Assertion & Reason

Question 11 Mark

Assertion : Fahrenheit is the smallest unit measuring temperature.
Reason : Fahrenheit was the first temperature scale used for measuring temperature.

Answer

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Question 21 Mark

Assertion : Melting of solid causes no change in internal energy.
Reason : Latent heat is the heat required to melt a unit mass of solid.

Answer

(e) Melting is associated with increasing of internal energy without change in temperature. In view of the reason being correct the amount of heat absorbed or given out during change of state is expressed $Q=m L$, where $m$ is the mass of the substance and $L$ is the latent heat of the substance.

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Question 31 Mark

Assertion : Specific heat capacity is the cause of formation of land and sea breeze.
Reason : The specific heat of water is more than land.

Answer

(a) The temperature of land rises rapidly as compared to sea because of specific heat of land is five times less than that of sea water. Thus, the air above the land become hot and light so rises up so because of pressure drops over land. To compensate the drop of pressure, the cooler air starts from sea starts blowing towards lands, so, setting up sea breeze. During night land as well sea radiate heat energy. The temperature of land falls more rapidly as compared to sea water, as sea water consists of higher specific heat capacity. The air above sea water being warm and light rises up and to take its place the cold air from land starts blowing towards sea and so et up breeze.

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Question 41 Mark

Assertion : The melting point of ice decreases with increase of pressure.
Reason : lce contracts on melting.

Answer

(a) With rise in pressure melting point of ice decreases. Also ice contracts on melting

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Question 51 Mark

Assertion : The molecules at $0^{\circ} \mathrm{C}$ ice and $0^{\circ} \mathrm{C}$ water will have same potential energy.
Reason : Potential energy depends only on temperature of the system.

Answer

(d) The potential energy of water molecules is more. The heat given to melt the ice at $0^{\circ} \mathrm{C}$ is used up in increasing the potential energy of water molecules formed at $0^{\circ} C$.

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Question 61 Mark

Assertion : Water kept in an open vessel will quickly evaporate on the surface of the moon.
Reason : The temperature at the surface of the moon is much higher than boiling point of the water.

Answer

(a) Water would evaporate quickly because there is no atmosphere on moon, due to which surface temperature of moon is much higher than earth (Maximum surface temperature of moon is $123^{\circ} \mathrm{C}$ ).

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Question 71 Mark

Assertion : Specific heat of a body is always greater than its thermal capacity.
Reason : Thermal capacity is the required for raising temperature of unit mass of the body through unit degree.

Answer

(d) Specific heat of a body is the amount of heat required to raise the temperature of unit mass of the body through unit degree. When mass of a body is less than unity, then its thermal capacity is less than its specific heat and vice-versa.

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Question 81 Mark

Assertion : Two bodies at different temperatures, if brought in thermal contact do not necessary settle to the mean temperature.
Reason : The two bodies may have different thermal capacities.

Answer

(a) When two bodies at temperature $T_1$ and $T_2$ are brought in thermal contact, they do settle to the mean temperature $\left(T_1+T_2\right) / 2$. They will do so, in case the two bodies were of same mass and material i.e., same thermal capacities. In other words, the two bodies may be having different thermal capacities, that's why they do not settle to the mean temperature, when brought together.

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Question 91 Mark

Assertion : Latent heat of fusion of ice is $336000 \mathrm{Jg}$.
Reason : Latent heat refers to change of state without any change in temperature

Answer

(b) The latent heat of fusion of ice is amount of heat required to convert unit mass of ice at $0^{\circ} \mathrm{C}$ into water at $0^{\circ} \mathrm{C}$. For fusion of ice$\begin{aligned}& L=80 \mathrm{cal} / \mathrm{gm}=80000 \mathrm{cal} / \mathrm{gm}=8000 \times 4.2 \mathrm{~J} / \mathrm{kg} \\& =336000 \mathrm{~J} / \mathrm{kg} .\end{aligned}$

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Question 101 Mark

Assertion : A beaker is completely filled with water at $4^{\circ} \mathrm{C}$. It will overflow, both when heated or cooled.
Reason : There is expansion of water below and above $4^{\circ} \mathrm{C}$.

Answer

(a) Water has maximum density at $4^{\circ} \mathrm{C}$. On heating above $4^{\circ} \mathrm{C}$ or cooling below $4^{\circ} \mathrm{C}$, density of water decreases and its volume increases. Therefore, water overflows in both the cases.

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Question 111 Mark

Assertion : When a solid iron ball is heated, percentage increase is its volume is largest.
Reason : Coefficient of superficial expansion is twice that of linear expansion where as coefficient of volume expansion is three time of linear expansion.

Answer

(a) As $\beta=2 \alpha$ and $\gamma=3 \alpha$, i.e., coefficient of volume expansion of solid is three time coefficient of linear expansion and 1.5 times the coefficient of superficial expansion, on heating a solid iron ball, percentage increase in its volume is largest.

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Question 121 Mark

Assertion : The temperature at which Centigrade and Fahrenheit thermometers read the same is $-40^{\circ}$.
Reason : There is no relation between Fahrenheit and Centigrade temperature.

Answer

(c) The relation between $F$ and $C$ scale is, $\frac{C}{5}=\frac{F-32}{9}$. If $F=C$ $\Rightarrow C=-40^{\circ} \mathrm{C}$ i.e., at $-40^{\circ}$ the Centigrade and Fahrenheit thermometers reads the same.

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Question 131 Mark

Assertion : The coefficient of volume expansion has dimension $K$.
Reason : The coefficient of volume expansion is defined as the change in volume per unit volume per unit change in temperature.

Answer

(a) As, $\gamma=\frac{\Delta V}{V \Delta T}$ i.e., units of coefficient of volume expansion is $K$

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Question 141 Mark

Assertion : A brass disc is just fitted in a hole in a steel plate. The system must be cooled to loosen the disc from the hole.
Reason : The coefficient of linear expansion for brass is greater than the coefficient of linear expansion for steel.

Answer

(a) Linear expansion for brass $\left(19 \times 10^{-4}\right)>$ linear expansion for steel $\left(11 \times 10^{-4}\right)$. On cooling the disk shrinks to a greater extent than the hole and hence it will get loose.

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