Science is easy

I've been a bit quiet again, not because there's anything catastrophically bad, or ecstatically good happening in the World of Bears, but rather because my mind has largely been frothing and ranting to itself about the stupidity of my customers who refuse to engage their brains. What I've been wanting to do is illustrate the utter idiocy with which I'm faced in a way that other people will understand, and yet I know that if I simply repeated, verbatim, what the customer has asked for, I'm not sure I'd get very far. I probably have two or three friends who would laugh immediately. And everyone else would look blankly at me, as though I were a few sandwiches short of a picnic. And then I thought, "hold on a minute PhysicsBear, you took an entire course in science communication at University, and you keep banging on about how anybody can understand science, this should be something you can explain."

So here goes....

I'm going to start with the easy stuff. The stuff that you know without evening knowing that you know it. Because there's a lot of science just hanging around, minding its own business, not causing any trouble, that you do know really. For instance - the boiling point of water* is 100 degrees Celsius. See, that's not so hard is it?

Now we'll move on to the next one. Humidity. There's water vapour in the air all the time. Water vapour is the gaseous form of water. Just like ice is the solid form of water. If there's water vapour in the air, that gives it what we normally call humidity. For instance, it's a damp day here in the fens, and the relative humidity** is 94%.

We've made a pretty good start already, and to get to the heart of what I'm going to explain, all we need to do is look hard at those two facts and think about their consequences.

1. The boiling point of water is 100°C

2. At 8°C there is 7.8g/m³ of gaseous water in the air**.

Can you see where I'm going yet? I'll help out. A substance does not need to be above its boiling point for some part of it to be in its gas phase, The air around you is definitely not above 100°C, and yet there is just as definitely gaseous water in that air. Admittedly, the higher the temperature, the more water there will be in gaseous form - if we had 94% humidity on a day where it happened to be 24°C instead of 8°C, then there'd be more like 20g/m³ of water vapour in the air, but the point remains - a substance doesn't have to be above its boiling point to be a gas. However, the closer to boiling point it gets, the more of it will be a gas.

Step one of today's lesson is now complete.

Step two is marginally less likely to be part of your everyday experience, but it might still be something you know, as it relies on one of those factoids that occasionally get batted around: if you try and boil and egg at the top of Everest, you will find it takes longer, because water at the top of Everest boils at approximately 70°C. And why is that? Is it because it's terribly excited to have climbed a big mountain? No. Is it because it's closer to the sun? No. Is it because gravity is trying to persuade it back down the mountain again, so it's in a hurry? No. It's because the air pressure is lower. The boiling point of water decreases with decreasing pressure. In fact, the boiling point of any liquid decreases with decreasing pressure.

Now, we can have a go at adding step one and step two together. It's getting pretty exciting isn't it?

Step 1 told us that a liquid doesn't have to be above boiling point for some portion of it to have turned to a gas and that the closer to a liquid's boiling point you get, the more of it will be in the gas phase.

Step 2 told us that a liquid's boiling point decreases at decreasing pressure.

So now we could take a wild deductive leap, and notice that as we lower the pressure, for any liquid, we'll lower the boiling point, and more and more of that liquid will move into the gas phase. We can take this to quite absurd levels. Sticking with water, if we drop the pressure to only 1% of atmospheric pressure, then water will boil at approximately 7°C. Drop it to 0.1% of atmospheric pressure and the boiling point has plunged to a distinctly chilly -24°C. And to be really silly, we'll drop the pressure to where the ion source of my mass spectrometer operates, which is 0.00001% of atmospheric pressure, at which point water boils at -101°C.

Given your, now enormous, wealth of knowledge about temperature, pressure, boiling point and liquids-turning-to-gases, I have no doubt that you've immediately spotted that a really vast amount of water will have turned to a gas at room temperature once its boiling point is a paltry -101°C.

So now I can tell you what my idiot customer has said.

He wishes to analyse a compound with a boiling point of 350°C. He has told us he wishes to heat our ion source to 350°C to make sure his compound remains as a gas.

Tell me, dear friends, do we really think that at 0.00001% of atmospheric pressure the boiling point of his compound is still 350°C? Do we even think that his compound needs to be above its boiling point for some of it to be a gas? Do we already, in a few short paragraphs, have a better grasp of the thermodynamics of gases than this alleged "expert"? And we haven't even started on the fact that some significant parts of my beautiful scientific instrument will melt at 350°C, thus rendering it utterly incapable of analysing anything at all, no matter what its boiling point. Do we now cease wondering why PhysicsBear could be found literally banging her head on her desk in frustration when she received another email from this muppet last week?



* For the pedants among you, and for the sake of future paragraphs, this is the boiling point of pure water at standard atmospheric pressure.

** Relative humidity is a technical term and in this case it means that the air is holding 94% of the maximum amount of water vapour that it can hold at the current temperature. That actually works out to be that there is about 7.8g of gaseous water in every cubic metre of air here at the moment.