Transition states between matter

[Bassalisk]

Take water for instance. It has a little vapor in liquid state. Today our professor explained boiling points and freezing points, but didn't go to molecular level, he only said that it needs more energy to actually boil water, or freeze it. Can u explain why more energy is needed to add to water to get it boiling? Process is distinctive by bubbles and all that. We all know that we see vapor before boiling point.

If the answer is wide, suggest a textbook that attacks this problem hard.

Thanks

 

[nasu]

Take water for instance. It has a little vapor in liquid state. Today our professor explained boiling points and freezing points, but didn't go to molecular level, he only said that it needs more energy to actually boil water, or freeze it.


Can u explain why more energy is needed to add to water to get it boiling? Process is distinctive by bubbles and all that. We all know that we see vapor before boiling point.

If the answer is wide, suggest a textbook that attacks this problem hard.

Thanks

More than what? Are you talking about vaporization versus boiling?

 

[Bassalisk]

I was told that it needs some extra energy to actually get the water to start boiling.

Those bubbles that occur are confusing me, why is this happening?

 

[alxm]

It's sort of a bad way of putting it. What happens is that you reach the boiling point, after which, adding additional energy does not lead to an increase in temperature but an increase in vaporization.

The reason for that is that, at the boiling point you've basically reached the 'breaking point' of the intermolecular bonds that characterize that phase (be it solid or liquid). The molecules that get more energy than that, leave the liquid phase and enter the gas phase, their additional energy becoming kinetic energy for the now-gaseous molecule.

To make an analogy, if you put some ping-pong balls in a bucket, the more violently you shake the bucket (higher temperature), the higher the balls will reach. But once you shake it so hard that they reach the rim of the bucket, they don't go any higher but rather start flying out.

Bubbles are a bit complicated. It comes about because the molecules not at the surface can't immediately go to the gas phase. Forming a bubble takes a certain amount of energy, due to the surface tension. This is a process called nucleation and occurs due to impurities and uneven heating, which allow enough sufficiently-hot molecules to get together and start a bubble.

 

[Bassalisk]

It's sort of a bad way of putting it. What happens is that you reach the boiling point, after which, adding additional energy does not lead to an increase in temperature but an increase in vaporization.

The reason for that is that, at the boiling point you've basically reached the 'breaking point' of the intermolecular bonds that characterize that phase (be it solid or liquid). The molecules that get more energy than that, leave the liquid phase and enter the gas phase, their additional energy becoming kinetic energy for the now-gaseous molecule.

To make an analogy, if you put some ping-pong balls in a bucket, the more violently you shake the bucket (higher temperature), the higher the balls will reach. But once you shake it so hard that they reach the rim of the bucket, they don't go any higher but rather start flying out.

Bubbles are a bit complicated. It comes about because the molecules not at the surface can't immediately go to the gas phase. Forming a bubble takes a certain amount of energy, due to the surface tension. This is a process called nucleation and occurs due to impurities and uneven heating, which allow enough sufficiently-hot molecules to get together and start a bubble.

so after 100 degrees water does not get hotter but rather all energy invested after helps boiling further?

And why do we have vapor even before boiling point? Is it because at some point molecules accumulate quanta of energy so they detach from liquid and go into air?Then another question: what happens when WATER freezes? its one of the rare liquids that gets less dense when solid.

 

[alxm]

so after 100 degrees water does not get hotter but rather all energy invested after helps boiling further?

As long as the pressure over the water is constant, yes. Otherwise the temperature and boiling point will increase with temperature.

And why do we have vapor even before boiling point? Is it because at some point molecules accumulate quanta of energy so they detach from liquid and go into air?

The energy is Boltzmann/Maxwell-Boltzmann distributed, so yes, some of them have enough energy to leave anyway. And some of the vapor will have sufficiently low energy to condense. It's an equilibrium.

Then another question: what happens when WATER freezes? its one of the rare liquids that gets less dense when solid.

Same thing as with all substances, it forms a more ordered crystal structure. It's just that water's crystal structure is less dense than its liquid phase. That's due a combination of the fact that that it forms hydrogen bonds and its geometry. Water molecules require fairly specific and well-ordered orientations to form strong hydrogen bonds, so they do not bond as strongly even though they're closer together in water, since they're not oriented correctly.
http://www.uic.edu/classes/bios/bios100/lectures/02_15_hydrogen_bonding-L.jpg" says it all, really.

 

[Bassalisk]

As long as the pressure over the water is constant, yes. Otherwise the temperature and boiling point will increase with temperature.


The energy is Boltzmann/Maxwell-Boltzmann distributed, so yes, some of them have enough energy to leave anyway. And some of the vapor will have sufficiently low energy to condense. It's an equilibrium.


Same thing as with all substances, it forms a more ordered crystal structure. It's just that water's crystal structure is less dense than its liquid phase. That's due a combination of the fact that that it forms hydrogen bonds and its geometry. Water molecules require fairly specific and well-ordered orientations to form strong hydrogen bonds, so they do not bond as strongly even though they're closer together in water, since they're not oriented correctly.
http://www.uic.edu/classes/bios/bios100/lectures/02_15_hydrogen_bonding-L.jpg" says it all, really.

From top to bottom everything is more clear! THANKS!