Water evaporation

Water evaporates at all temperatures.

...provided the wet-bulb temperature of the surrounding air is less than the dry-bulb temperature, i.e. the relative humidity is less than 100%.

Otherwise it condenses.

The water would still evaporate, but it becomes an equilibrium process under the conditions you note.

Quite.

If water could only evaporate at 100C How do we cool by sweating?

Dear Mr. Brainwave,

You have referred "If water could only evaporate at 100C How do we cool by sweating?

Pl. note that Water will BOIL ONLY AT ONE TEMPERATURE and not at different temp. When you say boiling at 100 Deg.C, it corresponds to Atmospheric Pressure of 1.03 Kg/CM^2 If the Pressure goes up, the Boiling Point will go up and viseversa.

But Evaporation can happen at all Temperatures, as long as the AIR is not saturated, vapour is accepted by the surrounding air, and wet bulb temp. is low - which relates to RELATIVE HUMIDITY.

Pl. refer PSYCHROMETRIC CHART and HUMIDIFICATION Topics in Thermo Dynamics.

DHAYANADHAN.S

Evaporation (or condensation) of a liquid (or solid) is related to its vapor pressure at a given temperature. When the temperature raises the vapor pressure to ambient (typically atmospheric), it boils with any additional heat applied.

http://en.wikipedia.org/wiki/Vapor_pressure

Regards,

Greg

It dose not boil but it evaporates as the ralitve humadety of air is always less then the spilled water

So, evaporation is simply a term to describe boiling that takes place at less than 1 atmosphere saturation temperature.

The relative humidity of air often gets to 100%. This is the precursor to the various meterorolgical phenomena known as mist, fog, rain, snow, hail...

http://www.simetric.co.uk/si_steam.htm

Do any primary schools teach Science anymore?

Quite.

Why would anyone want to become a teacher and get AU$40,000 a year when you can work in a mine driving trucks for AU$80,000?

Too many do gooders and have you seen the cost of public liability insurance of lat plus because no teacher in their right mind wants to pass on any knowledge that might come back to bite them.

Water has surface tension. Molecules of water have energy and vibrate in place bouncing off adjoining water molecules. When the enrgy of a particular water molecule becomes enough to break through the surface tension it goes out into the air. Temperature, humiduty and pressure are priamry factors that will affect the surface tension of water. As the surface tension goes down or temp of water goes up evaporation will increase. Evaporation can begin at low temp, but will not be very fast. As temp goes up evaporation rate increases.

So what would be the effect of adding a detergent, like washing-up liquid, to the evaporating or boiling water?

The soap molecules weaken or break up the surface tension of the water, making it evaporate more quickly. The soap, however, doesn't evaporate, leaving a film on the surface (floor) which must be mopped up with clean water. Wax has the opposite effect and causes the surface tension to hold the water in beads or droplets which slows the evaporation.

Then we have the slowest evaporation of them all: sublimation. Even ice will disappear eventually without passing through the liquid phase. Same principle: a few molecules having sufficient energy to escape do so. The others remain behind and jostle each other until statistics favor a few more getting bounced just right for attaining escape velocity, and so on.

Not long ago someone posted a blog entry: Wanted: Simple Science Experiments for Kids. In Post #20 of that blog I describe a simple experiment which demonstrates water "evaporating" from a puddle. My kids loved it!

Then, because of a tendency to form an equilibrium, vapour would condense more quickly too. Just as long as the boiling temperature/boiling pressure relationship doesn't change...

Dear Mr. PW Slack,

Still the water will escape as long as RELATIVE HUMIDITY is NOT 100 Percent. Lesser the elative Humidity faster is the water evaporation at the surface.

DHAYANANDHAN.S, INDIA.

Temperature is a measure of the average heat energy of molicules. Individual molicules rapidly oscillate between high and low energy levels as they exchange heat through friction and radiation. Water can evaporate at room temperature because the molicules on the surface of the water occationally absorb enough heat energy to transition from liquid to vapor. In other words, when the average temperature of water is at room temperature, some of the molicules are actually freezing while others are boiling.

For a really cool (quite literally) experiment, put a Styrofoam cup filled with room-temperature tap water in a bell jar and pump it down. As the air pressure in the jar drops below the vapor pressure of water (about 20 Torr at 25°C), the water will begin to boil. Generally about 1/3 to 1/2 of the water boils away (because the most energetic water molecules are escaping en masse at this low pressure), leaving the cooler slow-pokes behind. When enough energetic molecules leave, the colder molecules predominate and the water freezes.

So, basically, you boil water 'til it freezes. Of course, none of your friends will believe you if you tell them this, so, to be on the safe side, you should probably just keep it between you and God.

Cheers!

-e

PS: You can also do this with liquid nitrogen to make solid nitrogen. I haven't tried it with liquid helium, but I'd be interested in how cold I could get it using this technique. For one thing LN₂ is about as expensive as beer. Liquid helium costs about the same as a good whiskey.

Dear Guest,

If RELATIVE HUMIDITY of SURROUNDING AIR is 100 percent, then water will NOT DISAPPEAR by EVAPORATION.

DHAYANANDAN.S, INDIA.

Dear Friend,

"how water on floor gets disappeared even before the temperature reaching 100 degree celsius......."

For water getting DIAPPEARED on the floor - Temp. need not be 100 Deg.C - which is the Boiling Point of water at atmospheric pressure.

Water DISAPPEARS because, the water gets EVAPORATED slowly, on account of UN-SATURATED CONDITION of AIR, which is DETERMINED BY WET BULB TEMPERATURE.

If air is saturated, water will remain as it is for more time.

DHAYANANDHDAN.S