Hard Water kills Soap

Demonstration Experiment on Video

Hard Water kills Soap

Objective: The Influence of Hard Water on the Washing Process

Peter Keusch

German version

Supermarket product:
curd soap

Chemicals:
CaCl₂ · 6 H₂0

Glass wares:
gas collecting cylinder h = 20 cm, d = 5 cm
beaker 100 mL
Pasteur pipette

Experimental procedure:

Using a Pasteur pipette, some drops of aqueous calcium chloride solution are added to a cylinder containing an aqueous solution of curd soap.

Result:

A white precipitate is formed.

Video clip (Download RealPlayer .rm-Datei)

Discussion and background:

Calcium ions react with soap anions (e.g. stearate anions) to form sparingly soluble "calcium soaps" (e.g. calcium stearate)

Hard water contains a greater quantity of Ca ²⁺ and Mg ²⁺ ions. The reaction of the earth alkali ions with soap produces precipates which create an insoluble residue on fabrics and can also discolor the fabric. White fabrics may become grayed or yellowish and dingy. Hard water can also leave a film on hair and glasswares. Hardness of the water is very important in industrial uses, because it forms scale in heat exchange equipment, boilers, and pipe lines.

Soap is less effective in hard water. The amount of soap needed for the complete precipitation of Ca ²⁺ can not be used in the washing process. Moreover extra soap is required to disperse the precipitates in the suds. The harder the water, the more soap it takes to disperse the precipitates in the washing liquor.

Modern synthetic detergents have excellent resistance to hard water. They contain "builders" which minimize the effects of hard water. The most common "builder" used to be sodium trimetaphosphate. The phosphate keeps Ca ²⁺ and Mg ²⁺ ions in solution by forming a soluble complex with them, thus softening the water and preventing the formation of insoluble calcium soaps.

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