Sometimes , tight bottle metal cap is dipped into hot water to open it, why ?

When the cap of a screw is dipped in hot water there will be unequal expansion of metal cap and glass. Since, the cap expands before the heat reaches the bottle and hence the metal cap will be loosened. 

This is because of pressure difference outside the glass jar and inside the glass jar, this pressure difference makes the gasses inside the glass jar expand as air expands on heating, the warm water make the gases in the glass jar expand thus pushing the metal lid to open up in the warm water.Simply leave your cap bottle under running hot water. Or you can pour hot water into a bowl and soak your bottle cap inside. After a short while, the bottle cap will expand and you can open the cap with ease! Just be careful not to scald your hands especially if the cap is metal.

you could fill a bowl with hot water and place the jar upside down in the water, leaving it there for around 30 seconds. The hot water will help expand the metal lid (more so than the glass), making it easier to remove.Applied to a metal jar lid, this implies that the inside diameter of the lid will increase, tending to loosen the lid. Directing the stream of hot water at the center of the lid heats the metal more than the glass, so that you get a greater difference in the expansion of the top and the jar.