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Author Topic: Why do clothes dry on the washing line?  (Read 14034 times)

Offline simeonie

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Why do clothes dry on the washing line?
« on: 09/01/2006 21:09:27 »
When you leave your clothes out on the line to dry after being washed (rather when my mum does it for me) why do they dry? It is never anywhere near 100 C so how does the water evaporate.

The reason why I am asking this is because I know my science teacher told us but I REALLY can't remember and when I can't remember something it buggs me for ages. So please someone explain. If no one can explain I will go and ask on my next lesson and post.

Thanks!!!! Simon
« Last Edit: 27/06/2009 15:12:27 by BenV »


 

Offline ukmicky

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Re: Why do clothes dry on the washing line?
« Reply #1 on: 09/01/2006 22:24:09 »
Why do clothes dry, Der....coz their wet...Damn silly question that

Michael                 HAPPY NEW YEAR                    
« Last Edit: 09/01/2006 23:16:18 by ukmicky »
 

Offline vj_tu

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Re: Why do clothes dry on the washing line?
« Reply #2 on: 10/01/2006 16:42:45 »
I think clothes become dry as water evaporates--changing phase from liquid to gas,it's endothermic reaction thus uses energy from surrounding(higher temperature)to do this job.
 

Offline neilep

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Re: Why do clothes dry on the washing line?
« Reply #3 on: 10/01/2006 16:59:05 »
Water does not have to boil to evaporate...just as vi_tu above says....what about water that is ...say 20 degrees Centigrade and you blast it with a constant flow of air that is ..say..15 degrees centigrade..will that NOT help in evaporation then ?

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Offline simeonie

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Re: Why do clothes dry on the washing line?
« Reply #4 on: 10/01/2006 17:15:54 »
No one has answered my question. Why do the clothes dry when it iscnt 100 degrees centigrade? And I don't see why an endothermic reaction has anything to do with it.

I am surprised no one knows. I will try and remember to ask my teacher.

« Last Edit: 27/06/2009 14:41:12 by BenV »
 

Offline neilep

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Re: Why do clothes dry on the washing line?
« Reply #5 on: 10/01/2006 17:24:48 »
Hey Simon...give us a chance eh ?...It's the same thing when you leave a cup of water out..it'll eventually evaporate into the air. If the air is hotter than the water then the surface tem of the water gets excited and all the little water atoms jump off and float up into the air:

here is an article that I have blatantly copied and pasted here:

Evaporation
Evaporation is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state.

The thermal motion of a molecule must be sufficient to overcome the surface tension of the liquid in order for it to evaporate, that is, its kinetic energy must exceed the work function of cohesion at the surface. Evaporation therefore proceeds more quickly at higher temperature and in liquids with lower surface tension. Since only a small proportion of the molecules are located near the surface and are moving in the proper direction to escape at any given instant, the rate of evaporation is limited. Also, as the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases.

If the evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapour above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapour increases. When the process of escape and return reaches an equilibrium, the vapour is said to be "saturated," and no further change in either vapour pressure and density or liquid temperature will occur.

Gas has less order than liquid or solid matter, and thus the entropy of the system is increased, which always requires energy input. This means that the entropy change for evaporation (#916;Hevaporation) is always positive.

Forced evaporation is a process used in the separation of mixtures, in which a mixture is heated to drive off the more volatile component (e.g. water), leaving behind the dry, less volatile, component.

It is a misconception that at 1 atm, water vapour only exists at 100C. Water molecules are in a constant state of evaporation and condensation flux near the surface of liquid water. If a surface molecule receives enough energy, it will leave the liquid and turn into vapour pending an allowable vapor pressure. At 100C, water will boil at STP.
Factors influencing rate of evaporation

    * Concentration of the substance evaporating in the air. If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
    * Concentration of other substances in the air. If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.
    * Temperature of the substance. If the substance is hotter, then evaporation will be faster.
    * Flow rate of air. This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. In addition, molecules in motion have more energy than those at rest, and so the stronger the flow of air, the greater the evaporating power of the air molecules.
    * Inter-molecular forces. The stronger the forces keeping the molecules together in the liquid of solid state the more energy that must be input in order to evaporate them.

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Offline chris

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Re: Why do clothes dry on the washing line?
« Reply #6 on: 10/01/2006 22:53:23 »
This is a really good question.

It's all to do with why things are liquids in the first place and what happens when they turn into gases. Liquids consist of lots of a molecule packed in tightly together. A gas, on the other hand, consists of a small number of molecules which are very spread out.

Now first lets answer your question about things having to boil to evaporate. If that were true we'd never have any rain because rain is merely water evaporating from the ocean surface, re-condensing in the air as clouds, and then dropping back to earth again. Although people occasionally describe the ocean as 'boiling' it's usually because it's rough rather than hot.

Okay, so given that the sea is never more than about 30 degrees, and nor are Simon's clothes on the washing line, why do they dry ?

Well, liquid water consists of a collection of water molecules which are all shaped like tiny boomerangs consisting of an oxygen atom with two hydrogen atoms stuck to it. Some people say that the molecule looks a bit like Mickey Mouse's head !

These molecules are sticky because the oxygen loves electrons, so it's slightly negative, making the hydrogens slightly positive. As a result the slightly negative part of one molecule attracts the slightly positive part of another, and the two are weakly bound together. This is referred to as hydrogen bonding.

To evaporate, a molecule of water must break free from the clutches of the surrounding molecules and float away into the air. One way it can do this is if it is given enough energy to escape. This works because heating a liquid causes the molecules it contains to begin to move around much more quickly. The higher the temperature the faster they move. If you put your hand into a hot liquid it feels hot because the molecules are slamming into your skin much harder than they did in a cooler solution.

Now when energy is added to a group of molecules in a liquid it is not shared out equally amongst everyone. The molecules randomly collide with each other so that some end up moving very fast, whilst others might be instantaneously close to a standstill.

This means that some molecules will pick up enough energy to break free from the attractive grasp of their surroundings and evaporate - in other words become spread out as a gas. You can see this happening in your coffee cup. It's not boiling but there is still steam - water vapour - coming off the top.

So in the content of the wet washing, where do they get the energy from to evaporate. The answer is from collisions with the air molecules that bash into your laundry on the line, together with a blast of heat from the sun.

Chris

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Offline Solvay_1927

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Re: Why do clothes dry on the washing line?
« Reply #7 on: 14/01/2006 01:46:04 »
What a clear and useful explanation - thanks Chris.
 

SPDSupport

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Re: Why do clothes dry on the washing line?
« Reply #8 on: 16/01/2006 09:56:53 »
When you leave your clothes out on the line to dry after being washed (rather when my mum does it for me) why do they dry?

Mine normally dont it!..lol normally chucks it down when i hang my washing out!
 

Offline tony6789

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Re: Why do clothes dry on the washing line?
« Reply #9 on: 27/01/2006 20:11:50 »
The sun warms them causeing the water to evaporate, conclusion: dry clothes
 

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Re: Why do clothes dry on the washing line?
« Reply #9 on: 27/01/2006 20:11:50 »

 

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