evaporation vs boiling

[inaccensa]

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what is the difference between evaporation and boiling?

 

[rocuronium]

EDIT: This is not correct. Please see responses below.

Boiling occurs when the vapour pressure of a liquid is equal to the atmospheric pressure surrounding it. This occurs at a specific temperature called the boiling point (e.g. 100C is the boiling point for water).

Evaporation is similar to boiling in that it results in a liquid turning into a gas. The difference, however, is that evaporation occurs at temperatures below the boiling point. Some molecules will spontaneously obtain enough energy to move into the gaseous state.

Evaporation is slower than boiling, but its rate increases as the temperature moves closer to the boiling point.

 

[RogueUnicorn]

Boiling occurs when the vapour pressure of a liquid is equal to the atmospheric pressure surrounding it. This occurs at a specific temperature called the boiling point (e.g. 100C is the boiling point for water).

Evaporation is similar to boiling in that it results in a liquid turning into a gas. The difference, however, is that evaporation occurs at temperatures below the boiling point. Some molecules will spontaneously obtain enough energy to move into the gaseous state.

Evaporation is slower than boiling, but its rate increases as the temperature moves closer to the boiling point.

lolwut

 

[inaccensa]

Boiling occurs when the vapour pressure of a liquid is equal to the atmospheric pressure surrounding it. This occurs at a specific temperature called the boiling point (e.g. 100C is the boiling point for water).

Evaporation is similar to boiling in that it results in a liquid turning into a gas. The difference, however, is that evaporation occurs at temperatures below the boiling point. Some molecules will spontaneously obtain enough energy to move into the gaseous state.

Evaporation is slower than boiling, but its rate increases as the temperature moves closer to the boiling point.

gr8 thanks. So water is continuously being evaporated into gaseous phase, correct. i remember that the when vapor pressure is equal or greater than atm pressure, boiling occurs. However, if the atm pressure is greater than the vapor pressure and the vapor pressure inturn is greater than the partial pressure, evaporation takes place. What partial pressure are we talking about? what is the difference between the partial pressure and vapor pressure?

 

[rocuronium]

So water is continuously being evaporated into gaseous phase, correct.

Correct.

what is the difference between the partial pressure and vapor pressure?

Partial pressure refers to the pressure that an individual gas would exert if it were alone in a container. Vapour pressure is the pressure of a vapour in equilibrium with its non-vapour phases.

However, if the atm pressure is greater than the vapor pressure and the vapor pressure inturn is greater than the partial pressure, evaporation takes place.

I think you are creating confusion by including partial pressure in the statement above. If a liquid's vapour pressure is less than atmospheric pressure, evaporation takes place. Period.

The use of 'partial pressure' in this context is confusing, but is sometimes used to refer to the vapour pressure of water. I would concentrate on understanding the relationship between vapour pressure and atmospheric pressure.

 

[MAHDI]

Boiling occurs when the vapour pressure of a liquid is equal to the atmospheric pressure surrounding it. This occurs at a specific temperature called the boiling point (e.g. 100C is the boiling point for water).

Evaporation is similar to boiling in that it results in a liquid turning into a gas. The difference, however, is that evaporation occurs at temperatures below the boiling point. Some molecules will spontaneously obtain enough energy to move into the gaseous state.

Evaporation is slower than boiling, but its rate increases as the temperature moves closer to the boiling point.

I am confused , what is the vapor pressure while we talk about a liquid

 

[plzNOCarribbean]

lolwut

+1 I agree that didn't make any sense. I would not go by that description, with all do respect, OP.

 

[plzNOCarribbean]

I am confused , what is the vapor pressure while we talk about a liquid

The vapor pressure is simply the pressure above a liquid (its the pressure above a liquid that is contributed by that liquid). If you a CLOSED container of pure liquid A, with no solutes in it, then the vapor pressures is the pressure above the liquid when the molecules escape from the liquid phase and go into the gaseous phase. At the same time, some of those gaseous molecules that are above the liquid will be moving (due to their kinetic energy) and collide back with the liquid. When the rate of the molecules leaving the liquid phase and entering the gas phase equals the rate of the gas phase molecules entering the liquid phase, ie an equilibrium exists between the two processes, is when the vapor pressure gets its value from.

Also, boiling/evaporation = same thing. The boiling point is the temperature at which liquid molecules begin to leave the liquid phase and enter the gaseous phase. It is true, which is what I think the first poster was trying to say, that this occurs when the vapor pressure of the liquid equals atmospheric pressure.

This is why the boiling point of a liquid decreases when you climb a mountain because as you go higher in altitude, the atmospheric pressure decreases. This means that the vapor pressure needed to equal that atm pressure needs to be less than normal, which corresponds to a lower BP. this can also be observed on a simple phase diagram.

 

[plzNOCarribbean]

Also, another thing you should understand is when you have two liquids mixed together, or when you add solute to a an otherwise pure liquid. Both of these instances result in a DECREASE in the vapor pressure.

In the first example, when you mix two liquids, the vapor pressure contributed to the total pressure above the liquid decreases for each component. That is, each liquid is contributing less to the total vapor pressure. The individual vapor pressure contributed by each liquid is equal to the mole fraction of that liquid x the partial pressure of that PURE liquid, Po. So, for example, if you mix liquid A with liquid B, the vapor pressure contributed by liquid a would be:

Va= Xa x Po

where Xa is the mole fraction of liquid a which is moles A/moles A + moles B
and Po is the partial pressure of Pure A. The same method can be used to find the vapor pressure contributed by liquid B, and when you add the two together you get the total pressure above the liquid contributed by each component of the mixture.

In case #2, the vapor pressure decreases due to the addition of a solute. The reason is because solute molecules take up space on the surface of the liquid, and so they decrease the surface area of the liquid. Thus, when you heat the liquid there are fewer liquid molecules entering the gaseous phase since some of the surface of the liquid is occupied by a salt particle, which has a very very high MP and will not enter the gaseous phase. This is why the VP is depressed upon the addition of solute

 

[rocuronium]

I remember discussing this topic after my original response. I realized that what I had posted was not correct, and I thought that I had made a comment in my post. I guess I'll do it again!

 

[vayntraubinator]

Simple Guidelines given by EK for evaporation and boiling.

Evaporation:

When P atmosphere>P Subst. liquid vapor>Partial Pressure of Subst. Above

Boiling:

When Pl Subst. iquid vapor> or = P atmosphere and > Partial Pressure Above