
b) Photosynthesis
c) Decomposition
d) Decay
Solution: b) Photosynthesis

Q2. The source of energy in an ecosystem is
a) Sun
b) Decayed bodies
c) Green plants
d) Sugar
Solution: a) Sun

Q3. Energy enters in a food chain through
a) Primary consumers
b) Secondary consumers
c) Tertiary consumers
d) Producers
Solution: d) Producers

Q4. Energy enters in a food chain through
a) Primary consumers
b) Secondary consumers
c) Tertiary consumers
d) Producers
Solution: Consumer


Q1. State two characteristics which a source of energy must have
1. An energy source should be easy to use and safe to handle.
2. An energy source should be affordable, convenient to store, and transfer.

Q2. Name the two groups in which various sources of energy are classified. States on what basis are they classified.
Different energy sources are divided into two categories: non-renewable or renewable sources of energy and non-renewable or conventional sources of energy.
These sources are categorised according to their usefulness and accessibility.

Q3. What is meant by the renewable and non-renewable sources of energy? State two differences between them, giving two examples of each
Renewable energy: comes from natural resources that continually supply us with power.
Non-renewable: Energy sources that have built up in nature over a very long time and cannot be easily replenished when used up are referred to as non-renewable sources of energy.

Q4. Select the renewable and non-renewable sources of energy from the following:
1. Coal
2. Wood
3. Water
4. Diesel
5. Wind
6. Oil
Renewable: Wood, Water and Wind
Non-renewable: Coal, Diesel and Oil

Q5. Why the use of wood as a fuel is not advisable although wood is a renewable source of energy?
Trees provide wood for construction. Therefore, in order to use wood as fuel, trees must be chopped down.
Additionally, burning wood produces a lot of smoke that pollutes the air.

Q6. Name five renewable and three non-renewable sources of energy.
Below are the renewable sources of energy:
1. Sun
2. Wind
3 .Flowing water
4. Tides
5. Nuclear fuel
Non-renewable source of energy are:
1. Coal
2. Petroleum
3. Natural gas

Q7. Explain in brief.
(i) What is tidal energy?
(ii) What is ocean energy?
(iii) What is geothermal energy?
(i) Tidal energy: Tidal energy is the force that drives tides’ increasing and lowering water levels. To capture tidal energy and generate electricity, dams are built over a small sea entrance. It is not a significant source of energy, though, as there is not enough energy in the rise and fall of the tides to produce significant amounts of power.
(ii) Ocean energy: Ocean water has two types of energy:
1. Ocean thermal energy: Ocean thermal energy is the energy made accessible by the temperature differential between the ocean’s surface and deeper layers of water. Ocean thermal energy conversion power plant (OCTEC power) is a device that uses this energy to generate electricity.
2. Oceanic wave’s energy is the kinetic energy that fast-moving oceanic (or sea) waves contain. Although models for producing power from ocean waves have been created, this technology has not yet found widespread use.
(iii) Geothermal energy: This term refers to the heat energy that the Earth’s rocks contain. The subsurface water is heated and converted to steam by the hot rocks found in hot spots deep beneath the Earth. Between the rocks, this steam is crushed at a tremendous pressure. To create electricity, holes are dug deep into the Earth up to the hot areas in order to collect the steam through pipelines and use it to turn turbines attached to the armature of an electric generator.

Q8. What is the main source of energy for Earth?
On Earth, the sun is the primary energy source.

Q9. What is solar energy? How is the solar energy used to generate electricity in a solar power Plant?
Solar energy is the energy derived from the Sun.
A solar power plant is a structure that uses the sun’s thermal energy to produce electricity. It is made up of several concave reflectors, each with a focus of black-painted water pipes. Reflectors focus the sun’s heat energy on pipes, causing the water inside the pipes to start boiling and emitting steam. The steam created in this way fuels the steam turbine that turns a generator into energy.

Q10. What is a solar cell? State two uses of solar cells. State whether a solar cell produces a.c. or dc. Give one disadvantage of using a solar cell.
A solar cell is a type of electrical device that uses the photovoltaic effect to convert light energy directly into electricity. Typically, semiconductors like silicon and gallium are used to create solar cells, along with certain impurities. When sunlight is shone upon a solar cell, a potential difference between its surfaces is created. As a result, a current flow through the circuit linked between the semiconductor’s opposing sides.
The following are two applications for solar cells:
1. They have minimal ongoing maintenance costs and endure for a very long time.
2. They are especially helpful in isolated, hard-to-reach regions where electricity lines cannot be installed. DC is generated by a solar cell (direct current).
The following list includes one drawback of solar cells:
(i) A solar panel’s purchase price is appropriately high at first.

Q11. State two advantages and two limitations of producing electricity from solar energy
Below advantages of using solar panels:
1. They don’t pollute the environment in any way.
2. Solar panels practically never need to be maintained.
3. They endure for a very long time.
4. They don’t need to be maintained.
5. They are appropriate for hard-to-reach locations where power lines cannot be installed.
Below are the disadvantages of using solar panels:-
1. Solar panels have a suitably high starting cost.
2. Solar energy is inefficiently converted to electricity.
3. DC power generated by solar panels cannot be used for many domestic tasks directly.

Q12. What is wind energy? How is wind energy used to produce electricity? How much electric power is generated in India using the wind energy?
Wind energy is the kinetic energy contained in moving vast quantities of air. A wind generator generates electricity from wind energy by using a wind mill as the driving force. At the moment, wind energy is used to produce more than 1025 MW of electricity in India.

Q13. State two advantages and two limitations of using wind energy for generating electricity.
Below are the Advantages of using wind energy:
1. It is pollution-free in every way.
2. It is a non-stop source.
Below are the Limitations of using wind energy:
1. It costs money to set up a wind farm.
2. The construction of a wind farm requires a sizable amount of land.

Q14. What is hydro energy? Explain the principle of generating electricity from hydro energy. How much hydroelectric power is generated in India?
The term “water energy” or “hydro energy” refers to the kinetic energy that flowing water possesses. The high dam at a hydroelectric power plant collects the water flowing through high-altitude rivers (or reservoir). The water turbine, which is situated close to the bottom of the dam, is then given permission to receive the water from the dam. The armature of an electric generator or dynamo is linked to the shaft of the turbine. Only 23% of the world’s electricity is now produced via hydropower.

Q15. State two advantages and two disadvantages of producing hydro electricity
Below are the Advantages of producing hydroelectricity:
1. There is no environmental contamination as a result.
2. It is a renewable energy source.
Below are the Disadvantages of producing hydroelectricity:
1. Local flora and animals are killed or destroyed as a result of the construction of dams over rivers.
2. The biological equilibrium in the river’s downstream regions is upset.

Q16. What is nuclear energy? Name the process used for producing electricity using the nuclear energy.
Slow neutrons cause a heavy nucleus to divide into two almost equal light nuclei, releasing an enormous amount of energy in the process. The overall amount of mass produced during this nuclear fission process is smaller than the entire sum of mass reactants. The mass that is lost is transformed into energy. Nuclear energy is the form of energy so released.
Principle: The coolant, which subsequently travels through the coils of a heat exchanger containing water, absorbs the heat energy created as a result of the carefully regulated chain reaction of nuclear fission of uranium-235 in a nuclear reactor. Heat exchanger water becomes heated and turns into steam as a result. In order to generate electricity, the steam is utilised to move a turbine, which in turn turns a generator’s armature in a magnetic field.

Q17. What percentage of total electrical power generated in India is obtained from nuclear power plants? Name two places in India where electricity is generated from nuclear power plants.
Only approximately 3% of the electricity produced in India as of right now comes from nuclear power plants.
Nuclear energy is used to generate power at Tarapur, Maharashtra, and Narora, Uttar Pradesh.

Q18. State two advantages and two disadvantages of using nuclear energy for producing electricity
Below are the Advantages of using nuclear energy:
1. Nuclear fuel can create a huge quantity of energy from a very tiny amount.
2. Nuclear fuel continues to emit energy for several years after it is placed into a nuclear power plant.
Below are the Disadvantages of using nuclear energy:
1. Because very dangerous radioactive radiations are created throughout the process, it is not a clean source of energy.
2. The garbage pollutes the environment.

Q19. State the energy transformation in the following:
(i) Electricity is obtained from solar energy.
(ii) Electricity is obtained from wind energy.
(iii) Electricity is obtained from hydro energy.
(iv) Electricity is obtained from nuclear energy.
(i) Light energy to electrical energy
(ii) Mechanical energy to electrical energy.
(iii) Mechanical energy to electrical energy.
(iv) Nuclear energy (or heat energy) to electrical energy.

Q20. State four ways for the judicious use of energy.
Below are the Four ways for the judicious use of energy:
1. Only utilise fossil fuels, such as coal, petroleum, and natural gas, under restricted circumstances and only when no other viable alternative energy sources are available.
2. Energy waste should be prevented.
3. Efforts must be made to use energy for collective or communal goals.
4. Tree cutting must be prohibited, and more and more young trees must be encouraged to sprout up.

Q21. What do you mean by degradation of energy? Explain it by taking two examples of your daily life.
The gradual decrease of useful energy due to friction etc. is called the Degradation of energy refers to the progressive loss of usable energy caused by friction and other factors.
1. The majority of the heat energy from the fuel is radiated out into the atmosphere when we cook meals over fire. We cannot utilise the energy that was emitted.
2. The majority of electrical energy is lost as heat energy when electrical equipment are powered by electricity.

Q22. The conversion of part of energy into an unuseful form of energy is called ___________
The conversion of part of energy into an unuseful form of energy is called the degradation of energy.



Q1. The ultimate source of energy is:
a) wood
b) wind
c) water
d) sun
Solution: d) Sun

Q2. Renewable source of energy is :
a) coal
b) fossil fuels
c) natural gas
d) sun
Solution: d) Sun


Q1. What do you mean by the greenhouse effect?
The greenhouse effect is a phenomenon that causes a planet’s surface and lower atmosphere to warm up by absorbing longer-wavelength infrared radiation that is released from the planet’s surface.

Q2. Name three greenhouse gases.
Methane, water vapour, and carbon dioxide are examples of greenhouse gases.

Q3. Which of the following solar radiations pass through the atmosphere of Earth: X -rays, ultraviolet rays, visible light rays or infrared radiation?
Short-wave infrared radiation and visible light waves both travel through the earth’s atmosphere.

Q4. What results in the increase of carbon dioxide contents of earth’s atmosphere?
Due to the expansion of industry, the burning of fossil fuels, and the loss of forests, the concentration of carbon dioxide in the earth’s atmosphere has grown.

Q5. Name the radiations which are absorbed by the greenhouse gases.
The greenhouse gases absorb long-wavelength infrared light.

Q6. What would have been the temperature of earth’s atmosphere in absence of greenhouse gases in it?
The average temperature of the world would be -18 if greenhouse gases weren’t there.

Q7. State the effect of greenhouse gases on the temperature of earth’s atmosphere.
On average, greenhouse gases warm the Earth’s surface by roughly 15.5°C (or 60°F).

Q8. What do you mean by global warming? Explain the term ‘global warming’.
The term “global warming” refers to the rise in average effective temperature close to the earth’s surface as a result of an increase in greenhouse gas concentrations in the atmosphere.
Global warming: The earth’s surface temperature rises as “greenhouse gas” concentrations rise. “Global warming” refers to the increase in the average temperature of the earth’s surface.

Q9. What causes the rise in atmospheric temperature?
The quantity of greenhouse gases in the earth’s atmosphere has grown as a result of industrialisation, deforestation, and excessive fossil fuel consumption. The rise in atmospheric temperature is a result of this increase in greenhouse gas concentrations.

Q10. State the cause of the increase of greenhouse effect.
The greenhouse effect has increased owing to an increase in greenhouse gases brought on by factors such as industrialisation, deforestation, natural gas exploration, burning of biomass, and increased usage of appliances like refrigerators.

Q11. What will be the effect of global warming at the poles?
As temperatures rise in the poles, snow and ice will melt, resulting in flooding in coastal nations. As the icebergs covering the black land and oceans melt, more heat radiation from the sun will be absorbed by the exposed dark land and oceans, intensifying the greenhouse effect.

Q12. State the effect of global warming in coastal regions.
As a result of global warming, the ice and snow at the poles will melt, flooding coastal nations.

Q13. How will global warming affect the sea level?
Sea level will increase on coastal wet places as a result of glaciers and polar ice melting. Many large coastal towns would be submerged by seawater as a result of it raising the global sea level.

Q14. How will global warming affect agriculture?
The patterns of wind, rainfall, etc. will drastically shift as a result of global warming. As a result, the agricultural yield will be minimal.

Q15. State two ways to minimize the impact of global warming
1. Use of fossil fuel-free power plants to produce electricity rather than power from renewable sources of energy.
2. Population control through welfare reforms, family planning, and women’s emancipation.

Q16. What is a carbon tax? Who will pay it?
The term “carbon tax” refers to a fee determined by a factory’s turnover, employee hours, and industry-related carbon emissions.
Industries are required to cover this tax. This will motivate businesses to employ energy-efficient methods.



Q1. The greenhouse gas is:
a) Oxygen
b) Nitrogen
c) Chlorine
d) Carbon dioxide
Solution: d) Carbon dioxide

Q2. The increase of carbon dioxide gas in the atmosphere will cause
a) Decrease in temperature
b) Increase in temperature
c) No change in temperature
d) Increase in humidity.
Solution: b) Increase in temperature

Q3. Without the greenhouse effect, the average temperature of Earth’s surface would have been:
a) -18°C
b) 33°C
c) 0°C
d) 15°C
Solution: a) -18°C

Q4. The global warming has resulted in:
a) the increase in yield of crops
b) the decrease in sea levels
c) the decrease in human deaths
d) the increase in sea levels
Solution: d) The increase in the sea level

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