Asac Posted September 13, 2006 Share Posted September 13, 2006 My Chem teacher aroused the question today, 'Why or how does water boil at room temperature?' that is if you were to leave a cup of water in a room with a temp of lets say 25degreesC and come back in two weeks the water would be evaporated(at least to some point). To change into a gas water is boiled and at 100degreesC it turns into vapor no? So how does water evaporate or chaneg states at 25 degressC rather than 100DC ? Link to comment Share on other sites More sharing options...
YT2095 Posted September 13, 2006 Share Posted September 13, 2006 it doesn`t Boil at room temp, at least not without a very VERY low pressure (think Vacuum distilation). what Does happen is that excided gas molecules in the air collide with the semi excited water molecules and knock them out by energy transferance. Link to comment Share on other sites More sharing options...
swansont Posted September 13, 2006 Share Posted September 13, 2006 To add to what YT said, the molecules don't all have the same energy — there is a distribution. The more energetic ones can escape, and this is why evaporation removes so much energy; you preferentially lose the most energetic molecules. But in this case the liquid will re-equilibrate with the container/surroundings, so there is a source of energy to keep the water evaporating. Link to comment Share on other sites More sharing options...
BobbyJoeCool Posted September 13, 2006 Share Posted September 13, 2006 So why does the sun expediate the process? It can't make it reach 100°C, can it? Or does it not make it faster... Link to comment Share on other sites More sharing options...
ecoli Posted September 13, 2006 Share Posted September 13, 2006 So why does the sun expediate the process? It can't make it reach 100°C, can it? Or does it not make it faster... The sun is an energy source to the system. The heat excites the molecules, which increases the rate of evaporation. Link to comment Share on other sites More sharing options...
Externet Posted September 13, 2006 Share Posted September 13, 2006 Hi. Be careful not to confuse the terms 'vapor' and 'steam', often frequently misused. The second is the one created over 100 ºC. The first happens at any conditions, as clearly, clouds are not at that temperature. Miguel Link to comment Share on other sites More sharing options...
ecoli Posted September 13, 2006 Share Posted September 13, 2006 Hi.Be careful not to confuse the terms 'vapor' and 'steam'' date=' often frequently misused. The second is the one created over 100 ºC. The first happens at any conditions, as clearly, clouds are not at that temperature. Miguel[/quote'] Actually clouds aren't water vapor. It's liquid water condensed on small particles in the atmosphere. The droplets stay airborn, however, because they are so small. Link to comment Share on other sites More sharing options...
BobbyJoeCool Posted September 13, 2006 Share Posted September 13, 2006 The sun is an energy source to the system. The heat excites the molecules, which increases the rate of evaporation. AH! Most helpful! Link to comment Share on other sites More sharing options...
Asac Posted September 13, 2006 Author Share Posted September 13, 2006 Thank you all, that does make sense Link to comment Share on other sites More sharing options...
JHAQ Posted September 22, 2006 Share Posted September 22, 2006 A liquid boils when its vapor pressure equals the pressure of its environment . Link to comment Share on other sites More sharing options...
JHAQ Posted September 22, 2006 Share Posted September 22, 2006 P S it is higher boiling than periodic table analogues like H2S because of hydrogen bonding Link to comment Share on other sites More sharing options...
aj47 Posted September 22, 2006 Share Posted September 22, 2006 P S it is higher boiling than periodic table analogues like H2S because of hydrogen bonding Yes Link to comment Share on other sites More sharing options...
jdurg Posted September 25, 2006 Share Posted September 25, 2006 Yes, it is very important to understand what boiling actually is. Boiling is when the vapor pressure of the liquid (that is, the pressure of the molecules of vapor that it is constantly giving off) is equal to the atmospheric pressure. A liquid is normally a liquid because the atmospheric pressure around the liquid is higher than the pressure of the vapor that the liquid gives off. This causes the molecules to remain in the liquid. However, there is an equilibrium that sets up between the liquid phase and the vapor phase. There are constantly molecules of the liquid that will escape from the surface of the liquid and form a vapor. As the temperature rises, the number of molecules that have the energy needed to escape the liquid rises as well. Therefore, the vapor pressure of the liquid rises as more vapor is formed. When you reach the boiling point of the liquid, the pressure that the escaping vapor is providing is equal to the atmospheric pressure. As a result, there is no real push against the liquid to keep it as a liquid. The molecules within have enough energy and they rapidly leave the surface of the liquid. If you lower the pressure around the liquid, then you alter the equillibrium of the liquid/vapor relationship and allow more molecules to escape as vapor. As a result, the boiling point drops because now you don't need as much energy to leave the surface of the liquid. Link to comment Share on other sites More sharing options...
chem1a Posted December 7, 2006 Share Posted December 7, 2006 interestingly if you have a fan in your freezer and place ice cubes infront of the fan you will notice over a period of weeks the ice cubes shrinking .... they certainly are not at 100c but are very slowly evaporating Link to comment Share on other sites More sharing options...
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