Precipitation reactions

 

• 4 test tubes with metal salt solutions (prepared by the teacher or you will receive salt solution you have to transfer in the test tubes (around 3 mL needed of each salt solution). Do not forget to label the test tubes).
• 1 M NaOH (~ 10 ml per group)
• Pipettes

 

1. Add about 0.5 mL of 1 M NaOH to each metal salt solution using the pipette.
2. For each metal salt, write down your observations on the following criteria:
   • Is a precipitate visible after adding 0.5 mL NaOH each time?
   • Color of precipitation
3. Repeat step 1 four more times and note which metal salts cause the precipitate to dissolve again by adding NaOH several times.

 

 

Hint 1:

Concentration gradients can falsify the result. To avoid concentration gradients, the test tubes should be closed with a stopper and shaken after each addition of NaOH.

Hint 2:

Precipitation is the settling out of a sparingly soluble solid from a solution. However, a fine solid distributed in the solution also counts as precipitate (see figure).

 

 

Compare your observations with the following solution variants. In which of the following variants are all three statements completely correct:

Solution A (code: 3)

• The aluminum salt precipitates blue-green. 
• For sodium chloride, no precipitation can be observed at any time.
• The precipitation of the iron salt remains even after multiple additions of NaOH.

Solution B (code: 7)

• The iron salt precipitates grey-green.
• No precipitation can be observed for the aluminum salt at any time.
• The precipitation of the manganese/barium salt* is white

Variante C (Code: 6)

• For sodium chloride, no precipitation can be observed at any time.
• The precipitate of the aluminum salt redissolves after several additions of NaOH.
• The precipitation of the manganese/barium salt* remains even after multiple additions of NaOH.

 

* You will be given either a manganese or a barium salt, depending on the availability at your school.