QotW: Why does my washing dry below boiling point?

12 September 2017

Question

If water or H2O is a solid as ice below zero degrees celsius, a gas above 100 degrees celsius, and a liquid between this range, why then does my washing dry when the air temperature is below 100 degrees?

Answer

Alex Ashcroft asked Thomas Ouldridge from the Imperial College London to hang Norm’s question out to dry…

Thomas - It is true that pure water will normally be a gas called water vapour only above 100 degrees celsius, but temperature isn’t the only factor at play here. The surrounding pressure also impacts when a substance like water can be a gas. The higher the pressure, the higher the temperature required for the gas to be stable.

Alex - Gases, like overfilled balloons, often can’t handle the pressure. But why is this?

Thomas - To exist as a gas, water molecules have to be widely spaced out. High enough pressure will tend to squeeze them back into their more compact liquid form. The more you heat water, the more energy you give to the individual molecules and the harder they can push back against their surroundings. Above 100 degrees celsius, but not below, water molecules can push back hard enough against the pressure of the atmosphere for pure water to stay as a gas of widely spaced molecules.

Alex - It’s all about the pressure then. So what’s going on when our clothes dry?

Thomas - Well, let’s consider a puddle for example. You might think it it should stay as a liquid because the temperature is below 100 degrees and the atmosphere is pushing down on it. However, we don’t only have water molecules involved; the system isn’t pure. The air above the surface of the puddle contains many other molecules such as nitrogen and oxygen. These extra molecules can actually help to push back against the surrounding atmosphere effectively lowering the pressure that must be supported by the water molecules themselves if they form a gas. It’s like many people helping to lift a weight rather than rather than just one.

In fact, there’s so much more nitrogen and oxygen they take almost all of the burden of the atmospheric pressure, and this is important. It means that any water molecules that have enough energy to escape from the puddle don’t face the full might of the atmospheric pressure so they don’t immediately liquidise. This is why some water vapour can survive in the atmosphere thanks to the hard work of the other gases, and thus we can explain why evaporation happens, and why puddles or clothes dry under normal conditions. Of course, only a certain amount of water vapour can actually be supported by the other gases, which is why things don’t evaporate immediately, and why movement of air is important if you want things to dry faster.

Alex - If you want to see this in action for yourself, like your wrist and blow on it. It dries almost immediately compared to if you don’t blow.

Thanks to Thomas Ouldridge for ironing that out. Next time we’ll answer David’s question...

Science has attempted to try and measure the intelligence of other primates, dolphins, birds, and many other species of animals. My question is: how does your averege ordinary, average, pet dog measure up in comparison?

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