NCERT Solutions for Class 7 Science Chapter 4 Heat

1. State similarities and differences between the laboratory thermometer and the clinical thermometer.

Ans: Similarities include the use of mercury and the measurement of temperature.

Differences include clinical thermometers have a kink to prevent the temperature from dropping, but laboratory thermometers do not have a kink. 

In comparison to clinical thermometers, the range of a laboratory thermometer is extremely broad.

2. Give two examples each of conductors and insulators of heat.

Ans: 

3. Fill in the blanks:

a. A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of ________. 

Ans: Conduction.

b. Temperature is measured in degrees _________.

Ans: Celsius.

c. No medium is required for transfer of heat by the process of _________.

Ans: radiation.

d. Clothes of _________ colours absorb heat better than clothes of light colours.

Ans: black 

e. The hotness of an object is determined by its _________.

Ans: Temperature.

f. Temperature of boiling water cannot be measured by a __________ thermometer.

Ans: clinical 

4. Match the following:

i. Land breeze blows during                                                    a. summer

ii. Sea breeze blows during                                                     b. winter

iii. Dark coloured clothes are

preferred during                                                                        c. day

iv. Light coloured clothes are

preferred during                                                                        d. night

Ans:

5. Discuss why wearing more layers of clothing during winter

Ans: When more than one layer of clothing is worn, air gets trapped between them. Heat does not transmit well through air. As a result, the insulation improves and we are more comfortable.

6. Look at Fig. 4.2. Mark where the heat is being transferred by conduction, by convection and by radiation.

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Ans:

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7. In places of hot climate, it is advised that the outer walls of houses be painted white. Explain.

Ans: White-surfaced objects radiate more heat and absorb less. As a result, less outside heat would enter the house.

8. One liter of water at 3000C is mixed with one liter of water at 5000C. The temperature of the mixture will be

a. 8000C

b. More than 5000C

c. 2000C

d. Between 3000C and 5000C

Ans: d. The temperature lies between 3000C to 5000C. As it is a mixture of waters boiled at two different temperatures.

9. An iron ball at 4000C is dropped in a mug containing water at 4000C the heat will

a. Flow from iron ball to water.

b. Not flow from iron ball to water or from water to iron ball.

c. Flow from water to iron ball.

d. Increase the temperature of both.

Ans: b. Not flow from iron ball to water or from water to iron ball. It is due to the fact that both the substances, the water and the iron have the same temperature.

10. A wooden spoon is dipped in a cup of ice cream. Its other end

a. becomes cold by the process of conduction.

b. becomes cold by the process of convection.

c. becomes cold by the process of radiation.

d. does not become cold.

Ans: d. does not become cold. Heat conducts poorly through a wooden spoon. Its opposite end does not grow cold even if it is dipped in a cup of ice cream.

11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that

a. Copper bottom makes the pan more durable.

b. Such pans appear colourful.

c. Copper is better conductor of heat than the stainless steel.

d. copper is a better conductor of heat than the stainless steel.

Ans: c. Copper is a better conductor of heat than stainless steel. Copper bottoms are commonly included with stainless steel cookware. Copper is a superior heat conductor than stainless steel, which could be the reason.

NCERT Solutions for Class 7 Science Heat – Free PDF Download

Aspirants will be able to download CBSE Class 7 Science Chapter 4 as per their convenience and requirements. They can also refer the notes to their friends to make learning more gratified. Students, through the notes provided, will be able to gain crucial insight into the chapters. NCERT Solution for class 7 science chapter 4 is prepared by the best teachers and easy to understand.

Chapter 4 – Heat

4.1 Hot and Cold

While performing our daily tasks, we often need to touch a number of objects, among which some are cold and hot. Obviously, an iron rod exposed to the sun for longer period becomes hot and an ice-cream stored in the refrigerator stays cool. Without noticing, we keep playing between hot and cold. Now, the degree of temperatures varies depending on the situation and objects. So, how is it possible to determine which objects are hotter than others? Naturally, by touching the object it is possible to figure out, but finding accuracy in temperature can become a problem.

4.2 Measuring Temperature

Have you ever seen a thermometer? It is quite natural to get your hands on one of them when someone in your family suffers from fever. A thermometer is used for measuring the temperature of our body. There are different types of thermometer available. What we see commonly in our house is a clinical thermometer. It is used for measuring the body’s temperature only. A clinical thermometer looks like a cylindrical pipe made of glass. A bulb containing mercury stays attached at the bottom. Outside the bulb, you can notice a small shining thread of mercury. If it’s hard to locate, considering rotating the thermometer slightly to see the meter. It is usually seen through the two meeting end of the thermometer’s radius. The scale used for the clinical thermometer is measured in °C.

When reading a thermometer, make sure to note the difference in temperature indicated by two bigger marks. Also, you can note the number of divisions. If a bigger mark reads one degree and it is divided into five divisions, it indicates 1/5 = 0.2 °C. Before using a thermometer, give it some nice jerks so that the mercury accumulates down at the bulb and prevent wrong readings. Now, place the thermometer under the tongue or underarm. After one minute precisely, take out the thermometer and note down the temperature. It is our natural body temperature. Our normal body temperature should be around 37°C.

Important Note: Thermometer must be prevented from receiving falling as it is made of glass. Also, it can be harmful if the mercury leaks out. Do not hold a thermometer by the bulb when reading.

4.3 Laboratory Thermometer

Now, let’s understand how is it possible to take readings from other objects. For this, we need laboratory thermometer. The teacher can probably show you a laboratory thermometer from the chemistry lab. Here, the range of measuring scale differs from that of a clinical thermometer. A lab thermometer’s measuring scale typically ranges from -10°C to 110°C. Each reading should be noted to discern correct information of an object.

Interesting Facts

The thermometers what we see today are an advanced form of traditional thermoscopes. It was first invented by the famous scientist Galileo Galilei. At first, he used a device that looked like an open tube filled with liquid. However, despite evolution, some of the numerical scales for measuring temperatures are still the same. The Fahrenheit and Celsius scales are mostly used for measuring different types of temperatures. Interestingly, there were more than 35 scales designed by 1700s, but finally, conclusions were made by Daniel Fahrenheit and Swedish physicist Anders Celsius. Daniel Fahrenheit assigned the boiling point of water at 212 degrees and the freezing point at 32 degrees. In 1742, Anders Celsius developed the centigrade scale and therefore, assigned the boiling point of water at 0 degrees and the melting point of ice at 100 degrees. The new age Celsius scale is generally popular with 0 degrees at the cold end and 100 degrees at the hot end. Remember, Fahrenheit and Celsius’s scale are same when each comes at the point of -40 degrees.

Other popular scales that are used for laboratory works are Kelvin and Rankine scales. In 1848, British inventor Lord Kelvin, officially known as William Thompson proposed a scale using the Celsius measurement. However, it was keyed to absolute zero (-273.15°C). Looking at the Rankine scale, it was invented by Scottish engineer William Rankine. He, in his research, used the Fahrenheit’s scale and keyed down to absolute zero (-459.67 °F). It is important to keep in mind that when it comes to theoretical temperature, absolute zero is the coldest reading in any substance that does not possess any heat energy.

4.4 Transfer of Heat

Have you ever thought why a frying pan becomes hot when placed on a source of flame? This happens due to the transfer of heat. The heat from the source passes to through the frying pan and cooks the food inside it. Gradually, it starts cooling down when removed from the source of heat. Again, why does it cool down? The heat escapes from the pan by transferring through the floor or anything that touches it. Therefore, it is clear that heat flows from hotter objects to cooler ones. The same happens even in the case of nature that builds the cycle of heat and rain.

4.5 Kinds of Clothes We wear in Summer and Winter

It is common for us to wear light coloured clothes in the time of summer and dark coloured clothes during winter. The reason behind this is its scientific cause. Anything which is light coloured absorbs less heat and reflects more. During the daytime, consider standing in front of a building that is painted white. It’s good if it is newly painted and the colour is glossy. You will notice that standing in front of the building for a long time isn’t possible. Also, the interior of the building remains relatively cooler when compared with other buildings. This is because light colours absorbs less heat and reflect more. On the contrary, the occupants in the building may feel the shiver during winter months. As light colours are unable to absorb more heat, they remain cool even during cold weather. For this reason, it is said to always wear dark coloured clothes during winter so that the heat inside our body cannot escape. It also helps us in preventing the natural and colder breeze from entering in and making our body cold.

Woollen Clothes Keep us Warm in Winter

Why do woollen clothes provide warmth in winter seasons? This is because wool is a bad conductor of heat. The fibres of wool help in keeping the air trapped inside and thus, keep the cold out. The trapped air stops the movement of heat to cold surroundings. Therefore, it is competent in keeping us warm. In a winter night, if you use a couple of blankets to make yourself warm, it will be a lot effective when compared with someone who is using a single blanket. This is not because of mere number count. The fact is, the air which is trapped between the two blankets, keeps you warm and prevents cold to enter in. 

Exercise 4.5 total Solutions: 16 Questions (11 short questions and 5 long questions).

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