[3 Mark Question Answer]

Question 13 Marks

A molten metal of mass $150 g$ is kept at its melting point $800^{\circ} C$. When it is allowed to freeze at the same temperature, it gives out $75,000 J$ of heat energy.
(a) What is the specific latent heat of the metal?
(b) If the specific heat capacity of metal is $200 Jkg ^{-1} K ^{-1}$, how much additional heat energy will the metal give out in cooling to $-50^{\circ} C$ ?

Answer

$
\text { Mass of metal }=150 g
$
Specific latent heat of metal
$L=\frac{Q}{m}=\frac{75000}{150}=500 Jg ^{-1}$
Specific heat capacity of metal is $200 Jkg ^{-1} K ^{-1}$.
Change in temperature $=800-(-50)=850^{\circ} C$ (or $850 K$ ).
$
\Delta Q=m c \Delta T=0.15 \times 200 \times 850=25500 J
$

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Question 23 Marks

Explain the following:

The surrounding become pleasantly warm when water in a lake starts freezing in cold countries ?

Answer

The reason is that the specific latent heat of fusion of ice is sufficiently high, so when the water of lake freezes, a large quantity of heat has to be released and hence the surrounding temperature becomes pleasantly warm.

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Question 33 Marks

Why do bottled soft drinks get cooled, more quickly by the ice cubes than by the iced water, both at 0℃?

Answer

This is because 1 g of ice at 0oC takes 336 J of heat energy from the bottle to melt into water at 0oC. Thus, bottle loses an additional 336 J of heat energy for 1 g of ice at 0oC than for 1 g iced water at 0oC. Therefore, bottled soft drinks get cooled, more quickly by the ice cubes than by iced water.

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Question 43 Marks

Ice cream appears coder to the mouth than water at 0℃. Give reason.

Answer

This is because 1 g of ice at 0°C takes 336 J of heat energy from the mouth to melt at 0°C. Thus, mouth loses an additional 336 J of heat energy for 1 g of ice at 0°C than for 1g of water at 0°C. Therefore, cooling produced by 1 g of ice at 0°C is more than for 1g of water at 0°C.

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Question 53 Marks

Water boils at 120 °C in a pressure cooker. Explain the reason

Answer

The boiling point of a liquid increases with the increase in pressure and decreases with the decrease in pressure.
The boiling point of pure water at one atmospheric pressure (= 760 mm of Hg) is 100 °C.
In a pressure cooker, the water boils at about 120 °C to 125 °C due to increase in pressure, as the steam is not allowed to escape out of it.

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Question 63 Marks

A mass of $50 g$ of a certain metal at $150^{\circ} C$ is immersed in $100 g$ of water at $11^{\circ} C$. The final temperature is $20^{\circ} C$. Calculate the specific heat capacity of the metal. Assume that the specific heat capacity of water is $4.2 Jg ^{-1} K^{-1}$.

Answer

$\text { Heat liberated by metal }= m \times s \times \triangle t =50 \times s \times(150-20)$
Heat absorbed by water $= m _{ w } \times s _{ w } \times \Delta t =100 \times 4.2 \times(20-11)$
Heat energy lost= heat energy gained
$50 \times s \times(150-20)=100 \times 4.2 \times(20-11)$
$S =0.58 J g ^{-1} K ^{-1}$

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Question 73 Marks

$0.5 kg$ of lemon squash at $30^{\circ} C$ is placed in a refrigerator which can remove heat at an average rate of $30 Js ^{-1}$. How long will it take to cool the lemon squash to $5^{\circ} C$ ? Specific heat capacity of squash $=4200 Jkg ^{-1} K ^{-1}$

Answer

Change in temperature $=30-5=25 K$.
$
\Delta Q = mc \Delta T
$
$
\Delta Q =0.5 \times 4200 \times 25=52500 J
$
$
t=\frac{\Delta Q}{P}=\frac{52500}{30}=1750 s
$
$
t=29 \min 10 sec
$

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Question 83 Marks

Calculate the amount of heat energy required to raise the temperature of $100 g$ of copper from $20^{\circ} C$ to $70^{\circ} C$. specific heat capacity of copper $=390 Jkg ^{-1} K ^{-1}$.

Answer

Mass of copper $m=100 g =0.1 kg$
Change of temperature $\Delta t=(70-20)^{\circ} C$
Specific heat of capacity of copper $=390 Jkg ^{-1} K ^{-1}$
Amount of heat required to raise the temperature of $0.1 kg$ of copper
is
$
\begin{aligned}
& Q=m \times \Delta t \times c \\
& =0.1 \times 50 \times 390 \\
& =1950 J
\end{aligned}
$

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Question 93 Marks

By imparting heat to a body its temperature rises by 15° C. what is the corresponding rise in temperature on kelvin scale?

Answer

The size of 1 degree on the Kelvin scale is the same as the size of 1 degree on the Celsius scale. Thus, the difference (or change) in temperature is the same on both the Celsius and Kelvi scales.
Therefore, the corresponding rise in temperature on the Kelvin scale will be 15K.

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Question 103 Marks

Differentiate between heat and temperature.

Answer

Heat	Temperature
The kinetic energy due to random motion of the molecules of a substance is known as its heat energy.	The quantity which determines the direction of flow of heat between two bodies kept in contact is called temperature.
S.I. unit joule (J).	S.I. unit kelvin (K).
It is measured by the principle of calorimetry.	It is measured by a thermometer.

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Question 113 Marks

Name the material of which it is made of. Give two reasons for using the material stated by you.

Answer

It is made up of thin copper sheet because:

(i) Copper is a good conductor of heat, so the vessel soon acquires the temperature of its contents.

(ii)Copper has low specific heat capacity so the heat capacity of calorimeter is low and the amount of heat energy taken by the calorimeter from its contents to acquire the temperature of its contents is negligible.

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Question 123 Marks

A liquid X has specific heat capacity higher than the liquid Y. Which liquid is useful as heat reservoir to keep juice bottles without freezing?

Answer

The specific heat capacity of liquid X is higher than that of Y. So, for same mass and same heat energy, the rise in temperature for X will be less than that of Y.

As a heat reservoir to keep juice bottles without freezing, the liquid needs to give out large amount of heat before reaching freezing temperatures. Hence, liquid X is ideal for this function.

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Question 133 Marks

Water property of water makes it an effective coolant?

Answer

By allowing water to flow in pipes around the heated parts of a machine, heat energy from such part is removed. Water in pipes extracts more heat from surrounding without much rise in its temperature because of its large specific heat capacity. So, Water is used as an effective coolant.

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Question 143 Marks

Two metallic blocks P and Q of masses in ratio 2: 1 are given the same amount of heat. If their temperature rise by the same amount, compare their specific heat capacities.

Answer

Let $C_p$ and $C q$ be the specific heat capacities of blocks $P$ and $Q$ respectively,
We know that,
$\begin{aligned} & \frac{Q}{m \times \Delta t} \\ & \therefore\left(\frac{C_p}{C_q}\right)=\left(\frac{\frac{Q}{2 m \times \Delta t}}{\frac{Q}{m \times \Delta T}}\right)=\frac{1}{2}\end{aligned}$
Hence, the required ratio is $1: 2$.

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