Question 1

Q1: (iii) A smoke detector is a device that detects smoke and sounds an alarm. Suppose you are fitting a smoke detector in your room. The most suitable pl...

Accepted Answer

The correct option is (c) On the ceiling. Smoke is warmer than the surrounding air, so it rises. Therefore, the smoke detector should be placed on the ceiling.

Question 2

Q1: (ii) Pins are stuck to a metal strip with wax and a burning candle is kept below the rod, as shown in Fig. 7.15. Which of the following will happen? (...

Accepted Answer

The correct option is (b) Pins I and II will fall earlier than pins III and IV. Heat is transferred by conduction. The pins closest to the heat source will fall first.

Question 3

Q1: Choose the correct option in each case. (i) Your father bought a saucepan made of two different materials, A and B , as shown in Fig. 7.14. The materi...

Accepted Answer

The correct option is (d) A is a poor conductor and B is a good conductor of heat. The handle (A) of a saucepan is made of a poor conductor of heat, so it is easy to hold the saucepan, whereas the base (B) is made of a good conductor of heat for faster cooking.

Question 4

Q10: The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.

Accepted Answer

The water cycle is the continuous movement of water on, above, and below the surface of the Earth. It involves evaporation, condensation, precipitation, and collection. Water evaporates from oceans, lakes, and rivers, rises into the atmosphere, and condenses to form clouds. These clouds then release...

Question 5

Q2: A shopkeeper serves you cold lassi in a tumbler. By chance, the tumbler had a small leak. You were given another tumbler by the shopkeeper to put the...

Accepted Answer

Yes, this arrangement will help to keep the lassi cold for a longer time. The air trapped between the two tumblers acts as an insulator, reducing the heat transfer from the surroundings to the cold lassi. This is because air is a poor conductor of heat.

Question 6

Q3: State with reason(s) whether the following statements are True [T] or False [F]. (i) Heat transfer takes place in solids through convection. [ ] (ii)...

Accepted Answer

(i) False [F]. Heat transfer in solids mainly takes place through conduction. (ii) True [T]. Convection involves the actual movement of particles to transfer heat. (iii) False [F]. Sandy materials allow more seepage of water than clay materials because the spaces between sand particles are larger. (...

Question 7

Q4: Some ice cubes placed in a dish melt into water after sometime. Where do the ice cubes get heat for this transformation?

Accepted Answer

The ice cubes get heat from the surroundings. The surrounding air, the dish, and any other objects in contact with the ice cubes transfer heat to the ice, causing it to melt.

Question 8

Q5: A burning incense stick is fixed, pointing downwards. In which direction would the smoke from the incense stick move? Show the movement of smoke with...

Accepted Answer

The smoke from the incense stick will initially move downwards for a short distance due to gravity. However, since the smoke is warmer than the surrounding air, it will eventually rise upwards due to convection. (See Figure 7.3 in textbook for a similar example).

Question 9

Q6: Two test tubes with water are heated by a candle flame as shown in Fig. 7.16. Which thermometers (Fig. 7.16a or Fig. 7.16b) will record a higher tempe...

Accepted Answer

The thermometer in Fig. 7.16a will record a higher temperature. In Fig 7.16a, the thermometer is placed directly above the flame, and the water near the bottom of the test tube will get heated up and becomes hot. It expands, becomes lighter, and rises. The water on the sides of the beaker is compara...

Question 10

Q7: Why are hollow bricks used to construct the outer walls of houses in hot regions?

Accepted Answer

Hollow bricks are used to construct the outer walls of houses in hot regions because the air trapped in the hollow spaces acts as an insulator. Air is a poor conductor of heat, so it reduces the transfer of heat from the outside environment into the house, keeping the interior cooler.

Question 11

Q8: Explain how large water bodies prevent extreme temperature in areas around them.

Accepted Answer

Large water bodies prevent extreme temperatures in areas around them due to water's high specific heat capacity. This means water takes a longer time to heat up and cool down compared to land. During the day, water absorbs heat, keeping the surrounding areas cooler. At night, water releases heat, ke...

Question 12

Q9: Explain how water seeps through the surface of the Earth and gets stored as groundwater.

Accepted Answer

Water seeps through the surface of the Earth through a process called infiltration. When rainwater falls on the ground, it percolates through the soil and rocks. The spaces between soil and rock particles allow water to seep downwards. Water can infiltrate more readily if the spaces between soil and...

NCERT Solutions