Science Questions

How do clothes dry?

Sun, 5th Jul 2009

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Question

Durgesh Dubhashi asked:

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?

Answer

Chris -   It’s a good question.  Water has energy.  So, in other words, at any given temperature, the water molecules are vibrating or moving around, washing machineproportional to the temperature of the water and when we give energy to water sufficient to raise the temperature to 100 degrees, what that means is that the molecules of water are vibrating or moving around sufficiently fast, that they can readily break the attraction that’s holding them onto other water molecules because water is sticky and this enables them to escape and get out into the atmosphere as vapor.

Kat -   But you don’t dry your clothes at 100 degrees so what’s going on here?

Chris -   Absolutely, not.  What you are doing though if you, say put them in a tumble dryer or hang them on the line is that you are putting some heat into the clothes or just because they’re at ambient temperature.  They’re not absolute zero.  The atoms and molecules therefore have some energy.  Now, because the energy is not shared equally amongst all the atoms or molecules and anything, in other words, if I come up to you and I shake your hand, I can give you some energy.  When the molecules are bashing into each other, sometimes some of them will end up transiently with a load of energy from lots of other molecules bashing into them and others will have much less.  This means that occasionally, you’ve got the odd molecule there that has sufficient levels of energy that it can break the bonds holding onto other molecules and it can escape.  The reason its slower to dry at less than 100 degrees or however hot you want to make is because obviously, it takes longer for those interactions to occur so that the odd molecule gets enough energy to escape and that’s why the sea for instance, can evaporate water when sunlight falls on it and warms up the ocean without having to boil itself.  It’s just much slower.  If you put a pot on the stove, you give lots more energy to lots more atoms and molecules all at once and as a result, more of them have more energy more of the time and therefore, they’re able to evaporate, and that’s the reason.

Ben -   Do you reach any equilibrium between water in the clothes and humidity in the air?  I assume that when there’s more wind blowing then you’ve got lower humidity in the air because it’s more of its moving parts.

Chris -   Yes.  I mean around the item that you’re drying, the air that’s in contact with the clothing will become slightly higher saturation of water.  So, in order to maintain the gradient, in other words, water wants to move from an area where there is lots of water to an area where there is much less water.  If you have full winds blowing, this is moving away any molecules of water that get off of the clothing and into the surrounding air very quickly and therefore, you maintain that gradient.  So, the molecules want to move more readily away from the clothing.

Kat -   So, this explains why tumble dryers are great because they’re hot and they’re sort of blowing air around and tumbling things about?

Chris -   Absolutely! 

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Water evaporates all the time. It does not need to reach boiling temperature to do so. Herman Melville, Mon, 29th Jun 2009

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. Ophiolite, Mon, 29th Jun 2009

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? Durgesh Dubhashi, Sat, 5th Sep 2009

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). lyner, Sat, 5th Sep 2009

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? Durgesh Dubhashi, Mon, 7th Sep 2009

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. lyner, Mon, 7th Sep 2009

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