Hard water

Hard water is water that contains dissolved calcium and magnesium ions. These ions react with soap to form scum but do not affect detergents. Malay: air liat · Chinese: 硬水.

Water picks up calcium and magnesium ions as it flows over rocks such as limestone. When soap is used in this water, the calcium and magnesium ions react with the soap and turn it into an insoluble scum, so the soap is wasted and does not lather well. A detergent avoids this problem because its calcium and magnesium salts are soluble, so a detergent keeps cleaning in hard water. This single behaviour, scum with soap but not with detergent, is the reason hard water matters in this chapter.

Example. In a limestone area the tap water is hard. A bar of soap used there produces little lather and leaves a grey ring on the bath, while a detergent-based shower gel lathers normally. Boiling can remove some kinds of hardness by driving off the dissolved gas, but the ions from other sources remain until they are softened chemically.

Confusion to avoid. Do not confuse hard water with dirty water. Hard water can be perfectly clean and clear; the “hardness” refers only to the dissolved calcium and magnesium ions, not to any dirt. Also remember that hard water troubles soap, not detergent, so do not blame a detergent for scum.

Hard water links soap, scum and the advantage of detergents, so it is central to the soap-versus-detergent comparison that appears so often in SPM Chemistry questions for this chapter. When you answer, name the two ions responsible, calcium and magnesium, state that they react with soap to form insoluble scum, and finish by noting that detergents are unaffected because their calcium and magnesium salts are soluble. Setting the three points out in that order gives a clear, complete explanation rather than a vague statement that soap “does not work” in hard water.