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Standard VII

Physics

Heat

Why does the ...

Question

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

Water has large specific heat.

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Conductivity of ice is low.

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Temperature of the earth at the bottom of the lake is high.

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Conductivity of ice is high.

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Solution

The correct option is B
Conductivity of ice is low.

The bottom of a lake do not freeze in severe winter. The reason is that ice is a poor conductor of heat hence once the surface is frozen no further heat is liberated or absorbed by water beneath ice. Therefore water below ice never freezes.

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Similar questions

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

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

Q. The temperature of water at the bottom of a lake whose upper surface has frozen to ice would be around ______________.

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)

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

The correct option is B Conductivity of ice is low. The bottom of a lake do not freeze in severe winter. The reason is that ice is a poor conductor of heat hence once the surface is frozen no further heat is liberated or absorbed by water beneath ice. Therefore water below ice never freezes.

The correct option is B
Conductivity of ice is low.

The bottom of a lake do not freeze in severe winter. The reason is that ice is a poor conductor of heat hence once the surface is frozen no further heat is liberated or absorbed by water beneath ice. Therefore water below ice never freezes.