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

Question

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

Wyatt Adkins · Accepted Answer

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

Thus, all you need to do to test for dihydrogen is light up the bubbles that emerge from a zinc metal mixture in hydrochloric acid; the mixture should ignite (safely) with a whoosh sound, as indicated by the previous equation.

We use the following equation to represent the oxidation of zinc metal by hydrochloric acid:

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

With acetic acid, the reaction would be a little slower:

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

To get the metal and acid mixture going, you would need to give it a little prodding with a heat gun.

In order to demonstrate the carbonate anion's presence, a little more work would be required:

#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.

Mateo Aguilar · Answer

undefined The discriminating tests for the presence of hydrogen gas, carbonate dianion, and zinc metal are:

1. Hydrogen gas: The pop test, where a lighted splint produces a squeaky pop sound when introduced into a container of the gas.

2. Carbonate dianion: The effervescence test, where bubbling occurs when a carbonate-containing substance is added to dilute hydrochloric acid, resulting in the release of carbon dioxide gas, which turns limewater milky upon passing through it.

3. Zinc metal: The displacement reaction test, where zinc metal displaces hydrogen ions from an acid solution, producing hydrogen gas, which can be tested using the pop test.