Chapter 08: Qualitative analysis
Chapter 8 :
Chapter 8 - Qualitative analysis
Qualitative analysis
To identify the substances present in the mixture
How do we identify the ions? We precipitate the ions out in the form of an insoluble salt. A precipitation reaction is carried out to help us identify the ions present.
Cation test: Usually NaOH or NH₃(aq) is used to determine the identity of the cation present in solution. Recap that in the solubility table most hydroxides are insoluble except for group I hydroxides and a few sparingly soluble Group II hydroxides. The precipitate formed when NaOH or NH₃(aq) is added is a metal hydroxide. This is because another way to represent aqueous ammonia is NH₄OH.
| Metal cation | Observations using NaOH(aq) | Observations using NH₃(aq) |
|---|---|---|
| Zn²⁺ | White precipitate, soluble in excess to form a colourless solution |
White precipitate, soluble in excess to form a colourless solution |
| Al³⁺ | White precipitate, soluble in excess to form a colourless solution |
White precipitate, insoluble in excess |
| Pb²⁺ | White precipitate, soluble in excess to form a colourless solution |
White precipitate, insoluble in excess |
| Ca²⁺ | White precipitate, insoluble in excess | No precipitate formed |
| NH₄⁺ | No precipitate formed, produces NH3 gas on warming | - |
| Fe²⁺ | Dirty green precipitate, insoluble in excess | Dirty green precipitate, insoluble in excess |
| Fe³⁺ | Reddish-brown precipitate, insoluble in excess | Reddish-brown precipitate, insoluble in excess |
| Cu²⁺ | Light blue precipitate, insoluble in excess | Light blue precipitate, soluble in excess to form a dark blue solution |
| Metal cation |
|---|
| Zn2+ |
| Observations using NaOH(aq) |
| White precipitate, soluble in excess to form a colourless solution |
| Observations using NH3(aq) |
|
White precipitate, soluble in excess to form a colourless solution |
| Metal cation |
|---|
| Al3+ |
| Observations using NaOH(aq) |
| White precipitate, soluble in excess to form a colourless solution |
| Observations using NH3(aq) |
|
White precipitate, insoluble in excess |
| Metal cation |
|---|
| Pb2+ |
| Observations using NaOH(aq) |
| White precipitate, soluble in excess to form a colourless solution |
| Observations using NH3(aq) |
|
White precipitate, insoluble in excess |
| Metal cation |
|---|
| Ca2+ |
| Observations using NaOH(aq) |
| White precipitate, soluble in excess to form a colourless solution |
| Observations using NH3(aq) |
|
White precipitate, insoluble in excess |
| Metal cation |
|---|
| NH4+ |
| Observations using NaOH(aq) |
| No precipitate formed, produces NH3 gas on warming |
| Observations using NH3(aq) |
| - |
| Metal cation |
|---|
| Fe2+ |
| Observations using NaOH(aq) |
| Dirty green precipitate, insoluble in excess |
| Observations using NH3(aq) |
| Dirty green precipitate, insoluble in excess |
| Metal cation |
|---|
| Fe3+ |
| Observations using NaOH(aq) |
| Reddish-brown precipitate, insoluble in excess |
| Observations using NH3(aq) |
| Reddish-brown precipitate, insoluble in excess |
| Metal cation |
|---|
| Cu 2+ |
| Observations using NaOH(aq) |
| Light blue precipitate, insoluble in excess |
| Observations using NH3(aq) |
| Light blue precipitate, soluble in excess to form a dark blue solution |
Recap: What are the three amphoteric oxides? ZnO, PbO, Al₂O₃.
The hydroxides of these elements are amphoteric as well. That is why the precipitates of these metals dissolve in excess NaOH to form a colourless solution.
The amphoteric hydroxides, which can react with both acids and alkalis, will react with OH⁻ ions and will thus dissolve when excess NaOH is added.
TIP: When memorizing, just memorise the metal cations that forms colored precipitates only, which are Fe²⁺, Fe³⁺ and Cu²⁺. The rest will form white precipitate, with the exception of
- ammonium ion when OH⁻ is added and
- calcium ion when aqueous ammonia is added.
| Compounds to know | Colour |
|---|---|
| CuCO₃ | Green |
| CuO | Black |
| ZnO | Yellow when hot, white when cold |
| Compounds to know | Colour |
|---|---|
| CuCO₃ | Green |
| CuO | Black |
| ZnO | Yellow when hot, white when cold |
Anion test
| Anion in solution | Test | Observation |
|---|---|---|
| Carbonate, CO₃²⁻ |
Add dilute acid. |
Effervescence of a colourless, odourless gas is observed.
CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l ) The gas evolved forms white precipitate when bubbled through aqueous calcium hydroxide (limewater). This reaction is a neutralisation reaction as carbon dioxide is an acidic oxide and can react with calcium hydroxide which is basic. CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) +H₂O(l ) |
| Chloride, Cl ⁻ or Iodide, I⁻ |
Add dilute nitric acid followed by silver nitrate. |
A white precipitate is formed if chloride ion is present in solution.
Ag+ (aq)+ Cl -(aq) → AgCl (s) A yellow precipitate is formed if iodide ion is present in solution. Ag+ (aq) + I- (aq) → AgI (s) |
| Nitrate, NO₃⁻ |
Add NaOH and aluminium foil. Heat the mixture. |
A colourless gas with a pungent smell is liberated.
Gas turns moist red litmus paper blue. |
| Sulfate, SO₄²⁻ |
Add dilute nitric acid followed by barium nitrate. |
A white precipitate is formed if sulfate ion is present in solution.
Ba2+ (aq) + SO42- (aq) → BaSO4 (s) |
| Anion in solution |
|---|
|
Carbonate, CO32− |
| Test |
| Add dilute acid. |
| Observation |
|
Effervescence of a colourless, odourless gas is observed.
CO32- (aq) + 2H+ (aq) → CO2(g) + H2O (l ) The gas evolved forms white precipitate when bubbled through aqueous calcium hydroxide (limewater). This reaction is a neutralisation reaction as carbon dioxide is an acidic oxide and can react with calcium hydroxide which is basic. CO2(g) + Ca(OH)2(aq) → CaCO3(s) +H2O(l ) |
| Anion in solution |
|---|
|
Chloride, Cl − or Iodide, I− |
| Test |
| Add dilute nitric acid followed by silver nitrate. |
| Observation |
|
A white precipitate is formed if chloride ion is present in solution.
Ag+ (aq) + Cl- (aq) → AgCl (s) A yellow precipitate is formed if iodide ion is present in solution. Ag+ (aq) + I- (aq) → AgI (s) |
| Anion in solution |
|---|
|
Nitrate, NO3− |
| Test |
|
Add NaOH and aluminium foil.
Heat the mixture. |
| Observation |
|
A colourless gas with a pungent smell is liberated.
Gas turns moist red litmus paper blue. |
| Anion in solution |
|---|
|
Sulfate, SO42− |
| Test |
| Add dilute nitric acid followed by barium nitrate. |
| Observation |
|
A white precipitate is formed if sulfate ion is present in solution.
Ba2+ (aq) + SO42- (aq) → BaSO4 (s) |
For chloride, iodide and sulfate ions, precipitation method is used to precipitate out the anion in the form of insoluble salts. Remember solubility table?
| Salt | Insoluble salts |
|---|---|
| Chlorides (Cl ⁻) and Iodides (I⁻) |
AgCl , AgI PbCl ₂ , PbI₂ |
| Sulfates (SO₄²⁻) | BaSO₄, CaSO₄, PbSO₄ |
| Salt | Insoluble salts |
|---|---|
| Chlorides (Cl ⁻) and Iodides (I⁻) |
AgCl , AgI PbCl ₂ , PbI₂ |
| Sulfates (SO₄²⁻) | BaSO₄, CaSO₄, PbSO₄ |
For iodide and chloride anion, apart from using silver nitrate, lead(II) nitrate can also be used to precipitate the anion in the form of lead(II) iodide or lead(II) chloride.
For sulfate anion, apart from using barium nitrate, calcium nitrate and lead(II) nitrate can be used to precipitate the anion as calcium sulfate or lead(II) sulfate respectively.
For chloride and sulfate anion test, an acid (usually nitric acid) will be added before adding the other aqueous reagent to precipitate out the anion. Why is nitric acid added to the sample?
Imagine if sample A contains both carbonate and chloride ions. If we did not add nitric acid at first and proceed to adding aqueous silver nitrate, precipitate will be observed. However, this precipitate will be a mixture of silver carbonate and silver chloride because both salts are insoluble. It will not be accurate to say “sample A contains chloride ions only” in the case where the addition of acid step is skipped, because we did not eliminate the possibility that A has carbonate ions.
Thus, we have to first ensure that sample does not contain any carbonate ions, since most carbonate salts are insoluble and will precipitate out when barium nitrate or silver nitrate is added. This confirmation is done by adding an acid and observing if there will be any effervescence.
Identification test of gases
| Gas | Properties | Identification test | Effect on moist litmus paper |
|---|---|---|---|
| Hydrogen gas (H₂) | Colourless and Odourless | Extinguishes a lighted splint with a pop sound. | Neutral gas.
Thus, litmus paper remains unchanged. |
| Oxygen gas (O₂) | Colourless and Odourless | Rekindle a glowing splint. | Neutral gas.
Thus, litmus paper remains unchanged. |
| Ammonia gas (NH₃) | Colourless and pungent | Turns moist red litmus paper blue. | Alkaline gas.
Thus, moist red litmus paper turns blue. |
| Carbon dioxide gas (CO₂) | Colourless and Odourless | Forms a white precipitate when bubbled through limewater. | Acidic gas.
Thus, moist blue litmus paper turns red. |
| Chlorine gas (Cl ₂) | Greenish-yellow gas and choking | Turns moist blue litmus paper red, then bleaches it. | Acidic gas.
Thus, moist blue litmus paper turns red then bleaches it. |
| Hydrogen chloride (HCl ) | Colourless and pungent | N.A | Acidic gas.
Thus, moist blue litmus paper turns red. |
| Sulfur dioxide (SO₂) | Colourless and choking | Turns purple acidified potassium manganate (VII) colourless. | Acidic gas.
Thus, moist blue litmus paper turns red. |
In acids and bases topic, remember that non-metallic oxides (except for monoxides) are acidic oxides. Thus, acidic oxides will turn moist blue litmus paper red.
Only chlorine gas has bleaching effect (causes the litmus paper to turn white).
Question 1:
Tests were conducted on unknown substance X and the following results were obtained.
| Test | Observations | |
|---|---|---|
| 1 | To an aqueous solution of X, add aqueous sodium hydroxide in excess. | No precipitate formed. |
| 2 | To an aqueous solution of X, add dilute hydrochloric acid. | A gas is evolved, giving a white precipitate with limewater. |
| Test | Observations | |
|---|---|---|
| 1 | To an aqueous solution of X, add aqueous sodium hydroxide in excess. | No precipitate formed. |
| 2 | To an aqueous solution of X, add dilute hydrochloric acid. | A gas is evolved, giving a white precipitate with limewater. |
Which of the following could solid X be?
A calcium nitrate
B zinc nitrate
C lead(II) carbonate
D potassium carbonate
Question 2:
Fig 1.1 describes of the substances that result from the chemical reactions of metal R.
Continue Reading Chapter 9: Mole Concept & Chemical Calculations