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#

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.

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Answer 2

Mateo Aguilar

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

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

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.