Soap Test for Water Hardness

Different samples of water give various amount of foam and some turn cloudy.

Scientific name: Mineral salts dissolved in water react with sodium stearate from soap.


To Test for Water Hardness, Soap Test

by science4schools

Testing for Water Hardness

by Lisa Bronner

The Water Softener Soap Test

by TotalSoftWater


Wear eye protection goggles and gloves.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

calcium salt(aq) + sodium stearate (soap)(aq) → calcium stearate (scum)(s) + sodium salt(aq) Ca(HCO3)2 → CaCO3 + CO2 + H2O

Step-by-step instruction

  1. Soap test for water hardness is quite simple. All we need for this experiment is a test-tube with water sample and soap ethanol solution.
  2. Add a few drops of soap solution to the water sample.
  3. Seal the test-tube with a stopper and shake it vigorously.
  4. If there is a lot of soap foam and bubbles the water is soft, otherwise it’s - hard. The addition of soap to hard water samples leads to the formation of a cloudy precipitate.
  5. To demonstrate the difference in water softness/hardness it is useful to prepare a few samples such as rain water, tap water and sea water or their substitutes. It is also interesting to show away how to make hard water softer.
  6. For the model experiment we need to prepare temporarily and permanently hard water. The temporarily hard water is the solution of calcium hydrocarbonate. To obtain this solution bubble carbon dioxide through the solution of calcium hydroxide till the milky-white solution of calcium hydroxide does not clear. As permanently hard water the solution of calcium sulfate is used.
  7. Deionized water could simulate rain water, temporarily hard water – tap water. A mixture of temporarily and permanently hard water in ratio 1:1 could be considered as sea water.
  8. Pour the prepared water samples into the test tubes. Place them in the following order: rain water (or deionized water), tap water (or temporarily hard water), sea water (mixture of temporarily and permanent hard water), hard water.
  9. Add an equal amount of soap solution to each test tube and shake them vigorously. Observe the difference in foam formation and increase of turbidity in a row.
  10. Fill the beaker with tap water (temporarily hard water).
  11. Heat it to boil. After a few minutes turn off the heating and leave the sample to cool down.
  12. Repeat the soap test. The amount of foam increases and solution is not as cloudy as it was earlier. The water becomes softer.

Scientific background

Hard water and soft water differ in content of mineral salts. The less the concentration of mineral salts the water comprises the softer it is. The main components which form water hardness are dissolved calcium and magnesium salts. Soap contains sodium stearate which reacts with calcium and magnesium salts forming insoluble stearates. Presence of calcium and magnesium stearates turns the solution cloudy. The effect of soap addition becomes more pronounced with the increase of water hardness. It is possible to lower water hardness by boiling since some mineral salts such as calcium hydrocarbonate decompose under heating.


Published on 05 August 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst