TEST OF PRACTICAL KNOWLEDGE QUESTION
(a) Name one laboratory equipment used for
(i) keeping salts dry:
(ii) converting vapour to liquid during distillation
(ii) bubbling a gas into a liquid
(b) (i) What technique would you use to purity a sample of sodium chloride contaminated with ammonium chloride?
(i) Given sodium hydroxide solution, outline tne procedure you would use to determine whether or not all the ammonium chloride in (b)(i) above had been removed
(c) State what is observed on carrying out each of the following exercises.
(i) Adding few drops of methyl orange indicator to lime juice:
(ii) Adding few drops of concentrated \( \mathrm{HNO}_3 \) to acidified \( \mathrm{FeSO}_4 \) solution
(iii) Exposing a fresh precipitate of silver chloride to sunlight for 30 minutes
(iv) Adding zinc dust to dilute \( \mathrm{CuCl}_2 \) solution;
(v) Adding dilute \( \mathrm{H}_2\mathrm{SO}_4 \), to \( \mathrm{Pb}(\mathrm{NO}_3)_2 \) solution
(a) Laboratory equipment
- (i) Keeping salts dry: a desiccator (containing a drying agent such as anhydrous calcium chloride or silica gel).
- (ii) Converting vapour to liquid during distillation: a (Liebig) condenser.
- (iii) Bubbling a gas into a liquid: a gas-washing (Dreschel) bottle, or a delivery tube dipping into the liquid.
(b)(i) Purifying NaCl contaminated with NH4Cl
Use sublimation. On gentle heating, ammonium chloride sublimes (turns directly to vapour and re-solidifies on a cool surface), leaving pure sodium chloride behind, because NaCl does not sublime.
(b)(ii) Testing whether all the NH4Cl has been removed
Place a portion of the residue in a test tube, add sodium hydroxide solution and warm gently. Hold a piece of moist red litmus paper at the mouth of the tube.
- If a pungent gas that turns moist red litmus blue (ammonia) is evolved, some ammonium chloride is still present.
- If no ammonia smell and no colour change occur, all the ammonium chloride has been removed.
(c) Observations
| Exercise | Observation |
|---|
| (i) Methyl orange added to lime juice | Indicator turns red/pink (lime juice is acidic). |
| (ii) Conc. HNO3 added to acidified FeSO4 | Pale green solution turns yellow/brown as Fe2+ is oxidised to Fe3+ (a brown ring/colouration). |
| (iii) Fresh AgCl exposed to sunlight for 30 min | White precipitate gradually turns grey then black (photo-decomposition to silver metal). |
| (iv) Zinc dust added to dilute CuCl2 | Blue colour fades to colourless and a reddish-brown deposit of copper appears (Zn displaces Cu). |
| (v) Dilute H2SO4 added to Pb(NO3)2 | A white precipitate of lead(II) tetraoxosulphate(VI), PbSO4, forms. |
(a) Laboratory equipment
- (i) Keeping salts dry: a desiccator (containing a drying agent such as anhydrous calcium chloride or silica gel).
- (ii) Converting vapour to liquid during distillation: a (Liebig) condenser.
- (iii) Bubbling a gas into a liquid: a gas-washing (Dreschel) bottle, or a delivery tube dipping into the liquid.
(b)(i) Purifying NaCl contaminated with NH4Cl
Use sublimation. On gentle heating, ammonium chloride sublimes (turns directly to vapour and re-solidifies on a cool surface), leaving pure sodium chloride behind, because NaCl does not sublime.
(b)(ii) Testing whether all the NH4Cl has been removed
Place a portion of the residue in a test tube, add sodium hydroxide solution and warm gently. Hold a piece of moist red litmus paper at the mouth of the tube.
- If a pungent gas that turns moist red litmus blue (ammonia) is evolved, some ammonium chloride is still present.
- If no ammonia smell and no colour change occur, all the ammonium chloride has been removed.
(c) Observations
| Exercise | Observation |
|---|
| (i) Methyl orange added to lime juice | Indicator turns red/pink (lime juice is acidic). |
| (ii) Conc. HNO3 added to acidified FeSO4 | Pale green solution turns yellow/brown as Fe2+ is oxidised to Fe3+ (a brown ring/colouration). |
| (iii) Fresh AgCl exposed to sunlight for 30 min | White precipitate gradually turns grey then black (photo-decomposition to silver metal). |
| (iv) Zinc dust added to dilute CuCl2 | Blue colour fades to colourless and a reddish-brown deposit of copper appears (Zn displaces Cu). |
| (v) Dilute H2SO4 added to Pb(NO3)2 | A white precipitate of lead(II) tetraoxosulphate(VI), PbSO4, forms. |
