2012: Advent Chemistry: Silicon Dioxide

by on December 21, 2012

Yesterday we took a whirlwind tour through liquid crystals. Today, let us consider another strange phase of matter – glass.

Technically speaking, ‘glass’ is actually the name of a phase just like ‘solid’ and ‘gas’ are phases, but in common usage glass means a mixture composed mainly of silicon dioxide.

Silicon dioxide, often referred to simply as ‘silica’, is what chemists call “isoelectronic” with carbon dioxide. It has the same number of electrons in equivalent orbitals, because silicon sits just below carbon in the Periodic Table. It’s in the same group, so has some of the same properties. But silicon dioxide is a solid at comfortable temperatures for humans. It makes up the vast majority of the Earth’s rocks, and therefore of the Earth.

Glass isn’t, as is commonly thought, a supercooled liquid. It’s a non-crystalline solid. Silica takes on a lot of forms, including several crystalline arrangements, and glass is its amorphous solid form. It’s both brittle and rigid, so it really can’t be considered a liquid. It doesn’t have a normal phase transition when it melts – it looks like a continuous softening rather than a distinct transition to liquid – which is where the confusion comes in.

Crystals form when a material cools and becomes a solid. In effect, the material freezes. But crystals aren’t all the same size. The faster you cool something, the smaller the crystals will be, which is why ice-cream made very fast is the best kind. If you cool some things, like silica, fast enough the crystals are so small they don’t exist. Molecules have to arrange themselves in strict order to make crystals and if you cool them fast enough, they don’t have time to line themselves up before they’re too cold to do it. The resulting solid is non-crystalline – in some cases, it’s a glass.

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