Silicon dioxide

Meera Senthilingam

This week, computers, breast augmentation and a trip to the beach. Confused? Here's Brian Clegg:

Brian Clegg

Ask the person in the street about how we use silicon and (if you are lucky) they will tell you about computer chips featuring silicon wafers. (If you are unlucky you will hear of breast implants - but they will be thinking of silicone, a polymer that admittedly features silicon, but has a rather confusing name.) However, important though silicon chips are, they are tiny fly speck when compared to the many applications of silicon in its oxide form.

Silicon dioxide is a simple molecule - as the name suggests, two oxygen atoms to one of silicon, and is sometimes referred to by the common name silica. This is neither a rarity in nature nor a substance that is difficult to get hold of. I can pretty well guarantee that you have sat on silicon dioxide, found it in your shoes, even suffered rather crunchy food because of it. A common form of silicon dioxide is sand.

Though chemically identical to the sand form, it is also possible for silicon dioxide to form much more extensive crystals. Sand grains range in size between a twentieth of a millimetre and two millimetres, but silicon dioxide can crystallize in large lumps, where it has a milky transparency. In this form it is known as quartz. Unlike quartz, sand is usually a mix of different minerals, giving it a more varied colouration, but silica usually dominates. In its sand and quartz forms, silica is the most common mineral on earth.

There's a lot of sand out there. When Archimedes wanted to demonstrate a new number system for handling big values, he naturally turned to counting sand grains. The biggest number in the Greek system at the time was a myriad - ten thousand - but Archimedes wanted to show that it was possible to go far beyond this. Addressing his book, The Sand Reckoner, to the king of Syracuse, he began:

There are some, king Gelon, who think that the number of the sand is infinite in multitude; and I mean by the sand not only that which exists around Syracuse and the rest of Sicily but also that which is found in every region whether inhabited or uninhabited. Again there are some who, without regarding it as infinite, yet think that no number has been named which is great enough to exceed its magnitude.

But Archimedes knew better. Using a bit of a geometry and a newly invented number system he estimated the number of grains of sand it would take to fill the known universe - in effect our solar system - coming up with a value between 10⁵¹ and 10⁶³ depending on whether you thought the Earth or the Sun was at the centre of things.

In reality, filling the universe with sand isn't exactly practical, but this is a substance that has proved much more versatile than merely a material for building sandcastles. Sand is used in huge quantities in construction, whether in the manufacture of bricks or in the familiar sand-and-cement mix of mortar or concrete. Sand finds its way into sandpits and sandbags. Mixed with soil, it makes for better drained crops in agriculture, and blasted against stonework it removes dirt and grime.

The larger crystals of silicon dioxide in their quartz form may seem less useful, more decorative than practical, but quartz came into its own with the discovery of its piezoelectric capabilities. Quartz can act as an interface between physical force and electrical current.

This effect works both ways. Put pressure on a quartz crystal and it generates electricity, or put a voltage across it and it will vibrate, producing regular oscillations that can be used as a timer. Quartz oscillators have been used since the 1920s to make accurate clocks, and are still used today in watches and even computers.

If sand and quartz were silicon dioxide's only contribution to civilization they would be sufficient, but by processing sand a whole new material can be produced - one that takes the properties that make quartz attractive to the next level. Usually with some impurities like carbonates or metal oxides added to lower the formation temperature, silicon dioxide is the main component of glass.

For over 5000 years human beings have been making glass. Once the technology was available to heat materials to around 1500 Celsius, it was possible to transform silicon dioxide into a smooth, often transparent form that can be blown into shape or rolled out to produce vessels and windows. Though we've become more advanced in our glass technology, even now - thousands of years after it was first made - we have nothing better to drink from or to use for glazing.

There is a common idea that glass remains a liquid after it hardens. It is often pointed out that medieval window glass is thicker at the bottom than at the top, suggesting that over the years it has gradually run down under the pull of gravity. But this is a myth. Because of the way it was made, medieval glass panes were usually thicker at one end than the other. The window makers sensibly put the fatter part at the bottom for stability. Glass isn't a crystalline solid, but it is a solid nonetheless.

As you sip your wine from a glass, looking through a glass window set in a concrete wall above a sandy beach, it's hard to deny that silicon dioxide may be a humble little compound - but it's one that we've built our world on.

Meera Senthilingam 

So a new-found appreciation for the compound behind those beautiful windows we see in our churches. That was science writer Brian Clegg raising a glass to the chemistry of silicon dioxide. Now, next week, a sweet, numbing, but also practical compound.

Simon Cotton

Ethene, or ethylene, is a remarkable molecule. It has got a sweet smell and has been used as an anaesthetic; in fact it has been suggested that it was ethene that made the Oracle of Delphi go into trances before emerging from her cubicle in the rock to make her pronouncements. And it is ethene that is emitted by ripe fruit and which also helps fruit to ripen. 

But the most significant use of ethene is in making poly(ethylene), which we in the United Kingdom commonly call polythene.

Meera Senthilingam
And to find out the multiple uses for polythene, as well as its complicated discovery, join Simon Cotton in next week's Chemistry in its element. Until then, thank you for listening. I'm Meera Senthilingam.