How to Test for Calcium Hydroxide

Written by oxana fox
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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.

Skill level:
Moderately Easy

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Things you need

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

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Instructions

  1. 1

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

  2. 2

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

  3. 3

    Compare the colour 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. 4

    Fill a pipette with the sulphuric acid solution.

  5. 5

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

  6. 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 and warnings

  • Wear gloves and goggles when handling hydrochloric acid.

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