Qualitative Analysis of Salts

What is qualitative analysis?

Qualitative analysis of a salt Analysis is a chemical technique used to identify the ions present in a salt by analysing its physical and chemical properties and hence determine the identity of the salt.
It determines only the presence or absence of a particular ion in a given salt. This method does not determine how much of a particular ion is present.
For example, a student did a chemical analysis on a sample of salt X. His results showed the presence of sodium ions and bromide ions. His conclusion was the salt must be sodium bromide.
The procedure for testing salts in the laboratory consists of the following general steps.
1. Make initial observations of the physical properties of the salt.
2. Study the action of heat on the salt.
3. Make aqueous solution of the salt to test for anions and cations present.
4. Carry out confirmatory tests for cations and anions.

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Examining the colour and solubility of salts in water

Preliminary examination on the physical properties of a salt such as colour and solubility in water can help to indicate that certain cations and anions might be present.
Observations of physical properties only allow you to make inferences on the possibility of the presence of certain ions in the salts. This is because it is possible to have more than one salt sharing the same physical properties. For example:
(a) Iron(II) ions, nickel(II) ions and chromium(III) ions dissolve in water to produce green solutions.
(b) Sodium chloride and potassium carbonate are white solids. Both solids dissolve in water to produce colourless solutions.
Hence, observations of physical properties of salts cannot be used to confirm the identities of the ions. Its main purpose is to help us narrow down the choices of cations and anions present in a salt. Chemical tests still have to be carried out to confirm these ions.

Table lists the colours of some common salts.

Salt	Colour
Salt	Solid	Aqueous solution
Potassium salts Sodium salts Ammonium salts Aluminium salts Calcium salts Lead(ll) salts Zinc salts (with colourless anions)	White	Colourless
Carbonate salts Chloride salts Nitrate salts Sulphate salts (with colourless cations)	White	Colourless
Iron(II) salts: Iron(II) chloride Iron(II) nitrate iron(II) sulphate	Green	Green
Iron(III) salts: Iron(III) chloride Iron(III) nitrate Iron(III) sulphate	Brown	Brown
Copper(II) salts: Copper(II) chloride Copper(II) nitrate Copper(II) sulphate	Blue	Blue
Copper(II) carbonate	Green	(Insoluble)

Table shows the solubility of some common compounds.

Compound	Solubility in water
Sodium, potassium and ammonium salts	All are soluble
Nitrate salts	All are soluble
Ethanoate salts	All are soluble
Chloride salts	All are soluble except AgCl, HgCl and PbCl₂
Sulphate salts	All are soluble except BaSO₄, PbSO₄ and CaSO₄
Carbonate salts	All are insoluble except Na₂CO₃, K2CO₃ and (NH₄)₂CO₃
Metal oxides	All are insoluble except Na₂O, K₂O and CaO (slightly soluble)
Metal hydroxides	All are insoluble except NaOH, KOH and Ba(OH)₂

Example: Preliminary examination of solid X gave the following observations. Identify solid X. Explain your answer.

Green solid that insoluble in water
Dissolved in dilute acid with effervescence to form a blue solution

Solution:

The colour of an aqueous solution of a substance gives a more accurate inference about the nature of the ion present. Blue solution indicates presence of Cu²⁺ ion.
When a metal reacts with an acid, effervescence due to hydrogen gas liberated is observed. Effervescence due to carbon dioxide gas is seen when an acid reacts with a carbonate salt. Most metals are grey solids. Hence, the green solid X has to be a carbonate salt. X is copper(II) carbonate.

Test for gases

Gases are often produced from reactions carried out during laboratory tests on salts. Gases can be evolved when
(a) salts are heated.
(b) salts are reacted with acids or alkalis.
For example:
(a) Heating a carbonate salt produces carbon dioxide gas.
(b) Reacting a metal with dilute acids produces hydrogen gas.
By identifying the gases evolved, it is possible to infer the types of cations or anions present in a salt.
A gas can be identified by its colour, smell, effect on litmus paper and reactions with special reagents.

Gas	Characteristic
Ammonia Method: Bring a piece of moist red litmus paper to the mouth of the test tube. Observation: Red litmus paper turns blue.	Colourless gas Pungent smell Alkaline gas Turns moist red litmus paper blue Forms dense white fumes with hydrogen chloride Produced by heating a mixture of ammonium salt and alkali NH₄Cl(s) + NaOH(aq) → NaCl(aq) + NH₃(g) + H₂O(l)
Oxygen Method: Lower a glowing wooden splint into the test tube. Observation: Glowing wooden splint is relighted.	Colourless gas No effect on litmus paper Supports combustion Relights a glowing wooden splint Produced by heating a chlorate(V) or nitrate salt 2KClO₃(s) → 2KCl(s) + 3O₂(g) 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g)
Carbon dioxide Method: Bubble the gas through limewater. Observation: Limewater turns milky (chalky).	Colourless gas Acidic gas Turns moist blue litmus paper red Turns limewater milky Produced by heating a metal carbonate or acid- carbonate reaction CaCO₃(s) → CaO(s) + C02(g) CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Hydrogen Method: Bring a lighted wooden splint to the mouth of the test tube. Observation: Gas burns with a ‘pop’ sound.	Colourless gas No effect on litmus paper Forms an explosive mixture with air Burns with a ‘pop’ sound Produced by an acid-metal reaction Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Chlorine Method: Bring a piece of moist blue litmus paper to the mouth of the test tube. Observation: Blue litmus paper turns red. It is then bleached.	Greenish-yellow gas Pungent smell Acidic gas Turns moist blue litmus paper red Bleaches litmus paper Produced by heating a mixture of manganese(IV) oxide and concentrated hydrochloric acid MnO₂(s) + 4HCl(aq) → MnCl₂(aq) + Cl₂(g) + 2H₂0(l)
Sulphur dioxide Method: Bubble the gas through acidified K2Cr207 solution or acidified KMn04 solution Observation: The orange K₂Cr₂O₇solution turns green or the purple KMnO₄ solution turns colourless.	Colourless gas Pungent smell Acidic gas Turns moist blue litmus paper red Reduces orange acidified dichromate(VI) ion to green chromium(III) ion Decolourises purple acidified manganate(VII) ion Produced by an acid-sulphite reaction Na₂SO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + SO₂(g)
Hydrogen chloride Method: Use a glass rod to bring a drop of concentrated ammonia solution to the mouth of the test tube. Observation: Dense white fumes are formed.	Colourless gas Forms fumes in moist air Acidic gas Turns moist blue litmus paper red Forms dense white fumes with ammonia gas Produced by heating a mixture of common salt and concentrated sulphuric acid H₂SO₄(aq) + NaCl(s) → NaHSO₄(aq) + HCl(g)
Nitrogen dioxide Method: Bring a piece of moist blue litmus paper to the mouth of the test tube. Observe the colour of gas evolved. Observation: Blue litmus paper turns red. Brown gas is evolved.	Brown gas Pungent smell Acidic gas Turns moist blue litmus paper red Reacts with water to form colourless solution Produced by heating a nitrate salt 2Cu(NO₃)O₂(s) → 2CuO(s) + 4NO₂(g) + O₂(g)

Qualitative Analysis of Salts

Examining the colour and solubility of salts in water

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