Qualitative Analysis I Flashcards
For a starting memory (14 cards)
Recall the cations that form white precipitate salts.
Ca2+ Zn2+
Al3+
Ag+ Ba2+
Excess NaOH is added to a solution to test for Cu2+.
Is a precipitate formed?
If so, what is its colour?
Yes. A light blue precipitate is formed.
This light blue precipitate is insoluble (does not dissolve) in excess sodium hydroxide.
Excess NH4OH is added to a solution to test for Cu2+.
Is a precipitate formed?
If so, what is its colour?
No. While a light blue precipitate is formed without excess,
if we add into the solution with excess NH4OH,
the initial light blue precipitate reacts with NH4OH and dissolves to form a dark blue solution.
(a) What are two alkalis we can use to test for Fe2+ and Fe3+ cations?
(b) If precipitates are formed, what colour are they?
(c) Are the precipitates soluble in excess alkali?
(a) Aqueous Sodium hydroxide and Aqueous ammonia.
(b) Fe2+: green precipitate
Fe3+: red-brown precipitate
(c) Both precipitates are insoluble in excess sodium hydroxide and excess ammonia.
Note: Fe2+ green precipitate turns brown on standing because it oxidizes in exposure to O2 in air.
What color precipitate is formed when Al3+ compound is tested with little NaOH?
Does the precipitate dissolve in excess NaOH?
A white precipitate is formed. (Al(OH)3)
It dissolves in excess NaOH to form a colourless solution.
(Memorise for now or ask: What type of reaction this is?)
What color precipitate is formed when Al3+ compound is tested with limited NH4OH?
Does the precipitate dissolve in excess NH4OH?
A white precipitate is formed.(Al(OH)3)
It is insoluble in excess aqueous ammonia.
What color precipitate is formed when Ca2+ compound is tested with excess NaOH?
Does it dissolve in excess NaOH?
A white precipitate is formed. (Ca(OH)2)
It does not dissolve in excess NaOH.
What color precipitate is formed when Ca2+ compound is tested with excess NH4OH?
No precipitate is formed.
A little NaOH is added to an aqueous solution with Zn2+ cations.
Does a green precipitate form?
Later, excess NaOH is added.
Does the precipitate dissolve or not?
No. A white Zn(OH)2 precipitate is formed.
Yes. The precipitate dissolves to give a colourless solution.
How do we test for the carbonate anion?
What are the observations seen during the experiment?
Add any dilute acid.
Carbon dioxide gas, which is formed, can be detected by bubbling it through limewater.
As the bubbled CO2 gas reacts with limewater,
a white precipitate will be formed.
How do we test for the chloride cation (Cl-) in a solution?
What observations will we see?
Add acidified silver nitrate into the solution.
A white precipitate (of AgCl2) will be formed from the reaction.
Test for nitrate anion? (NO₃-)
What are the observations of the test?
Add aqueous sodium hydroxide,
then add aluminium foil and warm gently.
Effervescence observed. A gas is evolved which turned moist red litmus paper blue.
Gas evolved is ammonia gas.
Bonus:
Why is nitric acid used with silver nitrate, in the test for chloride anions?
Silver nitrate (AgNO₃) reacts with chloride ions to form silver chloride (AgCl), a white precipitate.
However, silver ions (Ag⁺) can also react with carbonate or hydroxide ions to form silver carbonate (Ag₂CO₃) or silver hydroxide (AgOH), which would also give precipitates — potentially misleading.
Adding nitric acid (HNO₃) neutralizes these interfering anions:
CO₃²⁻ + 2H⁺ → CO₂ (gas) + H₂O
OH⁻ + H⁺ → H₂O
So, the nitric acid removes competing anions.
Test for sulphate anion?
What is the colour of the precipitate formed (if any)?
Add acidified aqueous barium nitrate or barium chloride.
(Ba(NO₃)₂ or BaCl2)
