So Christmas has been and gone and we’re all forlornly looking at pine-needles around the tree and the mountainous pile of recycling in the kitchen, promising ourselves that we’ll eat nothing but salad come January first. But in the meantime, let’s take a bit of time out from the sales, watching Christmas telly and eating endless chocolates (I’m pretty sure anything eaten between December 24th-31st doesn’t count) and think about all the chemistry we’ve done over the last few days – yay!

Cracker snaps contain silver fulminate.

Pulling crackers
Pulled a cracker over Christmas? Of course you have, and probably more than one. Did you wonder what caused the bang and the strangely appealing chemically smell? Of course you didn’t, but never fear, I shall tell you anyway. It was probably silver fulminate, AgCNO. This particular chemical is a primary explosive, but not a particularly useful one due to its extreme sensitivity. It’s so sensitive to any kind of shock (including the touch of a feather, a drop of water, or even just a particularly loud noise) that it’s completely impossible to collect more than the most minute amount without it blowing up unexpectedly. It was first prepared by Edward Charles Howard in 1800, who was working on preparing fulminates. None of them are stable, and one has to wonder if he had any eyebrows or eardrums by the time he’d finished. Anyway, silver fulminate has found one sort of practical use, and that’s in novelty snaps like the ones in crackers. There’s a tiny amount of silver fulminate on one piece of cardboard, and an abrasive on the other. When you pull, the two rub against each other and BANG! Paper hats, plastic toys and bad jokes abound. What happened after an explosion at a French cheese factory? All that was left was de brie.

Release the pressure and carbonic acid converts into water and carbon dioxide. Quickly.

Opening bottles of fizzy stuff
Most people are probably already vaguely aware that the bubbles are carbon dioxide, but there’s more to it than that, oh yes. Have you ever noticed that the liquid in the bottle looks completely bubble-less until you actually open it? If not, check next time. It’s really quite amazing. Why is this? Well, there’s a bit of chemistry going on. Brace yourself for an equation:

CO₂ + H₂O ⇌ H₂CO₃

There on the left you have carbon dioxide and water, and on the right something called carbonic acid. The double arrow thingy means the reaction is reversible, and the thing about reactions like this is that they will sit there quite happily, perfectly balanced, until something happens to change them. In the case of fizzy bottles, opening them will do that. It lets out the carbon dioxide and that causes the reaction to make yet more water and carbon dioxide in an attempt to compensate. That’s where all the bubbles come from, and it’s also why fizzy drinks taste peculiarly sweet if they’re left to go flat – like all acids (testing this is not recommended, but trust me) carbonic acid tastes sour and when it gets used up the sweetness due to sugars and sweeteners starts to take over. Contrary to popular belief, putting a spoon in the bottle will do absolutely nothing whatsoever to stop your champers from going flat. Sticking some kind of air-tight stopper in it, on the other hand, will definitely help.

The blue flame is due to complete combustion.

Setting fire to the christmas pudding
Or rather, the generous splash of alcohol you’ve just poured on it. Have you noticed that the flame is a lovely blue colour, very different from the warm yellow of coal and candles? That’s because when you burn alcohol, specifically ethanol, CH₃CH₂OH, you get something called complete combustion. This happens when there’s enough oxygen to only produce carbon dioxide and water as products. Ethanol has an oxygen atom built in, so it burns more completely than hydrocarbon fuels like coal and candle wax, which tend to produce carbon atoms (also known as soot) and carbon monoxide as well. The reason the flame is blue rather than yellow is because that yellow colour is caused by carbon atoms getting so hot that they glow. By definition, in complete combustion there’s no carbon, so no yellow. Instead the gas molecules in the flame get so hot they start glowing instead, giving off blue light. All together now, oooooh!

Alpha-pinene gives christmas trees their smell.

Sniffing a Christmas tree
What is that lovely smell? Mostly a molecule called pinene, specifically alpha-pinene. It’s a funny-looking thing isn’t it? Looks a bit like a waiter rushing with a full drinks tray. Anyway, there are two forms of this molecule: alpha and beta.  Alpha is the most common one in nature, particularly in conifers (which Christmas trees are). Peculiarly, it somehow manages to be both an insect repellant while also, apparently, being used by insects as a chemical communication system. I don’t know how this works, ask an entomologist.

Christmas lights owe their glow to tungsten.

Switching on the Christmas lights
These days, LED lights are slowly taking over, but there are still enough filament bulbs kicking around in boxes of decorations that they’ll probably persist for a few years yet. Electricity consumption be dammed, they do make a much prettier glow. And why is that? It’s partially due to tungsten, element number 74. It has the highest melting point of all the elements (there’s a handy fact for your next trivia quiz) and as a result it is, or at least used to be, used to make the filament in incandescent light bulbs. Heat it up and it starts to glow long before it reaches its melting point of 3422 ^oC. The bulbs are also filled with an inert gas, usually krypton (nothing whatsoever to do with Superman, sorry), which stops the tungsten from reacting with the oxygen that would be present in ordinary air. In fact, filling a bulb with krypton makes it even brighter and longer-lasting than just pumping all the air out leaving a vacuum, because the krypton helps to disperse the heat.

So there you go, just a few of the many, many bits of chemistry you’ve done so far this Christmas. Enjoy the rest of the chocolates, have a happy New Year, and to those out there with January mock exams coming up, good luck!