Naked Science Forum
General Science => Question of the Week => Topic started by: katieHaylor on 23/08/2017 14:57:41
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Norm asks:
If water is a solid below 0 degrees celsius, and a gas above 100 degrees celsius, why then does my washing dry when the air temperature is below 100 degrees?
What do you think?
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Perhaps water molecules are being absorbed by air, being drawn from an area of higher concentration to an area with lower concentration. Kind of like a sponge working a molecular level.
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It's because water has a finite vapor pressure. Even below the boiling point, water slowly evaporates until it saturates the atmosphere. At that point, it reaches an equilibrium where evaporation and condensation happen at roughly equal rates.
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Even on Mars and comets far from the Sun, where the temperature is below 0°C, water has some vapor pressure, and some water molecules will jump out of the solid state, and become a vapor. But the rate is very low.
If your washing is solid ice outside, probably best to put it in a dryer.
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I heard the Eskimos beat the ice out of their clothes and make them dry.
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One way to think about this is to consider the energy in the particles of water. Water molecules resemble miniature boomerangs with oxygen atoms at the apex and hydrogen atoms for the arms. The electron-loving oxygen is slightly more negative than the hydrogens, deforming the charge distribution across the molecule.
This facilitates a process called "hydrogen bonding" whereby the positive hydrogens are attracted to the negative oxygen of another water molecule. These interactions hold water together as a liquid and account for many of its unusual properties.
When we give energy to water, the particles - individual water molecules - are constantly "sharing out" and exchanging the energy between them.
You can think of it as a bit like a bunch of children sharing out a bag of sweets, although they don't share out the sweets equally between themselves; instead the sweets are randomly distributed around the group, and the children constantly trade the sweets among themselves too, at random.
Now, if we return to the water molecules, which are like the children with their sweets, this process means that, by chance, some water molecules will occasionally end up with a very large amount of energy, while others will have much less. And What we call the "temperature" of something is actually a measure of the average energy of the particles.
The effect of holding this energy is to make the particles vibrate or run around - a bit like a child who's eaten too many sweets! And if we give a water particle enough energy, it can vibrate sufficiently hard to overcome the attraction of the hydrogen bond holding it to an adjacent water molecule.
This means that, sometimes, a water molecule with a large amount of energy will have enough energy to break the bonds holding it to the next door molecules and escape as water vapour.
So even when you wet towel is hanging on the washing line, it is still losing water because, by chance, the odd water molecules will shake sufficiently hard to detach themselves from the surrounding water molecules, produce some water vapour and this will drift away into the air and not come back.
Hey presto, your washing dries, but it need not be 100°C for it to happen...
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The reason I like the sponge analogy rather than random molecules jumping ship is clothes dry faster in a breeze, ie more surface area for the sponge so to speak, Explains more with less effort.
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The reason I like the sponge analogy rather than random molecules jumping ship is clothes dry faster in a breeze, ie more surface area for the sponge so to speak, Explains more with less effort.
What's the sponge analogy?
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It was described in reply #1.
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It was described in reply #1.
Ah yes, now I see it!
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Julius Sumner Miller posed a similar question. Why did his mother's his washing still dry in sub zero temperatures?
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Julius Sumner Miller posed a similar question. Why did his mother's his washing still dry in sub zero temperatures?
More importantly, what was his answer?
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This question has now been answered on our show, and you can listen to it here: https://www.thenakedscientists.com/podcasts/naked-scientists-podcast/drug-discovery-future-pharma (https://www.thenakedscientists.com/podcasts/naked-scientists-podcast/drug-discovery-future-pharma)
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Another take on it is why does water stay as water.To overcome its liquid form water requires energy to release its molecular links, these are governed by pressure watert is under, temerature of the environment, and the volume the water has to fill. Because water is governed by the various gas laws, ie at a set temperature pressure and volume the water molecules seek to fill the space as is expected. Due to the water being in a confined space(humidity neighberhood) at a low temperature, and pressure some of the water canot be liberated into gas state, and remains as liquid. Thats why you get freeze drying.
Water has some strange properties, ice being one of them, various temperature freezing points due to pressure and temperature glassy water etc