Maharashtra State Board Textbook Solutions
Standard VII – Science
Chapter 9 – Heat
1. Fill in the blanks with the proper word from the brackets.
(radiation, white, conduction, blue, convection, bad conductor, good conductor, black, reflection)
(a) Maximum heat is absorbed by a ….…. coloured object.
Ans: Black
(b) ………… of heat does not require a medium.
Ans: Radiation
(c) Conduction of heat takes place through a ………. substance.
Ans: Good conductor
(d) The shining surface in a thermos flask decreases the outgoing heat by ……. .
Ans: Reflection
(e) Cooking utensils are made from metals due to their property of …….
Ans: Conduction
(f) The earth receives heat from the sun by ………
Ans: Radiation
2. What will absorb heat?
Steel spoon, wooden board, glass vessel, iron griddle (tava), glass, wooden spoon, plastic plate, soil, water, wax
Ans:
All the given materials absorb heat, but the rate and effect differ depending on whether they are good or bad conductors of heat.
(i) Steel spoon and iron griddle (tava) are good conductors, so they absorb heat quickly and become hot.
(ii) Water absorbs heat slowly and gets heated gradually.
(iii) Wooden board, wooden spoon, and plastic plate are bad conductors, so they absorb heat very slowly and do not become hot quickly.
(iv) Glass and soil are also poor conductors of heat; they absorb heat slowly and may crack on sudden heating.
(v) Wax absorbs heat and melts when heated.
3. Write answers to the following questions.
(a) How does a fever get lowered by putting a cold compress on the forehead of a patient?
Ans:
A cold compress absorbs heat from the patient’s body. Heat flows from the hotter body (forehead) to the cooler compress. As a result, the body temperature decreases and the fever is reduced.
(b) Why are the houses in Rajasthan painted white?
Ans:
White colour reflects most of the heat falling on it. Rajasthan has a very hot climate, so houses are painted white to reflect heat and keep the interiors cool.
(c) What are the modes of heat transfer?
Ans:
There are three modes of heat transfer:
- Conduction: Transfer of heat from the hotter part to the cooler part of a solid.
- Convection: Transfer of heat through liquids and gases by movement of particles.
- Radiation: Transfer of heat without any medium.
(d) Explain which mode of heat transfer causes sea breezes and land breezes.
Ans:
Sea and land breezes are caused by convection currents.
(i) During the day, land heats up faster than the sea. The hot air above land rises and cooler air from the sea moves towards land. This creates a sea breeze.
(ii) At night, land cools faster than the sea. The warm air above the sea rises and cooler air from land moves towards the sea. This creates a land breeze.
Thus, both are caused due to convection.
(e) Why is the outer coat of the penguins of Antarctica black?
Ans:
(i) Antarctica is a very cold region.
(ii) Black colour absorbs more heat.
(iii) The black outer coat of penguins absorbs maximum heat from sunlight.
Therefore, it helps them to stay warm in cold conditions.
(f) Why are heaters fitted near the floor and air conditioners near the ceiling of a room?
Ans:
(i) Warm air is lighter and rises upward. So, heaters are placed near the floor so that warm air spreads throughout the room.
(ii) Cold air is heavier and moves downward. So, air conditioners are placed near the ceiling to cool the room effectively.
(iii) This circulation happens due to convection currents.
4. Give scientific reasons.
(a) An ordinary glass bottle cracks when boiling water is poured into it, but a borosil glass bottle does not.
Ans:
(i) Different materials expand differently when heated.
(ii) Ordinary glass expands more and unevenly on sudden heating, causing it to crack.
(iii) Borosilicate glass expands very little and can withstand high temperatures.
Therefore, borosilicate glass does not crack easily.
(b) The telephone wires which sag in summer become straight in winter.
Ans:
(i) Materials expand on heating and contract on cooling.
(ii) In summer, wires expand due to heat and sag.
(iii) In winter, they contract due to cold and become straight.
(c) Dew drops form on the grass in winter.
Ans:
(i) Air contains water vapour.
(ii) In winter, temperature decreases.
(iii) The water vapour condenses on cool surfaces like grass.
Thus, dew drops are formed.
(d) In winter, why does an iron pillar feel colder than a wooden pole?
Ans:
(i) Iron is a good conductor of heat, so it quickly draws heat from our body, making it feel colder.
(ii) Wood is a bad conductor, so it does not draw heat quickly.
Therefore, iron feels colder than wood in winter.