Sparkly bathroom prevented by natural chemical equilibria

I have had the pleasure of cleaning the bathroom today. I have used a number of different products over the years to find a chemical that will remove the annoying marks from the tiles, taps and shower screen. The hard water means that no matter how clean and sparkly I get the taps, they are only two showers away from being once again covered in the annoying white powder.

 

Crystals of limescale (http://en.wikipedia.org/wiki/File:Kesselstein_k.jpg)

There is a lot of interesting chemistry involved in limescale. It is essentially deposits of insoluble calcium carbonate left behind from the hot water. But how does it get into the water in the first place and why doesn't it stay in it?

Limescale initially starts as limestone, chalk and other similar sedimentary rocks that formed from the bodies of tiny calcium containing sea creatures that lived in warm shallow tropical seas millions of years ago.

See the BBC clip at this learning zone site.

Since their formation these rocks have been subjected to weathering and in particular weathering involving rainwater. As water flows over the rocks the calcium carbonate in the rocks reacts with the water and carbon dioxide dissolved in the water to become soluble calcium hydrogen carbonate Ca(HCO3)2. This chemical then gets swept along with the water into rivers, lakes and reserviors eventually ending up as our drinking water.

The rocks themselves become classic geological formations including, limestone pavement, potholes and underground cave systems.

Once in our domestic water this is where the chemistry gets interesting. The Ca(HCO3)2 when dissolves, exists as ions; Ca2+ and HCO3-. The HCO3- is in equilibrium with the carbonate ion, CO32-, CO2 and H2O. Additionally the dissolved CO2(aq) is in equilibrium with CO2(g).

When water gets hot less CO2 dissolves in it and it escapes as a gas. This shifts the CO2 equilibrium to the right removing the dissolved CO2 . This is turn shifts the HCO3- /CO32- equilibrium to the right causing many more CO32- ions to be made. As these are made in larger and larger amounts they combine with the Ca2+ ions to form a white precipitate of solid CaCO3 (calcium carbonate); the limescale itself.

The only way to really prevent this is to use softened water, water can be softened using sodium ions that replace the calcium ions in the water so the calcium carbonate can't be formed. This can't be used for human consumption though as it contains too much sodium that has its own health implications. It is also very expensive.

So, until a better alternative comes along I will continue the battle to maintain my sparkly bathroom.

Today it looks like I am winning, tomorrow I'm not to sure.

 

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