Living Science 2020 2021 Solutions for Class 7 Science Chapter 5 - Heat And Its Effect

Answer:

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

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

Answer:

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

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.
 

Answer:

Hotness or coldness of a body is measured in terms of the 'Temperature' of the body. Unit of temperature is $°\mathrm{C}$.

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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. 

Answer:

Kelvin temperature scale is used for scientific purposes. 

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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. 

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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. 

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(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.

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.

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.

Answer:

(c) 32^o F
Equation to convert ^oC to ^oF is as follows:
$°\mathrm{C}$ = ($\frac{5}{9}$)($°\mathrm{F}$ - 32)
0  = ($\frac{5}{9}$)($°\mathrm{F}$ - 32)
$°\mathrm{F}$ = 32

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.

Answer:

(b) 18°F fall in temperature
Assume,

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(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.

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.

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.

Answer:

For scientific work, the Kelvin temperature scale is used.

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.

Answer:

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

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.

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.

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.

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The correct formula is F = ($\frac{9}{5}$) C + 32. It is used to convert temperature on a Celsius scale to temperature on a Fahrenheit scale.

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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.

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

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.

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.

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.

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.

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

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

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.

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.

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.

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.

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.

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.

Answer:

Equation to convert ^oC to ^oF $\Rightarrow$  ^oF = ($\frac{9}{5}$) ^oC + 32
When ^oC = 25
^oF = ($\frac{9}{5}$)25 + 32
^oF = 45 + 32
^oF = 77

Equation to convert ^oF to ^oC $\Rightarrow$  ^oC = ($\frac{5}{9}$)(^oF - 32)
^oC = ($\frac{5}{9}$)(86 - 32)
^oC = ($\frac{5}{9}$)(54)
^oC = 30

Answer:

Let the temperature be x^oC, then on the Fahrenheit scale the temperature will be 2x^oF.
Now, the relation between C and F is: C = ($\frac{5}{9}$)(F-32)
Putting the value of C = x and F = 2x in the above equation
We get,   x = ($\frac{5}{9}$)(2x-32)
               x = $\frac{10}{9}$x - $\frac{160}{9}$
               - $\frac{1}{9}$x = - $\frac{160}{9}$
                 $\frac{1}{9}$x =  $\frac{160}{9}$
                  x= 160
Hence, when the temperature is 160^oC, then on the Fahrenheit scale it will be 320^oF.

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.

• Sterilise the thermometer using an antiseptic solution before and after its use.
• Ensure that the mercury level is below 35^oC 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.

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.

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 = [($\frac{9}{5}$)C₂ + 35 ] - [($\frac{9}{5}$)C₁ + 35]
1 = ($\frac{9}{5}$)C₂  - ($\frac{9}{5}$)C₁
1 = ($\frac{9}{5}$)[C₂ - C₁]
$\frac{5}{9}$ = C₂ - C₁
0.56 = C₂ - C₁
Hence, for the same temperature increase, the increase on the Celsius scale is only by 0.56^o, while that on the Fahrenheit scale is by 1^o. Hence, 1^o rise in the Celsius scale is greater than the 1^o rise on the Fahrenheit scale.

Answer:

A clinical thermometer has the temperature range from 35^oC to 42^oC. But the boiling point of water is 100^oC. 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.

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.

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.

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.