Identification of gases and ions

Identifying Gases

Oxygen (Glowing wooden splinter)

A glowing wooden splinter is inserted into the test tube that contain the gas.

The gas rekindles the glowing wooden splinter.

Hydrogen (Lighted Wooden Splinter)

A lighter wooden splinter is brought close to the mouth of the test tube that contain the gas.

A “pop” sound is produced.

Carbon dioxide (Lime Water)

The gas is directed to flow through lime water.

The lime water turn chalky.

Sulphur Dioxide (Potassium Dichromate(VI))

The gas is directed to flow through potassium dichromate(VI) solution.

The orange colour of potassium dichromate(VI) solution become green.

Chlorine (Moist Litmus Paper)

Moist blue litmus paper is inserted into the test tube that contain the gas.

The blue litmus paper turn red and then white.

Ammonia Gas (Moist litmus paper)

Moist red litmus paper is inserted into the test tube that contain the gas.

The red litmus paper turn blue.

Identifying Anions – Carbonate

The carbonate salts react with acid produce carbon dioxide and water. This chemical property of carbonate is used to test the presence of carbonate in a salt.

CO32– + 2H+ → H2O + CO2

During the test, some dilute hydroChloric acid / nitric acid /sulphuric acid is added to the carbonate salt.

qualitative cont

If the salt contain carbonate, effervescence occurs.

If the gas given off is passed through lime water, the lime water will turns chalky.

This indicates that the gas is carbon dioxide, and hence the salt contain carbonate.

Identifying Anions – Sulphate

All salts of sulphate are soluble in water, except lead(II) sulphate, barium sulphate and calcium sulphate.

The insolubility of barium sulphate is used to test for the presence of sulphate in a salt.

2 cm3 of dilute hydrochloric / nitric acid is added to 2 cm3 of sulphate solution. This is to check whether carbonate is presence in the solution or not because carbonate may give the same result as sulphate.

If not effervescence, then 2 cm3 of barium chloride is added into the solution.

If sulphate ions are presence, a white precipitate will form. The precipitate is barium sulphate.

This is actually the double decomposition reaction that you have learned in preparation of insoluble salt.

Ba2+ + SO42– → BaSO4

Identifying Anions – Chloride

All the salts of chloride are soluble in water except lead(II) chloride, silver(I) chloride and mercury chloride.The insolubility if silver(I) chloride is used in the test of presence of chloride.

2 cm³ of dilute nitric acid is added to 2 cm3 solution of chloride ions. This is the check if carbonate ions are presence because carbonate ions may give the same result.

If there is no effervescence, 2 cm³ of silver nitrate solution is then added into the mixture.

A white precipitate will form if chloride is presence in the salt.

The precipitate is silver chloride

Ag+ + Cl– → AgCl

Identifying Anions – Nitrate

Test 1

Add dilute sodium hydroxide and a little aluminum powder. If a nitrate is present, ammonia gas is produced. The gas can be identified as it turns moist red litmus paper blue.

Test 2

About 2cm3 of dilute sulphuric acid is added into the solution that wants to be tested and then followed by 2cm3 iron (II) sulphate solution.

A few drops of concentrated sulphuric acid are carefully drop through the inclined side of the test tube without shaking the test tube.

A brown ring will form in the middle of the solution.

Explanation: Iron (II) sulphate reduce nitric acid (from the reaction between nitrate ion and concentrated sulphuric acid) to nitrogen monoxide. Afterwards, nitrogen monoxide combines with iron (II) sulphate to form the compound FeSO4.NO which is brown in colour (brown ring).

Identifying Cations – Test with Sodium Hydroxide and Ammonia Solution

Cations can be identified by their reaction with aqueous sodium hydroxide and aqueous ammonia.

Sodium hydroxide and aqueous ammonia produce hydroxide ion which will react with most anion to form precipitate.

NaOH + H₂O → Na+ + 2OH– + H+

NH3 + H₂O → NH4+ + OH–

Different cations like aluminium Al3+, calcium Ca2+, copper(II) Cu2+, iron(II) Fe2+, iron(III) Fe3+, lead(II) Pb2+, zinc Zn2+ produce different coloured precipitates, which may or may not dissolve in excess alkali.

Zn(OH)2, Al(OH)3 and Pb(OH)3 dissolve in excess NaOH solution, this is because Zn(OH)2, Al(OH)3 and Pb(OH)3 are amphoteric, they can react with NaOH to form salt and water.

Zn(OH)2 + 2NaOH → Na2ZnO2 + 2H2O

Al2(OH)3 + 3NaOH → Na3Al2O3 + 3H2O

Pb(OH)2 + 2NaOH → Na2PbO2 + 2H2O

Table below shows the summary of the precipitate form by different cation.

NaOH(ak)   NH3(ak)

Na+   –        –

Ca2+ White precipitate. –

Mg2+ White precipitate. White precipitate.

Al3+  White precipitate.

Dissolve in excess NaOH solution.White precipitate.Zn2+White precipitate.

Dissolve in excess NaOH solution.White precipitate.

Dissolve in excess NH3 solution.Pb2+White precipitate.

Dissolve in excess NaOH solution.White precipitate.Fe2+Dirty green precipitate.Dirty green precipitate.Fe3+Red brown precipitate.Red brown precipitate.Cu2+Blue precipitate.Blue precipitate.

Dissolve in excess NH3 solution and form a blue solution.NH4+—

Identifying Cations – Test with Chloride Ions

Out of the 10 cations, only lead(II) ions will form a precipitate with chloride ions.

This is because lead(II) chloride is insoluble in water.

The chemical reaction is a double decomposition reaction.

Pb2+ + 2Cl– → PbCl2

Lead(II) chloride will dissolve in hot water.

HCl or NaCl

Na+   –

Ca2+ –

Mg2+ –

Al3+  .-

Zn2+ –

Pb2+ White precipitate.

Dissolve in hot water

Fe2+  –

Fe3+  –

Cu2+ –

NH4+ –

Identifying Cations – Test with Sulphate Ions

Out of the 10 cations, only calcium ions and lead(II) ions will form a precipitate with sulphate ions.

This is because both calcium sulphate and lead(II) sulphate are insoluble in water.

The chemical reaction is a double decomposition reaction.

Pb2+ + SO42- → PbSO4

Ca2+ + SO42- → CaSO4

H2SO4 or Na2SO4

Na+   –

Ca2+ White precipitate.

Mg2+ –

Al3+  –

Zn2+ –

Pb2+ White precipitate.

Fe2+  –

Fe3+  –

Cu2+ –

NH4+ –

Identifying Cations – Test with Carbonate Ions

All ions, except sodium ions and ammonium ions will form precipitate with carbonate.

This is because sodium carbonate and ammonium carbonate are soluble in water.

Na2CO3

Na+   –

Ca2+ White precipitate.

Mg2+ White precipitate.

Al3+  White precipitate.

Zn2+ White precipitate.

Pb2+ White precipitate.

Fe2+  Green precipitate.

Fe3+  Brown precipitate.

Cu2+ Blue precipitate.

NH4+ –

Identifying Cations – Test with Iodide Ions

Iodide ions will form precipitate with lead(II) ions and copper(II) ions.

You only need to know the reaction between lead(II) ions and iodide ions.

The yellow precipitate formed will dissolve in hot water.

Pb2+ + 2I– → PbI2

KI

Na+   –

Ca2+ –

Mg2+ –

Al3+  –

Zn2+ –

Pb2+ Yellow precipitate. Dissolve in hot water

Fe2+  –

Fe3+  A red brown solution formed.

Cu2+ White precipitate form in brown solution

NH4+ –

Identifying Cations – Tests to Distinguish Iron(II) and Iron(III) ions.

Some Tests to Distinguish Fe2+ ion From Fe3+ Ion

The presence of Fe2+ ion and Fe3+ ion in a salt can be confirmed by using solution of potassium hexacyanoferrate (II), solution of potassium hexacyanoferrate (III) and potassium thioCyanate.

Table below shows the observation of the tests.

Reagent       Observation Ion presents

Solution of potassium hexacyanoferrate (II)  Light blue precipitate      Fe2+

Dark Blue precipitate      Fe3+

Solution of potassium hexacyanoferrate (III) Dark blue precipitate      Fe2+

Greenish brown solution Fe3+

Potassium thioCyanate    Pinkish solution    Fe2+

Blood red solution Fe3+

Assessment (Post your answer using the Stoplearn.com box below for evaluation and discussion)

How can the following gas be tested for

1. Oxygen
2. Hydrogen
3. Chlorine
4. Hydrogen Sulphide

What are the heating effects of the following

1. Carbonate salts
2. Nitrate salts