Try this microscale practical to explore the properties of elements in Groups 1 and 2 as they form various precipitates

In this experiment, students add drops of sullfate and carbonate solutions to Group 1 or 2 metal ions and see whether any precipitates form. They observe that no precipitates form in Group 1, indicating that Group 1 carbonates and sulfates are soluble, while the behaviour of Group 2 is more variable.

The practical should take approximately 20 minutes.

Equipment

Apparatus

  • Student worksheet (available for download as a PDF or editable Word document below)
  • Clear plastic sheet (eg ohp sheet)

Chemicals

Note

Solutions should be contained in plastic pipettes – see the accompanying guidance on apparatus and techniques for microscale chemistry.

  • Magnesium nitrate, 0.5 mol dm–3
  • Calcium nitrate, 0.5 mol dm–3
  • Strontium nitrate, 0.5 mol dm–3
  • Barium nitrate, 0.2 mol dm–3
  • Lithium bromide, 1 mol dm–3
  • Sodium chloride, 0.5 mol dm–3
  • Potassium bromide, 0.2 mol dm–3
  • Sodium carbonate, 0.5 mol dm–3
  • Sodium sulphate, 0.5 mol dm–3

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Wear eye protection throughout.
  • The following chemicals are skin/eye IRRITANTS:
    • Magnesium nitrate, MgNO3.6H2O(aq), 0.5 mol dm–3
    • Calcium nitrate, Ca(NO3)2.4H2O(aq), 0.5 mol dm–3
    • Strontium nitrate, Sr(NO3)2.4H2O(aq), 0.5 mol dm–3
    • Barium nitrate, Ba(NO3)2, 0.2 mol dm–3
    • Sodium carbonate, Na2CO3.10H2O, 0.5 mol dm–3
  • The following chemicals are low hazard:
    • Sodium sulphate, Na2SO4(aq), 0.5 mol dm–3
    • Sodium chloride, NaCl(aq), 0.5 mol dm–3
    • Lithium bromide, LiBr(aq), 1 mol dm–3
    • Potassium bromide, KBr(aq), 0.2 mol dm–3

Procedure

  1. Cover the worksheets with a clear plastic sheet.
  2. Put two drops of each of the metal ion solutions in each box of the appropriate row.
  3. Add two drops of each of the anion solutions to the appropriate columns.
  4. Observe and interpret your observations.

Teaching notes

Observations

There should be no precipitates in Group 1, indicating that all Group 1 carbonates and sulphates are soluble.

For Group 2, magnesium sulphate is soluble while strontium and barium sulphates are insoluble. Calcium sulphate is particularly interesting because although it is only sparingly soluble its solubility is much higher than is expected from the solubility product. This is due to ion pairing of the calcium and sulphate ions in aqueous solution. No precipitate will be seen.

The concepts of solubility product and ion pairing may be too complex for most pre-16 students.

Tips

Students might think that the Group 1 part of this experiment is rather dull. However, they can be told that chemistry experiments that seem to produce no visual results may nevertheless still produce useful information!

Students should also observe that all the precipitates are white not coloured. The accompanying solubility data will be useful.

Solubility data

The table below shows solubility in grams per 100 cm3 of water at 20 °C (except where indicated with a superscript).

 Carbonate Hydroxide Sulfate Fluoride 
Magnesium  0.0106  0.0009  73.8  0.007628 
Calcium  0.0014  0.185  0.209  0.001618 
Strontium  0.0011  0.41  0.0113  0.01227 
Barium  0.002  5.6  0.00022  0.1225 

Source: CRC handbook of chemistry and physics, 74th edn., 1993–4.

 

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