Heat | Aster Classes

Q.4 Answer the following (Any one)


Q.3. Answer the following ( Any two)


Q.2. Answer the following (any three)


(B). Choose the correct alternatives.


Q.1. (A) Answer the following questions.


CHAPTER 7, heat, 9th std, science , physics, tamilnadu board,

I. Choose the correct answer:

Question 1.

Calorie is the unit of

(a) heat

(b) work

(c) temperature

(d) food

Answer:

(a) heat

Question 2.

SI unit of temperature is

(a) fahrenheit

(b) joule

(c) Celsius

(d) kelvin

Answer:

(d) kelvin

Question 3.

Two cylindrical rods of same length have the area of cross-section in the ratio 2:1. If both the rods are made up of same material, which of them conduct heat faster?

(a) Both rods

(b) Rod-2

(c) Rod-1

(d) None of them

Answer:

(c) Rod-1


Question 4.

In which mode of transfer of heat, molecules pass on heat energy to neighbouring molecules without actually moving from their positions?

(a) Radiation

(b) Conduction

(c) Convection

(d) Both B and C

Answer:

(b) Conduction


Question 5.

A device in which the loss of heat due to conduction, convection and radiation is minimized is

(a) Solar cell

(b) Solar cooker

(c) Thermometer

(d) Thermos flask

Answer:

  1.  (a) Heat
  2.  (d) kelvin
  3.  (c) Rod-1
  4.  (b) Conduction
  5.  (d) Thermos flask

II. Fill in the blanks.

  1. The fastest mode of heat transfer is ……………….
  2. During day time, air blows from ………… to …………….
  3. Liquids and gases are generally …………….. conductors of heat.
  4. The fixed temperature at which matter changes state from solid to liquid is called ………………

Answer:

  1. radiation
  2. land, sea
  3. bad
  4. melting point

III. Assertion and Reason type questions.

Mark the correct choice as:

(a) If both assertion and reason are true and reason is the correct explanation of assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of assertion.

(c) If assertion is true but reason is false.

(d) If assertion is false but reason is true.

Question 1.

Assertion(A): Food can be cooked faster in copper bottom vessels.

Reason (R): Copper is the best conductor of heat.

Answer:

(a) If both assertion and reason are true and reason is the correct explanation of assertion.

Question 2.

Assertion(A): Maximum sunlight reaches earth’s surface during the afternoon time.

Reason(R): Heat from the sun reaches earth’s surface by radiation.

Answer:

(b) If both assertion and reason are true but reason is not the correct explanation of assertion.

Question 3.

Assertion(A): When water is heated up to 100°C, there is no raise in temperature until all water gets converted into water vapour.

Reason(R): Boiling point of water is 10°C.

Answer:

(c) If assertion is true but reason is false.


IV. Answer briefly.

Question 1.

Define conduction.

Answer:

The process of transfer of heat in solids from a region of higher temperature to a region of lower temperature without the actual movement of molecules is called conduction.

Question 2.

Ice is kept in a double-walled container. Why?

Answer:

Ice is kept in a double-walled container so as to prevent melting of ice from the heat absorbed present in the immediate surroundings. The vacuum present in between the two walls prevents the transfer of heat from the first to the second wall and hence the ice remains in the solid form for a longer time period.

Question 3.

How does the water kept in an earthen pot remain cool?

Answer:

An earthen pot consists of small pores from which the water inside the pot constantly seeps out and gets evaporated due to the presence of high temperature around it. The evaporation process requires heat which is acquired from the surface of the pot, hence making the water and the pot cooler. 

Question 4.

Differentiate convection and radiation.

Answer:

S.No.ConvectionRadiation
1.Flow of heat through a fluid from places of higher temperature to places of lower temperature by movement of the fluid itself.Flow of heat from one place to another by means of electromagnetic waves.
2.Convection needs matter to be present.Radiation can occur even in vacuum.
3.Convection seen in daily life:Hot air balloons, breeze, wind, chimney.Radiation in daily life:White or light coloured cloths, highly polished surface of airplane, helps to reflect most of the heat radiation from the sun.

Question 5.

Why do people prefer wearing white clothes during summer?

Answer:

People prefer white or light coloured clothes during summer as they are good reflectors of heat and hence, they keep us cool.

Question 6.

What is specific heat capacity?

Answer:

  • The specific heat capacity of a substance is defined as the amount of heat required to raise the temperature of 1 kg of the substance by 1°C or 1 K.
  • Q = mC∆T, where Q is the quantity of heat required to raise the temperature and m is the mass of the body and AT is the change in temperature of the body.
  • The SI unit of specific heat capacity is Jkg– 1 K– 1. The most commonly used units of specific heat capacity are J/kg°C and J/g°C.

Question 7.

Define thermal capacity.

Answer:

  • Heat capacity or thermal capacity is defined as the amount of heat energy required to raise the temperature of a body by 1°C. It is denoted by ‘C’.
  • C = Q/t, where C’ is the heat capacity, ‘Q’ is the quantity of heat required and ‘f’ is rise in temperature.
  • SI unit of heat capacity is J/K. It is also expressed in cal/°C, kcal/°C or J/°C.

Question 8.

Define specific latent heat capacity.

Answer:

  • Specific latent heat is the amount of heat energy absorbed or liberated by unit mass of a substance during change of state without causing any change in temperature.
  • Specific latent heat is given as L = Q/m, where ‘Q’ is the amount of heat energy absorbed or liberated and ‘m’ is mass of a substance during its change of phase at a constant temperature.
  • The SI unit of specific latent heat is J/kg.

V. Answer in detail.

Question 1.

Explain convection in daily life.

Convection in daily life:

(i) Hot air balloons: Air molecules at the bottom of the balloon get heated by a heat source and rise. As the warm air rises, cold air is pushed downward and it is also heated. When the hot air is trapped inside the balloon, it rises.

(ii) Breeze: During day time, the air in contact with the land becomes hot and rises. Now the cool air over the surface of the sea replaces it. It is called sea breeze. During night time, air above the sea is warmer. As the warmer air over the surface of the sea rises, cooler air above the land moves towards the sea.

(iii) Winds: Air flows from area of high pressure to area of low pressure. The warm air molecules over hot surface rise and create low pressure. So, cooler air with high pressure flows towards low pressure area. This causes wind flow.

(iv) Chimneys: Tall chimneys are kept in kitchen and industrial furnaces. As the hot gases and smoke are lighter, they rise up in the atmosphere.

Question 2.

What are the changes of state in water? Explain.

Answer:

Any matter around us can be in three forms: solid, liquid and gas, called as states of matter. Depending upon the temperature, pressure and transfer of heat, matter is converted from one state to another and is known as change of state in matter. There are different such processes in the change of state in matter.

For example;

  • Water molecules are in liquid state at normal temperature. When water is heated to 100°C, it becomes steam or vapour which is a gaseous state of matter. The process by which a
    liquid is converted to vapour by absorbing heat is called boiling or vaporization.
  • The temperature at which a liquid changes its state to gas is called boiling point.
  • On reducing the temperature of the steam it becomes water again. The process by which a vapour is converted to liquid by releasing heat is called condensation. On reducing the temperature of water further to 0°C, it becomes ice which is a solid state of water.
  • The process by which a liquid is converted to solid by releasing heat is called freezing. The
    temperature by which a liquid changes its state to solid is called freezing point. Ice on
    heating, becomes water again by absorbing heat, a process known as melting.
  • Dry ice changes directly to gaseous state without becoming liquid. This process is called
    sublimation.

Thus, water changes its state when there is a change in temperature.


Question 3.

How can you experimentally prove that water is a bad conductor of heat? How is it possible to heat water easily while cooking.

Experiment to prove that water is a bad conductor of heat:

Answer:

Take a glass tube and drop an ice cube wrapped in wire gauze in it.

Now fill 3/4th of this tube with water and place it above the burner as shown in the figure.

You can observe that the water boils at the edge and the ice present in the bottom of the tube has not melted indicating that heat has not reached the bottom where the ice cube is present. This proves that water is a bad conductor of heat.

It is easy to heat water easily or quickly while cooking. This is because, while cooking the vessel or pan is usually covered with a lid.

This leads to three things;

  1. Radiation from the hot water is reflected back into the pan rather than being emitted
  2. Free convection is effectively eliminated, and
  3. Evaporative cooling’ is also eliminated.
    This in turn allows the water to be heated more easily.

VI. Problems:

Question 1.

What is the heat in joules required to raise the temperature of 25 grams of water from 0°C to 100°C? What is the heat in Calories? (Specific heat of water = 4.18 J/g°C).

Solution:

Given m = 25 g, ∆T = (100 – 0) = 100°C

Or in terms of Kelvin (373.15 – 273.15) = 100K,

C = 4.18J/g°C

Heat energy required, Q = m × C × ∆T = 25 × 4.18 × 100 = 10450 J


Question 2.

What could be the final temperature of a mixture of 100 g of water at 90°C and 600g of water at 20°C.

Solution:

To find final temperature: ∆Q = mc

lOOg of water originally at 90°C will loose an amount of heat,

∆Q = mc ∆T

∆Q = 100 × c × (90 – T)

The same amount of heat will be absorbed by 600g of water originally at 20°C to raise its temperature to T.

∆Q = 600 × c × (T – 30)

600C (T – 20°) = 100C (90° – T)

6T – 120° = 90° – T

6T + T = 120° + 90°

7T = 210° ⇒ T = 210/7

T = 30°C


Question 3.

How much heat energy is required to change 2 kg of ice at 0°C into water at 20°C? (Specific latent heat of fusion of water = 3,34,000J/kg, Specific heat capacity of water = 4200Jkg– 1 K– 1 ).

Solution:

Total heat = Heat required to convert 2Kg of ice into water at 0°C + Heat required to convert 2Kg of water at 0°C to 2Kg of water at 20°C

Heat = m (hfw) + mc∆T

Here, m(mass of ice) = 2Kg

hfw (specific latent heat of water) = 3,34,000J/Kg

C (specific heat capacity of water) = 4200JKg– 1K– 1

AT (Temperature difference) = 20°C

Therefore, Heat required = (2 × 334000) + (2 × 4200) (20 – 0)

= 668000 + 8400 (20)

= 668000+ 168000

Heat required = 8,36,000 J


Question 1:

Fill in the blanks and rewrite the sentence.

a. The amount of water vapour in air is determined in terms of its ……….. .

b. If objects of equal masses are given equal heat, their final temperature will be different. This is due to difference in their …………….. .

c. During transformation of liquid phase to solid phase, the latent heat is ………… .

ANSWER:

a. The amount of water vapour in air is determined in terms of its absolute humidity.
b. If objects of equal masses are given equal heat, their final temperature will be different. This is due to differences in their different specific heat capacity.
c. During transformation of liquid phase to solid phase, the latent heat is latent heat of fusion.

Question 2:

Observe the following graph. Considering the change in volume of water as its temperature is raised from 0°C, discuss the difference in the behaviour of water and other substances. What is this behaviour of water called?

ANSWER:

Most of the substances expand on heating and contract on cooling. Whereas, from the graph, it is clearly visible that water shows a distinct and peculiar behavior between 0oC to 4oC.
Water, instead of expanding, contracts between 0oC to 4oC. After 4oC, it shows the normal behavior of expansion as is shown by other substances.
Thus, at 4oC, water posses maximum density and minimum volume. 
This behavior of water between 0oC to 4oC is known as anomalous behavior of water.

Question 3:

What is meant by specific heat capacity? How will you prove experimentally that different substances have different specific heat capacities?

Take three balls of iron, copper and lead of equal mass and put them in boiling water for some time.
Then, take them out of the water and measure their temperature.
All of them will be at temperature 100oC. Now, put them immediately on the thick slab of wax.
Note the depth that each of the ball goes into the wax.
The ball which absorbs more heat from the water will give more heat to wax.
More wax will thus melt and the ball will go deeper in the wax. It will be observed that the iron ball goes deepest into the wax.
Lead ball goes the least and copper ball goes to intermediate depth.
This shows that for equal rise in temperature, the three balls have absorbed different amounts of heat.
This means that the property which determines the amount of heat absorbed by a ball is different for the three balls.
This property is called the specific heat capacity.

Question 4:

While deciding the unit for heat, which temperature interval is chosen? Why?

ANSWER:

While deciding the unit for heat (which is calorie), the temperature interval chosen is 14.5oC-15.5oC.
We know that the amount of heat released or absorbed by a body is given as∆Q =ms ∆T
Now, we also know that one calorie is defined as the amount of heat required to raise the temperature of 1 g of water through 1oC.
Thus, for 1 calorie of heat energy, the specific heat capacity of water should be 1 cal g-1 oC-1.
It is found experimentally that the specific heat capacity of water is 1 cal g-1 oC-1 when the temperature range is 14.5oC-15.5oC.

Question 5:

Explain the following temperature vs time graph.

ANSWER:

In the given graph, line AB represents conversion of ice into water at constant temperature.
When ice is heated, it melts at 0oC and converts into water maintaining constant temperature of 0oC.
This constant temperature, at which the ice converts into water is called the melting point of ice.
Also, during this transition, the ice absorbs heat energy.
This heat energy is utilized for weakening the bonds between the atoms or molecules in the ice to transform itself into liquid.
This heat energy absorbed by ice, at constant temperature, to convert it into liquid is called the latent heat of fusion.
Once all the ice is transformed into water, the temperature of the water starts rising. It increases upto 100oC.
Line BC in the graph represents rise in temperature of water from 0oC to 100oC.
Thereafter, even though heat energy is supplied to water, its temperature does not rise.
The heat energy is absorbed by water at this temperature and is used to break the bonds between molecules of the liquid and convert the liquid into gaseous state.
Thus, during transformation from liquid phase to gas phase, heat energy is absorbed by the liquid, but its temperature does not change.
The constant temperature at which the liquid transforms into gaseous state is called the boiling point of the liquid.
The heat energy absorbed at constant temperature during transformation of liquid into gas is called the latent heat of vaporization.

Question 6:

Explain the following:

a. What is the role of anomalous behaviour of water in preserving aquatic life in regions of cold climate?

ANSWER:

The anomalous behaviour of water is that it contracts from 0oC to 4oC and beyond 4oC it expands.
Thus, the density of water is maximum at 4oC.
When the surrounding temperature falls, the water in oceans and rivers cools down and say the temperature of water reaches 4oC.
Thus, the water reaches its maximum density at this temperature. 
Below this temperature (4oC), the water layer on the surface expands due to anomalous behaviour of water because of which its density decreases.
Thus, this colder layer remains on top  and converts into ice which acts as an insulator and does not allow the temperature of water layer below it to fall below 4oC.
In this way, a livable temperature is maintained for the aquatic life under the oceans and rivers due to the anomalous behaviour of water.
b. How can you relate the formation of water droplets on the outer surface of a bottle taken out of refrigerator with formation of dew?

ANSWER:

In both the given processes, the temperature of the air near the surface of bottle or the leaves (in case of dew formation) decreases to dew point.
Thus, the air becomes saturated with water vapour.
As a result, water vapour converts or condenses into tiny water droplets which appears on the surface of bottle or leaves.

c. In cold regions in winter, the rocks crack due to anomalous expansion of water.

ANSWER:

Water has the property of expanding below 4oC.
Thus, in cold regions when the temperature falls below 4oC, the water content present in rocks expands.
Due to this expansion of water or increase in volume of water, the rocks cracks.

Question 7:

Answer the following:

a. What is meant by latent heat? How will the state of matter transform if latent heat is given off?

ANSWER:

Latent heat of a body is the amount of heat required to change the state of unit mass of the body from solid to liquid or from liquid to gas without any change in temperature.
If latent heat is given off, then the body in liquid state will transform to solid state and the body in vapour state will transform to liquid state.

This means the internal energy of the matter decreases when latent heat is given off.

b Which principle is used to measure the specific heat capacity of a substance?

ANSWER:

Principle of heat exchange is used in the calorimetry method to determine the specific heat capacity of a substance.
c. Explain the role of latent heat in the change of state of a substance?

ANSWER:

According to the kinetic model, the total energy of a molecule is the sum of kinetic energy due to its motion (which depends on temperature) and its potential energy (which depends on the force of attraction between the molecules and the separation between them.
During the phase change of a substance, its temperature does not increase and hence its kinetic energy.
But the potential energy increases or decreases depending on the type of phase transformation (for example, while melting of ice, the separation between the molecules of ice increases and hence its potential energy). 
Thus, some energy is required in increases or decreasing the separation between the molecules.
Thus, the heat supplied during phase transformation is used up in increasing or decreasing the potential energy and this heat energy is known as latent heat.
d. On what basis and how will you determine whether air is saturated with vapour or not?

ANSWER:

On the basis of amount of water vapour present in the air, it can be either saturated or unsaturated.
If the amount of water vapours exceeds the amount that the air can contain, then it is called saturated.
If the amount of water vapour is less than the limit of the amount that air can contain then it is called unsaturated.
We can determine whether the air is saturated with vapour or not in terms of relative humidity.
If the relative humidity is 100%, the air will be saturated otherwise not.

Question 8:

Read the following paragraph and answer the questions.

If heat is exchanged between a hot and cold object, the temperature of the cold object goes on increasing due to gain of energy and the temperature of the hot object goes on decreasing due to loss of energy.
The change in temperature continues till the temperatures of both the objects attain the same value. In this process, the cold object gains heat energy and the hot object loses heat energy.
If the system of both the objects is isolated from the environment by keeping it inside a heat resistant box (meaning that the energy exchange takes place between the two objects only), then no energy can flow from inside the box or come into the box.
i. Heat is transferred from where to where?
ii. Which principle do we learn about from this process?iii. How will you state the principle briefly?
iv. Which property of the substance is measured using this principle?

ANSWER:

(i) Heat is transferred from the object at higher temperature to the object at lower temperature.
(ii) We learn the principle of heat exchange from this process.
(iii) Principle of heat exchange states that the heat energy lost by hot object is always equal to heat gained by cold object provided that the system of both the objects is isolated.
(iv) Specific heat of an object can be measured using this principle.

Question 9:

Solve the following problems:

a. Equal heat is given to two objects A and B of mass 1 g. Temperature of A increases by 3°C and B by 5°C. Which object has more specific heat? And by what factor?

b. Liquid ammonia is used in ice factory for making ice from water. If water at 20°C is to be converted into 2 kg of ice at 0°C, how many grams of ammonia are to be evaporated? (Given: The latent heat of vaporization of ammonia= 341 cal/g)

c. A thermally insulated pot has 150 g ice at temperature 0°C. How much steam of 100°C has to be mixed to it, so that water of temperature 50°C will be obtained? (Given : latent heat of melting of ice = 80 cal/g, latent heat of vaporization of water = 540 cal/g, specific heat of water = 1 cal/g °C)

d. A calorimeter has mass 100 g and specific heat 0.1 kcal/ kg °C. It contains 250 gm of liquid at 30°C having specific heat of 0.4 kcal/kg °C. If we drop a piece of ice of mass 10 g at 0°C, What will be the temperature of the mixture?

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