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Author Topic: Why H2O exist as gas(water vapor) and liquid(water) under same conditions??  (Read 12056 times)

Offline EmberCult

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How come it exist as water and vapor too under same conditions,
Like in is in glass and stays as liquid and at the very same place it is present as vapor(gas) in air?

Please explain!


 

Offline chiralSPO

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It's even worse than that. Under some conditions, there can be solid, liquid and gaseous water all in equilibrium at the same time!

All water consists of water molecules (H2O). These molecules are so incredibly small that a typical glass of water has about 5x1024 molecules. They are always moving around, and their speed is determined by the temperature. The hotter they are, the faster they move. But it is very, very unlikely that they all move at the same speed. At any given temperature there is a whole distribution of speeds, from very slow to very fast--as the temperature increases the distribution shifts so the average speed increases, and the distribution also gets wider.

So what does this have to do with liquids and gases?

Water molecules are sticky (at least they stick to each other). When moving slowly enough water molecules tend to stick to one another, but as they get faster and faster, this overcomes the attraction, and they can break away. At any reasonable temperature there are always some water molecules that have enough speed to break away from the rest (even if each molecule only has a one in a billion chance to escape, and there are 1024 molecules, then 1015 molecules can still get away).

Molecules in the gas phase also move at a distribution of speeds that is temperature dependent. So some of them are moving slowly enough that the stickiness can overcome their freedom--but it can only form a liquid if there are other water molecules around to stick to. So a slow-moving water molecule can crash into a bowl of water and get stuck, or if there are enough slow-moving water molecules in the gaseous phase, they can start to crash into oneanother, and condense into a liquid when there was no liquid to start with.

So if we have a glass of water in the air, there is a constant stream of molecules leaving the surface and evaporating, but there are also water molecules in the air that go into the glass (or they just bounce back out). If the rate at which water molecules leave the liquid phase in the glass is the same as the rate of molecules entering the liquid phase in the glass, we call that "equilibrium." If the water in the glass is heated up, more molecules will leave, and go into the gas, so equilibrium is lost until there are enough molecules in the gas phase that the liquid phase is bombarded with enough slow-moving water molecules that the equilibrium is restored (the more water molecules are in the gas phase, more have a chance of (re)joining the liquid phase).
 

Offline RD

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All the water molecules involved do not have the same kinetic energy : there is a distribution of energy amongst the molecules ...


http://en.wikipedia.org/wiki/Maxwell%E2%80%93Boltzmann_distribution

The thermometer temperature is the average energy value ,
like the distribution of human height, most people are close to average height , but there are a few giants and dwarves ... 

« Last Edit: 10/10/2014 00:29:30 by RD »
 

Offline lightarrow

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It's even worse than that. Under some conditions, there can be solid, liquid and gaseous water all in equilibrium at the same time!
Even worse: not only for water, but for most of chemical elements and compounds  ;)

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lightarrow
 

Offline chiralSPO

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Yes, this is true for pretty much everything. The Boltzmann distribution is universally applicable, and most molecular substances can exist as solid liquid and gas.
 

Offline CliffordK

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You might look up "Partial Pressures" of liquids. 

Essentially all liquids exert a certain force of evaporation based on the temperature and pressure.

Say you have a sealed partly full water bottle (with air above it).  The air above the water will reach an equilibrium with a percent of vapor based on the temperature and pressure of the container. 

On the other hand, leave a bowl of water on the counter top, and the vapor above the bowl is constantly changing.  Assuming the humidity isn't 100% (maximum amount of vapor at any given temperature), then the bowl will slowly evaporate. 

The boiling point is a unique point where the partial pressure is equal to the pressure of the vessel (or atmospheric pressure).  In a sealed vessel, the pressure will increase to compensate.  In an open vessel, or one where the pressure is kept fixed, then the substance will rapidly boil away.
 

Offline lightarrow

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You might look up "Partial Pressures" of liquids. 

Essentially all liquids exert a certain force of evaporation based on the temperature and pressure.

Say you have a sealed partly full water bottle (with air above it).  The air above the water will reach an equilibrium with a percent of vapor based on the temperature and pressure of the container. 
But it's not this the condition to measure water partial pressure (the one which becomes equal to 1 atm at T = 100C): there must be no air inside the container, just water.

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