The length of the foam (lather) will be the longest in test tube A, which contains only distilled water. This is because lather forms easily in soft water. Distilled water is soft water i.e. it doesn’t contain any salts and hence lather will form easily in test tube A.
Tube B and C contain dissolved salts making them hard waters. They react with soap solution and form scum because of the calcium and magnesium salts with little lather.
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