Living Science 2019 Solutions for Class 7 Science Chapter 5 Heat And Its Effect are provided here with simple step-by-step explanations. These solutions for Heat And Its Effect are extremely popular among Class 7 students for Science Heat And Its Effect Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Living Science 2019 Book of Class 7 Science Chapter 5 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Living Science 2019 Solutions. All Living Science 2019 Solutions for class Class 7 Science are prepared by experts and are 100% accurate.

Page No 50:

Question 1:

Heat is a form of energy. What kind of energy is it?

Answer:

Heat energy is basically the energy of the movement of molecules. 

Page No 50:

Question 2:

When you heat a substance, several changes can be observed. Talk about three of these changes.

Answer:

When a substance is heated, the following changes can be observed:

(1) Change in size and shape
(2) Change in state 
(3) Change in color

Page No 50:

Question 3:

The expansion in all solids of equal size when their temperature is increased by the same amount is the same. Do you agree? Give reasons.

Answer:

No, different solids expand by a different amount for the same increase in temperature. Expansion depends upon the nature of the material.

Page No 50:

Question 4:

Arrange in order of increasing expansion on heating: liquids, solids, gases.

Answer:

Molecules are tightly packed in solids as compared to liquids and gases, similarly, molecules of a liquid are bound more as compared to gases. That's why when solids are heated they expand less as compared to liquids and liquids expand less as compared to gases. In gases, molecules are not bound at all, so they expand maximum upon heating.

Hence, the order of expansion can be written as: Gases > Liquids > Solids.
 



Page No 53:

Question 1:

What measure do we use to compare the hotness or coldness of a body?

Answer:

Hotness or coldness of a body is measured in terms of the 'Temperature' of the body. Unit of temperature is °C.

Page No 53:

Question 2:

Between Celsius and Fahrenheit scales which one is more convenient to use? Why?

Answer:

Between Celsius and Fahrenheit scales, the Celsius scale is more convenient to use. Because the Celsius scale has 100 divisions in total. That's why its easier to measure, calculate and convert the temperature on the Celsius scale. 

Page No 53:

Question 3:

Which temperature scale is used for scientific work?

Answer:

Kelvin temperature scale is used for scientific purposes. 

Page No 53:

Question 4:

Which property of matter is used to measure temperature? Matter in which state is more commonly used?

Answer:

Expansion of matter is used to measure the temperature. Solids expand very little, gases expand too much, therefore expansion of liquid is commonly used to measure the temperature. 

Page No 53:

Question 5:

Name the instrument used to measure temperature. What is the special name given to the instrument used to measure body temperature?

Answer:

The instrument used to measure the temperature is known as the thermometer. The instrument used to measure body temperature is generally known as a clinical thermometer. 



Page No 54:

Question 1:

Heat causes

(a) change of state
(b) change of temperature
(c) expansion
(d) all of these

Answer:

(d) all of these

When we heat water, it starts boiling, i.e., it starts expanding. With further heat, it produces steam, i.e., water changes from its liquid state to its gaseous state.

Page No 54:

Question 2:

Heat cause expansion because

(a) heat occupies space
(b) of increase in the movement of molecules
(c) matter tries to escape from the heat
(d) none of the above is true−it is not yet known why heat causes expansion.

Answer:

(b) of the increase in the movement of molecules
On heating, the movement of molecules increases; this increases the average distance between the molecules, thereby causing expansion.

Page No 54:

Question 3:

Which of these will expand the most on heating?

(a) gold
(b) water
(c) mercury
(d) air

Answer:

(d) air

Molecules in the air are not bound to each other; hence, the average distance between the molecules will increase considerably when heated. Therefore, air will expand more on heating.

Page No 54:

Question 4:

0°C is the same temperature as

(a) 0°F
(b) 212°F
(c) 32°F
(d) 100°F

Answer:

(c) 32o F
Equation to convert oC to oF is as follows:
°C = (59)(°F - 32)
0  = (59)(°F - 32)
°F = 32

Page No 54:

Question 5:

A bimetallic strip is made of metals M1 and M2. On cooling through the same temperature, M1 contrast more than M2. When the strip is heated, it will

(a) bend towards M1
(b) bend towards M2
(c) not bend at all
(d) direction of bending cannot be predicted

Answer:

(b) bend towards M2
Because M1 contracts more than M2on cooling, M1 will also expand more than M2 on heating. Hence, the strip will bend towards M2.



Page No 55:

Question 6:

A 10°C fall in temperature is equal to

(a) 10°F fall in temperature
(b) 18°F fall in temperature
(c) 10°F rise in temperature
(d) 18°F rise in temperature

Answer:

(b) 18°F fall in temperature
Assume,

  Initial Temperature Final Temperature
In Celcius Scale C1 C2
In Fahrenheit Scale F1 F2

oC2 - oC1 = -10oC
As, oC = (59)(oF-32)
oC2-oC1 = (59)(oF2-32) - (59)(oF1-32)
-10oC = (59)(oF2-oF1)
-18oC = oF2-oF1
The negative sign implies that in the Fahrenheit scale temperature decreases by 18oF.

Page No 55:

Question 7:

Temperature is measured by expansion on heating. Expansion in which of these is most commonly used?

(a) solid
(b) liquid
(c) gas
(d) none of these

Answer:

(b) liquid

Expansion in solids is minimum whereas it is very high in gases. Hence, measurement of temperature can be done easily through the expansion in liquids as in liquids expansion is neither too much nor too less.

Page No 55:

Question 8:

When you touch a cold object

(a) heat flows from your body to the object
(b) heat flows from the object to your body
(c) temperature flows from your body to the object
(d) temperature flows the object to your body

Answer:

(a) heat flows from your body to the object

Heat is a form of energy, it can flow from one object to the other. However, temperature cannot flow from one object to the other. A cold object contains minimal heat energy as compared to our body, so when we touch a cold object, heat from the higher energy region, i.e., our body, flows to the lower energy region, i.e., the cold object.

Page No 55:

Question 1:

Heat is a form of __________

Answer:

Heat is a form of energy.
This is because heat can be converted into other forms of energy, and many other forms of energy can be converted into heat energy.

Page No 55:

Question 2:

For scientific work, the ________ temperature scale is used.

Answer:

For scientific work, the Kelvin temperature scale is used.

Page No 55:

Question 3:

The apparatus commonly used in the laboratory to show expansion on heating is the ring and _________ apparatus.

Answer:

The apparatus commonly used in the laboratory to show expansion on heating is the ring and ball apparatus.

In this experiment, the ball cannot pass through the ring due to its expansion, but the same ball can pass through the same ring when it is cooled down.

Page No 55:

Question 4:

A solid is heated from 0° to 100°C. Its volume increases from V1 to V2. It is now cooled to 0°C. Will its volume at 0°C be V1 or V2?

Answer:

At 0oC, the volume of the solid will be V1 again.
The molecules of solids are tightly packed. Therefore, on heating to 100oC, the molecules just vibrate at their specific positions, thereby increasing the volume. Once the solids are cooled back to 0oC, the molecules stop vibrating, thereby returning to their specific positions.

Page No 55:

Question 5:

Which of these expand the most on heating−solids, liquids or gases?

Answer:

On heating, gases expand the most.
Because the molecules of gases are not bound to each other, the vibration of molecules increases when gases are heated. This increases the average distance between the molecules, resulting in their expansion. In liquids and solids, molecules are bound to each other and restrict the expansion as compared to those in gases.

Page No 55:

Question 6:

All solids expands by the same amount when heated through the same temperature increase. True or false?

Answer:

False. Different solids have different arrangements of molecules; hence, their molecular bonding will also be different. This brings a difference to the amount of expansion in different solids at the same temperature.

Page No 55:

Question 7:

If overhead electric cables are being laid in summers, should they be tightened up, or left a little loose?

Answer:

If overhead electric cables are laid in summers, they should be left a little loose. This will save the cables from tearing when they start contracting in winters.

Page No 55:

Question 8:

Which is the correct formula: F = (95)C + 32 or C = (95)F + 32?

Answer:

The correct formula is F = (95) C + 32. It is used to convert temperature on a Celsius scale to temperature on a Fahrenheit scale.

Page No 55:

Question 9:

Temperature is usually measured by measuring expansion in gases since gases expand the most. True of false?

Answer:

False. Temperature is usually measured through the expansion in liquids. Gases expand the most; therefore, they fail to give the correct reading. Again, solids expand the least; therefore, they too fail to give the correct value of temperature.

Page No 55:

Question 10:

What are the upper fixed points of the Celsius and the Fahrenheit scales?

Answer:

The upper fixed point of the Celsius scale is 100oC, at which pure water starts boiling. On the Fahrenheit scale, the upper fixed point is 212F, which is equivalent to 100oC on the Celsius scale.

Page No 55:

Question 11:

A ________ thermometer is used to record body temperature.

Answer:

A clinical thermometer is used to record body temperature.
The range of temperature in the thermometer varies from 35°C to 42°C. Outside this range of body temperature, no human can survive.

Page No 55:

Question 1:

List four effects that heat produces.

Answer:

Four effects of heat are as follows:

  1. Change in temperature - When water is heated, its temperature increases and it starts boiling.
  2. Change in State - On boiling water, steam is evolved, i.e., the its liquid state changes to its gaseous state.
  3. Expansion - While heating water, some of it overflows due to expansion, i.e., increase in volume.
  4. Chemical change - When vegetables are boiled in water while cooking, they become soft and their taste changes. Thus, a chemical change occurs in the vegetables.

Page No 55:

Question 2:

What does a bimetallic strip consist of?

Answer:

A bimetallic strip consists of two metal strips, one of iron and the other of brass, bolted together firmly. The two metals expand at different rate on heating. Brass expands more than iron on heating, thereby bending the bimetallic strip towards the iron strip. And, after returning to the room temperature, the strip straightens as before.

Page No 55:

Question 3:

A steel bridge expands in summer. What precautions are taken to make sure this expansion does not damage the bridge?

Answer:

A steel bridge expands in the summers. One of its end is fixed and the other end is made to rest on rollers with enough space for expansion. This allows the bridge to expand easily without taking any damage.

Page No 55:

Question 4:

How are temperature and hotness of a body related?

Answer:

Hotter the body, higher is its temperature. Hence, hotness of a body is directly proportional to its temperature.

Page No 55:

Question 5:

What do you mean by 'upper fixed point'?

Answer:

Upper fixed point is the temperature at which pure water boils at the sea level. In the Celsius scale, the upper fixed point is 100oC, and in the Fahrenheit scale, it is 212oF.

Page No 55:

Question 6:

Why does a clinical thermometer have a kink in its stem?

Answer:

As soon as the thermometer is taken out of our mouth, i.e., from a hotter region to a colder region, the mercury in it starts contracting and tends to flow back into the bulb. The kink in the stem prevents the mercury from falling back immediately, thereby giving us the accurate reading of our body temperature.



Page No 56:

Question 1:

Explain why a substance expands on heating.

Answer:

When a substance is heated, its molecular movement increases. Due to this, the average distance between the molecules also increases, thereby increasing the volume of the substance, i.e., expanding the substance. However, the movement of molecules depends upon the state of matter. Solids have tightly packed molecules; hence, least movement of molecules takes place leading to minimal expansion. In liquids, the molecules are less tightly packed as compared to solids; hence, they expand more than solids. Molecules of gases have no bonding at all; therefore, gases expand the most among the three.

Page No 56:

Question 2:

Give one example where expansion on heating is put to good use. Explain the working with the help of a labelled diagram.

Answer:

Automatic fire alarm is an example of expansion on heating, which is put to good use. The fire alarm works on the principle that different solids expand differently when heated. An automatic fire alarm consists of a bimetallic strip which have two different metals strips, iron and brass, bolted together firmly. Brass expands more than iron on heating. Therefore, when fire will reach this bimetallic strip, the brass will expand more thereby bending it towards the iron strip. Due to this bending of the strip, the circuit for the electric bell is completed and it starts ringing as the fire alarm. Once the strip cools down, it will return to its original shape, i.e., straighten up to break the circuit and stop the alarm.

Page No 56:

Question 3:

Explain why gases expand more than liquids and solids.

Answer:

Gases comprise loosely packed molecules as compared to that of liquids and solids. Therefore, when the gases are heated, there is a huge vibration in their molecules, thereby increasing the average distance between them. In case of solids and liquids, there is a strong bonding between their molecules; therefore on heating, these molecules vibrate but less comparatively to gases where the bonding between molecules is almost nil. Hence, in solids and liquids, expansion will take place but less than gases.

Page No 56:

Question 4:

Explain with the help of a labelled diagram an experiment to show that liquids expand when heated.

Answer:

Take a flask filled with a liquid up to its brim. Take a rubber cork with a hole in it and insert a glass tube in it. Now, fit the cork tightly on the mouth of the flask. You will notice that the the level of the liquid will rise in the tube. Note down the level in the tube and start heating the flask. Initially, the liquid level will drop, but after a few moments it will start rising. At first the liquid level will fall due to the expansion of the flask on heating; then when this heat reaches the liquid, it starts expanding. Only at this time you will notice that the level of liquid is rising in the tube. After cooling back the liquid to its normal temperature, the level will drop again.

Page No 56:

Question 5:

Expansion on heating can be a nuisance. Explain giving two examples.

Answer:

Expansion due to heating can be a nuisance. This is explained below with the help of the following examples:
    1.  Railway tracks are made up of iron which expands during the summers. Hence, if no gap is left between two conjoined tracks, they will bend and result into serious accidents.


    2.  A steel bridge also expands during the summers. This is the reason why one end of a steel bridge is fixed and the other rested on rollers with enough space on its front to allow it to expand easily. If this is not done, it will only cause serious damage to the bridge.

Page No 56:

Question 6:

Convert: 25 °C to °F and 86 °F to °C

Answer:

Equation to convert oC to oF   oF = (95oC + 32
When oC = 25
oF = (95)25 + 32
oF = 45 + 32
oF = 77

Equation to convert oF to oC   oC = (59)(oF - 32)
oC = (59)(86 - 32)
oC = (59)(54)
oC = 30

Page No 56:

Question 7:

Calculate the temperature at which the reading on the Fahrenheit scale is double the reading on the Celsius scale.

Answer:

Let the temperature be xoC, then on the Fahrenheit scale the temperature will be 2xoF.
Now, the relation between C and F is: C = (59)(F-32)
Putting the value of C = x and F = 2x in the above equation
We get,   x = (59)(2x-32)
               x = 109x - 1609
               - 19x = - 1609
                 19x =  1609
                  x= 160
Hence, when the temperature is 160oC, then on the Fahrenheit scale it will be 320oF.

Page No 56:

Question 8:

What precautions will you take while measuring temperature with a laboratory thermometer? What extra precaution will you take while measuring body temperature with a clinical thermometer?

Answer:

Precautions that should be taken while using a laboratory thermometer for measuring temperature are as follows:

  • It should be properly washed before and after use.
  • It should be kept upright while reading the temperature.
  • Its bulb should be properly dipped into the liquid or perfectly touched to the surface of the material whose temperature is being measured.
  • Its bulb should not touch the surface of the container in which the material or liquid has been placed.
  • It should be handled with care, as it is made up of a glass tube.
Precautions that should be taken while using a clinical thermometer for measuring body temperature are as follows:
  • Sterilise the thermometer using an antiseptic solution before and after its use.
  • Ensure that the mercury level is below 35oC before measuring the temperature.
  • Take the reading keeping the level of mercury along the line of sight.
  • Avoid holding it by its bulb while reading the temperature.

Page No 56:

Question 1:

You are planning to go to a very cold place where the night temperature drops to -50 °C.
Which thermometer will you take with you−mercury of alcohol? Why?

Answer:

To measure the temperature of cold places where the night temperature drops to -50°C, we will use the alcohol thermometer. This is because alcohol freezes at -115°C, while mercury freezes at -38°C.

Page No 56:

Question 2:

Which is greater−a 1° rise in the Celsius scale or a 1° rise in the fahrenheit scale?

Answer:

1° rise in the Celsius scale is greater than a 1° rise in the Fahrenheit scale. It is shown in the calculation below.
F2 - F1 = [(95)C2 + 35 ] - [(95)C1 + 35]
1 = (95)C2  - (95)C1
1 = (95)[C2 - C1]
59 = C2 - C1
0.56 = C2 - C1
Hence, for the same temperature increase, the increase on the Celsius scale is only by 0.56o, while that on the Fahrenheit scale is by 1o. Hence, 1o rise in the Celsius scale is greater than the 1o rise on the Fahrenheit scale.

Page No 56:

Question 3:

Can a clinical thermometer be used to measure the temperature of boiling water? Why?

Answer:

A clinical thermometer has the temperature range from 35oC to 42oC. But the boiling point of water is 100oC. Therefore, it is not possible to measure the temperature of boiling water with a clinical thermometer, as it can break outside its range of temperature.

Page No 56:

Question 4:

Why is a laboratory thermometer not used to measure body temperature?

Answer:

A laboratory thermometer is designed to measure high temperature ranges; therefore, the divisions marked in between the degrees are not equal to that of a clinical thermometer which is specifically designed for measuring small temperature ranges. The divisions marked on a clinical thermometer are accurate to measure the body temperature perfectly.

Page No 56:

Question 5:

You have seen that an ordinary glass tumbler can crack if very hot or very cold water is poured into it.You have two glass tumblers made of ordinary glass−one with thick walls and the other with very thin walls. Which one is more likely ot crack when very hot or very cold water is poured into them?

Answer:

A thick walled tumbler will crack more easily than a thin walled tumbler when extremely hot or cold water is poured into them.This is because in a thick walled glass, it's outer wall will be less affected by the extreme temperature as compared to its inner wall. The inner wall will expand or contract easily, while the outer wall will remain unaffected causing the glass to break. But in a thin glass, both the inner and the outer wall will react to the high or low temperature and expand or contract almost at the same time. Hence, it is less likely to crack.

Page No 56:

Question 1:

​A railway engineer who looks after the maintenance of rails is quite fed up of the extensive maintenance that is required at the hundreds of joints in the rails. The gaps between short sections of rails have to be constantly checked. Also they cause the rail ride to be rough. He wants to design a railroad without these joints. However, he knows that expansion on heating is a fact of life and cannot be done away with.
He thinks of using much longer rails to reduce the number of gaps, and hence the maintenance. However, it is pointed out by a colleague that this would not work because longer rails also meant that the gaps had to be longer. This would make the ride rougher and could also prove to be dangerous–the train could derail at these joints.

Answer these questions.
1. Why would the gaps be longer if longer sections of rails are used?
2. Can you offer a solution that would do away with the gaps?

Answer:

(1) A gap between the sections of rails is necessary because due to heating rail tracks undergo linear expansion and their length is increased. Longer sections of rails mean longer expansion in them, hence longer gaps would be required.

(2) We can use long welded rails(LWR) and continuously welded rails(CWR). The expansion/contraction of LWR and CWR is taken care of by a device called Switch Expansion Joint (SEJ) at the ends. The welding of rails reduces maintenance efforts of the track.



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