g) Tests for ions and gases ​

2.37 describe tests for the cations: ​

Li+, Na+, K+, Ca2+ using flame tests ​

The technique is first of all to clean the end of a piece of platinum or nichrome wire by dipping it onto clean hydrochloric acid and then placing it in a roaring Bunsen flame. This procedure should be repeated until the wire no longer produces a colour in the flame. Then the end of the wire should be dipped again into fresh hydrochloric acid and then into the solid sample under test. The end of the wire should be placed in to a non-roaring, non-luminious Bunsen flame.

This will give this following results:

NH4+, using sodium hydroxide solution and identifying the ammonia evolved ​

Equation: $(NH4)2 SO4 + NaOH \rightarrow NH3 + Na2SO4 + H2O$

Add aqueous sodium hydroxide to the solid or solution, under test and warm the mixture. If ammonium ions are present, then a pungent-smelling gas is produced. The gas produced turns damp red litmus paper blue. It is ammonia, NH3.

Cu2+, Fe2+ and Fe3+, using sodium hydroxide solution ​

Of the common hydroxides, only sodium, potassium and ammonium, hydroxides dissolve in water to any extent. Most metal hydroxides are insoluble. That means that if you add sodium hydroxide solution to a solution containing the metal ions, you will get a precipitate of the metal hydroxide.

• Copper(ii) ions + sodium hydroxide -> blue precipitate
  When sodium hydroxide is added to compound containing copper ions, it will form a blue precipitate.

• Iron(ii) ions + sodium hydroxide -> green precipitate
  When sodium hydroxide is added to a compound containing Iron (II) ions, a green precipitate of Iron (II) oxide will form.

• Iron(iii)ions + sodium hydroxide -> brown precipitate
  When sodium hydroxide is added to a compound containing Iron (III) ions, a orange-brown precipitate of Iron (III) oxide is formed.

2.38 describe tests for the anions: ​

Cl-, Br- and I-, using dilute nitric acid and silver nitrate solution ​

Make a solution of your suspected chloride, bromide or iodide and add enough nitric acid to make it acidic. Then add few drops of silver nitrate solution.

P.S: The acid is added to react with and remove other substances which might also produce precipitates with silver nitrate solution.

• Chloride ions + nitric acid + silver nitrate -> white precipitate (silver chloride)
  Chloride ions react with silver nitrate to form a white precipitate of silver chloride.

• Bromide ions + nitric acid + silver nitrate -> cream precipitate (silver bromide)
  Bromide ions react with silver nitrate to from a cream precipitate of silver bromide

• Iodide ions + nitric acid + silver nitrate -> yellow precipitate (silver iodide)
  Iodide ions react with silver nitrate to form a yellow precipitate of silver iodide.

SO42-, using dilute hydrochloric acid and barium chloride solution ​

Test: To an aqueous solution of the solid under test, add dilute hydrochloric acid followed by a few drops of barium chloride solution.

Result: This will form a white precipitate (of barium sulphate).

Equation: $SO4^{2-} + HCl + Ba^{2+} \rightarrow$ white precipitate (barium sulphate)

CO32-, using dilute hydrochloric acid and identifying the carbon dioxide evolved ​

Test: To either the solid, or an aqueous solution of the solid, under test add dilute hydrochloric acid.

P.S: It is best to use dilute nitric acid. Some acid-carbonate combinations can produce an insoluble salt that coats the solid carbonate and stops the reaction, but this doesn't happen if you use nitric acid because all nitrates are soluble.

Result: This will for bubbles of gas (carbon dioxide). This gas will turn limewater milky.

Equation: Carbonate + acid -> salt + water + carbon dioxide

2.39 describe tests for the gases: ​

Hydrogen ​

Test: Hold a lighted splint into the gas
Result: It burns with a squeaky pop.

Equation: $2H2(g) + O2(g) \rightarrow 2H2O(l)$

Oxygen ​

Test: Hold a glowing splint into the gas.
Result: It will relight.

Carbon dioxide ​

Test: Pass the gas through lime water.

Result: It turns white due to the formation of calcium carbonate.

Equation: $CO2 (g) + Ca(OH)2 (aq) \rightarrow CaCO3 (s) + H2O (l)$

If carbon dioxide is passed in excess, the solution turns clear again due to the formation of soluble calcium hydrogen

Equation: $CaCO3 (s) + H2O (l) +CO2 (g) \rightarrow Ca(HCO3)2$

Ammonia ​

Test: Hold a damp red litmus paper into the gas.
Result: The colour of the paper turns blue.

PS: Ammonia is the only alkaline gas.

Test2: Hold a glass rod soaked in concentrated hydrochloric acid.
Result2 : A white fume will form near the glass rod
Equation: $NH3 (g) + HCl(g) \rightarrow NH4Cl$

Chlorine ​

Test: Hold a damp blue litmus paper into the gas.
Result: It turns red before it bleaches(turns white).

Colour Chemistry ​