Homework Help: Chemistry

Posted by candy on Saturday, January 31, 2009 at 9:46am.

1) When calcium is reacted with cold water, the equation is as follows: -
Ca(s) + H2O(l) -->Ca(OH)2(aq) + H2(g)
Isn't Ca(OH)2 insoluble? Why is the state symbol aqueous? When I conducted the experiment in a test tube with cold water, I got a white suspension (precipitate).

2) When I placed a piece of magnesium foil in a test tube of cold water,a very slow stream of hydrogen gas bubbles was observed. When writing the equation for the reaction, should I write
i) Mg(s) + 2H2O(l)-->Mg(OH)2(s) + H2(g)

or

ii)Mg(s) + H2O(g)-->MgO(s) + H2(g)

since we know that magnesium reacts with steam to form magnesium oxide as in ii)?

Thanks!

Chemistry - DrBob222, Saturday, January 31, 2009 at 1:19pm
Ca(OH)2 is slightly soluble in water with approximately 1 g Ca(OH)2 dissolving in a liter of water. In COLD water it is less soluble. But this is how "lime water" is made (by dissolving Ca(OH)2 in water) and that is the old fashioned white wash that was put on trees in yards and on fences. That practice has gone out of style but was popular 60 or 70 years ago. As for the second question, the first equation you have written is correct for water. The second equation is written as it is because steam is used and there isn't enough water present for the MgO to dissolve to Mg(OH)2. However, if the MgO formed by the steam was allowed to dissolve in water, it would form
MgO(s) + H2O(l) ==> Mg(OH)2(aq) although Mg(OH)2 is less soluble in water than is Ca(OH)2. But you know MgO does dissolve in water for that is the principal ingredient in MOM (milk of magnesia).

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