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Analyse the make-up of a Compound

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A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Look up the following tests for cations, anions, gases, hydrocarbons etc. if necessary:
Identify Cations
Identify Anions
Collect and Identify Gases
Identify Hydrocarbons and Others

Questions
1. How would you test for ammonium ions?
2. How would you test for carbonate ions?

Answers
  • Show Solutions
    1. Add aqueous sodium hydroxide. Ammonia is produced that will turn damp litmus paper blue.
    2. Add dilute acid. Carbon dioxide is produced which is bubbled through limewater. The limewater will turn milky.
Method:
1. Place a little ammonium carbonate in a boiling tube.
2. Heat using the blue flame of a Bunsen burner until no further change takes place.
Questions
What did you observe when you heated the first sample of ammonium carbonate?

Answers
  • Show Solutions
    1. Effervescence is observed and fizzing is heard.
    2. Droplets of a clear, colourless liquid form at the top of the test tube.
    3. The solid ammonium carbonate disappears.



Place another sample of ammonium carbonate in the tube.
1. Heat the tube gently and hold a piece of moist red litmus paper next to the delivery tube.
2. Continue heating and use the delivery tube to bubble the gas produced through limewater.
3. Stop heating and test any liquid at the top of the boiling tube with cobalt chloride paper. Questions
1. What happened to the litmus paper?
2. What conclusion can you draw?
3. What happened to the limewater?
4. What conclusions can you draw?
5. What happened to the cobalt chloride paper?
6. What conclusions can you draw?
7. Write an equation to show what happens when ammonium carbonate is heated.
8. How does this explain your observations in the first part of the experiment?

Heating ammonium chloride When ammonium chloride is heated, the following reaction takes place:
NH4Cl ⇆ NH3 + HCl
ammonium chloride ⇆ ammonia + hydrogen chloride

The white solid (ammonium chloride) changes to gas and then re-forms at the top of the tube as a white solid.
9. What does the symbol ⇆ mean?


Answers
  • Show Solutions
    1. The litmus paper turn blue.
    2. A basic gas is produced which in this case is ammonia gas.
    3. Limewater turned milky.
    4. Carbon dioxide gas was produced.
    5. Cobalt chloride paper changed from blue to pink.
    6. This indicates the presence of water.
    7. Ammonium carbonate → ammonia + carbon dioxide + water
    (NH4)2CO3(s) → 2 NH3(g) + CO2(g) + H2O(g)
    8. The effervescence and fizzing is due to the release of ammonia gas and carbon dioxide gas.
    The clear, colourless liquid is condensed water vapour.
    The solid disappears because the ammonium carbonate decomposes into ammonia, carbon dioxide gas and water vapour.
    9. The symbol ⇆ means that the reaction is reversible.

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