Properties of group 2 elements

In this experiment, students will be observing and interpreting the changes when drops of solutions of various anions are added to drops of solutions of group 2 element cations.

This practical should take 20 minutes. 

Equipment

Apparatus

• Student worksheet
• Clear plastic sheet (eg OHP sheet or clear plastic wallet)

Chemicals

• 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
• Sodium hydroxide, 1 mol dm^–3
• Sodium fluoride, 0.5 mol dm^–3
• Sodium chloride, 0.5 mol dm^–3
• Potassium bromide, 0.2 mol dm^–3
• Potassium iodide, 0.2 mol dm^–3
• Sodium carbonate, 0.5 mol dm^–3
• Sodium sulfate, 0.5 mol dm^–3

Health, safety and technical notes

• Read our standard health and safety guidance
• Students must wear splash resistant goggles to BS EN166 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 are CORROSIVE (see CLEAPSS Hazcards HC019b and HC019d)
• Sodium hydroxide solution, NaOH(aq), 1 mol dm^–3 is CORROSIVE (see CLEAPSS Hazcard HC091a).
• Magnesium nitrate,  MgNO₃.6H₂O(aq), 0.5 mol dm^–3 is an IRRITANT (see CLEAPSS Hazcard HC059b), 
• Sodium carbonate, Na₂CO₃.10H₂O, 0.5 mol dm^–3 is an IRRITANT (see CLEAPSS Hazcard HC095a).
• Sodium sulfate,  Na₂SO₄,0.5 mol dm^–3, Sodium chloride,  NaCl(aq), 0.5 mol dm^–3, Sodium fluoride, NaF(aq), 0.5 mol dm^–3, Potassium iodide, KI(aq), 0.2 mol dm^—3,  Barium nitrate,  Ba(NO3)2, 0.2 mol dm^–3 and Potassium bromide, KBr(aq), 0.2 mol dm^–3, are of low hazard (see CLEAPSS Hazcard HC098b, HC047b, HC089, HC011, HC047b).

Procedure

1. Cover the worksheet with a clear plastic sheet.

2. Put one drop of magnesium solution into each box in the magnesium ions row.

3. Repeat using calcium solution in the next row, then strontium solution in the next row and barium solution in the last row.

4. Add one drop of fluoride solution to each drop in the fluoride ions column. Observe what happens.

5. Repeat step 4 using each of the other solutions of anions in the subsequent columns. Observe each reaction carefully and record your observations.

Observations

Magnesium

No precipitates should be seen. All the compounds are colourless and soluble at these concentrations.

Calcium

An immediate white cloudiness is seen with the carbonate ions. No precipitates are seen with chloride, bromide or iodide but a cloudiness is seen with fluoride (due to its high lattice energy CaF₂ is insoluble).

Calcium hydroxide

This is clear at first but when left for a few minutes the drops become hazy as calcium carbonate is formed by absorbing carbon dioxide from the air: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Calcium sulphate also appears clear (the solubility product is not realised at these concentrations due, possibly, to ion-pairing).

Strontium

The sulfate and carbonate are insoluble, and a white cloudiness is seen. For the sulfate, the precipitate forms slowly. The halides are all soluble, except for the fluoride. The hydroxide is clear at first, but becomes hazy – similar to calcium.

Barium

The sulfate and carbonate give immediate white precipitates. The halides are soluble, except for the fluoride. The hydroxide is (like calcium and strontium) clear at first, becoming hazy due to the formation of barium carbonate.

Student questions

1. What explanations can you give for your observations?