Chemistry (Metals)

1.when calcium hydroxide is poured into sea water, it dissociates into calcium (II) ions (Ca2+) and hydroxide ions (OH-).
2. The hydroxide ions will bind with Magnesium ions more than Calcium ions (because magnesium hydroxide is more stable than calcium hydroxide, detail AL/DSE Chemistry)
3. Magnesium hydroxide is insoluable in water (detail AL/DSE chemistry)
4. So it is precipitated out (forms solid)

ref.: calcium hydroxide and magnesium ions are both ionic compound, for your benefit, it is a compound form by the transfer of some electons from a metal to a non-metal (this defination is not accurate because it can't even hold here! But is sufficient for you to understand) so that they from ions with opposite charges. So they attract. This attraction is known as ionic bond

magnesium hydroxide is NOT more stable than calcium hydroxide
but magnesium hydroxide is more INSOLUBLE than calcium hydroxide
it is the real reason that magnesium hydroxide precipitate is formed

2011-04-28 19:45:03 補充：
in fact, Ca(OH)2 is more stable than Mg(OH)2
because Ca is more reactive than Mg, and the ionic compound formed will be more stable