Peter Borrows brings us another excursion into local chemistry. 
In this issue: calcium and its carbonate

Cliffs of Dover

Source: istockphoto

Calcium carbonate deposits on a large scale

Calcium is the fifth most abundant element on the Earth and a large proportion of that is in the form of calcium carbonate. If you live in a hard water area, you will be familiar with the deposits of scale in pipes and fur in kettles caused by the decomposition, especially on heating, of soluble calcium hydrogen carbonate (bicarbonate). 

Ca(HCO3)2(aq) → CaCO3(s) + H2O(l) + CO2(aq) 

Chalk, marble and cement

Chalk is ca 95 per cent calcium carbonate but limestone contains more clay. Calcium carbonate is also found in some (calcareous) sandstones, where the grains of sand (silicon dioxide) are cemented together with calcium carbonate, as a quick test with dilute acid will readily confirm. Even if these rocks do not outcrop in your high street, they may still be found there as building stones, especially marble - though much of this is often imported. The building stones and bricks will be held together with mortar, a mixture of lime with cement, sand and water. Cement is a mixture of calcium silicates and aluminates made by heating limestone with clay in a kiln.1 Some buildings are likely to be made of concrete, which is made by mixing cement with aggregate (gravel and sand) and water. 

The Clergy House, in Alfriston in West Sussex, was the first property to be owned by the National Trust. The floor is compacted chalk which was then sealed by pouring sour milk over it. This was re-done as recently as 10 years ago. The lactic acid (2-hydroxypropanoic acid) reacted with the carbonate to give insoluble calcium lactate.  

CaCO3(s) + 2CH3CHOHCO2H(aq) → Ca(CH3CHOHCO2)2(aq/s) + H2O(l) + CO2(g) 

The floor now looks as polished as marble. 

Cleopatra's cocktail

If you walk past a jeweller's window, look at the pearls (calcium carbonate) on display - and think of the story told by the classical author Pliny.2 He tells of a bet made between Cleopatra and Anthony about how much she could spend on a banquet. While conceding that Cleopatra's feast was very good, Anthony maintained that it wasn't that exceptional. At that point, Cleopatra called for a cup of vinegar (ethanoic acid), removed one of her pearl earrings and dropped it in. The pearl dissolved: 

CaCO3(s) + 2CH3CO2H(aq) → Ca(CH3CO2)2(aq) + H2O(l) + CO2(g) 

She then drank the solution of calcium ethanoate and won her bet because the pearl was the largest known. But don't encourage your students to try this at home with their mothers' pearls.