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1. The presence of carbonate ions in the tap water leads to a reaction with the dissolved calcium ions to form a white calcium carbonate precipitate which coats the lime particles and inhibits further dissolution. Calcium carbonate, with a Ksp of 4.8 x 10 ^-9, is far less soluble than calcium hydroxide, which has a Ksp of 4.7 x 10 ^-6, so we would expect it to precipitate first when there is a high concentration of calcium ions in solution. The reaction would be

{latex}
\small
$$
Ca^{ - 2} (aq) + CO_3^{ + 2} (aq) = CaCO_3 (s)
$$
{latex}

This may also be thought of in terms of a replacement reaction in which hydroxide is exchanged for carbonate on the surface of the solid lime, as in figure 1. Then the surface of the particles becomes much less soluble and the lime contained within them is effectively lost. The team is unsure of the specific chemical mechanism at work here.

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