# Definition of boiling point of water

As we all know at normal boiling point, vapor pressure of water is equal to atmospheric pressure. Vapour pressure of water is pressure exerted by its vapor(in dynamic equilibrium with water) on the surface of water.

We also know that liquid water doesn't change it's phase untill it reaches 100°C except evaporation is always there.

So, can I assume that just before it starts boiling (in saturated water), the entire 1 atmospheric pressure is only due to the part of water which is evaporated before? As boiling is not started yet!!

• Atmospheric pressure is a constant at the particular location. Can you please rephrase your question? Aug 25 '18 at 12:33
• Suppose we have cylinder, piston contains water at 60°C. We continuously supply heat and get saturated water at 100°C. Upto this instant no boiling has occurred but evaporation was surely there in picture. Now dq amount of heat will start boiling. Now at boiling point vapor pressure of water must be equal to atmospheric pressure. My question is from where does this vapour come from? at the very beginning of boiling. is this constitutes only the evaporated vapour? I am bit confused because of the fact that evaporation is extremely slow. Aug 25 '18 at 12:46
• 1. Do include that into your question. 2. What is the pressure in that piston? If it is vacuum, then sure the water will boil immediately without any heating. Aug 25 '18 at 12:50
• @user185991, at the boiling point (leave out the word "normal"), the vapor pressure of water is equal to the ambient pressure. If you have a beaker of water under a bell jar, and expose it to vacuum, the ambient pressure is vacuum pressure, not atmospheric pressure. Also note that liquid water can change its phase (i.e., boil) at pressures below atmospheric pressure, and at temperatures below 100 C. Note - your last assumption is invalid. The 1 atmosphere of pressure on the water in an open container is provided by the air above the container. Aug 26 '18 at 0:32
• @user185991, as a class room physics demonstration, I have placed a beaker of 25 C water under a bell jar, pulled a vacuum on it, and caused it to boil at 25 C. As it boiled, its temperature dropped, because the heat to boil the water had to come from the water itself. For an example, see youtube.com/watch?v=glLPMXq6yc0 Aug 26 '18 at 0:37