They recently contacted me to provide an answer to the following question:
"Water is solid below 0 degrees C, a liquid from 0 to 100 degrees C, and a gas above 100 degrees C. Then why does washing and the roads dry when the temperature isn't 100 degrees C?"
I think the answer's actually quite subtle, for such a deceptively simple-sounding question. My recorded answer can be downloaded from here. Full transcript below!
Interviewer: I asked Thomas Ouldridge from Imperial College London to hang Norm’s question out to dry
Tom: It is true that pure water will 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 to be a gas. You've probably all heard of LPG - this stands for liquified petroleum gas. The chemicals in LPG would normally be a gas, but by keeping LPG in high-pressure cannisters it can be stored as a liquid.
So gases, like [insert humorous reference here], often can't handle pressure. But why is this?
To exist as a gas, water molecules have to be widely spaced out. High enough pressure will simply squeeze them back into their more compact liquid form.
The more you heat the water, the more energy you give to the molecules and the harder they 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.
Well, personally I tend to hang my washing inside the earth’s atmosphere, and below boiling point, and it still dries. So what’s going on?
Well, let’s consider a puddle, for example. You might think 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.
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 if they form a gas. It’s like many people helping to lift a weight rather than just one.
In fact, there’s so much more nitrogen and oxygen that they take almost all of the burden of the atmospheric pressure.
This is important: 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 gasses, and we can explain why evaporation happens and puddles (or clothes) dry under normal conditions.
Only a certain amount of water vapour can be supported by the other gasses though, which is why things don’t evaporate immediately, and why movement of air is important if you want things to dry faster.
If you want to see this in action for yourself, lick your wrist and blow on it. It dries almost immediately compared to if you don’t blow. So that’s evaporation covered, but how is that different from boiling?
When water reaches 100 degrees at atmospheric pressure, it has so much energy that the vapour no longer needs the help of other gases to be stable. In our analogy, the water vapour is now like a weightlifter that is strong enough to support the “weight” of the atmosphere on its own.
At this point, bubbles of vapour can form within the liquid itself, converting liquid to gas much faster than slow evaporation from the liquid surface.