# B Why does water evaporate when it's less than 212 F outside?

#### fourthindiana

When one boils a pot of water on the stove, the water starts to boil into steam at 212 degrees Fahrenheit. The stove provides the heat both to make the water 212 degrees, and the stove provides the heat to change the state of the liquid water to vapor.

However, we all know that water (or any other liquid, for that matter) evaporates when it is cold outside, as long as the temperature is 33 degrees or higher.

I know that it takes a relatively large amount of heat just to change the state of liquid water to vapor without increasing the temperature of water at all. If there is a puddle of 50 degrees Fahrenheit liquid water on the ground outside, and if the temperature outside is 50 degrees Fahrenheit, any increase in temperature would just increase the temperature of the liquid water, not cause a change of state of the liquid water to vapor. Therefore, when it is 50 degrees Fahrenheit outside, why does liquid water evaporate?

#### Drakkith

##### Armchair Scientist
Staff Emeritus
2018 Award
From wiki:

Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase when it reaches its boiling point.[1] The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other based on how they collide. When a molecule near the surface absorbs enough energy to overcome the vapor pressure, it will "escape" and enter the surrounding air as a gas.[2] When evaporation occurs, the energy removed from the vaporized liquid will reduce the temperature of the liquid, resulting in evaporative cooling.

#### fourthindiana

From wiki:

No.

From your source: "Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase when it reaches its boiling point"

If evaporation occurs as a liquid reaches its boiling point (212F for water), then why does water evaporate at 50 degrees Fahrenheit?

#### Drakkith

##### Armchair Scientist
Staff Emeritus
2018 Award
If evaporation occurs as a liquid reaches its boiling point...
It doesn't occur only at the boiling point. It can occur at any temperature above freezing. Further down in the wiki article:

For molecules of a liquid to evaporate, they must be located near the surface, they have to be moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular forces.[4] When only a small proportion of the molecules meet these criteria, the rate of evaporation is low. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures. As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid decreases.
I believe the first sentence in the article is simply mistaken. Evaporation does not required the liquid as a whole to reach its boiling point.

#### sophiecentaur

Gold Member
Evaporation occurs only on the surface. Boiling can cause a change of state anywhere in the liquid. (bubbles)
The liquid on the surface has what is called a Vapour Pressure. Dalton's Law of Partial Pressures says that the pressure in a mixture of gases is equal to the sum of the partial pressures of each of its constituents. If you have a fixed volume of air (no circulation to the atmosphere) over a water surface, the air pressure will be equal to the pressures of the Oxygen, Nitrogen etc. individually and also the pressure of the gaseous water vapour. Water vapour will leave the surface until equilibrium is reached and the vapour pressure will balance the pressure causing molecules to leave. If you refresh the air over the surface, vapour will keep being lost by evaporation and the temperature will drop due to the lost KE of the escaped molecules. Clothes dry quicker on a warm windy day - higher temperature and constantly refreshed air (with lower partial pressure of water) over the surface.
Vapour pressure is a function of temperature and it is the same as Atmospheric Pressure at 100°C (so called boiling point). At 100°C (212°F) the internal pressure will cause a faster loss of molecules and all the liquid can be converted to vapour even under the surface as bubbles - as long as you keep supplying heat.
If you reduce the pressure of the air over the surface (vacuum pump or up a mountain), boiling will occur at a lower temperature.

#### russ_watters

Mentor
From your source: "Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase when it reaches its boiling point"

If evaporation occurs as a liquid reaches its boiling point (212F for water), then why does water evaporate at 50 degrees Fahrenheit?
I believe the first sentence in the article is simply mistaken. Evaporation does not required the liquid as a whole to reach its boiling point.
I'd call it poorly written. The key is that the temperature of any substance is related to the average kinetic energy of the substance. Some molecules have higher kinetic energy and some lower. Those molecules with a kinetic energy equal to vaporization for 212F water/steam will evaporate in liquid water that is overall below 212F. The energy balance bears this out as the energy loss from evaporation per unit mass is higher at lower temperatures, to include the energy required to raise the temperature to 212F. A steam table will show this.

#### Chestermiller

Mentor
Who says that ice doesn’t evaporate? Water, including ice, will evaporate if the partial pressure of water vapor in the atmosphere above the condensed phase is less than the equilibrium vapor pressure of water at the temperature of the condensed phase.

#### phinds

Gold Member
Who says that ice doesn’t evaporate?
Technically, it doesn't. It sublimates.

#### Chestermiller

Mentor
Technically, it doesn't. It sublimates.
Semantics. I call it evaporation. Potatoes - potahtoes.

#### phinds

Gold Member
Semantics. I call it evaporation. Potatoes - potahtoes.
An odd point of view here on PF to say that technical terms don't matter and that it's ok to give them your own definition.

#### Chestermiller

Mentor
An odd point of view here on PF to say that technical terms don't matter and that it's ok to give them your own definition.
Would calling it by the correct name change the mechanistic description I provided?

#### phinds

Gold Member
Would calling it by the correct name change the mechanistic description I provided?
Naturally not, but I still think it's better when we all use the agreed on words to describe phenomena.

#### DarioC

Perhaps this doesn't belong here but I have found myself amazed at this: I have a carbon steel small paring knife and when I have washed it I can smell the fairly strong odor of "rusted" iron coming from it. I can imagine iron oxide molecules evaporating off the metal surface. I find that rather astounding.

#### jbriggs444

It can occur at any temperature above freezing.
And below. Supercooled liquid water still evaporates as long as the partial pressure of water vapor in the surroundings is sufficiently low.

#### DarioC

I have a tray of ice cubes in my freezer that have been there for months and some of the cubes are as much as 75% reduced in size. The temperature is about minus 20 degrees C.

#### Drakkith

##### Armchair Scientist
Staff Emeritus
2018 Award
I have a tray of ice cubes in my freezer that have been there for months and some of the cubes are as much as 75% reduced in size. The temperature is about minus 20 degrees C.
Interesting. The ice in my ice maker tends to freeze together and grow into one massive ice clump instead of slowly dissipating. I assume this is because the air in my house is usually fairly humid and the moisture gets into the freezer every time I open the door and then condenses and freezes. Not to mention the fact that the ice maker itself adds moisture to the inside every time it squirts water into the trays to make new ice.

#### phinds

Gold Member
I have a tray of ice cubes in my freezer that have been there for months and some of the cubes are as much as 75% reduced in size. The temperature is about minus 20 degrees C.
Absolutely normal for a refrigerator that has a good defrosting mechanism. They are designed to cause sublimation so ice doesn't build up and they have the unfortunate side-effect of sublimating your ice cubes.

#### 256bits

Gold Member
Perhaps this doesn't belong here but I have found myself amazed at this: I have a carbon steel small paring knife and when I have washed it I can smell the fairly strong odor of "rusted" iron coming from it. I can imagine iron oxide molecules evaporating off the metal surface. I find that rather astounding.
Probably not the iron molecules themselves, nor even the iron oxide,
https://phys.org/news/2006-10-iron.html
"That we are smelling the metal itself is actually an illusion."
But a chemical reaction with the metal and your body secretions. from the skin.
The key component is called 1-octen-2-one, which smells fungal-metallic even when highly diluted. The precursors to the odor molecules are lipid peroxides, which are produced when oils on skin are oxidized by certain enzymes or other processes (e.g. under UV light). These lipid peroxides are then decomposed by the doubly negative iron ions, which are consequently reduced to triply negative iron ions. When touching objects made of iron, the required doubly negative ions are formed when perspiration on the skin corrodes the iron
.
The researchers were also able to characterize another iron-type smell: carbon- and phosphorus-containing cast iron and steel develop a metallic-garlic odor when exposed to acids. Until now, metallurgists ascribed this to the gas phosphine (PH3). However, at breathable concentrations, pure phosphine (also known as a pesticide) is basically odorless. The true culprits are organophosphines, especially those champions among intensively smelly compounds like methylphosphine and dimethylphosphine. Their structure is like that of a phosphine molecule in which one or two of the hydrogen atoms are replaced with methyl (CH3) groups.

#### Mister T

Gold Member
When one boils a pot of water on the stove, the water starts to boil into steam at 212 degrees Fahrenheit. The stove provides the heat both to make the water 212 degrees, and the stove provides the heat to change the state of the liquid water to vapor.
Note that you are describing an equilibrium situation where the liquid and the gas co-exist. (By the way this occurs only at pressures of 101 kPa.)

You're asking about a different situation. Different because you don't have the two phases co-existing.

#### LURCH

So it seems that the key to understanding this is that temperature is the average kinetic energy, and there is a great deal of variation between the individual molecules. How much variation would you say is typical, between molecules in, for example, a glass of water setting on a table at room temperature? Is that in the range of a 50% difference, a few hundred percent, orders of magnitude, or what? (Never really occurred to me to wonder about that before. )

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