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Author Topic: What is happening when wet clothes dry?  (Read 23477 times)

Offline thedoc

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What is happening when wet clothes dry?
« on: 13/09/2013 23:26:03 »
This seems to be a simple phenomenon but I have a question on it.  How do clothes dry or even how does water in any place dry out without heating it up?  It’s obviously that water boils and evaporates but how does water just vanish off your clothes when they’re wet?
Asked by Durgesh Dubhashi


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« Last Edit: 13/09/2013 23:26:03 by _system »


 

Herman Melville

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Water evaporates all the time. It does not need to reach boiling temperature to do so.
 

Offline Ophiolite

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So how come water dries from clothes at well below this temperature? Does it have any connection with atmospheric pressure?
It is connected with vapour pressure, which is determined by the amount of water in the atmosphere and humidity.

When the air is 100% saturated with water vapour there will be a dynamic equilibrium where as much water condenses out of the air onto the clothes, as evaporates from the clothes into the air. If humidity is less than 100%, then there will be more evaporation than condensation and the clothes will become drier.

If the humidity is very low,  either because the air is hot and can hold more water, or because there is just very little moisture in it, then the evaporation will proceed more quickly. If there is a wind this will speed the drying up because 'fresh' air, with low humidity is constantly passing over the clothes.
 

Offline thedoc

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Offline Durgesh Dubhashi

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Thank you, I get this out for Water. But does the same principle work out for all other liquids e.g. petrol. What exactly is Humidity? Is it only the Percentage of Water Vapour or a mixture of many other?
 

lyner

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The molecules in a liquid at any temperature are all moving at different speeds (vibrating). The faster ones will have enough energy to leave the surface, leaving the slower ones behind. In a covered container, there is a time when as many molecules are returning to the surface as are leaving it. If the container and the space over it just contain water (i.e. if you have 'evacuated' it) you will end up with the space above the liquid just filled with water vapor at a pressure which is called the vapour pressure (the hotter, the higher the pressure). At 100C the vapour pressure happens to be 1 Atmosphere and all the molecules will leave the water when the ambient pressure is 1 Atmosphere. If you allow the container to be bigger, you will reduce the pressure and the water will boil at a lower temperature.
In still air, the molecules will leave the surface until there are as many leaving as returning. The humidity just above the surface of the water is said to have a humidity of 100% and the clothes will dry very slowly because they are relying on the evaporated molecules just diffusing away through the air. When you hang clothes out to dry, the wind keeps taking the saturated air away (or, rather, removing the water molecules above the surface) so more will leave the surface in a given time. It's a popular misconception that air is a sponge, absorbing water at different rates, according to the temperature. That idea is 'almost' consistent with reality but, in fact, the water molecules in the air are only another component of the mixture of gases which constitute the gas. Some very bright Scientists still think in terms of the air as a sponge - that will be largely because they haven't needed to think it through! Humidity is a bit of a flawed concept which works so well in practice that it is used nearly all the time. It is used in just the way you suggest, DD.
The same thing applies to all liquids (petrol etc.) but at different temperatures. The oil which is used in vacuum pumps is chosen to have an extremely low vapour pressure so that it doesn't boil inside the pump.

Why does water boil at 100C? That's because the Celsius scale chose to call the temperature at which water boils 100C when the air pressure is 1 standard atmosphere. It's no coincidence; it was chosen that way so that people could calibrate a new thermometer easily (0C was chosen for a similar reason).
 

Offline Durgesh Dubhashi

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Well in that case shall I expect that at any given instance of time with no external heat supplied in a Air tight(non-vaccum or Vaccum)container the Quantity of Water or any other Liquid remains Constant?
 

lyner

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Correct - except that, during evaporation, heat is lost from the liquid because the faster molecules are the ones which get away - lowering the liquid's temperature. Heat will tend to flow in from the surrounding air so you do get some external heat supplied. That is until the surrounding air is 'saturated' and you reach equilibrium temperature, too.
 

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