Thursday, 18 April 2013

Chemistry at the Movies

This post is part of the Chemistry at the Movies blog carnival started by SeeArrOh of Just Like Cooking. Head over there for a list of other posts.

A while back, a friend and I grabbed a few drinks and settled in to watch the classic movie G.I. Joe: the Rise of COBRA. For those of you who somehow missed this historic motion picture event, it's an adaptation of the eponymous range of toys (called Action Man in the UK). The film is geared at selling more toys: the villains are cartoonish, the action sequences involve all kinds of fancy vehicles, and the only real attempt at acting is probably Christopher Ecclestone's dire version of a Scottish accent. Leave it to Tennant, Chris.

There's a fair amount of sci-fi in this movie, and for the most part it'd be silly to look at its plausibility. City-eating nanobots? Basically magic. However, one scene bears a second look. Preferably a slack-jawed look, with eyes opened wide in horror at the dumb. This movie contains an error so fundamental it makes you wonder what the writers have spent their lives doing.

(Yes, I am going to pick holes in G.I. Joe. No, I have no shame.)

Towards the end our intrepid heroes escape from COBRA's base, which sits on the ocean floor beneath the Arctic ice. Creatively, the villains activate the base's self-destruct sequence and detonate explosive charges in the ice above. The heroes have to escape in cool-looking submarines, weaving in and out of the enormous blocks of ice that are plunging through the water around them.

If you find yourself staring wordlessly at your screen at this point, fear not: you are not alone.

If you're not sure what the problem is, it's time for a home chemistry experiment. Get yourself a drink and drop an ice cube in it. Spoiler: ice does not sink in water.

Water is a weird substance in many ways, but it's so ubiquitous that we overlook its strange behaviour. It's probably the liquid most people handle most often, so we might assume its properties are 'normal'. I remember arguing with my high school science teacher that no, liquids expand when they freeze - after all, pipes burst in winter, right?

Ice is something like 8-9% less dense than liquid water, and hence water expands when it freezes - unlike the majority or other liquids. The most common explanation I've come across is to do with organisation of hydrogen bonds: in the solid, the bonds are quite literally frozen into place, and adopt a stable and roughly constant length. In the liquid, the bonds are constantly breaking and re-forming as molecules tumble around each other, and on average are shorter than in the solid. 

We can exploit this to freak out first-year students and lab visitors. Water's isotopic cousin, D2O, is about 10% denser than the vanilla flavour. This is in part due to the heavier mass of D vs. H - more mass in the same volume gives a denser substance - but also because the hydrogen bonds between D2O molecules are slightly stronger and hence shorter.

The upshot of this is that you can make ice sink... if you enrich it with deuterium. 

I couldn't find a figure for annual production of D2O, but given that the volume of arctic ice is something like 2-3 million km3 I'm not sure this is a viable option for Cobra Commander.

If you enjoyed this be sure to visit Just Like Cooking and read the other posts in the Chemistry at the Movies collection.


  1. That's awesome. When I am really rich, I'll put D2O cubes in my drinks.
    Also I had no idea GI Joe and Action Man were the same dude. Life fail for me.

  2. How dare you besmirch the reputation of this almost Hitchcockian masterpiece! If you listen to Duke approximately 100 minutes into the movie he clearly warns the others to get out of the way because a whole lot of ice AND STEEL is on the way. Additionally, the icepack is riddled with tunnels which, once filled with seawater, will increase the density of the system further. Taking the density of polar surface sea water as 1.03 g/cm^3, ice as 0.917 g/cm^3 and steel as 8.0 g/cm^3, we can calculate that the volume of steel in the icebase needs to be about 1.5% steel. This is a pretty significant amount but, considering the immense wealth of Clan McCullen, well within the realm of possibility.

    1. I have a confession to make. I watched this film back-to-back with a Bollywood classic, and we accidentally left the subtitles on. They synced up surprisingly well with G.I. Joe. There's nothing more hilarious than two jarheads swapping Bollywood lyrics: "don't tease, just please - let's shikdum!" makes an excellent substitute for "lock 'n load".

      As a consequence I missed 90% of the dialogue.

      It didn't occur to me that they might have hollowed out the ice cap, and it certainly didn't occur to me that the writers might have thought that through.

      Is the ice actually filled with tunnels? I thought the base was on the sea floor, not actually in the ice.