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

(ii) What will absorb heat?

Steel spoon, wooden board, glass vessel, iron griddle (tava), glass, wooden spoon, plastic plate, soil, water, wax

Ans: All will absorb heat depending on whether it is a good or bad conductor of heat.

(i) Steel spoons and iron griddles (tava) are good conductors of heat, so they will become hot.

(ii) Water is a moderately good conductor, and hence it will take up heat slowly and become hot.

(iii) A wooden board, a wooden spoon, and a plastic plate are bad conductors of heat, so they will not only become hot but catch fire.

(iv) Glass vessels, glass, and soil are also bad conductors of heat, so they may crack on heating.

(v) Wax is also a bad conductor of heat, but after taking up heat, it will melt.

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 is made up of material that absorbs heat. Thus, by putting it on the forehead of a patient suffering from high fever, the heat flow takes place from the head of the patient to the cold compress. Therefore, a cold compress absorbs heat from the patient’s body and lowers the body temperature.

(b) Why are the houses in Rajasthan painted white ?

Ans: White colours are reflectors of heat. Thus, Rajasthan, a very hot city, has houses with walls painted white so that most of the heat from the sun is reflected back to the atmosphere. This keeps the houses cool from the inside.

(c) What are the modes of heat transfer ?

Ans: There are three modes of heat transfer:

Conduction: Conduction is a process in which heat is transferred from the hotter end of an object to its cooler end.

Convection: The process of transferring heat through a fluid (liquid or gas) is called convection.

Radiation: Radiation is a method of heat transfer that does not require a medium.

(d) Explain which mode of heat transfer causes sea breezes and land breezes.

Ans: 

(i) During daytime, the air above the land gets heated by the sun to a higher temperature than the air above the sea.

(ii) The hot air above the land has less density than the cool air above the sea. So air above the land moves upward and is replaced by comparatively cooler air moving from the sea, forming convection. In this way, the sea breeze is set up.

(iii) Similarly, during the night, the air above the land is cooler than the air above the sea. The air above the sea, being hotter, moves upward and is replaced by comparatively cooler air moving from the land, forming convection currents. In this way, land breezes are set up.

Thus, sea and land breezes are formed due to convection.

(e) Why is the outer coat of the penguins of Antarctica black?

Ans: 

(i) In Antartica, most of the time the temperature is less than 0⁰ C.

(ii) As black absorbs more heat than it reflects, the outer black coat of the penguins absorbs most of the heat radiation falling on it.

(iii) Hence, to survive in such cold weather, the outer coat of penguins in Antarctica is black.

(f) Why are heaters fitted near the floor and air conditioners, near the ceiling of a room?

Ans: 

(i) The air warmed by the heater near the floor rises easily due to its lower density, and the cold air near the ceiling comes down and replaces it due to its higher density.

(ii) The density difference of air at upper and lower heights sets in convection currents by transferring heat.

(iii) Similarly, air conditioners fitted near the ceiling blow cold air that settles down due to its higher density, while warm air near the floor rises up 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) Solids expand on heating. However, different solids expand to different extents.

(ii) An ordinary glass expands more than a borosil glass for the same temperature rise.

Hence, an ordinary glass bottle cracks when boiling water is poured into it, but a borosilicate glass bottle does not.

(b) The telephone wires which sag in summer become straight in winter.

Ans: 

(i) All substances expand on heating and contract on cooling.

(ii) In summer, when temperatures rise, the telephone wires expand, causing them to sag. In winter, the temperature decreases. This change in temperature brings the particles of the wire closer to each other.

Hence, the telephone wires, which sag in summer, become straight in winter.

(c) Dew drops form on the grass, in winter.

Ans: 

(i) There is water vapour in the air.

(ii) The water-holding capacity of the air depends upon the temperature.

(iii) In winter, when the temperature drops, the water holding capacity also decreases. If the temperature recedes much more, then the water vapour condenses and the dew drops are formed.

(d) In winter, why does an iron pillar feel colder than a wooden pole ?

Ans: 

(i) Iron being a good conductor of heat, it transfers heat with the surroundings during winter and feels colder.

(ii) Wood being a bad conductor of heat, no heat transfer takes place through it.

Hence, wooden poles do not feel cold in the winter.