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CNMS ICSE - 7 Std
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Answers to textbook exercises

Prepared by: learnloophq@gmail.com

Chapter: 05. Heat

⁠

CONNECT

What is the temperature in degrees Celsius on Monday?
Answer: The exact temperature reading for Monday’s thermometer is not specified in the provided image description. However, any reading in Fahrenheit (
math
⁠
) can be converted to Celsius (
math
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) using the formula:
math
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​(For example, if the thermometer reads 80°F, it corresponds to approximately 26.7°C).
Which day was the hottest?
Answer: Based on the available data, Tuesday was the hottest day with a temperature of approximately 99°F (37.2°C), which is significantly higher than Sunday’s temperature of 70°F (21.1°C).
⁠

SCIENCE TALK

What are the qualities of a good thermometric liquid?
Answer: A good thermometric liquid should:
Have a uniform and high rate of thermal expansion.
Have a high boiling point and a low freezing point so it remains a liquid over a wide temperature range.
Be easily visible (opaque or highly reflective/shiny).
Not stick to the walls of the glass capillary tube.
Be a good conductor of heat to quickly register temperature changes.
Why is mercury used in thermometers?
Answer: Mercury is used because:
It is a shiny, silvery liquid that is easily visible through the glass stem.
It does not wet or stick to the walls of the capillary tube.
It is a good conductor of heat, ensuring rapid and accurate temperature measurements.
It expands uniformly and stays liquid over a very broad temperature range (freezes at -39°C and boils at 357°C).
Why is alcohol used in thermometers?
Answer: Alcohol is used because it has a very low freezing point (-114°C), allowing it to measure extremely cold temperatures where mercury would otherwise freeze.
What are the steps to measure the temperature of a person?
Answer:
Wash the thermometer bulb with an antiseptic solution or clean water.
Hold the thermometer firmly and give it a few quick jerks to bring the mercury level down below the constriction (under 35°C or 95°F).
Place the bulb under the person’s tongue (or in the armpit) and keep it there for about one to two minutes.
Remove the thermometer and read the temperature at eye level where the mercury thread ends.
⁠

CHECK YOUR PROGRESS - 1

Which of the following liquids is not used in thermometers?
Answer: c. water
Which of the following is not an effect of heat?
Answer: c. reduction in temperature
The degree of hotness or coldness of a body is called
Answer: a. temperature.
⁠

MY QUESTION TIME - 1

Frame a question on each of the following topics and find their answers.
a. temperature
Question: What is the standard SI unit of temperature, and which temperature scale has no negative values?
Answer: The standard SI unit of temperature is Kelvin (K), and the Kelvin scale starts at absolute zero (0 K) and has no negative values.
b. thermal expansion
Question: Why are small gaps left between the metallic rails of a railway track?
Answer: Gaps are left to allow room for the rails to expand (linear thermal expansion) on hot summer days without buckling or bending.
c. conduction
Question: Why does a metal spoon left in a hot bowl of soup become hot after a few minutes?
Answer: This occurs due to conduction, where heat energy is transferred from the hot soup to the cooler metal spoon from molecule to molecule without the actual movement of the spoon’s material.
⁠

REASON CORNER

A thick glass tumbler cracks when boiling hot water is poured over it. Why?
Answer: Glass is an insulator and a poor conductor of heat. When boiling hot water is poured into a thick glass tumbler, the inner wall of the glass absorbs heat and expands rapidly, while the outer wall remains cool and does not expand at the same rate. This uneven expansion creates immense thermal stress, causing the glass to crack.
Mud houses with thatched roofs feel cool in summer and warm in winter than concrete houses. Why?
Answer: Mud is a poor conductor of heat. Additionally, thatched roofs contain trapped pockets of air, which is also an excellent insulator. In summer, they block external heat from entering the house, keeping it cool. In winter, they prevent the indoor warmth from escaping, keeping it warm. Concrete structures, by contrast, readily absorb and release heat.
Why do birds puff up their feathers on a cold day?
Answer: When birds puff up their feathers, they trap a large volume of air close to their skin. Since air is a poor conductor of heat (an insulator), it prevents their body heat from escaping into the cold environment, helping them stay warm.
Why does a marble floor feel hot in summer than a carpeted floor?
Answer: Marble is a good conductor of heat, which means it rapidly transfers heat from its surface to our feet when stepped on during hot summer days. A carpet, being a poor conductor (insulator), slows down the transfer of heat to our feet, making it feel less hot.
⁠

CHECK YOUR PROGRESS - 2

Which of the following is a bad conductor of heat?
Answer: b. rubber
Which method of heat transfer does not need a medium?
Answer: c. radiation
The increase in the volume of a solid is called
Answer: c. cubical expansion
Bimetallic strips are used in
Answer: c. both of these
Arrange the following according to the heat-conducting capacity.
Answer: b. silver, copper, aluminium
Which of these is NOT an insulator?
Answer: a. mercury
⁠

MY QUESTION TIME - 2

Frame a question on each of the following topics and discuss their answers.
a. convection
Question: Why are air conditioning units typically installed near the ceiling of a room?
Answer: Warm air in a room rises because it is less dense. When an air conditioner is installed near the ceiling, it cools this warm rising air. The cooled air becomes denser and descends, setting up a continuous convection current that cools the entire room effectively.
b. radiation
Question: How does heat from the Sun reach the Earth across millions of miles of empty space?
Answer: Heat from the Sun travels to the Earth via radiation as electromagnetic waves (radiant energy), which can travel through a vacuum and do not require any physical medium.
⁠

PREPPING FOR PISA

Based on the passage given above, mark the incorrect statements.
Answer:
b. The upper mantle is denser than the lower mantle.
c. The process of transfer of heat is non-repetitive.
Which of the following factors will decrease the convection current in the mantle?
Answer: d. Change in neither temperature nor density of the upper and lower mantle.
⁠

EVALUATION

A. Choose the correct option.
A clinical thermometer is marked from
Answer: c.
math
⁠
to
math
⁠
.
Conduction takes place in
Answer: a. solids only.
Convection occurs in
Answer: a. fluids.
A thermos flask prevents loss or gain of heat by
Answer: d. all three methods.
Four pins A, B, C and D are attached to a metal rod with the help of wax. A lighted candle is placed below the metal rod as shown in the figure. Identify the correct order in which the pins will fall (first to last).
Answer: a. A, B, C, D
⁠
B. Fill in the blanks.
The handle of a kettle is made of a bad conductor of heat.
All metals are good conductors of heat.
Dark / Black coloured bodies are good absorbers of heat.
Radiation is the fastest method of transfer of heat.
The boiling point of water and the freezing point of water on Fahrenheit scale is 212°F and 32°F, respectively.
⁠
C. Write true (T) or false (F) against the following statements.
A medium is not required for transfer of heat by radiation.
Answer: True (T)
In a thermos flask, the space between the two walls of the vessel contains air.
Answer: False (F)
The process of conversion of a liquid into a gas is called sublimation.
Answer: False (F)
Heat is a kind of energy that flows from a hotter body to a colder body.
Answer: True (T)
Conduction cannot happen in vacuum.
Answer: True (T)
⁠
D. Define the following:
conduction
Answer: The process of transfer of heat in solids from a region of higher temperature to a region of lower temperature without the actual movement of the molecules.
radiation
Answer: The process of transmission of heat in which heat energy directly travels from a hotter body to a colder body, without requiring or heating the intermediate medium.
insulator
Answer: A substance that does not allow heat to pass through it easily.
vaporisation
Answer: The process in which a liquid changes into its vapor or gaseous state at a fixed temperature.
convection
Answer: The mode of transfer of heat in a liquid or a gas by the actual bulk movement of its molecules from hotter (less dense) to colder (more dense) regions.
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E. Short-answer-type questions.
Define heat. Give its unit.
Answer: Heat is an invisible form of energy that flows from a hotter body to a colder body, causing the sensation of hotness or coldness. Its standard SI unit is the Joule (J). Another common unit is the calorie (cal).
What is the mathematical relation between Fahrenheit and Celsius scales?
Answer: The relationship is expressed as:
math
⁠
where
math
⁠
is the temperature in degrees Celsius and
math
⁠
is the temperature in degrees Fahrenheit.
List any three effects of heat.
Answer:
Change in temperature (rise in temperature).
Change in size (thermal expansion).
Change in state of matter (e.g., melting of solids, vaporization of liquids).
Differentiate between conductors and insulators and give two examples for each.
Answer:
Conductors: Materials that allow heat to pass through them easily and quickly. Examples: Copper, Aluminium.
Insulators: Materials that do not allow heat to pass through them easily. Examples: Wood, Rubber.
What do you understand by thermal equilibrium?
Answer: Thermal equilibrium is a state reached when two contacting bodies of different temperatures exchange heat energy until they attain the exact same temperature, at which point net heat flow between them stops.
⁠
F. Long-answer-type questions.
Differentiate between convection, conduction and radiation.
Answer:
Conduction: Heat transfer primarily in solids where thermal energy is passed from molecule to molecule via vibrations without actual molecular displacement. Requires a material medium.
Convection: Heat transfer in fluids (liquids/gases) where heated molecules expand, become less dense, and physically move upward, carrying heat with them. Requires a material medium.
Radiation: Heat transfer that occurs directly from a hotter body to a colder body in straight lines at the speed of light. It does not require a material medium and can travel through a vacuum.
Draw a labelled diagram of a thermos flask. Describe its working.
Answer:
(For the diagram, please refer to the labeled parts: Stopper/Cork, Double-walled glass vessel, Vacuum space, Silvered inner walls, Protective outer case, and Shock absorber/insulating support).
Working:
The vacuum between the double glass walls eliminates heat transfer via conduction and convection because there are no molecules to transfer the heat.
The insulating materials (glass vessel, cork stopper, rubber support) prevent heat from transferring into or out of the flask by conduction.
The highly polished, silvered inner walls reflect radiant heat back into the liquid, minimizing heat transfer by radiation.
Write a few applications of conductors and insulators.
Answer:
Applications of Conductors:
Cooking utensils are made of metals like aluminium and copper to conduct heat rapidly to the food.
Copper pipes are used in car radiators to quickly dissipate heat from the engine.
Applications of Insulators:
Utensil handles are covered with plastic or wood to allow safe handling without burning.
Woolen clothes are worn in winter because wool and its trapped air act as insulators to retain body heat.
Ice blocks are covered with sawdust or blankets to prevent outside heat from reaching and melting the ice.
Explain conduction of heat at the level of molecules.
Answer: In solids, molecules are packed closely in fixed positions. When one end of a solid is heated, the molecules at that end absorb heat energy and begin vibrating rapidly. As they vibrate, they collide with their cooler, neighboring molecules, transferring some of their kinetic/thermal energy. These neighbors then vibrate faster and collide with the next set of molecules. This process continues progressively along the length of the solid, conducting heat to the colder end without any actual movement of the molecules from their original positions.
⁠

Higher Order Thinking Skills (HOTS)

The weather report predicted the minimum and maximum temperatures to be 35°F and 40°F, respectively. What type of clothes would you wear?
Answer: a. sweater, scarf (35°F to 40°F is roughly 1.7°C to 4.4°C, which represents near-freezing, cold winter conditions).
X and Y are two insulated cylindrical tanks with cooling systems placed inside them, as shown in the image given below. In which tank will the water become cold faster?
Answer: a. Tank X (Since cold water is denser than warm water, placing the cooling system at the top causes the chilled water to sink to the bottom, pushing warmer water up to be cooled. This creates effective convection currents that cool Tank X rapidly. In Tank Y, the chilled water remains at the bottom, making cooling a much slower process).
Why should room heaters be placed at the floor level of a room?
Answer: Room heaters heat the air near the floor. As this air becomes hot, it expands, becomes lighter, and rises toward the ceiling. The cooler, denser air near the ceiling sinks to the floor to be heated, setting up convection currents that warm up the entire room efficiently.
Beakers A and B are made of plastic and copper, respectively. Same amount of ice at 0°C is kept in both the beakers. These beakers are further kept in a big vessel containing water at 35°C. In which beaker will the ice melt faster?
Answer: b. Beaker B (Copper is an excellent conductor of heat, allowing the thermal energy from the 35°C water to flow rapidly into the ice. Plastic is an insulator, which slows down heat transfer).
⁠

Learning by Doing

Based on your observation, arrange the rods in the increasing order of their conductivity.
Answer: Glass rod < Iron rod < Copper rod
⁠

Integrate

Using the Internet, list some of the animals that travel long distances to escape harsh weather conditions.
Answer: Some notable migratory animals include:
Arctic Tern
Monarch Butterfly
Siberian Crane
Caribou (Reindeer)
Humpback Whale
Gallery
 
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