By Penny Carmichael, on 19 February 2013
-Article by Abigail Mountain
For many, Professor Andrea Sella’s lectures are the highlight of the CPS calendar. And with good reason too! Renowned for his explosive (literally) demonstrations and electric charisma, there’s no wonder he’s the media maverick of the chemistry department. Last year he wowed us with “Spooklights”, a talk which I found incredible. So no pressure…
“Strange Ice” – coinciding with the centenary of X-ray diffraction – guided us through the weird and wonderful properties of ice. The most bizarre, but so taken for granted on a day-to-day basis, is the phenomenon that ice floats on water. But almost every other solid phase of a compound is denser than its liquid counterpart. In fact it is such an oddity that Professor Sella challenged the audience to name another material exhibiting this property (our resident “iceman” Dr Christoph Salzmann was exempt from the challenge, however). He made the amusing observation that society actually views the normal behaviour between solid and liquid phases of a substance as alien, with warning labels of olive oil-bergs as an example.
Dripping away at the front of the auditorium was an enormous block of crystal clear ice which had been cut through by a wire over the course of the day. You could see where the wire had sliced a seam through the block but also that the ice had healed itself, resealing behind the wire. It isn’t the pressure underneath the wire which causes the cutting, however. A process called pre-melting is the culprit – also the reason we’re able to ice-skate and ski.
Moving on to the structure of ice, Sella posed the question “why are snowflakes six-sided?”. This must be related to the way in which ice is built up. What about other crystals? Calcite – or fondly called Iceland spar – is a beautifully clear mineral that is easily split into rhombs and is birefringent, creating a double image of anything underneath it. The fact that it splits so perfectly into identically angled rhombs illustrates the concept of the unit cell, something which fascinated René Just Haüy. If you keep splitting the crystal you’ll eventually get down to the smallest reproducible building block.
X-ray diffraction of crystals, discovered by Max von Laue, allowed us to probe their internal structure and observe these building blocks. The ice we use everyday (ice Ih) has a hexagonal arrangement, containing large empty channels that explain its low density. Although the oxygen arrangement throughout the crystal is fixed, the placements of the hydrogen atoms need only satisfy two rules: each oxygen must be covalently bonded to two of them and hydrogen bonded to two others; only one H atom can be found between each pair of O atoms. This allows for a plethora of hydrogen arrangements and leads to a highly disordered internal structure, despite the highly ordered long range crystal. There are in fact fifteen polymorphs of ice, many of which have been discovered by the aforementioned department iceman.
Andrea then told us about the amazing ways we can observe ice in the sky. A 22°-halo can be seen at the end of your little finger when you put your arm out at arm’s length and, holding you hand open, hold your thumb over the sun. These and other beautiful optical phenomena are due to sunlight being refracted by hexagonal ice crystals in the atmosphere. For more information and to see some incredible images visit http://www.atoptics.co.uk
I can’t possibly fit everything that we learnt into such a short post. As always, Professor Sella impresses the audience with his polymathic knowledge and entertaining delivery. I hope I’ll still be around next year to see his talk, otherwise I’ll be making a special trip. If you missed it, details of when he’ll be doing it again can be found on his blog: http://solarsaddle.wordpress.com