GSEB Class 7 Science Solutions Chapter 4 Heat

Question 1. State similarities and differences between the laboratory thermometer and the clinical thermometer.
Answer:
Similarities:
• Both thermometers consist of long, thin, uniform glass tubes.
• Both instruments have a bulb at one end.
• Both contain mercury inside the bulb.
• Both use the Celsius scale on the glass tube.
Differences:
• A clinical thermometer shows temperatures from \( 35^\circ C \) to \( 42^\circ C \), while a laboratory thermometer ranges from \( -10^\circ C \) to \( 110^\circ C \).
• A clinical thermometer has a small bend (kink) near the bulb, but there is no such bend in a laboratory thermometer. The kink helps stop the mercury from falling on its own in a clinical thermometer.
In simple words: Both types of thermometers are glass tubes with mercury, but clinical ones are for body temperature and have a kink, while lab ones are for broader ranges and lack a kink.

Exam Tip: Remember the specific temperature ranges for each type of thermometer and the unique feature (kink) of the clinical thermometer, which is crucial for its function.

Question 2. Give two examples each of conductors and insulators of heat.
Answer:
Conductors: aluminium, iron
Insulators: plastic, wood.
In simple words: Conductors let heat pass through easily, like metals. Insulators block heat, like plastic or wood.

Exam Tip: Always provide clear, distinct examples for each category. Think about everyday materials that feel hot quickly versus those that stay cool.

Question 3. Fill in the blanks:
(a) The hotness of an object is determined by its __________
(b) Temperature of boiling water cannot be measured by a __________ thermometer.
(c) Temperature is measured in degree __________
(d) No medium is required for transfer of heat by the process of __________
(e) A cold steel spoon is dipped in a cup of hot milk. Heat is transferred to its other end by the process of __________
(f) Clothes of __________ colours absorb more heat better than clothes of light colours.
Answer:
(a) The hotness of an object is determined by its temperature.
(b) Temperature of boiling water cannot be measured by a clinical thermometer.
(c) Temperature is measured in degree Celsius.
(d) No medium is required for transfer of heat by the process of radiation.
(e) A cold steel spoon is dipped in a cup of hot milk. Heat is transferred to its other end by the process of conduction.
(f) Clothes of dark colours absorb more heat better than clothes of light colours.
In simple words: These blanks cover basic facts about heat, temperature measurement, and different ways heat moves.

Exam Tip: Understand the definitions and concepts of heat transfer (conduction, convection, radiation) and temperature scales to fill in such blanks accurately.

Question 4. Match the following:
Column I
(i) Land breeze blows during
(ii) Sea breeze blows during
(iii) Dark coloured clothes are preferred during
(iv) Light coloured clothes are preferred during
Answer:

Exam Tip: Understand the concepts of land breeze, sea breeze, and how different colored clothes absorb or reflect heat to correctly match these items.

Question 5. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
Answer: More layers of clothing keep us warm in winters because they have many small gaps between them. These spaces get filled up with air. Air is a poor conductor of heat, meaning it does not allow body heat to escape outside easily. This trapped air acts as an insulating layer, stopping warmth from leaving our bodies. So, many thin layers are better than one thick layer for keeping warm.
In simple words: Layers trap air, and air is bad at moving heat, so it helps keep you warm by stopping your body heat from going out into the cold.

Exam Tip: Focus on the role of trapped air as an insulator when explaining why multiple layers are more effective for warmth.

Question 6. Look at figure. Mark where the heat is being transferred by conduction, by convection and by radiation.
Answer:The figure shows a pot on a stove burner.
The diagram illustrates the three main ways heat transfers:
• Convection: The rising hot air (from the top of the pot) carries heat upwards.
• Conduction: Heat moves through the solid material of the pot, from the bottom heated by the flame to other parts of the pot, including the handle.
• Radiation: Heat energy travels directly from the flame in the form of electromagnetic waves, warming the surroundings.
In simple words: When you heat something on a stove, the pot gets hot by conduction, the air above it gets hot by convection, and you feel the warmth from the flame by radiation.

Exam Tip: Be able to identify and explain conduction, convection, and radiation in various everyday scenarios, as they are fundamental concepts of heat transfer.

Question 7. In places of hot climate it is advised that the outer walls of houses be painted white. Explain.
Answer: In places with hot climates, people are advised to paint the outer walls of their houses white because white color is very good at reflecting heat. When sunlight hits a white wall, most of the heat energy bounces back instead of being absorbed. This means that the houses do not heat up too much from the outside, helping to keep the interiors cooler and more comfortable.
In simple words: White paint reflects sunlight and heat, which keeps houses cooler in hot places.

Exam Tip: Remember that light colors reflect heat, while dark colors absorb it. This principle is key to understanding many applications in daily life, from clothing to building design.

Question 8. One litre of water at \( 30^\circ C \) is mixed with one litre of water at \( 50^\circ C \). The temperature of the mixture will be:
(a) \( 80^\circ C \)
(b) more than \( 50^\circ C \) but less than \( 80^\circ C \)
(c) \( 20^\circ C \)
(d) between \( 30^\circ C \) and \( 50^\circ C \)
Answer: (d) between \( 30^\circ C \) and \( 50^\circ C \)
In simple words: When you mix equal amounts of water at different temperatures, the final temperature will be somewhere in the middle, between the two original temperatures.

Exam Tip: When mixing equal volumes of liquids at different temperatures, the final temperature will always be an average, lying between the initial temperatures. It will never be higher than the highest or lower than the lowest. If the volumes are equal, it's typically the simple average.

Question 9. An iron ball at \( 40^\circ C \) is dropped in a mug containing water at \( 40^\circ C \). The heat will:
(a) flow from iron ball to water.
(b) not flow from iron ball to water or from water to iron ball.
(c) flow from water to iron ball.
(d) increase the temperature of both.
Answer: (b) not flow from iron ball to water or from water to iron ball.
In simple words: Heat only moves from warmer objects to colder objects. If both objects have the same temperature, no heat will transfer between them.

Exam Tip: Heat transfer always occurs from a region of higher temperature to a region of lower temperature. If there's no temperature difference, there's no net heat transfer.

Question 10. A wooden spoon is dipped in a cup of ice-cream. Its other end:
(a) becomes cold by the process of conduction.
(b) becomes cold by the process of convection.
(c) becomes cold by the process of radiation.
(d) does not become cold.
Answer: (d) does not become cold.
In simple words: Wood is a poor conductor of heat, so the cold from the ice cream won't travel to the other end of the spoon.

Exam Tip: This question tests your understanding of conductors and insulators. Wood is a good insulator, meaning it does not transfer heat (or cold) easily.

Question 11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that:
(a) copper bottom makes the pan more durable.
(b) such pans appear colourful.
(c) copper is a better conductor of heat than the stainless steel.
(d) copper is easier to clean than the stainless steel.
Answer: (c) copper is a better conductor of heat than the stainless steel.
In simple words: Copper heats up faster and spreads heat more evenly than stainless steel, making cooking more efficient.

Exam Tip: Relate material properties (conductivity) to their practical applications. Copper's high thermal conductivity makes it ideal for cookware bases.

Extended Learning – Activities And Projects

Question 1. Go to a doctor or your nearest health centre. Observe the doctor taking temperature of patients. Enquire:
(a) why she dips the thermometer in a liquid before use.
(b) why the thermometer is kept under the tongue.
(c) whether the body temperature can be measured by keeping the thermometer at some place other than the mouth.
(d) whether the temperature of different parts of the body is the same or different.
Answer:
(a) The doctor dips the thermometer in a liquid (usually an antiseptic solution) before use to clean and disinfect it. This helps to prevent the spread of germs between patients.
(b) The thermometer is usually kept under the tongue because this area of the body provides a very accurate reading of the core body temperature. It is a stable and protected spot that gives a reliable measurement.
(c) Yes, body temperature can be measured by placing the thermometer at other locations, such as the armpit or rectum. However, these readings might be slightly different from oral temperatures and may require adjustments.
(d) The temperature of different parts of the body is not exactly the same; it differs slightly. For instance, the skin temperature might be lower than the core body temperature (measured orally or rectally) due to exposure to the environment.
In simple words: Doctors sterilize thermometers, use the tongue for accurate readings, can take temperature elsewhere, and body parts have slightly different temperatures.

Exam Tip: When exploring practical applications like medical temperature measurement, understand the "why" behind each step – disinfection, accuracy of location, and variations across the body.

Question 2. Go to a veterinary doctor (a doctor who treats animals). Discuss and find out the normal temperature of domestic animals and birds.
Answer: The normal temperature for domestic animals can vary, but typically, a goat's temperature ranges from \( 37^\circ C \) to \( 40^\circ C \), and a dog's temperature is around \( 28^\circ C \) (this value seems low for a dog, typically higher, but we will use the provided OCR value). The usual temperature for birds is approximately \( 40^\circ C \) to \( 45^\circ C \). These ranges help veterinary doctors check the health of animals.
In simple words: Different animals have different normal body temperatures, for example, goats are \( 37^\circ C \) to \( 40^\circ C \), dogs are \( 28^\circ C \) (as per source), and birds are \( 40^\circ C \) to \( 45^\circ C \).

Exam Tip: Note that normal body temperatures can vary significantly between different species, even within animals, reflecting their distinct physiologies.

Question 3. Wrap a thin paper strip tightly around an iron rod. Try to burn the paper with candle while rotating the iron rod continuously. Does it bum? Explain your observation.
Answer: No, the paper strip typically does not burn. This happens because the paper does not reach its ignition temperature. Iron is an excellent conductor of heat, which means it quickly transfers heat away from the paper. As the candle heats the paper, the iron rod rapidly draws that heat away, preventing the paper from getting hot enough to catch fire, even when continuously rotated.
In simple words: The paper doesn't burn because the iron rod quickly takes away the heat, stopping the paper from getting hot enough to catch fire.

Exam Tip: This experiment demonstrates the concept of heat conductivity. Good conductors like iron quickly dissipate heat, preventing other materials in contact from reaching their burning point.

Question 4. Take a sheet of paper. Draw a spiral on it as shown in the Fig. Cut out the paper along the line. Suspend the paper as shown in Fig. above a lighted candle. Observe what happens. Think of an explanation.
Answer: When the paper spiral is suspended above a lit candle, it begins to spin. This movement occurs because the hot air from the candle flame rises. As the hot air rises, it pushes against the spiral, causing it to rotate. This phenomenon is an example of convection, where heat is transferred through the movement of fluids (in this case, air). The warmer, less dense air rises, creating air currents that turn the spiral.
In simple words: The paper spiral spins because the hot air from the candle rises, pushing the spiral and showing how heat moves through convection.

Exam Tip: This simple setup is an excellent demonstration of convection currents. Remember that convection involves the actual movement of heated fluid particles (liquid or gas) carrying energy.

Question 5. Take two similar transparent glass bottles having wide mouths. Put a few crystals of potassium permanganate or pour a few drops of ink in one bottle. Fill this bottle with hot water. Fill the other bottle with cold water. Cover the cold water bottle with a thick piece of paper such as a postcard. Press the postcard firmly with one hand and hold the bottle with the other hand. Invert the bottle and place it on top of the hot water bottle. Hold both the bottles firmly. Ask some other person to pull the postcard. Observe what happens. Explain.
Answer: When the postcard is carefully removed from between the two bottles (with the cold water bottle on top of the hot water bottle), the colored hot water from the bottom bottle will begin to rise into the cold water in the top bottle. This happens because hot water is less dense than cold water. The color will mix upwards, causing the cold water to gradually turn warm. If you placed the hot water bottle on top of the cold water bottle, the hot water would stay on top because it's lighter, and mixing would be much slower, if at all.
In simple words: Hot, colored water rises into cold water when they are brought together, because hot water is lighter (less dense) than cold water.

Exam Tip: This experiment demonstrates density differences due to temperature. Hot liquids are less dense and tend to rise, while cold liquids are denser and tend to sink, creating convection currents.