Thermal Properties of Matter: Important Questions

Very Short Answer Questions [1 Mark Questions]

Ques. How is the triple point of a substance unique is used in modern thermometry?

Ans. In modern thermometry, the triple point of a substance is unique because it happens at a very specific point of pressure and temperature.

Ques. Why is milk poured into a cup of tea and mixed with a spoon? Is this an example of a reversible process?

Ans. No this is not an example of a reversible process because when milk is poured into tea, the work done at the time of mixing is converted into the heat of the milk, which we cannot get back in the previous situation.

Ques. What is a substance that contracts with heating?

Ans. Many substances which contract after heating are leather, rubber, and ice.

Ques. What is the reason why the boiling point of a liquid changes with temperature?

Ans. As the temperature increases, the vapor pressure increases because the energy needed for the liquid molecules to expand to the gas phase also increases. So conversion of liquid into gas requires the liquid molecules to expand to the gas phase also increases. 

Ques. What is the reason a wooden table fixed with iron nails becomes loose after some time?

Ans. In the open space the iron nails lose their burnish because it becomes rusted when. iron reacts with the moisture and air present in the environment.

Ques. What is the reason behind the hotness of a body?

Ans. The temperature of the body also rises when the external temperature increases but also when the internal temperature increases. The kinetic energy of the molecules constituting the body is the cause of its hotness.

Ques. Why does a fur coat itself a source of heat or merely an insulator?

Ans. The reason behind this is the fur coat is merely an insulator. The fur is the natural protector from the air and it protects from wind.

Ques. Which gas thermometers are more sensitive than mercury thermometers?

Ans. Gas thermometers are more sensitive than mercury thermometers because the coefficient of expansion of a gas is very large as compared to the coefficient of expansion of mercury. For the same temperature change, the gas would undergo a much larger change in volume as compared to mercury.

Ques. Explain why the coefficient of cubical expansion of water is zero at 4°C?

Ans. The coefficient of cubical expansion of water is zero at 4°C because the density of the water is maximum at 4°C.

Ques. What is the technique to measure a temperature of less than 100 K?

Ans. Germanium resistance thermometer. Is the technique to measure a temperature of less than 100 K.

Ques. Write the S.I. unit of heat capacity?

Ans. The S.I unit of heat capacity is Joule/Kelvin (JK^-1) is the S.I. unit heat capacity.

Ques. What is the thermal conductivity of materials like copper and silver?

Ans. The thermal conductivity of materials like copper and silver is 385 and 406 respectively.

Ques. Suppose the temperature of water in a beaker is around 60°C, determine its value on the Fahrenheit scale? (122°F)

Ans. Since it is given that temperature in Celsius scale = C = 60°C

• The temperature on the Fahrenheit scale = F =?
• F = 1.8C + 32
• F = 1.8 x 60 + 32 = 108 + 32
• F = 140°F

Ques. Define melting point of a substance?

Ans. Melting point of a substance is defined as amount of heat required to change the state of from solid to liquid.

Ques. What is the formula used for specific heat capacity?

Ans. The formula used for specific heat capacity is as follows as:

S=ΔQ/mΔT 

Ques. Define molar specific heat capacity?

Ans. Molar specific heat capacity is defined as the amount of heat required to raise the temperature of 1 mole of the substance by one degree celsius.

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Short Answer Questions [2 Marks Questions]

Ques. What is the reason the brake drum of an automobile gets heated up when the automobile moves down a hill at constant speed?

Ans. The reason for this brake drum is since the speed is constant so there is no change in kinetic energy. The loss in gravitational potential energy is partially the gain in the heat energy of the brake drum.

Ques. What is thermal conduction?

Ans. Thermal conduction is the process of the transfer of heat energy from one part of a solid to another part at a lower temperature without the actual motion of the molecules. It is also known as the conduction of heat.

Ques. A woolen blanket keeps the body warm. The same blanket if wrapped around ice would keep ice cold. Explain this in detail.

Ans. As we know the blanket is an insulator that traps the heat released by our body and it makes us warm and in the case of ice, it does not release heat so, there is nothing to trap. The woolen blanket keeps us warm by preventing the heat of the human body from flowing outside and hence our body remains warm. In the case of ice, the heat from outsiders is prevented from flowing inside and thus ice remains cold.

Ques. Let us take some Wooden charcoal and a metal piece of the same dimension that are heated in the same oven to the same temperature and then removed in the dark. Which will shine more and why?

Ans. The charcoal will shine more as it is a good absorber and emitter of heat, so it will emit more energy and this is the only reason that charcoal will shine.

Ques. What is the coefficient of thermal conductivity of a material?

Ans. Thermal conductivity of a material is the quantity of heat flowing per second across the opposite faces of a unit cube made of that material when the opposite faces are maintained at a temperature difference of 1K or 1°C.

Ques. What happens to the size of the hole if the metal disc is heated and there is a hole in a metal disc?

Ans. The size of the hole increases on heating the metal disc according to the thermal properties of matter.

Ques. What are the three modes of transmission of heat energy from one point to another point?

Ans. The three modes of transmission of heat energy are.

• Conduction- is the process of transfer of energy from one molecule to another by direct contact.
• Convection- is the process of movement of heat by a fluid such as water or air. 
• Radiation- is the process of transfer of heat by electromagnetic waves.

Ques. What is the reason for the thick glass tumbler cracking when boiling liquid is poured into it?

Ans. The inner surface of the glass will expand when boiling liquid is poured into it. Because of the low thermal conductivity of glass, the expansion of the outer surface of the tumbler is quite small. Due to uneven expansion of the outer and inner surfaces, the tumbler breaks.

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Long Answer Questions [3 Marks Questions]

Ques. What is the value of γ for polyatomic gas and hence determine its value for a triatomic gas in which the molecules are linearly arranged.

Ans. The number of degrees of freedom for a triatomic gas with a linear arrangement is 7.

The energy of a polyatomic gas having n degrees of freedom is given by

E = n × 1/2 kT × N = n/2 RT

\(\begin{aligned} &C_{v}=\frac{d E}{d T}=\frac{n}{2} R\\ &C_{p}=C_{v}+R=\frac{n}{2} R+R\\ &\ \ \ \ =\left(\frac{\mathrm{n}}{2}+1\right) \mathrm{R}\\ &\gamma=\frac{C_{p}}{C_{v}}=\frac{\frac{n}{2}+1}{\frac{n}{2}}=1+\frac{2}{n} \end{aligned}\)

We have the relation γ = 1 + f/2; (where f is the number of degrees of freedom)

γ =1 + 7/2 = 9/7

So, the value of γ is 9/7.

Ques. Explain the reason why the food in a hot case remains warm for a long time during winter, how?

Ans. As we know, according to the thermal properties of matter the hot case is a double-walled vessel. The space between the walls is evacuated in a good hot case.

• The food container placed inside the hot case is made of crowning steel, thus neither the outside low-temperature air can enter the container nor the heat from inside can escape through the hot case by conduction or convection.
• The highly polished shining surface of the food container helps in stopping the loss of heat due to radiation.
• Thus, in this way, the heat of the food is preserved for a long time and food remains hot in winter.

Ques. Beggars sleep on footpaths or in the open in winter with their hands and knees pulled inside and in the same way dogs too curl while sleeping in winter. Why and tell the reason for doing such action?

Ans. The radiation and emission of heat from a body at a given temperature depend on these factors.

• The difference in temperature between the body and the surrounding,
• The area of the body in contact with the surroundings and
• The nature of the body.

These are the factors which cause human beings' and animals' body heat to curl up to reduce the surface area in contact with cold air.

Ques. Why housing on hills is either made of wood or has wooden lining and walls. Why do people in plains where temperature variations are extreme in winter and summer do not use-wooden houses?

Ans. Actually on hills, people make wooden houses or put wooden lining on house walls to insulate the house from the extreme cold of winter because wood being a bad conductor of heat does not allow the heat of the house to escape out or exchange low outside temperature with the high temperature inside.

• In plain, the weather is very hot, and due to temperature variations.
• The thick walls of the houses insulate the people against the heat in summer and extreme cold in winter water will not collect on the rooftops.
• This is the reason why plain people do not use-wooden houses.

Ques. An iron ring is attached to the rim of a wooden wheel of a bullock cart by a blacksmith. The diameter of the rim is 5.243m and the diameter of the iron ring is 5.231m at 37 degrees Celsius. What should be the temperature of the ring to fit the rim of the wheel?

Ans. T1 = 37 degree celsius

LT₁ = 5.231m

LT₂ = 5.234m 

So,

LT₂ = LT₁ [1 + α₁ (T₂ – T₁)]

5.243m = 5.231m [1 + 1.2010^-5 K ^-1 (T₂ – 37 Δc)

Or, T₂ = 118^oc

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Very Long Answer Questions [5 Marks Questions]

Ques. People who drive on their own and they close all the glass windows of the car park it in the Sun, it remains very hot inside the car even after sunset. To keep inside the car, cooler insulating screens are put inside the car covering, the wind screen, and curtains are pulled on the glass of the windows. Explain in detail.

Ans. Its universal truth is that sunlight emits ultraviolet radiation which passes through the glass and it makes the car heated. The radiation emitted by inner parts of the glass cannot pass out of the glass, so the heat trapped inside raises the temperature of the interior.

• This is because glass is transparent to infrared radiations released from the sun, having a smaller wavelength than that emitted by the interior of the car which has a longer wavelength to which the glass of the car is opaque.
• So, it's necessary to put inside the car cover, the wind screen, and curtains are pulled on the glass of the windows.

Ques. Explain the following in detail:

(a) Coefficient of linear expansion (α)

(b) Coefficient of superficial expansion (β)

Ans. (a) The Coefficient of linear thermal expansion (denoted by “α”) is a property of a material that defines the ability of a plastic to expand under the effect of temperature elevation.

• α – It is defined as the change in the length per unit original length per unit change in temperature of the material of a solid rod.
• α = Δl / l₀ΔT

(b) The coefficient of superficial expansion is denoted by the symbol β (beta). The amount by which the unit area of a material increases when the temperature is raised by one degree is called the coefficient of superficial (i.e. area) expansion and is represented by β (Greek beta).

• β is the coefficient of superficial expansion and It can be explained like this also like the change in surface area per unit original surface area per unit change in the temperature of the material of the solid.
• i.e. β = ΔS / S₀ΔT

Ques. Define the various properties of a thermometric substance?

Ans. The properties of any materials which change uniformly with temperature and are used for measuring the temperature are called thermometric properties and the materials used for making thermometers are called thermometric substances.

• Few good thermometric substances are Mercury and alcohol, All gasses, Platinum, Thermocouples, Radiant energy.

Given below are the various properties of a thermometric substance.

• The thermometric substances should have a high boiling point and low freezing point to provide for the measurement of a wide range of temperatures.
• It should not be evaporative.
• It should be Constant with the temperature
• Its variation should be computable.
• Its rate of expansion should be uniform so that the calibration becomes easier.
• The coefficient of expansion of the substance should be high so that the thermometer may be sensitive.
• It should be available in a pure state.
• It should not stick to the side of the glass tube.
• It should have good thermal conductivity.

Ques. A container contains a mixture of 0.15kg of ice at 0^oc and 0.20kg of water at 50^oc. the resulting temperature is 6.7^oc. calculate the heat of fusion of ice.

Ans. Heat lost in water = msw( e_f - e_l )w 

= (0.20 kg) (4186 J kg^-1 k ^-1 ) (50.0^oc – 6.7 ^oc )

= 36250.76 J 

 Heat required to melt ice = m₂L₁ = ( 0.15kg) L₁

Heat required to increase temperature to ice water to final temperature = m₁sw (e_f - e₁)₁

= (0.15 kg) (4186 J Kg_-1 k_-1 ) (6.7 ^oc -0^oc)

= 4206.93 J 

Heat gained = heat lost

36250.76 J = (0.15 Kg )L_f + 4206.93 J 

L₁ = 2.234 J kg^-1

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