How to Test for Calcium Hydroxide

Calcium hydroxide, commonly called slaked lime, is an inorganic compound with the chemical formula Ca(OH)2. This compound is a base and is widely used in industry--for example, as a cleaning agent. Identifying calcium hydroxide might be a chemistry class assignment that requires two tests. The first one measures the pH (acidity) of the calcium hydroxide solution to ensure it is basic. The second test, using a dilute solution of sulphuric acid, determines the presence of calcium ions.

Things You'll Need

• Beaker
• pH paper
• Plastic pipette
• Sulphuric acid solution (5 percent)

Instructions

1. Pour about 5 milliliters of the solution to be tested into the beaker.

2. Dip the strip of the pH paper strip into the solution and then take it out. The pH paper will change its color.

3. Compare the color of the paper with the scale enclosed with the pH paper to assign the pH of the solution. A basic pH value such as 10 to 11 is indicative for calcium hydroxide.

4. Fill a pipette with the sulphuric acid solution.

5. Add five to ten drops of the solution from the pipette to the beaker.

6. Observe the solution in the beaker: If a white precipitate forms, it indicates the presence of calcium ions; this reaction follows the equation Ca(OH)2 + H2SO4 = CaSO4 (precipitate) + 2H2O.

Tips & Warnings

• Wear gloves and goggles when handling hydrochloric acid.

References

• Fun Science: Experiments with Acids and Bases
• Material Safety Data Sheet: Calcium hydroxide
• ThinkQuest: Common Laboratory Tests