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Zoe Louise Matthews | ASY-EOS | UK

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Chemist and Particle Physicist Chatter

I have mentioned before that my boyfriend is a chemist. Some very interesting conversations go back and forth between us, and as I know virtually no chemistry I learn a lot from him. This week, on my mother’s advice, I bought some “moisture-combating crystals” from the corner shop to try to reduce the damp air in some of the rooms. They attract water from the air, and it gathers in a tray underneath them. Phil was delighted to discover that they were calcium chloride, and wanted to tell me how they work.

Calcium chloride, CaCl_2, is very “hygroscopic” (meaning it attracts water) for good reason. It is quite “ionic”, meaning that despite being neutral overall, electron transfer has left it with quite distinct positive and negative charges. Water molecules, on the other hand, have a covalent bond (sharing electrons instead of transferring them) but because oxygen’s nucleus is more positively charged, more of the negative charge surrounds it, making water molecules polar. The chlorides, Cl-, attract the positive H side, where the 2+ charged Ca ion draws in the negative O side. This attraction of water “dipole” molecules will happen with any ionic substances. CaCl_2 is large enough to take on up to 6 water molecules around it, becoming calcium chloride hexahydrate. Quite a mouthful. The crystals eventually turn a fetching orange and we are advised by the packaging to throw them out and buy more. However, Phil has a cunning plan. Heating them to 30 degrees releases four of the six water molecules, making Calcium Chloride Dihydrate, and at 175 degrees one more is released, leaving Calcium Chloride monohydrate. Provided we can boil off the water in the oven, then, we can dry out the crystals and reuse them to almost full effectiveness.

Occasionally when discussing science with each other we get confused, because chemists and physicists often use quite different language. We use similar phrases to describe totally different things. I can talk about dipoles and mean virtual electron-positron pairs in a vacuum, whilst he considers only bonds of atoms sharing electrons as polar. We are both familiar with diffraction of light and electrons. If he says diffraction he is probably talking about using x ray diffractometers to learn about the structures of materials. However, when I talk about diffraction, I am probably talking about interactions between protons where particles called Pomerons are exchanged.

For Christmas, Phil bought a very “me” gift for me – a book called “The Science of Chocolate” by the Royal Society of Chemistry. It has been very enlightening so far, and there was one thing in particular that was very interesting – the Maillard Reaction, occurring during roasting. This is a reaction between an amino acid and a sugar, and is responsible for the caramel-like taste and brown colour of chocolate. Depending on the temperatures, and whether roasting the beans, nibs, or cocoa crumb/liquor, the flavour is different. In fact this reaction happens in many different foods, like in barley for beer, or in roasting coffee beans. One of Phil’s friends is working on the undesirable browning effect when the reaction occurs in fusty old potatoes.

I liked this because colour and flavour come up a lot in particle physics too. When we think about a particular type of quark – the “up” quark, with +2/3 charge, or “down” quark with -1/3, say – we call the type “flavour”. It makes sense because if you consider them as ingredients, you can see that the up and down quarks in different amounts can make protons (u, u, d) or neutrons (u, d, d). We can use them with their antimatter counterparts to make charged pions, (u, anti d) or (d, anti u). Then, if we introduce a new flavour, like the strange quark, with -1/3 charge and a heavier mass than the others, it can be used to make a range of new exotic particles, like kaons (u, anti s) for example, or Lambdas (uds). The other quark types, “charm”, “bottom” and “top”, increasingly massive, are referred to as “heavy flavour”.

Colour is another story. In fact, we never see quarks on their own, so we would never have known about this new kind of charge if it wasn’t for the Delta++ particle. Made up of three up quarks, (u, u, u), its properties are such that each of the up quarks seemed to need identical “quantum numbers” (spin and angular momentum, as well as charge). Now, this will ring alarm bells for any scientist. Phil knows about the dangers of electrons in orbitals – they have to have opposite spins or some angular momentum, something to make them different, because Pauli Exclusion Principle forbids any two identical fermions to exist in the same quantum state. This is why we know that quarks have an additional quantum number, which each quark has, but together in hadrons they become neutral. Why we call it colour I am not sure. Interestingly, its existence, and the fact that the carrier of the strong force, the gluon, also has it, underpins the difference in behavior of electromagnetic and strong forces.

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