2.39 describe tests for the gasses

i Hydrogen (H+)

1. Place lit splint in has, burning with a ‘squeaky pop’
2. 2H (g) + O2 (g) –> H2O (l)

ii Oxygen (O2)

1. Insert glowing splint and it should relight in the presence of oxygen

iii Carbon Dioxide (CO2)

1. Bubbles through limewater and it should turn cloudy / milky / white in the presence of oxygen
2. Ca(OH)2 (aq) + CO2 (g) –> CaCO3 (s) +H2O  (CaCO3 is the white precipitate of milky lime water)

iv Ammonia (NH4+)

1. Damp red litmus turns blue
2. NH3 (g) + H2O (l) ↔ NH4 + (aq) + OH- (aq)
3. The hydroxide (water) ions are needed to dissolve the ammonia gas to make ammonium liquid. The damp universal indicator turns blue-meaning it’s an alkali substance

v Chlorine (Cl-)

1. Damp litmus/universal indicator paper bleaches white
2. Damp or moist starch-iodide paper turns blue (Cl2 (g) + 2I- (aq) –> 2Cl – (aq) + I2 (aq) – the iodine formed turns starch blue)

2.38 describe the tests for these anions:

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

Testing for Halide ions by precipitation. Method:

1. Add dilute nitric acid
2. Add dilute silver nitric acid
3. Check the precipitate colour 

 

 

 

ii SO4 2- (sulphate ions) using dilute hydrochloric acid and dilute barium chloride solution

Method:

1. Add dilute hydrochloric acid
2. Add dilute barium chloride
3. Ba  2+ (aq) + SO4 2 (l)- –> BaSO4 2- (s)
4. Barium sulfate precipitate is white – hence the solid symbol at the end

iii CO3 2- , using dilute hydrochloric acid and identifying the carbon dioxide evolved

Method:

1. Add and acid to react with the solution under test
2. Collect the gas given off by collecting in a test tube
3. Test with lime water – if it goes milky, then there is presence of carbonate ions

• Carbonate + acid –> salt + water + carbon dioxide
• CO3 2- (aq) + 2H+ + (aq) –> CO2 (g) + H2O (l)
• It’s the hydrogen ions (H+) which make the solution acidic- therefore causing the solution to bubble and produce carbonate bubbles