Why Does Evaporation Cool You Down If It Uses the Sun's Energy?

[daniel_i_l]

I know that the conventional explanation for this is that the water needs energy to evaporate so it conviniantly takes the heat energy from your body in order to do this. But first of all, why does the water need to evaporate in the first place? Because of the sun? Then it should be getting energy from the sun not from your body? I hope that you guys understand the question, this always seemed so obvious to me but then when I thought it over I got confused.
Thanks!

 

[HallsofIvy]

The energy required to evaporate is from the water itself. Water "evaporates" when it changes from liquid into a gas. Remember that different molecules in a drop of water will have different speeds- the temperature is a function of the average speed. Even if the average molecular speed (i.e. temperature) is low enough that the water stays in liquid form, some of the molecules will have speed high enough to escape from the drop. Since it doesn't come back, that reduces the average molecular speed and so the temperature of the water.

 

[actionintegral]

So the water itself is the thing that cools you? The water gets cooler and then so do you? I think I understand now. I always incorrectly thought the water was sucking the heat out of your body somehow.

 

[Hootenanny]

So the water itself is the thing that cools you? The water gets cooler and then so do you? I think I understand now. I always incorrectly thought the water was sucking the heat out of your body somehow.

No, the water does not get cooler. It takes energy to change the state of the water (evaporate it). This energy comes from your skin, you transfer thermal energy from your skin, to the sweat this increases the internal energy of the sweat and it evaporates. I will say again, the sweat does not get cooler.

 

[actionintegral]

Now I am in trouble. The experts disagree. Mr. Halls of Ivy said that the most energetic water molecules would leave droplets - not by virtue of my body heat per se, but because the water vapor content of the surrounding air was lower. The energetic particles leaving the droplet would leave the cooler particles behind.

 

[Hootenanny]

Now I am in trouble. The experts disagree. Mr. Halls of Ivy said that the most energetic water molecules would leave droplets - not by virtue of my body heat per se, but because the water vapor content of the surrounding air was lower. The energetic particles leaving the droplet would leave the cooler particles behind.

Sorry I should have made my point clearer. I meant that the water which evaporates does not get cooler. I hadn't read HOI's post, I just skipped to yours. The water that evaporates does not get cooler, but the water that it left behind does. Apologies for the confusion.

 

[quark]

Perspiration is a mechanism by which body temperature, of warm blooded animals, is maintained. When the ambient temperature is greater than 98.4F, heat transfers from ambience to the human body and thus raises the body temperature. Body pushes out sweat from sweat glands, this water absorbs heat from human body and evaporates.

For water to evaporate from human body, the partial pressure of water vapor in the air should be less than that of saturated partial pressure of water at ambient temperature. In simple terms, RH should be lower than 100%. When RH is 100%, sweat will not evaporate and becomes visible on your body.

 

[daniel_i_l]

So do you get cooled down because the heat gets sucked out of your body, or because the water itself cools down, I've always read that it's the later, but if it's the former then why doesn't the energy source (sun) heat up the rest of the "cold" water and make it hot to?

 

[Hootenanny]

So do you get cooled down because the heat gets sucked out of your body, or because the water itself cools down, I've always read that it's the later, but if it's the former then why doesn't the energy source (sun) heat up the rest of the "cold" water and make it hot to?

It is the evaporation of the sweat that cools you. If the sweat didn't evaporate it wouldn't cool your body down, that is you generally feel hotter in a humid, the air is already saturated (or close to) with water, so your sweat cannot evaporate.

 

[daniel_i_l]

Yes but's that's my question, how does the evaporation cool you down? Thanks.

 

[russ_watters]

The first reply (and several others) contains the answer: the evaporating sweat carries away heat, leaving your skin cooler.

 

[Hootenanny]

I suggest that you re-read this

The energy required to evaporate is from the water itself. Water "evaporates" when it changes from liquid into a gas. Remember that different molecules in a drop of water will have different speeds- the temperature is a function of the average speed. Even if the average molecular speed (i.e. temperature) is low enough that the water stays in liquid form, some of the molecules will have speed high enough to escape from the drop. Since it doesn't come back, that reduces the average molecular speed and so the temperature of the water.

 

[quark]

We can check it by an example calculation. Consider a 60kg person in a 40deg.C ambience and got his body temperature raised to 40deg.C before perspiration starts (actually this is quite dynamic phenomenon and the body doesn't wait to calculate the quantity of sweat required).

Average specific heat of human body is about 3.5 kJ/kg deg.C.

For a 3deg.C drop of body temperature, quantity of heat to be removed is 60x3.5x(40-37) = 630 kJ. (quantity of to be evaporated is 630/2310 = 0.27 kgs.

If this quantity of heat is to be supplied by water (itself) at 40 deg.C, the quantity of water required is 630/(4.2 x (40-37)) = 50 kgs.

So the heat is transferred from the body to the evaporating water and you feel cool as heat is removed from the body.

This is different from cooling phenomenon of hot water in a container, where water gets heat from rest of the water and thus it gets cooled.

 

[rcgldr]

Water "evaporates" when it changes from liquid into a gas

I thought evaporation was just very small droplets of water being asorbed by the air, not generation of steam. Maybe vapor would be a better description than gas.

 

[Hootenanny]

I thought evaporation was just very small droplets of water being asorbed by the air, not generation of steam. Maybe vapor would be a better description than gas.

Yes, technically sweat doesn't evaporate is vaporises.

 

[russ_watters]

Huh? "Vaporize" and "evaporate" are two forms of the same word root, and the meaning of both is "to change into a vapor"

http://dictionary.reference.com/browse/evaporate
http://dictionary.reference.com/browse/vaporize

And a vapor is a gas - it is not suspended particles of liquid water. If it were just droplets of still liquid water , then there'd be no energy loss due to sweating.

http://dictionary.reference.com/browse/vapor

"The gaseous state of a substance that is liquid or solid under ordinary conditions."

 

[rcgldr]

And a vapor is a gas - it is not suspended particles of liquid water.

So water can be steam at room temperature, which is well below boiling point?

 

[russ_watters]

Yes. Check the phase diagram - it can exist as a liquid or vapor (or more often, both at the same time) at room temp:

http://www.lsbu.ac.uk/water/phase.html

 

[rcgldr]

Yes. Check the phase diagram - it can exist as a liquid or vapor (or more often, both at the same time) at room temp: http://www.lsbu.ac.uk/water/phase.html

Unless I'm mis-reading the graph, this only happens at very low pressure. At one atmosphere of pressure, or about 10^5 Pa, the chart shows vapor at 373K, the normal boiling point of water. I guess vaporization is a different type of action.

I had the impression that there wasn't much difference between mist, fog, or low level clouds (one's not composed of ice).

 

[russ_watters]

Pressures of mixed-gases are treated separately, as described by the concept of "partial pressure" (or vapor pressure): http://en.wikipedia.org/wiki/Partial_pressure

In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume.

In chemistry, the partial pressure of a gas in a mixture of gases or of a gas dissolved in a liquid is the pressure which that gas would have if all other components of the gas mixture or liquid suddenly vanished without its temperature changing.[1] The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. Gases will always flow from a region of higher partial pressure to one of lower pressure; the larger this difference, the faster the flow.

Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases (i.e., liquid or solid). Most often the term is used to describe a liquid's tendency to evaporate. It is a measure of the tendency of molecules and atoms to escape from a liquid or a solid. A liquid's boiling point corresponds to the point where its vapor pressure is equal to the surrounding atmospheric pressure.

Gases dissolve, diffuse, and react according to their partial pressures, and not necessarily according to their concentrations in a gas mixture. [emphasis added]

So, at 300K, in a sealed container, liquid water will evaporate until the vapor reaches a pressure of about 2kpa, then the air will be saturated and the water and water vapor in equilibrium.

 

[MalayInd]

I know that the conventional explanation for this is that the water needs energy to evaporate so it conviniantly takes the heat energy from your body in order to do this. But first of all, why does the water need to evaporate in the first place? Because of the sun? Then it should be getting energy from the sun not from your body? I hope that you guys understand the question, this always seemed so obvious to me but then when I thought it over I got confused.
Thanks!

First consider the water kept in an open container. Is evaporation taking place? The answer is yes.
The water body consists of molecules which are constantly in motion. All of them do not have the same speed. They are colliding with each other. During collision, energy of one molecle increases and the energy of other molecule decreases. If the energy increases to such an extent that it can overcome attraction by neighouring molecules, it leaves the water body and evaporation takes place. Since the more energetic molecule leaves, cooling takes place. Just as molecules are leaving, now and then, water molecules from the air are entering the container.
When we blow off a soup, we decrease the concerntration of air just above the cup so that more water molecules leave and cooling takes place.

KeepSmiling
Malay

 

[rcgldr]

vapor

So in vapor form, water in the air is composed of individual molecules and this is why it's considered a gas? What happens if water molecules collide in the air? In addition to evaporation, I understand that there is also some re-capturing of water molecules from the air back into the water, so evaporation, is generally as case of more molecules escaping than returning?

Would the speed of an escaping water molecule be about the same as the speed of boiling water (only to be later slowed by collision with air molecules)?

Once the air gets saturated, then the molecules link up to form mist, but it still hovers in the air, is this still considered the equivalent of a gas?

Are clouds considered to be a gas? What about clouds made up of ice crystals?

 

[3trQN]

Think about a dynamic equillibrium between the three phases, the solid body, the liquid sweat and the gas of the air. In a dynamic equillibrium there is a constant change from one form to the other at the junction ( or surface ) between the phases.

So in vapor form, water in the air is composed of individual molecules and this is why it's considered a gas? What happens if water molecules collide in the air?

Thats a good question. They just collide, but on average ( this is related to statistical mechanics and the distribution of the molecules in dynamic equillibrium which in turn is a property of entropy and thermodynamical laws ), on average the same number of molecules leave the liquid surface as enter it from the surrounding air. But this equillibrium takes time to establish, so initially liquid is lost from the surface of the skin and this is seen as a cooling. By keeping a good air flow over the surface you can keep disturbing the equillibrium in favour of cooling.

 

[russ_watters]

So in vapor form, water in the air is composed of individual molecules and this is why it's considered a gas?

Yes.

What happens if water molecules collide in the air?

Well the reason it is called a gas and not a suspension is that the molecules tend to have enough energy that if they collide, they bounce off of each other. But if the concentration increases enough to the point of saturation, they will start sticking together and you'll have liquid particles in suspension - aka fog (or clouds).

In addition to evaporation, I understand that there is also some re-capturing of water molecules from the air back into the water, so evaporation, is generally as case of more molecules escaping than returning?

Yes! Since the kinetic energy of the particles is a bell curve, you'll always have some condensing and some evaporating, (unless the air is completely devoid of water).

Would the speed of an escaping water molecule be about the same as the speed of boiling water (only to be later slowed by collision with air molecules)?

Yes. Though if it is "slowed down", it may just lose enough energy to condense back into a liquied.

Once the air gets saturated, then the molecules link up to form mist, but it still hovers in the air, is this still considered the equivalent of a gas?

No. It is particles of liquid water in a suspended mixture.

Are clouds considered to be a gas?

No. There is dust in there too, but it is otherwise the same as fog: suspended particles of liquid.

What about clouds made up of ice crystals?

Suspended particles of ice.

 

[DaveC426913]

This is a rough and admittedly somewhat juvenile analogy, but hopefully it will clarify how evaporation actually cools the skin.

Imagine a hypothetical area of the skin with water on it. The skin's molecules have a certain amount of energy and the water molecules do too. The energy in the jostling molecules (i.e. their heat/temperature) of the skin and of the water are both about the same (we'll say x). The sum of those two amounts of energy is 2x - the total energy in this area of the skin.

Water molecules and skin molecules jostle each other back and forth, in elastic collisions. The average energy 2x remains constant.

But if the water molecules get pushed, and, rather than pushing back, they use that energy to escape, the skin molecules do not get the rebound kick. The water molecules have taken much of the energy (1.5x) and stolen away with it. Without that kick, the skin molecules are left with low kinetic energy (0.5x).

Skin molecules with low kinetic energy = cold skin.

 

[Mk]

You guys keep spelling it wrong, its vapour!

 

[Hootenanny]

Huh? "Vaporize" and "evaporate" are two forms of the same word root, and the meaning of both is "to change into a vapor"

http://dictionary.reference.com/browse/evaporate
http://dictionary.reference.com/browse/vaporize

And a vapor is a gas - it is not suspended particles of liquid water. If it were just droplets of still liquid water , then there'd be no energy loss due to sweating.

http://dictionary.reference.com/browse/vapor

"The gaseous state of a substance that is liquid or solid under ordinary conditions."

In which case I stand corrected. My apologies