This experiment illustrates the periodicity of the alkaline earth metals, Group 2 (remember that a group on the periodic table is the same thing as a column). You will be looking for the formation of a precipitate when two solutions are mixed.  Concentrations are very important so use exactly the number of drops suggested.

PURPOSE

• To relate the formation of precipitates to position in the periodic table.
• To identify an unknown metal ion.

(1) Transparent plastic sheet

Various solutions with droppers

The data table

• The solution containing the chromate ion, CrO4^2-, contains hexavalent chromium, which is a known animal carcinogen.  Avoid skin contamination.  You may wish to use gloves.
• Goggles must be worn, and gloves are available.

1. Put a transparent plastic sheet over the data table.  You will mix solutions on the transparency.  Use one partner's copy of the data table to record your results.
2. Add a drop of the solution containing magnesium ions to each of the empty squares in the first horizontal row.  Repeat with the calcium ions in the second row, strontium in the third, bariumin the fourth and the unknown in the last row.  The order is not important.
3. Add a drop of sulfate solution to each of the solutions in the squares in the vertical column labeled SO4^2-. Continue with hydroxide, chromate, and carbonate solutions in their respective columns.
4. Record the appearance of precipitates.  Gently slide the data table from under the transparency, and record the presence of any precipitates you did not see before.
5. Identify the unknown by comparing your results to those of the known ions.
6. When you have finished the experiment, dispose of the chemicals according to your teacher's instructions.  Rinse and dry the transparency, and wash your hands.

DATA ANALYSIS

    1.    The formula of the unknown is ____________________.
 
 
 

    2.    This is called ____________________.
 
 
 

    3.    Which Group 2 ion forms thc most precipitates?
 
 
 

    4.    Which Group 2 ion is the largest?
 
 
 

    5.    What can you conclude about the size of an ion and the number of precipitates formed with reagents in this experiment?
 
 
 
 

    6.    Write the correct name and formula for each observed precipitate.
 
 
 
 

    7.    Suggest a process to physically separate Mg²⁺ and Ba²⁺ ions from a solution containing both ions.
 
 
 
 
 
 
 

Adapted from WINGS OF LEARNING c1990
[Group 2 Precipitates score sheet][MHS Chem page]