Sunday, July 14, 2013

The white tan on my cold chocolate…

In 1911, Robert Scott the Englishman and Roald Amundsen the Norwegian were on a race with each other- To be the first human to reach the South Pole. Scott, sent his caravan in November 1911; it was a support team, which dropped caches of food and fuel on the way out so that the small final team that would dash to the pole could retrieve them on the way back. After an arduous, foot journey, which lasted for months, five men led by Scott, arrived at the pole in January 1912. They were shocked and disappointed to see a brown tent and a Norwegian flag, fluttering at the pole. Amundsen had arrived a month before Scott. Disappointed, Scott and his men started their return journey. For weeks they were pinned down by the Antarctic weather, and they began facing starvation, scurvy, dehydration, hypothermia and gangrene. To their horror, the fuel, dropped by Scott's caravan, had leaked and most of the foodstuff was tainted by it. Without kerosene, the men could not cook, melt water to drink and they took ill one by one. Finally, by March 1912, Scott and his men perished due to exposure, eleven miles from the British base. The root cause of their troubles was tin. Scott, in his previous Arctic expeditions had discovered that the leather seals on his kerosene canister leaked badly and he routinely lost half his fuel. So for the South Pole expedition, the team replaced the leather with pure tin solders. Tin has been used since ancient times to make containers, since it is easy to shape. But when tin is used in its pure form, whitish rust begins to form on it. This white rust weakens and corrodes tin. Unlike iron rust, this is not a chemical reaction, but a rearrangement of tin atoms. Tin atoms can arrange themselves in two ways and when they get cold they shift from their strong "beta" form to the crumbly "alpha" form.
Imagine a crate of apples; the bottom layer is evenly spread out. Now the next layer on top can be arranged in two ways; one is place the apple right on top of another apple and make the layer. In terms of tin's atoms, this is one form or crystal structure. The other option is to nestle the second layer of apples into the spaces between the apples of the bottom layer and this is another way atoms can arrange. Both these arrangements, give the same metal different properties. What Scott's men tragically discovered was that an element's atoms can spontaneously shift from a weak crystal to a strong one or vice versa. Generally it needs extreme conditions to promote rearrangement (a classic example is graphite becoming diamonds).Tin rearranges itself at 13 degree Celsius and colder temperature accelerate the process. This condition is called leprosy, and the alpha-beta shift also causes audible sounds called 'tin scream'. This tin leprosy, also affects chocolates (chocolate leprosy!) The white tan that you notice when you open a chocolate packet from the fridge, is caused by the same alpha-beta shift, which once doomed Scott.

1 comment:

  1. Aarina5:46 pm

    Enjoyed reading through love. The information about the chocolate tan was very interesting, however, the term "chocolate leprosy" didn't go down too well with me"