NCERT Solutions Class 7 Science Curiosity Chapter 07 Heat Transfer in Nature

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Question 1(i). Choose the correct option in each case. Your father bought a saucepan made of two different materials, A and B, as shown in Fig. 7.14. The materials A and B have the following properties -
(a) Both A and B are good conductors of heat
(b) Both A and B are poor conductors of heat
(c) A is a good conductor and B is a poor conductor of heat
(d) A is a poor conductor and B is a good conductor of heat
Answer: (c) A is a good conductor and B is a poor conductor of heat
In simple words: The metal body of the pan lets heat pass through easily to cook food, while the handle is made of a different material to keep our hands safe.

Exam Tip: Remember that cooking utensils are made of conductors so heat can reach the food, while handles are made of insulators to protect our hands.

Question 1(ii). Pins are stuck to a metal strip with wax and a burning candle is kept below the rod, as shown in Fig. 7.15. Which of the following will happen?
(a) All the pins will fall almost at the same time
(b) Pins I and II will fall earlier than pins III and IV
(c) Pins I and II will fall later than pins III and IV
(d) Pins II and III will fall almost at the same time
Answer: (b) Pins I and II will fall earlier than pins III and IV
In simple words: Heat travels slowly through the metal bar from the hot side to the cold side. The pins closest to the fire will drop first because their wax melts first.

Exam Tip: Heat conduction in solid metals is always step-by-step, meaning parts closer to the flame heat up and melt the wax before the distant parts do.

Question 1(iii). A smoke detector is a device that detects smoke and sounds an alarm. Suppose you are fitting a smoke detector in your room. The most suitable place for this device will be:
(a) Near the floor
(b) In the middle of a wall
(c) On the ceiling
(d) Anywhere in the room
Answer: (c) On the ceiling
In simple words: Hot air and smoke are light, so they rise straight up to the top of the room. Putting the alarm high up ensures it detects the smoke quickly.

Exam Tip: Always associate smoke with convection currents. Hot gases expand, become lighter than the surrounding air, and always rise upwards.

Question 2. A shopkeeper serves you cold lassi in a tumbler. By chance, the tumbler had a small leak. You were given another tumbler by the shopkeeper to put the leaky tumbler in it. Will this arrangement help to keep the lassi cold for a longer time? Explain.
Answer: Yes, this setup will keep the drink cold for a longer time. The air caught in the space between the two cups does not let heat pass through easily. Since air is a very poor conductor of heat, it stops the warmth of the room from reaching the cold drink inside.
In simple words: The air trapped between the two cups acts like a blanket. It stops the heat from the room from getting in and warming up the cold drink.

Exam Tip: When explaining insulation, always mention that trapped air is a poor conductor of heat. This prevents external heat from transferring to the cold substance inside.

Question 3. State with reason(s) whether the following statements are True [T] or False [F].
(i) Heat transfer takes place in solids through convection. [ ]
(ii) Heat transfer through convection takes place by the actual movement of particles. [ ]
(iii) Areas with clay materials allow more seepage of water than those with sandy materials. [ ]
(iv) The movement of cooler air from land to sea is called land breeze. [ ]
Answer:
(i) False. Heat moves through solid objects mainly by conduction, not convection.
(ii) True. In liquids and gases, convection happens when the warm particles physically move from one place to another.
(iii) False. Sandy soil has bigger spaces between its grains, so water drains through it much faster than through tight clay soil.
(iv) True. A land breeze is the flow of cool air from the land towards the sea, which typically occurs during the night.
In simple words: (i) Solids pass heat through conduction. (ii) Convection needs moving particles. (iii) Water flows much faster through sand than clay. (iv) Land breeze blows from land to sea at night.

Exam Tip: For true or false questions that require a reason, state the correct fact clearly. Use key terms like "conduction", "particle movement", "pore space", and "night time".

Question 4. Some ice cubes placed in a dish melt into water after sometime. Where do the ice cubes get heat for this transformation?
Answer: The ice cubes take in heat from the objects around them. This includes the plate they are sitting on and the air in the room.
In simple words: Ice melts because it takes heat from the warm air and the dish around it.

Exam Tip: Always mention both solid contact (the dish) and fluid contact (the surrounding air) as sources of heat transfer when explaining melting.

Question 5. A burning incense stick is fixed, pointing downwards. In which direction would the smoke from the incense stick move? Show the movement of smoke with a diagram.
Answer: The smoke will travel in an upward direction. This is because the smoke is made of hot gases. Since hot air is lighter and less dense than the cold air around it, it rises up through convection.
In simple words: Even though the stick points down, the hot smoke is very light, so it floats straight up into the air.

Exam Tip: Make sure your diagram clearly depicts the incense stick pointing down and the smoke traveling upwards. Label the process as convection.

Question 6. Two test tubes with water are heated by a candle flame as shown in Fig. 7.16. Which thermometers (Fig. 7.16a or Fig. 7.16b) will record a higher temperature? Explain.
Answer: The thermometer shown in Fig. 7.16b will show a much higher temperature more quickly. When you heat water from the bottom, the warm water rises and the cold water sinks, creating convection currents that spread heat throughout the whole tube. In Fig. 7.16a, heating from the top only warms the top part of the water, and heat has to travel slowly downwards through conduction.
In simple words: Heating from below lets water circulate and warm up completely. Heating from above only warms the top surface because warm water does not sink.

Exam Tip: Explain that liquids are poor conductors. Convection currents must start from the bottom so the entire liquid can circulate and heat up.

Question 7. Why are hollow bricks used to construct the outer walls of houses in hot regions?
Answer: Hollow bricks contain pockets of trapped air, which acts as an insulator. This layer of air prevents the hot air outside from passing through the walls easily. As a result, very little outdoor heat gets inside, keeping the inside of the house cool.
In simple words: The air inside the bricks blocks the heat from coming in, which keeps the house nice and cool.

Exam Tip: To score full marks, always define "insulator" or explain how "trapped air" acts as a poor conductor of heat to limit thermal transfer.

Question 8. Explain how large water bodies prevent extreme temperature in areas around them.
Answer: Large bodies of water take much longer to warm up and cool down compared to solid ground. In the daytime, the water absorbs a lot of heat, which helps keep nearby land cooler. Once night falls, the water holds onto its warmth and releases it very slowly, keeping the coastal air from becoming too cold. This constant cycle, along with sea and land breezes, keeps the local temperature steady.
In simple words: Water does not change its temperature as fast as land. It stays cool during the day and warm at night, keeping nearby towns pleasant.

Exam Tip: Make sure to mention the difference in heating and cooling rates between land and water, and how sea and land breezes help balance the local climate.

Question 9. Explain how water seeps through the surface of the Earth and gets stored as groundwater.
Answer: Rainwater and surface water soak into the ground by moving through tiny spaces in the soil and rocks, a process known as infiltration. This water continues to trickle downwards until it reaches deep, filled layers called aquifers. These aquifers act as natural underground storage tanks. Water can pass through loose materials like sand much quicker than tight clay.
In simple words: Rainwater sinks into the tiny holes in the dirt. It travels deep down until it gets trapped in underground rocks, which we call groundwater.

Exam Tip: Define key terms like "infiltration" and "aquifers" clearly in your answer. Also explain how soil type affects the rate of water absorption.

Question 10. The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.
Answer: The water cycle is a continuous natural loop. It starts when the sun evaporates water into vapor, which then cools to form clouds through condensation. Finally, this moisture falls back to the ground as rain or snow through precipitation. This movement carries water from oceans and lakes into the sky, and then spreads it over land to refill our rivers, wells, and lakes.
In simple words: The water cycle moves water from the oceans up into the sky and brings it back down as rain to fill our rivers and lakes.

Exam Tip: To justify this statement, outline the three main steps of the water cycle: evaporation, condensation, and precipitation, and explain how they move water from seas to dry land.

Very Short Answer Type Questions

Question. What is conduction?
Answer: Conduction is how heat travels through a solid object from the warm end to the cool end, while the tiny particles of the object stay in their places.
In simple words: Conduction is heat moving along a solid thing, like a metal spoon in hot tea, without any parts of the spoon actually moving.

Exam Tip: The key phrase examiners look for is "without the movement of particles." Be sure to include this to secure full marks.

Question. Which materials are poor conductors of heat?
Answer: Some common materials that do not allow heat to pass through them easily are wood, plastic, wool, clay, and air.
In simple words: Materials like plastic, wood, and wool keep heat from passing through them.

Exam Tip: Providing a diverse list of examples, including both solids (like wood) and gases (like air), shows a strong understanding of insulators.

Question. Why is a metal pan used for cooking?
Answer: Metal pans are ideal for cooking because they conduct heat extremely well. They allow the heat from the stove to spread rapidly and evenly across the food.
In simple words: Metal gets hot very quickly and heats the food evenly all over the pan.

Exam Tip: Always use terms like "good conductor" and mention the "even distribution of heat" to explain why metals are preferred for cookware.

Question. Name the three modes of heat transfer.
Answer: The three main methods of heat movement are conduction, convection, and radiation.
In simple words: Heat travels in three ways: conduction, convection, and radiation.

Exam Tip: List all three processes clearly. Memorizing this trio is essential as they form the foundation of this chapter.

Question. Which process allows heat transfer from the Sun to Earth?
Answer: The Sun's heat reaches the Earth through the process of radiation.
In simple words: Radiation is how heat travels across empty space from the Sun to us.

Exam Tip: Remember that radiation is the only method of heat transfer that can travel through a vacuum (empty space) without needing any medium.

Short Answer Type Questions

Question. Why do pins attached to a metal strip fall one after the other during heating?
Answer: Heat moves step-by-step through the metal bar via conduction. As the warmth travels down the strip, it melts the wax holding each pin one by one, causing them to drop in order.
In simple words: The heat travels down the metal bar. As it reaches each pin, it melts the wax holding it, so the pins drop one after another.

Exam Tip: Make sure to use the word "conduction" and explain that the wax melts "sequentially" as the heat front moves forward.

Question. Why do we wear woollen clothes in winter?
Answer: Woolen fabric has many tiny spaces that trap air. Since air is an excellent insulator, it locks in our body heat and prevents it from escaping into the cold surroundings.
In simple words: Woolen clothes trap air close to our skin. This trapped air keeps our body heat from escaping so we stay warm.

Exam Tip: The double concept of wool trapping air, and air being a poor conductor of heat, is required to score full marks.

Question. What causes land and sea breeze?
Answer: Land warms up and cools down at a much faster rate than water. This difference in temperature creates winds: during the day, cool air blows from the sea to the land (sea breeze), while at night, the air moves from the land toward the sea (land breeze).
In simple words: Land heats up faster than the sea. This temperature difference creates winds that blow from the sea by day and from the land at night.

Exam Tip: Always clarify the direction of the wind (sea-to-land for sea breeze, land-to-sea for land breeze) and state when each occurs.

Question. How does a bukhari keep a room warm?
Answer: A bukhari relies on all three methods of heat transfer. Conduction warms up its outer metallic body, convection circulates the heated air around the room, and radiation sends warmth directly to people sitting nearby.
In simple words: A bukhari heats up its metal body, blows warm air around the room, and sends heat directly to you like campfire rays.

Exam Tip: To answer completely, explain how each of the three modes of heat transfer (conduction, convection, radiation) plays a distinct role in the heater's function.

Question. Why is it comfortable to wear white clothes in summer?
Answer: Wearing light or white clothing is more comfortable because these shades reflect most of the sunlight and heat. This prevents our bodies from absorbing too much warmth on hot days.
In simple words: White clothes bounce the sun's heat away from us, which keeps our bodies cooler.

Exam Tip: Use the word "reflect" when talking about light-colored clothes and contrast it with dark clothes, which "absorb" heat.

Descriptive Answer Type Questions

Question. Explain the process of convection in liquids using an activity.
Answer: To see convection in action, heat a beaker of water containing a crystal of potassium permanganate. As the candle warms the water at the very bottom, this heated water expands, becomes lighter, and rises. Meanwhile, the cooler and heavier water from the sides sinks to the bottom. This continuous, circular loop of moving water particles distributes the heat and is known as convection.
In simple words: When you heat a cup of water, the hot water at the bottom rises up, and the cool water at the top sinks down, making a circle.

Exam Tip: Use a clear diagram if possible, and make sure to highlight the circular motion of the fluid particles, which forms the convection current.

Question. How does the use of hollow bricks help in temperature control in houses?
Answer: Hollow bricks have empty spaces that trap air. Since air does not conduct heat well, it serves as a barrier against thermal movement. This means these bricks keep outdoor heat from entering during the summer, and keep indoor warmth from escaping during the winter, making homes highly energy-efficient.
In simple words: The empty spaces inside the bricks hold air. This air blocks heat, keeping the house cool in summer and cozy in winter.

Exam Tip: Explain both seasonal benefits: keeping the home warm in winter and cool in summer, and mention that this reduces energy bills.

Question. What is infiltration and how does it affect groundwater levels?
Answer: Infiltration refers to the way rainwater filters down through soil and stone layers. This water gathers in underground reservoirs known as aquifers. When we cover the land with concrete or draw out too much water, the ground cannot recharge properly, which leads to a drop in water levels and causes water scarcity.
In simple words: Infiltration is rain sinking into the ground to fill up underground water pools. If we pave the ground with concrete, the water cannot sink in, causing a water shortage.

Exam Tip: Define infiltration clearly, mention aquifers, and explain how human activities (like cementing surfaces) block this natural refill process.

Question. How does heat transfer through radiation differ from conduction and convection?
Answer: Radiation is unique because it can move heat through a vacuum via electromagnetic waves, without needing any material medium. In contrast, conduction and convection both require a solid, liquid, or gas medium to work, relying on direct contact or moving particles. Radiation is the only way heat from the Sun can cross space to reach us.
In simple words: Radiation does not need air or solids to travel - it can move through empty space. Conduction and convection must have physical materials to move heat.

Exam Tip: Draw a comparison table or list the differences clearly. Use the term "medium" as it is a crucial scientific keyword for this topic.

Question. Explain the water cycle using heat transfer processes.
Answer: The water cycle is completely driven by thermal energy. First, solar radiation heats up water on Earth, turning it into vapor via evaporation and transpiration. This warm, light vapor rises and cools down in the sky to form clouds, which is called condensation. Eventually, this moisture falls back down as rain or snow through precipitation. The water then sinks into the soil to recharge groundwater, keeping the cycle going.
In simple words: The sun heats up water and makes it rise into the sky. It cools into clouds and falls back down as rain, filling the ground and rivers again.

Exam Tip: Clearly link each step of the water cycle (evaporation, condensation, precipitation) to heat changes (gaining heat to rise, losing heat to condense).

Exploring Questions

Question. Why does smoke from incense or firewood rise upward?
Answer: When air gets hot, it expands, becomes lighter, and starts to rise. Smoke is made of warm, light gases that are less dense than the cool surrounding air, so it gets carried upwards by convection currents. We use this exact scientific idea to build chimneys that pull smoke out of homes.
In simple words: Hot smoke is much lighter than the cold air around it, so it floats straight up into the sky.

Exam Tip: Explain the concept of density - hot air is less dense than cool air, which causes it to rise and create convection currents.

Question. Why is water served in clay or porcelain cups instead of metal ones during summers?
Answer: Clay and porcelain are excellent insulators that do not conduct heat. They block the hot summer air from warming up your drink, keeping the water cool. Metal cups, on the other hand, are highly conductive and will quickly transfer heat from the air to your drink, making it warm.
In simple words: Clay cups block heat from the hot summer air, keeping your water cool. Metal cups let heat in very quickly.

Exam Tip: Contrast the insulating properties of clay/porcelain with the highly conductive properties of metals to present a complete argument.

Question. Why do we feel warm even when we sit at a distance from a fire?
Answer: We feel this heat because of radiation. Infrared rays travel directly from the fire through the air without needing any physical medium to carry them. These rays warm up our skin and clothes immediately upon contact.
In simple words: The fire sends out heat rays through the air. These rays hit our bodies and warm us up directly.

Exam Tip: Make sure to state that radiation does not need a medium, allowing heat to travel directly across the distance to warm our bodies.

Question. What are aquifers and why are they important for survival?
Answer: Aquifers are large underground layers of sand, gravel, or porous rock that hold fresh water. They are vital because they supply clean water for drinking, farming, and homes. However, if we pump out water faster than rain can refill them, these crucial reserves will dry up.
In simple words: Aquifers are natural underground water tanks made of rock and sand. They give us clean water to drink and grow food.

Exam Tip: Define what an aquifer is physically made of (porous rock/soil), list its main uses, and warn about the dangers of over-extraction.

Question. How do ice stupas in Ladakh help in water conservation?
Answer: Ice stupas are artificial towers of ice built to save winter water that would otherwise run off. As spring arrives, these towers melt slowly, providing a steady supply of fresh water during dry months for farming and daily household activities. This is a brilliant, nature-friendly invention.
In simple words: Ice stupas are big frozen ice towers built in winter. In spring, they melt slowly to give people water when it is very dry.

Exam Tip: Use the phrase "man-made ice towers" and explain their timing - storing water in winter and releasing it slowly during the dry spring months.

Question. What are the three methods of heat transfer explained in Class 7 Science Curiosity Chapter 7?
Answer: The three main methods of heat transfer are:
1. Conduction: This occurs inside solid materials, where thermal energy passes from particle to particle while the particles stay in their fixed spots.
2. Convection: This happens in fluids (liquids and gases), where the warm particles physically rise and circulate, carrying the heat with them.
3. Radiation: This is the movement of heat through empty space with no material medium required, such as feeling the warmth from a distant fire or the Sun. Often, these processes work in harmony, like when a metal pot on a stove heats up (conduction) and warms the water inside (convection).
In simple words: Heat moves through solids by conduction, through liquids and gases by convection, and through empty space by radiation. Sometimes they all work together.

Exam Tip: Define all three terms clearly with their respective states of matter (solids for conduction, fluids for convection, no medium for radiation).

Question. Why do people living in cold places use double-layered wooden walls, as discussed in Class 7 Science Curiosity Chapter 7?
Answer: In freezing regions like the Himalayas, houses are designed with two layers of wooden walls packed with mud or cow dung. Because these substances are bad conductors of heat, they prevent the indoor warmth from escaping outside. This smart setup locks the heat inside, keeping the home cozy during freezing winters and showing how heat science helps protect people from harsh weather.
In simple words: In cold places, walls have two layers filled with mud. This stops the warmth from escaping, keeping the inside of the house warm.

Exam Tip: Highlight that both wood and the filling materials (mud or cow dung) act as poor conductors (insulators) to trap indoor heat and prevent thermal loss.

Question. How does the water cycle relate to heat transfer, as taught in Class 7 Science Curiosity Chapter 7?
Answer: The water cycle is completely powered by heat transfer from the Sun. Solar radiation heats up water bodies and plants, causing evaporation and transpiration. The resulting water vapor is carried upwards by convection currents. As it climbs, the vapor loses heat, cooling and condensing into clouds. Eventually, this water falls back as precipitation (such as rain or snow) and seeps into underground aquifers, showcasing how heat moves and recycles our planet's water.
In simple words: The sun's heat evaporates water, and warm air currents carry it up into the sky. When it cools, it falls as rain, recycling the water we need.

Exam Tip: In your response, directly mention the specific heat transfer processes involved: radiation (to heat the water) and convection (to carry the water vapor upwards).

Question. What is the main thing to understand in Class 7 Science Curiosity Chapter 7?
Answer: The core theme of this chapter is mastering the three distinct modes of heat movement: conduction, convection, and radiation. These methods explain how thermal energy travels through different states of matter and empty space. Connecting these ideas to everyday situations like coastal breezes, cooking pots, or solar rays makes the entire topic easy to understand.
In simple words: The most important part is knowing how heat travels through conduction, convection, and radiation in our daily lives.

Exam Tip: Make sure you can define and differentiate conduction, convection, and radiation, as they form the core of almost all questions in this chapter.

Question. Is Class 7 Science Curiosity Chapter 7 full of tough science terms?
Answer: No, the chapter is actually very easy to understand because it uses everyday examples. While it introduces terms like conduction, convection, radiation, and infiltration, these are just names for things we see every day, like boiling water or feeling the sun's warmth.
In simple words: The science terms might look hard at first, but they just describe simple things we see around us every day.

Exam Tip: Do not feel overwhelmed by scientific terms. Try to relate each new vocabulary word to a real-life action you have observed, like cooking or staying warm.

Question. How can I remember the difference between conduction, convection and radiation in Chapter 7?
Answer: You can easily remember the differences by thinking of how the heat moves:
- Conduction: Think of heat "walking" along a solid (like a spoon in a hot cup).
- Convection: Think of heat "swimming" or "flying" in currents (like boiling water or rising smoke).
- Radiation: Think of heat "beaming" through empty space (like sunlight or heat from a campfire).
In simple words: Conduction is heat moving through solids. Convection is heat circulating in liquids or gases. Radiation is heat traveling as rays through space.

Exam Tip: Create a small mental picture for each type of heat transfer. A hot spoon for conduction, boiling water for convection, and solar rays for radiation.