Qualitative Analysis of Salts

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Qualitative Analysis of Salts

What is qualitative analysis?

  1. 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.
  2. 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.
  3. 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.
  4. 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

  1. 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.
  2. 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.
  3. 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.

SaltColour
SolidAqueous solution
Potassium salts
Sodium salts
Ammonium salts
Aluminium salts
Calcium salts
Lead(ll) salts
Zinc salts
(with colourless anions)		WhiteColourless
Carbonate salts
Chloride salts
Nitrate salts
Sulphate salts
(with colourless cations)		WhiteColourless
Iron(II) salts:
Iron(II) chloride Iron(II) nitrate iron(II) sulphate		GreenGreen
Iron(III) salts:
Iron(III) chloride
Iron(III) nitrate
Iron(III) sulphate		BrownBrown
Copper(II) salts:
Copper(II) chloride
Copper(II) nitrate
Copper(II) sulphate		BlueBlue
 Copper(II) carbonate Green(Insoluble)

Table shows the solubility of some common compounds.

CompoundSolubility in water
Sodium, potassium and ammonium saltsAll are soluble
Nitrate saltsAll are soluble
Ethanoate saltsAll are soluble
Chloride saltsAll are soluble except AgCl, HgCl and PbCl2
Sulphate saltsAll are soluble except BaSO4, PbSO4 and CaSO4
Carbonate saltsAll are insoluble except Na2CO3, K2CO3 and (NH4)2CO3
Metal oxidesAll are insoluble except Na2O, K2O and CaO (slightly soluble)
Metal hydroxidesAll 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

  1. 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.
  2. For example:
    (a) Heating a carbonate salt produces carbon dioxide gas.
    (b) Reacting a metal with dilute acids produces hydrogen gas.
  3. By identifying the gases evolved, it is possible to infer the types of cations or anions present in a salt.
  4. A gas can be identified by its colour, smell, effect on litmus paper and reactions with special reagents.
GasCharacteristic
Ammonia
Method:

  • Bring a piece of moist red litmus paper to the mouth of the test tube.
    Qualitative Analysis of Salts 1

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.
    Qualitative Analysis of Salts 2

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.
    Qualitative Analysis of Salts 3

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.
    Qualitative Analysis of Salts 4

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.
    Qualitative Analysis of Salts 5

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

Qualitative Analysis of Salts 6

Observation:

  • The orange K2Cr2O7 solution 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.

Qualitative Analysis of Salts 7

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.

Qualitative Analysis of Salts 8

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)

 

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