Science textbook solutions for Class 7 science Chapter 9 Heat

Important points to remember :
  1. The moisture  in the air cools down  due to the ice cubes in the glass  and collects on the outside of the glass.  
  2. A thermometer  is used to measure the temperature  of a substance. 
  3. Modes of heat transfer : Conduction, convection and radiation. 
  4. Transfer of heat from the hot part of an object  to the cold part, is called  conduction  of heat. 
  5. heat is  transferred by means of  currents, called convection currents. This process is called  convection of heat. 
  6. The transfer of heat that takes place in this way, without a medium, is called radiation. 
  7. In the two examples  above, heat reaches us mainly  by  radiation. Radiation of heat takes place  from many objects in nature such as trees, mountains, stones and roads.  
  8. A  camera has  been developed  which uses these  radiations to make our surroundings  visible at night.  It is called  an infrared  camera. 
  9. Sir James Dewar was a Scottish  scientist.  He made the first thermos flask in 1892. That is why it is called Dewar flask. Even  today,  a Dewar flask is used for keeping  a substance cold or hot.

Question 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 black coloured object.

b) Radiation of heat does not require a medium.

c) Conduction of heat takes place through a good conductor substance.

d) The shining surface in a thermos flask decreases the outgoing heat by reflection.

e) Cooking utensils are made from metals due to their property of conduction.

f) The earth receives heat from the sun by radiation.

Question 2: What will absorb heat?
Steel spoon, wooden board, glass vessel, iron griddle (tava) , glass, wooden spoon , plastic plate, soil, water, wax
Answer :
Steel spoon, iron griddle, water, soil and wax will absorb heat.

Question 3: Write answers to the following question.

a) How does a fever get lowered by putting a cold compress on the forehead of a patient ?
Answer : when you run a fever, your body temperature rises and so does your brain temperature. The cold compress starts cooling the blood temp down as it's entering and leaving the brain.

b) Why are the houses in Rajasthan painted white ?
Answer : To keep the houses cooler on the inside, we paint them white. White colors reflect almost all the heat falling on them, thus keeping the inside of the house cool and comfortable. Black color, for instance, absorbs virtually all the heat falling on it.

c) What are the modes of heat transfer ?
Answer : Heat can be transferred from one place to another by three methods: conduction in solids, convection of fluids (liquids or gases), and radiation through anything that will allow radiation to pass. The method used to transfer heat is usually the one that is the most efficient.

d) Explain which mode of heat transfer causes sea breezes and land breezes.
Answer : Convection causes land and sea breezes to occur. Convection is the heat transfer due to bulk movement of molecules within fluids such as gases and liquids, including molten rock (rheid). Convection takes place through advection, diffusion or both.

e) Why is the outer coat of the penguins of Antarctica black ?
Answer : The outer coat of the penguins of antarctic black as it absorbs more heat from surrounding and provide heat or keep the body warm.

f) Why are heaters fitted near the floor and air conditioners, near the ceiling of a room ?
Answer : HEATER is placed near the floor because hot air rises up and the room will be heatness will spread even. The density of air depends upon the temperature. So it is placed near the ceiling because the cool ( high density ) air tends to settle downwards.

Question 4: Give scientific reasons.
a) An ordinary glass bottle cracks when boiling water is poured into it, but a borosil glass bottle does not.
Answer : A glass is made from liquid sand (silicon dioxide). Normal glass is an insulator i.e, it is a poor conductor of heat.

When a very hot liquid is poured into a glass tumbler, the surface of the glass which is in contact with the hot liquid heats up and expands as per its coefficient of thermal expansion. But the outer layer remains cold and does not expand since glass is a poor conductor and does not transfer the heat quickly. This causes the differential thermal expansion which eventually leads to cracking.

Borosilicate glass or Borosil glass is made up of boron trioxide and silicon dioxide. Borosilicate glasses are known for having very low coefficients of thermal expansion which makes them resistant to thermal shock, more than any other common glass. Borosil glass has a melting point of 820°C.

So, Borosil glass does not crack when hot liquid is poured in it, but normal glass cracks.

b) The telephone wires which sag in summer become straight in winter.
Answer : During summer, the wires heat up and expand in length. Hence it sags down. During winter, the temperature drops and wires cool down. Due to decrease in temperature,it contracts in length and hence becomes straight.

c) Dew drops form on the grass in winter.
Answer : Dew drops are seen on grasses in winter because air contains moisture, especially on mornings as the temperature is less i.e the air is cool. at this temperature air cannot hold moisture in it. the excess moisture condenses on cool surfaces like grass, leaves etc in the form of dew drops.

d) In winter, why does an iron pillar feel colder than a wooden pole ?
Answer : An iron pillar feels colder than a wooden pole in winter because the thermal conductivity of iron is very high as compared to wood. But for iron pillar, the heat conducts away rapidly from your finger to the whole pillar and your finger then keeps trying to supply more heat until it is as cold as the iron.

# Use your brain power
1. Why do we wear woollen clothes  in winter ?
Answer : Well wear woolen clothes in winter because woolen clothes gives us warmth. We should wear light colored clothes in summer because light coloured clothes keeps us cool as they insulate the heat. And cotton clothes makes us comfortable in summers and cotton clothes absorb the sweat which we get mostly in summers.

2. Why  do we use white  clothes  in  summer  and dark or black clothes  in winter ? 
Answer : We prefer white coloured clothes in summer because white colour is poor absorber of heat and so they keep us cool during summer. On the other hand, during winter we prefer black and dark coloured clothes because black colour is good absorber of heat and thus they keep us comfortably warm during winter.

3. Why is there a gap at the joints of rails and of  cement concrete bridges?
Answer : In any bridge gap at joints are provided to allow for the expansion of rails/slabs which may due to the rise of temperature in summer. If we do no provide this gap then there may be a failure due to crushing of slab/rail over one another.

4. What is meant  by thermoware ?
Answer :
Thermoware is made of thermo which means related to heat and ware which refers to object. Thus thermoware refers to any object that doesn't allow heat to escape and and retains warmth.

4. Why is mercury or alcohol used in a thermometer ?
Answer : Mercury is the only one in liquid state at room temperature. It's used in thermometers because it has high coefficient of expansion. Hence, the slightest change in temperature is notable when it's used in a thermometer. It also has a high boiling point which makes it very suitable to measure higher temperatures.

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