THERMAL PROPERTIES OF MATTER CHAPTER # 09 PHYSICS 9TH - Short / Detailed Question Answers

 PHYSICS 9TH

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PHYSICS 9TH - Short / Detailed Question Answers
THERMAL PROPERTIES OF MATTER
CHAPTER # 09

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Q.1: Define heat. What is its unit?

Ans:
Heat:
Heat is a form of energy that transfers from a hot body to a cold body as a result of the difference in temperature between them.

Unit of Heat:
As heat is a form of energy, its S.I unit is joule. Its other unit is a calorie.

Q.2: Define temperature. Write its unit.

Ans:
Temperature:
It is a degree of hotness of a body. It determines the direction of the flow of heat from one body to the other body.

For example, a hot cup of tea is placed on a table; after some time, the tea in the cup becomes cold because the surrounding temperature is lower than that of the hot tea. Hence, heat flows from the hot cup to the surrounding.

Unit of Temperature:
The S.I unit of temperature is Kelvin. Its other units are Celsius and Fahrenheit.

Q.3: How many ways does heat transfer take place? Write the names of them.

Ans:
Heat transfers in three ways:

1. Conduction
2. Convection
3. Radiation

Q.4: What is a thermometer?

Ans:
Thermometer:
A thermometer is a device used to measure temperature.

For example, a clinical thermometer is used to measure the temperature of the human body.

Q.5: Write the names of different scales to measure temperature.

Ans:
Thermometers have different scales to measure temperature. There are three scales of temperature:

1. Celsius scale (Mostly used for environmental measurements)
2. Fahrenheit scale (Mostly used for clinical measurements)
3. Kelvin scale (Mostly used for industrial measurements)

Q.6: Write down the types of scales with which we can measure the temperature.

Ans:
Types of Scales:
There are three types of scales from which we can measure the temperature.

1. Celsius Scale or Centigrade:
   In this scale, the lower fixed point is at 0°C, which is the freezing point of water, and the upper fixed point is at 100°C, which is the boiling point of water. The interval between these two points is divided into 100 equal divisions or units. Each division is called a degree Celsius.

2. Fahrenheit Scale:
   In this scale, the melting point of ice is taken as the lower fixed point, marked as 32°F, and the boiling point of water is taken as the upper fixed point, marked as 212°F. There are 180 equal divisions or units between these points.

3. Kelvin Scale:
   In this scale, the melting point of ice is taken as 273K, and the boiling point of water is 373K. There are 100 equal units between these points. The zero of this scale, marked as 0K, starts from -273°C.

Q.7: How can we convert temperatures from one scale to another scale?

Ans:
These three scales of temperature are inter-convertible. Therefore, temperature measured in Celsius scale can be converted into Kelvin and Fahrenheit scales as follows:

• Conversion of temperature from Celsius scale to Kelvin scale:
  $K = ^\circ \text{C} + 273$

• Conversion of temperature from Celsius scale to Fahrenheit scale:
  $^\circ \text{F} = 1.8 \times ^\circ \text{C} + 32$

Q.8: What is heat capacity? Write its formula and units. Name the factor on which heat capacity depends.

Ans:
Heat Capacity:
Heat capacity is a term in physics that describes how much heat is added to a substance to raise its temperature by $1^\circ \text{C}$.

• Formula:
  Mathematically, $C = \frac{Q}{\Delta T}$
  Where:

  • $Q$ = amount of heat absorbed
  • $\Delta T$ = change in temperature

• Unit:
  The unit of heat capacity is J/K.

The factor on which heat capacity depends:
Heat capacity depends upon the nature of the material. For example, two beakers containing equal masses of water and oil are heated by the same gas burner for three minutes. It is observed that the temperature of oil may rise twice that of water.

Q.9: Define specific heat capacity. Write its expression and unit.

Ans:
Specific Heat Capacity:
When comparing the heat capacity of different substances, we are talking about their specific heat capacity. Hence, specific heat capacity can be defined as:

The amount of heat required to raise the temperature of 1 kg of a substance through $1^\circ \text{C}$ is called the specific heat capacity of that substance.

Expression:
Equation of specific heat capacity ‘$c$’ is as under:

$$C = \frac{C}{m} = \frac{1}{m} = \frac{Q}{\Delta T}$$ $$C = \frac{\Delta Q}{m \Delta T}$$

where “C” is a constant which depends upon the nature of the material of the body. This constant is called specific heat capacity or specific heat.

Unit:
Its S.I. unit is joule per kilogram per Kelvin $(J kg^{-1} K^{-1})$.
The given table shows the specific heat capacity of different substances of common use.

Q.10: Write down the factors on which specific heat capacity depends.

Ans:
Factors:

1. It depends on the nature of the substance and is entirely independent of its mass and the rise in temperature.
2. If “C” is small for a substance, the heat needed will also be small.
3. If “C” is large, the heat needed will also be large under the similar condition of mass and rise in temperature for all substances.

Q.11: What are the effects of large specific heat of water? OR Write examples of specific heat capacity from daily life experience.

Ans:
Effects due to large specific heat of water:

1. We know that water has a large specific heat; due to this quality, it plays an important role in everyday life.
   • A large amount of water in oceans helps to maintain the temperature ranges in their surroundings.
   • Water with coolant is used to reduce the temperature of an engine through the radiator of the vehicle.
   • Water also helps to maintain our body temperature.

Q.12: Define heat of fusion.

Ans:
Heat of Fusion:
The heat absorbed by a unit mass of a solid at its melting point to convert solid into liquid without a change of temperature is called “heat of fusion.”

Q.13: Define and explain the heat of vaporization.

Ans:
Heat of Vaporization:
Heat of vaporization is defined as: The amount of heat energy required to change the state of a substance from liquid to vapor form, without changing its temperature, is called "heat of vaporization."

Explanation:
When a beaker is filled with water and placed on a burner to boil, the temperature of the water gradually raises until it reaches 100°C. At this temperature, it starts to boil, meaning that bubbles of vapor form at the bottom and start to rise to the surface and escape in the form of steam. At this stage, the temperature of water (liquid) and water vapors (gas) is the same. Thus, the heat energy required to convert water from liquid to vapor state is known as "heat of vaporization."

Q.14: Write the experiments to determine the latent heat of fusion and latent heat of vaporization of ice and water respectively.

Ans:
Experiments, given below, determine the latent heat of fusion and latent heat of vaporization of ice and water respectively by sketching the temperature-time graph of heating ice. This experiment has two parts:

1. Conversion of ice into water
2. Conversion of water into steam

Experiment to Convert Ice (solid) into Water (liquid):
Take a container and place it on a stand. Put small pieces of ice in the container. Suspend a thermometer in the container to measure the temperature. Take a stopwatch to measure accurate time at different stages. Now place the container on the burner. The ice will start melting after absorbing heat. The temperature will remain the same up to 0°C until all the ice melts. Note the time $t_1$ and $t_2$, which the ice takes to melt completely into the water at 0°C. Supply heat continuously to the water at 0°C, and again note the time as its temperature starts to increase.

Note the time, which water in the container takes to reach its boiling point at 100°C from 0°C. Draw a temperature-time graph as shown in the given graph. Calculate the heat of fusion of ice from the data using the graph.

Experiment to Convert Water (Liquid) into Steam (Gas):
This is a continuation of the previous experiment. The container now contains boiling water. We continue to supply heat to the water until all the water converts into steam. Note the time during which water in the container completely changes into steam at its boiling point using the temperature-time graph. Calculate the heat of vaporization of water.

Q.15: Define and explain evaporation.

Ans:
Evaporation:
The process in which water changes from liquid to gas or vapor form is known as "evaporation." It is our common observation that wet clothes dry in the sun due to evaporation. The water in the wet cloth takes heat energy from the sun and gets evaporated. Similarly, the water taken from the sea is kept under the sun for a long period, leading to the evaporation of the water molecules, and as a result, common salt is formed, which is left as remnants in this process. We mostly notice that water placed in a pot disappears slowly. It is because of the evaporation process.

Evaporation Causes Cooling:
When evaporation occurs, the molecules of water with greater kinetic energy escape from its surface. Thus, the molecules of water with lower kinetic energy are left behind. This results in a decrease in the temperature of the water. Hence, evaporation causes cooling.

Q.16: Define volatile liquids.

Ans:
Volatile Liquids:
Some liquids have a low boiling point, due to which they change from liquid to vapor very easily at ordinary temperature. These liquids are called "volatile liquids."

Q.17: Describe the factors that influence surface evaporation.

Ans:
Factors Influencing Surface Evaporation:

1. Temperature: With the increase in temperature, the rate of evaporation also increases.
2. Wind Speed: The rate of evaporation also increases with the increase in wind speed.
3. Surface Area of Liquid: The rate of evaporation increases with the increase in surface area of the liquid.
4. Humidity: The rate of evaporation decreases with an increase in humidity.
5. Nature of Liquid: The nature of liquid also affects the rate of evaporation. Liquids with a lower boiling point have greater vapor pressure and evaporate more rapidly.
6. Solute Concentration: Salty water evaporates more slowly than pure water.

Q.18: What is the freezing point of ethanol on the Celsius scale?

Ans:
The freezing point of ethanol is 114.1°C.

Q.19: Define thermal expansion. Give some examples of thermal expansion in solids.

Ans:
Thermal Expansion:
Most solid materials expand on heating and contract on cooling because, on heating, the kinetic energy of their molecules increases. Therefore, changes take place in shape, area, and volume of the substances with the temperature change. This is called "thermal expansion," defined as: The expansion of a substance on heating is called thermal expansion.

Examples of thermal expansion in solids:

• Expansion in railway tracks in summer
• Expansion in electric wires in summer
• Expansion in bridges in summer

Q.20: Define linear expansion. Derive the expression $\Delta L = \alpha L \Delta T$.

Ans:
Linear Expansion:
The expansion in length of a solid object on heating is called linear expansion.

Derivation:
It is one-dimensional expansion as it occurs only along the length of the object. Suppose a rod of some material with original length $L$, at initial temperature $T$, is heated through a certain temperature $T'$, then its length increases and becomes $L'$. Therefore:

• Change in temperature $\Delta T = T' - T$ $(i)$
• Change in length $\Delta L = L' - L$ $(ii)$

It has been experimentally proven that change in length is directly proportional to the original length and change in temperature. Therefore:

$$\Delta L = (\text{Constant}) L \Delta T$$

This constant is denoted by $\alpha$ and is called the coefficient of linear expansion. It depends upon the nature of the material. Therefore, the equation can be written as:

$$\Delta L = \alpha L \Delta T$$

Q.21: Define volume expansion and derive its expression.

Ans:
Volume Expansion:
The expansion in the volume of a solid object on heating is called volume expansion. It is a three-dimensional expansion as it occurs along the length, width, and height of the object.

Derivation:
Consider a solid body having volume $V$, at some initial temperature $T$. When the body is heated, its temperature changes from $T$ to $T'$, and its volume becomes $V'$. Therefore:

• Change in temperature $\Delta T = T' - T$ $(i)$
• Change in volume $\Delta V = V' - V$ $(ii)$

It has been experimentally proven that change in volume is directly proportional to the original volume and change in temperature.

$$\Delta V=(\text{constant})\beta V\Delta T\text{(iii)}$$

This constant is denoted by “$\beta$” and is called the coefficient of volume expansion. It depends upon the nature of the material.
Therefore, equation (iii) can be written as:

$$\Delta V = \beta V \Delta T$$

Q.22: Show that $\beta = 3\alpha$. OR What is the relation between $\alpha$ and $\beta$?

Ans:
The coefficient of volume expansion of liquid is greater than solids. As the linear expansion occurs in one dimension, whereas volume expansion occurs in three dimensions. Hence, the coefficient of volume expansion “$\beta$” is three times the coefficient of linear expansion “$\alpha$”:

Therefore:

$$\beta = 3\alpha$$

Substance	Coefficient of Expansion (Per degree centigrade)
Aluminum	$25 \times 10^{-6}$
Brass or Bronze	$19 \times 10^{-6}$
Brick	$09 \times 10^{-6}$
Copper	$17 \times 10^{-6}$
Glass (Plate)	$09 \times 10^{-6}$
Glass (Pyrex)	$03 \times 10^{-6}$
Ice	$51 \times 10^{-6}$
Iron or Steel	$11 \times 10^{-6}$
Lead	$29 \times 10^{-6}$
Quartz	$0.4 \times 10^{-6}$
Silver	$19 \times 10^{-6}$

Q.23: Name some applications of thermal expansion.

Ans:
Applications of Thermal Expansion:

Thermal expansion of solids is useful in some daily life situations and in some cases, it creates problems. Some applications of thermal expansion are:

1. Bimetallic thermostat
2. Rivets
3. Car radiator coolant
4. Mercury in thermometer

Q.24: Define bimetallic thermostat. What are its uses?

Ans:
Bimetallic Thermostat:
A bimetallic thermostat is used to control the temperature of ovens, irons, water heaters, refrigerators, air-conditioners, and more. It is designed to bend when it becomes hot. Two metals with different coefficients of linear expansion are joined firmly to make it. When heated, the metal with a large coefficient of linear expansion causes the strip to bend, thus cutting off the current supply. The current supply to the circuit is restored when it cools down.

Q.25: What are rivets? Write their uses.

Ans:
Rivets:
Rivets are used in shipbuilding and other industries to join metal plates. A red-hot rivet is passed through holes in two metal plates and hammered until the ends are rounded. The rivet contracts on cooling and pulls the two plates tightly together.

Q.26: How can a metal rim be fixed on a wooden wheel?

Ans:
A metal rim can be fixed on a wooden wheel of a cart. The diameter of the metal rim is set slightly smaller than the diameter of the wooden wheel. The diameter of the metal rim increases on heating and can easily be put over the wooden wheel. It contracts on cooling and holds the wooden wheel tightly.

Q.27: What do you know about the real and apparent expansion of liquid?

Ans:
Real and Apparent Expansion of Liquids:
Consider a flask, filled with water up to level “a.” The flask is placed on a burner, as shown in the figure.
Heat starts to flow through the flask to the water, causing the flask to expand. Due to the expansion of the flask, the water level falls from point “a,” to level $L_1$, then to point “b,” to level $L_2$.
When water gets heated, it expands from point “b” beyond its original level. Thus, the expansion of water from level $L_1$ point “a” to level $L_2$ point “c” is called the "apparent expansion of water.” But in a real sense, the water on heating has undergone real expansion.

expanded from level $L_2$ point "b" to level $L_3$ point "c" which is the "real expansion of water".
Real expansion = $L_2$ to $L_1$ i.e., from point “b” to “C”, as shown in the figure.

(Figure of Real and Apparent expansion of liquids)

Q.28: Explain two types of thermal expansion.

Ans:
There are two types of thermal expansion:

1. Linear Thermal Expansion
2. Volumetric Thermal Expansion

Linear Thermal Expansion:
The expansion in the length of a solid object on heating is called linear expansion. It is one-dimensional expansion as it occurs only along the length of the object.

Derivation:
Suppose a rod of some material with original length $L$, at initial temperature $T$, is heated through a certain temperature $T'$, then its length increases and becomes $L'$. Therefore,

• Change in temperature = $\Delta T = T' - T$ .......... (i)
• Change in length = $\Delta L = L' - L$ .......... (ii)

It has been experimentally proven that the change in length is directly proportional to the original length and change in temperature. Therefore,

$$\Delta L = (\text{Constant}) \, L \, \Delta T$$

This constant is denoted by $\alpha$ and is called the coefficient of linear expansion. It depends upon the nature of the material. Therefore equation (iii) can be written as:

(Diagram: Linear Expansion)

Volumetric Thermal Expansion:
The expansion in the volume of a solid object on heating is called volume expansion. It is a three-dimensional expansion as it occurs along the length, width, and height of the object.

Derivation:
Consider a solid body having volume $V$, at some initial temperature $T$. When the body is heated its temperature changes from $T$ to $T'$ and its volume becomes $V'$. Therefore,

• Change in temperature = $\Delta T = T' - T$ .......... (i)
• Change in volume = $\Delta V = V' - V$ .......... (ii)

It has been experimentally proven that change in volume is directly proportional to the original volume and change in temperature.

This constant is denoted by $\beta$ and is called the coefficient of volume expansion. It depends upon the nature of the material. Therefore equation (iii) can be written as:

(Diagram: Volumetric Expansion)

Q.29: How would you find the specific heat of a solid?
Ans:
Method:
In this method, a certain amount of water of known mass and temperature is kept in a vessel called a calorimeter. Usually, we fill two-thirds of the calorimeter with water at room temperature. A known mass of substance (solid), whose heat is to be determined, is heated through a certain temperature and then put into the water contained in the calorimeter. According to the law of heat exchange, the heat is lost by the hot substance and gained by the water and calorimeter. We take the following observations:

Observations:

• Mass of the calorimeter and stirrer = $m_1$ kg
• Mass of the calorimeter + stirrer + $\text{H}_2\text{O}$ = $m_2$ kg
• Temperature of the calorimeter + $\text{H}_2\text{O}$ = $t_1$°C
• Temperature of the substance = $t_2$°C
• Temperature of the mixture = $t_3$°C
• Mass of calorimeter + stirrer + $\text{H}_2\text{O}$ + substance = $m_3$ kg
• Mass of $\text{H}_2\text{O}$ = $(m_2 - m_1)$ kg
• Mass of substance = $(m_3 - m_2)$ kg
• Specific heat of water = $C = 4200 \, \text{J/kg K}$
• Specific heat of Calorimeter made of copper = $C_1 = 390 \, \text{J/kg K}$
• Specific heat of substance = $C_2$

Calculation:
Now we calculate the heat lost and gained separately.

• Heat lost by the substance:
  $\text{Heat lost} = C_2 (m_2 - m_3) (t_2 - t_3)$

• Heat gained by the calorimeter and water:

  • Heat gained by the calorimeter:
    $= C_1 m_1 (t_3 - t_1)$
  • Heat gained by water:
    $= C (m_2 - m_1) (t_3 - t_1)$

So, the total heat gained by the calorimeter and water is:

$$\text{Heat gained} = C_1 m_1 (t_3 - t_1) + C (m_2 - m_1) (t_3 - t_1)$$

By using the Law of heat exchange,

$$\text{Heat lost} = \text{Heat gained}$$

$C_2 (m_1 - m_2) (t_2 - t_3) = C_1 m_1 (t_3 - t_1) + C (m_2 - m_1) (t_3 - t_1)$
$C_2 = \frac{C_1 m_1 (t_3 - t_1) + C (m_2 - m_1) (t_3 - t_1)}{(m_1 - m_2)(t_2 - t_3)}$

Q.30: Write down the freezing and boiling points of the following:
(i) Acetic acid
(ii) Benzene
(iii) Chloroform
(iv) Water

Ans:
Freezing and boiling points of different solvents:

Solvent	Freezing Point (°C)	Boiling Point (°C)
Water	0.0	100
Acetic acid	17.0	118.1
Benzene	5.5	80.2
Chloroform	-63.5	61.2
Ethanol	-114.7	78.4