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I read an article lately and it said that water turns into steam when it reaches its boiling point. But it led me to another question. Why does water boil and why does the water turn into gas when it boils?

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  • $\begingroup$ I'm hoping someone will answer this with a discussion of phase changes and the suddenness of divisions on phase diagrams. However this might be a duplicate of physics.stackexchange.com/q/227987/12613 $\endgroup$ – RedGrittyBrick Jan 26 '17 at 11:40
  • $\begingroup$ Note: Steam often refers to a suspension of liquid water droplets in air, rather than to water vapour which is invisible. when water boils it may not produce that kind of steam. $\endgroup$ – JMLCarter Jan 26 '17 at 12:14
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Water molecules have attractive forces between them and form the liquid state.

First of all start with a container with water liquid and a vacuum above the water liquid.
This container and the water within it is kept at a constant temperature.

Some the water liquid molecules will have enough kinetic energy to overcome the attraction of their neighbouring water liquid molecules and escape from water liquid surface and become water vapour.
There is a net migration with water liquid molecules becoming water vapour molecules.

As time goes on and the number of water vapour molecules increases but some of those water vapour molecules will hit the water surface and become part of water liquid.

Eventually there are sufficient water vapour molecules and a dynamic equilibrium will be set up where the rate at which water liquid is converted into water vapour is exactly the same as the rate at which water vapour is converted into water liquid.

The pressure of the water vapour when this condition is satisfied is called the saturated vapour pressure.

Increasing the temperature means that the average kinetic energy of the water molecules increases and so the probability of a water liquid escaping from the surface of the liquid is increased.
So the rate at which water liquid turns to into water vapour increases.
For a time there is a net migration from water liquid to water vapour until the increase in the density of water vapour is sufficient for a new dynamic equilibrium to be set up.
The saturated vapour pressure increases as the temperature increases.

Raising the temperature will thus increase the saturated vapour pressure until there comes a temperature when the density of the vapour is the same as the density of the liquid.
The boundary (surface) between the liquid and the vapour disappears and you have just one phase.
That temperature is called the critical temperature and here is one video showing this effect.
Water liquid does not exist above the its critical temperature of $374\,^\circ \rm c$


Now consider a situation where there is water liquid and air the whole system is kept at constant temperature and constant pressure.

Again water liquid molecules escape to form water vapour.
The density of water vapour molecules in the air increases and at the same time the rate at which water vapour molecules return increases.
Eventually a dynamic equilibrium is reached and the pressure of the water vapour in the air is the saturated vapour pressure.

The total pressure of the gas above the liquid surface is the sum of the pressure due to the air and the pressure due to the water vapour.
Increasing the temperature will increase the saturated vapour pressure and since the pressure is being kept constant that means that the contribution to the total pressure of the air molecules is reduced.
The density of air has been reduced in the space above the liquid surface.

If there is still liquid left there will come a temperature when the saturated vapour pressure is equal to, or very, very slightly more than, the pressure being exerted on the system and you now get a conversion of water liquid to water vapour not just at the surface but it becomes energetically favourable for new surface to be created within the bulk of the liquid - bubbles of vapour form and you have the onset of boiling.
More water liquid molecules are leaving the liquid than water vapour molecules are arriving at the liquid.

So boiling occurs when the saturated vapour pressure of the liquid is equal to the external pressure.
Changing the external pressure will change the boiling the boiling point of the liquid.


The difference between these two scenarios is the fact that for the first case the system was in a sealed container and the pressure increased as the temperature increased whereas in the second case the pressure was kept constant.

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Water as a liquid or gas ( vapour ) is essentially a bunch of molecules. At even room temperature , the water contained in a glass evaporates. What happens is that some of the water molecules in top layer of the water get a "kick" from the molecules present in the below layers. And they leave of to roam in the atmosphere. But some of the water molecules in air also come under the influence of the water in the jar. And they then get into the jar. There is an equilibrium. What goes out comes in. When you increase the temperature the number of molecules going out increases than the number of molecules coming in. At boiling point this difference in rate is great. That is why Boiling point changes with pressure !

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