In The Limelight.

Today's post will be on limestone because I have my Chemistry exam on Wednesday and I've hardly looked at this topic. It's a small one, but I think it'll be worth quite a few marks, knowing my luck.

So, the syllabus says I need to know about the thermal decomposition "of the carbonates of calcium, copper and sodium", but today I'm just going to focus on calcium carbonate because it has the most to do with limestone.

Thermal decomposition is when something gets so hot that it's forced to break down into the chemicals it's composed of, so in the case of Calcium Carbonate, you'd get Calcium Oxide and Carbon Dioxide. To get that to happen though, you have to raise the temperature to about 1200 degrees Celsius, which is mighty high.

But, we do it anyway because broken down Calcium Carbonate is pretty useful. You see, this Calcium Oxide is also called quicklime. We make it put crushing some limestone, then chucking it in this rotating drum with some air. Then we burn some fuel to get lots of heat (1200 degrees) and out comes CO2 and quicklime (Ca0). And this quicklime can be used for all sorts of things - we can put it in soil to neutralise acidity, we can use it to make steel from iron and some industries use it as a drying agent.

Then, if you add some water to the quicklime, you get an exothermic (gives out heat) reaction that gives you slaked lime. Which is also pretty useful - it can be used to neutralise soil acidity, to make mortar for buildings, to make pottery, and in a solution makes limewater to test for CO2.

But, limestone on its own is also really useful -here's some stuff it can do;

-can be crushed and used for road building,

-can be added as a powder to lakes to neutralise acidity,

-can be mixed with clay to make cement,

-can be used to extract iron in the blast furnace,

-can be heated with soda and sand to make glass and,

-can be used to neutralise acid gases give off by power stations.

Which is all great, and the trade for limestone is great - the UK quarries over 65 million tonnes per year, and there's loads more in the rest of the world. But, Quarrying is a bit of an environmental issue because they're great big holes in the ground which look pretty ugly and dirty.

Now, for some equations.

Here's the one for the thermal decomposition of calcium carbonate (limestone) -

CaCO3 ------> Ca0 + CO2

And this is the one for turning quicklime into slaked lime -

CaO + H2O ------> Ca(OH)2

Tahdah, thats limestone covered :)