The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. Why? Following are some options:
Correct Answer:
(C)
The density of water is maximum at 4°C
Generally, all liquids expand on heating, but water is an exception to this rule. If water is heated, its volume gradually decreases (This decrease in volume continues till the temperature rises to 4°C). At temperatures over 4°C water starts expanding. It then keeps expanding with the further rise in temperature, till finally at 100 °C and turns into steam.In other words, at 4°C, water has the least volume (occupies the least amount of space) and maximum density (is at its heaviest). This irregular expansion of water is called anomalous expansion. This anomalous expansion plays an important role by only freezing the upper layer in lakes and rivers. During winter months in colder countries, the outside or atmospheric temperature is very low – it drops to below freezing – and the upper layers of water in the lakes and ponds start cooling. When the temperature of the surface layers falls to 4°C, the water body acquires maximum density and sinks down. The water that sinks down displaces water below, and the lower layers of water simultaneously rise up. This also gets cooled to 4°C and again sinks down.When the temperature of the water body finally goes below 4°C, the density or heaviness of water decreases and as a result water does not sink down. The surface water finally freezes at 0°C while the lower part still remains at 4°C. The light frozen layer of ice floats on top.
Ques:
2
A needle sinks in water whereas a ship made of iron floats on it because:
Correct Answer:
(D)
Relative density of the needle is greater than that of water displaced by it
Relative density is defined as a ratio of the density of a particular substance with that of water. The relative density of pure water is 1. The substance having the relative density less than 1 will float in water, while a substance having the relative density more than 1 will sink in water. The ship is designed in such a way that its effective density (m/v) is less than the density of water and the weight of the water displaced by the ship is equal to its weight, so it floats upon the water. But the density of the iron needle is much larger than the water, so it sinks easily.
Ques:
3
When a ship enters the sea from a river, what happens?
Correct Answer:
(A)
It rises a little
When a ship enters a sea from a river, the ship is elevated due to the higher density of salty sea water than the river.
Ques:
4
While lifting a bucket of water from a well, we feel that the bucket :
Correct Answer:
(A)
Becomes heavier above the surface of water
According to the Archimedes' principle of flotation, object appears to be heavier in the air than inside the water or they appear to be less heavy in the water. It is because water exerts an upward force on the objects immersed in it. This reduction is equal to the weight of the fluid that body displaces.
Ques:
5
Pressure inside a soap bubble is –
Correct Answer:
(A)
More than atmospheric pressure
Soap bubbles are large because when soap is dissolved in water its surface tension is reduced. The pressure inside a soap bubble is more than atmospheric pressure.