Question

What are discriminating tests for the presence of hydrogen gas, carbonate dianion, and zinc metal?

Answer 1

`H_2(g) + 1/2O_2(g) rarr H_2O(l) + Delta`

Explanation:

So to test for dihydrogen, simply ignite the bubbles that come off a mixture of zinc metal in hydrochloric acid. It should ignite (safely) with an audible whoosh as shown in the former equation.

WE represent the oxidation zinc metal by hydrochloric acid by means of the following equation:

`Zn(s) + 2HCl(aq) rarr ZnCl_2(aq) + H_2(g)uarr`

The reaction with acetic acid would be a trifle more sluggish:

`Zn(s) + 2HOAc(aq) rarr Zn(OAc)_2(aq) + H_2(g)uarr`

You would need to give the mixture of metal and acid a bit of encouragement with a heat gun to get it going.

To show the presence of carbonate anion, you would have to go to a little bit more trouble:

`CO_3^(2-) + 2H^(+) rarr CO_2(g) + H_2O(l)`

Now carbon dioxide turns lime water milky due to a precipitate of `CaCO_3`:

`Ca(OH)_2(aq) + CO_2(g) rarr CaCO_3(s)darr + H_2O(l)`

Calcium hydroxide is only sparingly soluble in water to give lime water, `Ca(OH)_2(aq)`, and you really have to look to see the precipitate of calcium carbonate. In these circumstances, it is often useful to take a bottle of sparkling water, and add it to the lime water. Sparkling mineral water is certainly supersaturated with respect to `CO_2(g)`, and these means allow you to visualize a positive test.