Ever wondered what makes some ice cloudy? It’s tiny, trapped air bubbles which refract the light. This is particularly true in home-made ice cubes, because the water used is generally from the tap, which becomes aerated as it pours.
Water expands as it freezes, and freezes from the outside in, so the air is pushed to the centre of the cube, which is why ice cubes tend to have particularly opaque centres.
If you want to make perfectly clear ice, use bottled water, boil it briefly to drive out any air, and freeze it in the pan you’ve boiled it in to avoid incorporating any air.
Hot ice
Hot water freezes faster than cold water. Aristotle first noted this in the fourth century BC, but it was only accepted by modern science due to the persistence of a Tanzanian schoolboy called Erasto Mpemba who, in 1963, proved it by repeatedly demonstrating that a hot
ice-cream mixture set more quickly than cold. We still don’t know why.
Cold ice
Air is the key ingredient in ice cream; up to a quarter of the finished article is made from trapped particles of air. Beating the mixture incorporates more air but also prevents large ice crystals from forming, which would make it unpleasantly grainy and hard.
This difference in ice cream’s cooling texture has led Maria Brumm, an Earth scientist from Seattle, to describe it as a form of igneous or volcanic rock – under a microscope its structure is almost identical to that of lava.
Glass ice
Water doesn’t always freeze at 0C (32F): it needs something for its molecules to latch on to. Ice crystals form around “nuclei”, such as small particles of dust. If there are none of these, you can get the temperature of water down to -42C (-44F) before it freezes in a process known as “supercooling”.
It has to be done slowly, but cooling water extremely fast has a completely different effect. It bypasses the ice stage (which has a regular crystalline lattice structure) and transforms into a chaotic amorphous solid known as “glassy water” (so called because the random arrangement of molecules is similar to that found in glass).
To form “glassy water” you need to get the water temperature down to -137C (-215F) in a few milliseconds. You won’t find glassy water outside the lab on Earth, but it’s the most common form of water in the universe – it’s what comets are made from.
Lake ice
The most coveted ice at fancy dinner parties in late 19th-century London came from Lake Wenham in Massachusetts. The company, Wenham Lake Ice, had a shop in the Strand. Every day they put a fresh block of ice in the window with a newspaper behind it so that passers-by could marvel at how clear the ice was.
The shop window was regularly crowded with people staring at the ice. It was used by Queen Victoria and her entourage at Buckingham Palace and had the Royal Warrant. It was shipped to England insulated with sawdust. The first shipment of ice to Britain baffled customs officers who had no idea how to classify it; it was stuck at the border for so long all 300 tons of it melted.
Later, the Norwegians changed the name of Lake Oppegard near Oslo to Lake Wenham so they could tap into the market. By the Fifties most of the block ice sold in Britain was imported from Norway.
www.telegraph.co.uk
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