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#### Question 1:

When you put a metal key into its metal lock, you find the key is too tight. Which of the following methods will you adopt to make the key fit properly?
(a) Heat the key and the lock.
(b) Cool the key and heat the lock.
(c) Heat the key and cool the lock.
(d) Cool the key and the lock.

#### Answer:

(b) Cool the key and heat the lock.
Key contracts on cooling; similarly, lock expands on heating. This will make the key fit properly in the lock.

#### Question 2:

Rohan wants to test whether a white object or a black object would heat up faster in the sun. The picture shows you his experiment. These thermometers were left out in the sun for 30 minutes.
figure

Which of the following statements is true?
(a) Thermometer 1 reads the same as thermometer 3.
(b) Thermometer 2 shows a higher temperature than thermometer 3.
(c) Thermometer 3 shows higher temperature than thermometer 1.
(d) Thermometer 1 reads the same as thermometer 2.

#### Answer:

(b) Thermometer 2 shows a higher temperature than thermometer 3.
Black object absorbs more heat than the white object. Hence, temperature of black object is higher than the white object.

#### Question 3:

Deepak put twenty ice cubes of the same volume into four containers packed inside different materials. He recorded the time taken for the ice to melt in each case. The table shows his results. Which of the boxes should he choose to carry ice that he needs to take to a friend's house 120 km away?
Deepak's Observations

 Packing Material Melting a. thermocol 3 h 10 min b. paper 2 h 05 min c. sand 30 min d. sawdust 1 h 15 min

#### Answer:

(a) Ice in the thermocol box, takes the longest time to melt. Hence, Deepak should use thermocol material to carry ice to his friend's house.

#### Question 4:

Of the pairs of substances given below, which one contains only good conductors of heat?
(a) wood and iron
(b) iron and glass
(c) iron and copper
(d) copper and glass

#### Answer:

(c) iron and copper
Iron and copper metals are good conductors of heat.

#### Question 5:

Suraj placed a thermometer in a beaker of water and heated it from below.
figure
He found that heat causes
A. rise in temperature
B. expansion in liquids
C. fall in temperature
D. chemical change

a. Only A
b. A and B
c. A, B and D
d. Can't say from this experiment.

#### Answer:

b. A and B
On heating, water expands and its temperature increases. The heating of water is only a physical change.

#### Question 6:

The transfer of heat from molecule to molecule without the movement of molecules from one place to another is called
(a) conduction
(b) expansion
(c) convection
(d) radiation

#### Answer:

(a) conduction
Conduction is the transfer of heat from one molecule to another without the movement of molecules from one place to another.

#### Question 7:

A tight metal lid on a jam jar loosens when it is held upside down in hot water because the hot water causes the
(a) glass jar to contract.
(b) metal lid to contract.
(c) glass jar to expand more than the metal lid.
(d) metal lid to expand more than the glass jar.

#### Answer:

(d) metal lid to expand more than the glass jar
Metal is a good conductor of heat than the glass. So, metal lid expands more than the glass jar.

#### Question 8:

Ron tried out an experiment. He wore a wet sock on his right foot and a dry sock on his left foot. He sat with his feet stretched out in front of a fan. After some time his right foot felt very cold. This was because __________________
(a)  water was cold
(b) the wet sock radiated all the heat falling on it and made his foot feel cold
(c) the water from the sock evaporated due to the heat from his foot and the breeze from the fan, making his foot colder
(d) the water trapped in the sock allowed the heat from his foot to escape

#### Answer:

(c) the water from the sock evaporated due to the heat from his foot and the breeze from the fan, making his foot colder

Wet sock trapped heat from his foot and evaporated the water, causing his foot feel cooler. Also, the breeze from the fan made his foot more cooler.

#### Question 9:

Pranjal performed an experiment which is shown here.
figure

He found that pin 2 fell off first and pin 1 did not fall even after five minutes. Which item can be made of the same materials as the rods A and B?
(a) A-cooking vessel; B handle of a saucepan
(b) A-handle of an electric iron; B base of an electric iron
(c) A-handle of a saucepan; B-coffee mug
(d) A-base of an electric iron; B-handle of an electric iron

#### Answer:

(b) A-handle of an electric iron; B-base of an electric iron
Pin 1 did not fall because rod A is a bad conductor of heat. Pin 2 fell off first because rod B is a good conductor of heat.

#### Question 10:

Fill in the blanks using suitable words.
(a) A thick glass tumbler is taken. Boiling hot water is poured into it. The hot water is in contact with the ______________ layer of glass. By ______________ this layer becomes hot and _____________. Glass is a ______________conductor of heat. Thus the _______________ layer does not get heated quickly. It does not ________________ as much. This ______________ expansion of the layers causes the glass to crack.
(b) Burning coal is kept on a muslin cloth which is spread out on a copper plate. Copper is a ______________ conductor of ______________. It takes away the heat from the ______________. The ______________ of the muslin cloth does not rise enough for it to catch fire.
(c) Rajesh opened a gas tap on the kitchen range. he held a piece of wire netting above it and struck a match below the wire netting. He found the gas burnt below the wire netting but not aove it. the wire netting is made of _______________ which is a good ______________ of heat. It takes away the heat from the fire. Thus the _____________ above the netting did not catch fire.

#### Answer:

(a) A thick glass tumbler is taken. Boiling hot water is poured into it. The hot water is in contact with the inner layer of glass. By heating, this layer becomes hot and expand. Glass is a bad conductor of heat. Thus the outer layer does not get heated quickly. It does not expand as much. This different expansion of the layers causes the glass to crack.

(b) Burning coal is kept on a muslin cloth that is spread on a copper plate. Copper is a good conductor of heat. It takes away the heat from the muslin cloth. The temperature of the muslin cloth does not rise enough for it to catch fire.

(c) Rajesh opened a gas tap on the kitchen range. He held a piece of wire netting above it and struck a match below the wire netting. He found the gas burnt below the wire netting but not above it. The wire netting is made of galvanised iron, which is a good conductor of heat. It takes away the heat from the fire. Thus the air above the netting did not catch fire.

#### Question 11:

Which of the following statements are incorrect? Rewrite them correctly in your notebook.
(a) A Thermos flask that is used to store either cold or hot liquids for several hours minimises heat transfer by conduction and convection only.
(b) We receive heat from the sun by radiation.
(c) Dark objects are better absorbers and better radiators of heat.
(d) Air is a good conductor of heat.
(e) Solids expand more than liquids.
(f) Overhead telephone and electric wires are kept loose because it is difficult to stretch them.
(g) The silvered walls of a thermos flask prevent loss of heat by convection.

#### Answer:

(a) False
A Thermos flask that is used to store either cold or hot liquids for several hours minimises heat transfer by conduction, convection and radiation processes.

(b) True

(c) True

(d) False
Air is a bad conductor of heat.

(e) False
Solids expand less than liquids.

(f) False
Overhead telephone and electric wires are kept loose because these wires contract when temperature decrease.

(g) False
The silvered walls of a Thermos flask prevent loss of heat by radiation.

#### Question 12:

A scientist took an iron rod and a wooden rod and wrapped each of them in a piece of paper. These were then heated as shown below. It was found that the paper around the metal rod does not burn whereas the one around the wooden rod catches fire. Can you explain why?
figure

#### Answer:

Wood is a bad conductor of heat, whereas a metal is a good conductor of heat. When paper wrapped around a metal rod is heated, its heat energy is readily trapped by the metal rod, which prevents the temperature of paper to rise and so the paper does not catch fire. But when the paper wrapped around a wooden rod is heated, it gets heated up without distributing its heat to the rod, which increases the temperature of paper and the paper burns.

#### Question 13:

Look at these two buildings. Which one would heat up more in summer and why?
figure

#### Answer:

Dark-coloured object traps more heat energy than the white object. Hence, the dark-coloured building would heat up more in summer than the white building.

#### Question 14:

In which case will all the water in the test tube heat up? Why?
figure

#### Answer:

Water heats up because of convection. In the test tube, which is heated from the bottom, the water molecules in the bottom of the test tube absorb heat and become lighter. This causes them to rise up, where they transfer their energy to the colder molecules lying above. Hence, all the molecules get heated up. But, if the test tube is heated from above, the molecules lying at the bottom of the test tube do not heat up. This is because the heated molecules move above and do not heat the water molecules lying below.

#### Question 15:

Draw a neat, labelled diagram of a thermos flask. Explain how hot tea kept it it remains hot for a long time.

#### Answer:

Hot tea can transfer its heat to its surrounding by three ways, i.e., conduction, convection or radiation. Thermos flask has vacuum space between double wall of glass. This vacuum restricts the transfer of heat through conduction and convection mode because these two modes require molecules of medium to transfer heat. And the silvered surface reflects again the radiated heat into the flask. Hence, it provides restriction in transfer of heat through radiation mode. As all three modes of transfer of heat are restricted, the tea remains hot for a long time.

#### Question 16:

Match the phrases in column I with those in column II.

 Column I Column II a. maximum temperature of a city i. takes up heat b. ice melts ii. 3,000,000∘C c. temperature of a healthy person iii. 1600∘C d. temperature inside the sun iv. hot object to cold object e. temperature of a steel-making furnace v. 0∘C f. temperature on a cold winter day vi. measured in the day g. heat flow vii. gives out heat h. evaporation viii. 100∘C i. condensation ix. 47∘C j. temperature of boiling water x. 6∘C xi. 37∘C

#### Answer:

 Column I Column II a. maximum temperature of a city ix. 47∘C b. ice melts v. 0∘C c. temperature of a healthy person xi. 37∘C d. temperature inside the sun ii. 3,000,000∘C e. temperature of a steel-making furnace iii. 1600∘C f. temperature on a cold winter day x. 6∘C g. heat flow iv. hot object to cold object h. evaporation i. takes up heat i. condensation vii. gives out heat j. temperature of boiling water viii. 100∘C

#### Question 17:

Define conduction, convection and radiation. In what way is conduction
(a) similar to convection?
(b) different from radiation?

#### Answer:

Conduction is the process of transfer of heat from the hotter end of an object to its cooler end.
Convection is the process of transference of heat through a fluid (liquid or gas).
Radiation is the process of transfer of heat from one point to another point through radiation waves, which does not require any medium to propagate.

(a) Conduction is similar to convection as both the modes require a medium to transfer the heat.

(b)

 Conduction Radiation It requires a medium to transfer heat. It does not require any medium to transfer heat.

#### Question 18:

Explain in terms of molecules what happens when
(a) ice changes to water.
(b) liquid wax solidfies after cooling.

#### Answer:

(a) When ice changes to water, the water molecules, which was at a fixed position in the ice crystal, start to move around each other.

(b) Initially, liquid wax molecules are free to move around; hence, it flows. But after cooling, it solidifies and its molecules become immobile, i.e., fixed at a specific position.

#### Question 19:

Harish picked up a spade that had been laid out in the cold all night. Which would have been warmer−the wooden handle or the metal blade? Why?

#### Answer:

The wooden handle would have been warmer than the metal blade because metal is a good conductor of heat. So, in cold night, metal blade would transfer its heat to cold environment. But wood, being a bad conductor of heat, would not transfer its heat to cold surrounding and would remain warmer than the metal blade.

#### Question 20:

List six effects of heat.

#### Answer:

Effects of heat are as follows:

1. It causes expansion of the material that is heated.
2. It causes increase in temperature.
3. It causes change in state of matter.
4. It causes chemical changes.
5. It causes increase in the movement of particles of the material that is heated.
6. It causes water to kill harmful microorganisms that are present in it at high temperatures.

#### Question 21:

(a) State the difference between a clinical thermometer and a laboratory thermometer.
(b) Draw a neat labelled diagram of a clinical thermometer.
(c) Can a laboratory thermometer be used to measure our body temperature? Why or why not?

#### Answer:

(a)

 Clinical Thermometer Laboratory Thermometer A clinical thermometer is used to measure our body temperature. A laboratory thermometer is used to measure temperature of solutions or any object that is under investigation in laboratory. A clinical thermometer has smaller range as human body has small range of temperature. The range of laboratory thermometer is very high as temperature of objects may vary from 0oC to 100oC or above.

(b)

(c) A laboratory thermometer cannot be used to measure our body temperature because a laboratory thermometer is not as accurate as a clinical thermometer. A clinical thermometer has a small range but has a high accuracy. It measures temperature to the tenth of a degree, but a laboratory thermometer do not have a partial degree separations.

#### Question 22:

Give reasons for the following statements.
(a) Vivek could not unscrew the metal lid on a glass jam jar. But when he placed it upside down in hot water for a few seconds, he could remove the lid easily.
(b) Small gaps are left between the joints of concrete slabs on bridges.
(c) An experienced housewife will always place a metal spoon in a glass tumbler before pouring hot tea into it.
(d) A loosely knitted sweater will keep you warmer than a tightly knitted one.
(e) Tea pots have polished, bright surfaces while frying pans are black.
(f) Ice blocks are covered with sawdust.
(g) The bottom of a pond is cooler than the top during a hot summer day.

#### Answer:

(a)  Metal lid on glass jam jar is screwed tightly, but when Vivek placed it upside down in hot water for a few seconds, the metal lid, being a good conductor of heat, expanded in comparison to the glass, which is a bad conductor of heat; hence, he could remove the lid easily.

(b) A steel bridge expands during the summers. This is the reason small gaps are left between the joints of concrete slabs on the bridges. If this is not done, it will cause serious damage to the bridge.

(c) An experienced housewife will always place a metal spoon in a glass tumbler before pouring hot tea into it because metal spoon, being a good conductor of heat, absorbs most of the heat from tea and prevents glass tumbler to get excessive heat and the expansion of inner surface of glass that may cause breakage of the tumbler.

(d) A loosely knitted sweater has large spaces between them that are filled with air. Air, being an insulator of heat, keeps us warmer. Whereas a tightly knitted one has lesser space for air to be filled in between.

(e) Teapots have polished, bright surface to prevent the heat loss from the radiation mode by reflecting most of the radiation back into the pot. This keeps the tea hot for longer time.
Frying pans have black coating to absorb maximum content of heat from the fire. This is because black body absorbs maximum heat from its surrounding.

(f) Sawdust is a poor conductor of heat and thus creates an insulation for ice blocks from the warmer air outside. Hence, ice blocks are covered with sawdust.

(g) During a hot summer day, the water at the top of the pond absorbs heat from sun and gets heated, but at the bottom of the pond, water is cooler because hot water molecules, being lighter, move upwards and transfer their heat to above molecules, and then on cooling, they move to the bottom.

#### Question 23:

Look at the picture carefully. Which candle would go out first? Why?
figure

#### Answer:

The candle in the chimney with no air gap at bottom will go out first because on burning the candle, all the air in chimney gets heated and melts the wax of candle readily. The candle in the chimney with air gaps at bottom will last longer because, in this case, the lighter air is replaced by the cooler air from the bottom of the chimney, which makes the environment of candle cooler.

#### Question 24:

Can transfer of heat by convection occur in solids? Why or why not?

#### Answer:

The transfer of heat by convection mode cannot occur in solids because in convection mode, the molecules have to move from one place to another to transfer the heat, but in solids, molecules are not free to move from one place to another.

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