Myles Moore of Tasmania asks :-

Calcium carbonate forms in hard water which contains traces of disolved calcium. Where does the CO3 come from to form limescale and how can it be got rid of from porcelain urinals?

Dave Warren, a chemist at the University of Otago, responded.

Calcium carbonate is a very common mineral that forms several different types of crystals such as aragonite and calcite. It is found in eggshells, seashells and the amazingly complicated skeletons of zooplankton (although how these complex shapes are produced is poorly understood). Crystals of calcium carbonate can even be found in the urine of several animals including horses.

If you have ever blown through a straw into a tube of limewater at school, making a milky solution, then you have made calcium carbonate from the calcium in the lime and the carbon dioxide in your breath. Lime scale, commonly found in kettles or hot water pipes, is also a type of calcium carbonate and is caused by the presence of calcium and carbonate ions in water (producing ‘hard’ water). These ions are usually present as the result of rainwater dissolving limestone (all rainwater is very slightly acidic). Carbonate can also form when carbon dioxide gas is dissolved in water (hence the milky solution that forms in the limewater test).

When water gets hot the calcium carbonate becomes less soluble and thick deposits can build up on surfaces. Deposits can also form when water rich in these ions passes over a rough surface and this is the cause of stalagmites and stalactites in limestone caves.

Traditional ways of removing lime scale from kettles include soaking overnight in vinegar or lemon juice. Since carbonate is a base it reacts with the acid in these substances and dissolves producing carbon dioxide gas in a similar way to the reaction between baking soda and vinegar.

I think deposits on a porcelain urinal might benefit from a slightly more aggressive approach and would (carefully!) use a hydrochloric acid solution, (available from hardware stores as a concrete cleaner) and a wire brush to roughen the surface.