Required activity 4: Testing for cations: Group 2, NH4+

Question 1

whats the method for adding dilute sodium hydoxide?

Answer 1

a) Place about 10 drops of 0.1 mol dm–3 metal ion solution in a test tube.
b) Add about 10 drops of 0.6 mol dm–3 sodium hydroxide solution, mixing well.
c) Continue to add sodium hydroxide solution, dropwise with gentle shaking, until in excess

Question 2

whats the trend of the soluble down group 2?

Answer 2

group II hydroxides become more soluble down the group.

Magnesium hydroxide is classed as insoluble in water and will appear as a white precipitate.

Simplest Ionic Equation for formation of Mg(OH)2(s)
Mg2+ (aq) + 2OH-(aq) -> Mg(OH)2(s).

Calcium hydroxide is classed as partially soluble in water and will appear as a white precipitate (it may need more sodium hydroxide to be added before it appears compared to a magnesium solution.)

Simplest Ionic Equation for formation of Ca(OH)2 (s)
Ca2+ (aq) + 2OH-(aq)  Ca(OH)2 (s).

Strontium and barium salts will not form a hydroxide precipitate on addition of sodium hydroxide due to their high solubility. The solutions will be highly alkaline

Question 3

how would a suspension of magnesium hydroxide and calcium hydroxide appear in water?

Answer 3

A suspension of magnesium hydroxide in water will appear slightly alkaline (pH 9) so some hydroxide ions must therefore have been produced by a very slight dissolving

A suspension of calcium hydroxide in water will appear more alkaline (pH 11) than magnesium hydroxide as it is more soluble so there will be more hydroxide ions present in solution.

Question 4

results of aluminum salts for group 3

Answer 4

Aluminium salts will form a white precipitate of
aluminium hydroxide which will dissolve in excess
sodium hydroxide to form a colourless solution

Al(H2O)6]3+(aq) + 3OH-(aq) —> Al(H2O)3(OH)3 (s) + 3H2O (l)

Al(H2O)3(OH)3 (s) + OH-(aq ) —–> [Al(OH)4(aq) + 3H2O (l)

Question 5

what are the colours for transition metals?

Answer 5

Copper solutions form a blue ppt
[Cu(H2O)6]2+(aq) + 2OH-(aq) —–> Cu(H2O)4(OH)2 (s) + 2H2O (l)

iron (II) solutions form a green ppt
[Fe(H2O)6]2+(aq) + 2OH-(aq) —–> Fe(H2O)4(OH)2 (s) + 2H2O (l)

iron (III) solutions form a brown ppt
[Fe(H2O)6]3+(aq) + 3OH-(aq) —–> Fe(H2O)3(OH)3 (s) + 3H2O (l)

Question 6

what is the 4 step method for testing for ammonium ions ( NH4+)? and whate the result?

Answer 6

a) Place about 10 drops of 0.1 mol dm–3 ammonium chloride in a test tube.
b) Add about 10 drops of 0.4 mol dm–3 sodium hydroxide solution. Shake the mixture.
c) Warm the mixture in the test tube gently using a water bath.
d) Test the fumes released from the mixture by holding a piece of damp red litmus paper in the mouth of the test tube.

Results: alkaline ammonia gas is released which turns the red litmus paper blue

Question 7

whats the 3 step method of adding sulfate ions? whats the result?

Answer 7

a) Place about 10 drops of 0.1 mol dm–3 metal ion solution in a test tube.
b) Add about 10 drops of 1.0 mol dm–3 sulfuric acid (or other soluble sulfate solution.
c) Continue to add sulfuric acid solution, dropwise with gentle shaking, until in excess

The results in this test are an application of the trend that Group II sulfates become less soluble down the group

magnesium and calcium salts will not form a sulfate precipitate on addition of sulfate ions due to their high solubility.

Strontium and barium solutions will form white precipitates with addition of sulfate ions

Full equation : SrCl2(aq) + Na2SO4(aq) –> 2NaCl (aq) +SrSO4(s)

Ionic equation: Sr2+(aq) + SO42-(aq) –> SrSO4(s).

Ionic equation: Ba2+(aq) + SO42-(aq) –> BaSO4(s).