Water Evaporation

Factual Questions
1

Hi,

This happened when we went out as a team.
We were standing near a swimming pool and the temperature outside was somewhere around 12 Degree CELCIUS. It was nearly 7 A.M and the sun was shining pretty much (in-spite of the temperature).
We saw that from the water, there was vapour rising.
It astonished me that with the temperature so low, why was there vapor that was coming up from the water?
I thought it was water-Vapor and told the same but was not able to convince my self since I remember having read as a child and also as an adult that we need to have 100 Degree CELCIUS for water to vapourize.

Please explain this phenomenan.

Thank you for your assistance.

Yours,
Prasanna G

2

Temperature is an average indicator of the energy held within a substance. There will always be some molecules/atoms possessing enough energy to break free. Consider that freezing is 273 Kelvin, so a mere 12 degrees more is 285 K. Plenty of energy to have some evaporation. Of course once it hits the relatively colder air you’ll get the condensation you noticed.

3

Thank you very much for your reply.
We have also read that the teamperature needs to be 100 Degree Celcius for water to evaporate. So does it mean that in spite of the outside temperature being 12 Degree Celcius, there could have been some molecules that were 100 Degree Celcius due to which we could see Vapors?
If so, why is it that we saw this only in the morning just after sun-rise and not later in the day?

Than you for your assistance.

Prasanna G

4

Here’s a nice link that might help. Kinetic theory.

Basically within a liquid at a temperature T there is a distribution of molecular speeds. For cooler liquids the distribution is shifted closer to 0 m/s. When it’s warmer it shifts further up. Basically there will always be a number of molecules with enough kinetic energy to escape and enter a gaseous state. There’s no need for the liquid to boil. Heating the liquid helps speed evaporation since it shifts the distribution of molecular speeds closer to the point where a larger number can escape.

Think of a cup of water left on a table top. After a day or two some of the water will have evaporated, despite never being close to the boiling point.

5

Water vaporizes at all temperatures where it is a liquid, and even from hard frozen ice. You have seen the effect of vaporized water (humidity) in your daily life, from fog-breath and eyeglass condensation in the winter to the condensed humidity on a cold drink in the summer to freshly mopped floors drying at any time of year.

The vapor pressure of water is the steady-state pressure of vaporized water in a closed container. It is an equilibrium: when there is enough water in the air, it condesnses into a liquid as fast as as it evaporates into a vapor, regardless of the shape or size of the interface. Though it is often stated as a pressure, it can also be stated, as it is in the chart I linked, in units of water per unit of air. [You can do the conversion yourselve, using the gas laws and Avogadro’s number]

The vapor pressure increases with increasing temperature. At 100 degrees C, it happens to equal 1 standard atmosphere. At that temperature (or above) bubbles of vapor can form anywhere in the liquid, not just at the surface, and they will, because there is not enough pressure to force them back into the liquid. This is the definition of “boiling point” - the point at which the vapor pressure equals tha ambient pressure. A boiling point is not a universal, it is always relative to local conditions, but it’s a scientific convention to quote it at “standard atmopheric pressure at sea level (Earth)” [1 atm = 101,325 Pascals (N/m[sup]2[/sup]) = 760 Torr (mmHg) ]

Vapor pressure is also why water boils at a lower temperature at low pressures or high altitudes [an importnat factor in high altitude cooking/baking], or why water in a pressure cooker or car radiator [high pressure] can reach teperatures well above 100 C for many hours yet remain usefully liquid – and if the pressure seal in your pressure cooker or cooling system fails, the water rapidly boils away)

Relative humidity is a measure of the water in the air, as a %age of the amount the air would carry as a steady state, under the local conditions of temperature and pressure, and on Earth, it is rarely below 30% even in the driest desert, yet temperatures on Earth rarely exceed 100 C, so obviously a lot of water is evaporating all around the world.

What you saw over your swimming pool is the exact analog of condensation on a cold glass on a warm day. The relative humidity in the air is below 100%, but when the air cools to the temperature of the glass, the water condenses faster than it evaporates. In your case, water vapor from the warmer air near the water condensed in the cooler air above it (I venture to say the POOL was well above 12 C)

You can see the converse effect when you go in ot out in the winter, due to the different abilities of heated indoor air and cold outside air to hold water. Indoor air is “dry” unless humidified, because the same amount of water that is 100% relative humidity at 0 C outdoors is a parched 25% RH in the heated indoors. Over time, water in the house (and your breath) slowly raise the internal humidity to something more livable – which immediately condenses when it hits the cold outside air or your cold eyeglasses as you enter the house.

Incidentally, the act of vaporizing water takes MUCH more heat than it takes to raise water to the boiling point under ordinary circumstances (e.g. 2260 kJ/kg to turn it to vapor, vs. about 8 kJ/kg to raise water from 20 C to 100 C). This makes the evaporation of water an estrmely effective way to cool (e.g. the human body) and also keeps liquid water from exceeding its boiling point except under the most extreme circumstances – just a little water vapor will carry away enough heat to reduce the temperature to just below boiling.

But that’s another thread.

6

Maybe I can simplify this. Water at any temperature is always evaporating a little. Some molecules are always bouncing off the mass of liquid and joining the atmosphere as gas molecules - what we call evaporating. If you get water up to 100 degrees Celsius, none of it can remain in liquid form and it will all evaporate. That is its boiling temperature. The cooler you get water, the slower its molecules move and the less likely it will be that some of them will bounce off, but even at low temperatures some will always do so. In fact, even at below zero temperatures, some molecules will break away, what is technically called subliming, and join the atmosphere. This explains why, after a snow, even though the temperature never gets above zero, eventually the snow disappears.

7

While the main question has been answered, it also bears pointing out that what you saw was NOT water vapour, as that it invisible!

What you saw was small droplets forming as the water vapour condensed in the air.
[/nitpick]

8

Or for a more familiar experience, look at the ice cube trays in your frost-free refrigerator. The fan that keeps the cold, dry air moving through the freezer compartment prevents frost from forming, and your ice cubes sublimate away to nothing in just a few weeks, even though the temperature (we hope!) never gets above the freezing point.

9

On cold nights you can see a similar effect on wet roads - you get layers of mist rising off the road. The road is slightly warmer than the air above, and the water evaporates then condenses into tiny droplets of mist in the cold air above.

Remember, if you spill a glass of water and don’t mop it up, it will evaporate in a few hours, even though the temperature is only around 20-25 degrees C.

10

Just to add one thing. When the high velocity, i.e. high temperature, water molecules escape the surface those left behind in the liquid are lower velocity and lower temperature. Thus water cools from evaporation

11

[nitpick] the figure is 335.2 kJ/kg to raise water from 20 to 100 C. [/nitpick]