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MECHANISM OF BOILING

       What happens when just a cup of water is boiled.  Let us have a some deep look.

      On the surface of water, atmospheric air gives downward pressure.  Water is heated up.  The water molecules slowly loose themselves from their bonds and gain momentum  using heat energy and moves up.  The loose molecules accumulates as water vapour above the surface of water.  Then the vapour pressure slowly builds up and acts upward against air pressure.  When the vapour pressure equals or slightly greater than atmospheric pressure, the water vapour pushes itself into the atmosphere and evaporates.  Next, the loose molecules(gas form) inside the bulk of water bubbles up rapidly and evaporates easily.  This moment is what we call boiling.  The temperature at this moment is called boiling point.

      so boiling occurs when vapour pressure is equal to atmospheric pressure.

 Pressure cooking:
     In pressure cooker, the steam (vapour) is trapped for sometime allowing the pressure to build up.  Naturally the boiling temperature rises by a few degrees.  This higher temperature helps to cook the food fast. 

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