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Standard IX

Chemistry

Latent Heat

The bottom of...

Question

The bottom of a lake does not freeze in harsh winter even when the surface is all frozen. Why?

Low conductivity of ice

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Low conductivity of water

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High conductivity of water

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High conductivity of ice

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Solution

The correct option is A
Low conductivity of ice

Because of very low conductivity of ice i.e. the lowering of temperature during winter takes place at faster rate over the surface while to form an ice at depth, due to low conductivity of ice it is not possible. This is the reason the bottom of a lake does not freeze in harsh winter even when the surface is all frozen.

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Similar questions

Why does the bottom of a lake not freeze in severe winter even when the surface is all frozen?

Q. In a

10

m deep lake, the bottom is at a constant temperature of

4^{o} C

. The air temperature is constant at

- 4^{o} C

. The thermal conductivity of ice is

3

times that of water. Neglecting the expansion of water on freezing, the maximum thickness of ice will be

Q. In a

10

metre deep lake, the bottom is at a constant temperature of

4^{\circ} C

. The air temperature is constant at

- 4^{\circ} C

. The thermal conductivity of ice is

3

times that water. Neglecting the expansion of water on freezing, the maximum thickness of ice will be

Water is densest at 4 $^{\circ}$ C; this is the reason lakes do not completely freeze during extreme winters. Say, the atmosphere is at - $θ^{\circ} C (θ > 0)$ ,now the temperature of the lake's water starts to fall, and the denser water from the top sinks, getting the bottom layers up, and cooling them until the temperature reaches 4 $^{\circ}$ C at the upper surface. Now, further reduction in temperature actually makes the water less dense! Hence the colder water stays on top, starts to freeze, until the top layer of ice reaches - $θ^{\circ} C$ and the bottom layer at 0 $^{\circ}$ C . And this layer of ice is the only area through which the water below loses heat (by conduction), slowly increasing the thickness of the ice (because ice is a bad conduction), and for a very long time the water at the bottom of the lake remains at 4 $^{\circ}$ C! Now, Let the thickness of the layer of ice be $y_{1}$ , at an instant.How much time t, will it take to increase to thickness $y_{2}$ ?

$ρ \to$ density of the ice

K $\to$ thermal conductivity of ice.

L $\to$ Latent heat of fusion

$θ \to$ temperature of surface (as mentioned earlier)

Q. The temperature at the bottom of a pond of depth

L = 15 m

4^{\circ}

C. The temperature of the air, just above the layer of ice frozen at the pond's surface is

- 2^{\circ}

C for the past many days. The thermal conductivity of ice is three times that of water. Find the depth (in m) of unfrozen water below ice?

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