1. Why coffee runs up into a sugar lump (a small cube of sugar) when one corner of the sugar lump is held in the liquid?
(i) If sugar cube is dropped into liquid the outermost layer has to dissolve first, then next layer, then next until the whole sugar is dissolved.
(ii) The coffee runs up into the pores of sugar lump due to capillary action of the liquid.
2. Why two holes are made to empty an oil tin?
When oil comes out from a hole of an oil tin, pressure inside it decreased than the atmosphere. Therefore, the surrounding air rush up into the same hole prevents the oil to come out. Hence two holes are made to empty the oil tin.
3. We can cut vegetables easily with a sharp knife as compared to a blunt knife. Why?
Since the stress produced on the vegetables by the sharp knife is higher than the blunt knife, vegetables can be cut easily with the sharp knife.
4. Why the passengers are advised to remove the ink from their pens while going up in an aeroplane?
When an aero-plane ascends, the atmospheric pressure is decreased. Hence, the ink from the pen will leak out. So that, the passengers are advised to remove the ink from their pens while going up in the aero-plane.
5. We use straw to suck soft drinks, why?
When we suck the soft drinks through the straw, the pressure inside the straw becomes less than the atmospheric pressure. Due to the difference in pressure, the soft drink rises in the straw and we are able to enjoy it conveniently.