Scale is a hard, grayish-white or yellowish-white deposit commonly found on the inner surfaces of kettles, water heaters, and poorly maintained hot-water systems. Similar deposits can form inside old water pipes, on faucets, shower surfaces, and other areas where hard water evaporates.

The type of scale varies depending on the water chemistry and the surface where it forms.

The main component of scale deposited on heating elements in water heaters and similar appliances is often calcium carbonate, which can precipitate when hard water is heated. Hard water contains dissolved calcium and magnesium, which are the principal minerals responsible for water hardness.

Calcium carbonate is less soluble under many conditions associated with heating, which can cause mineral deposits to form on heated surfaces. As water is heated and some of it evaporates, dissolved minerals can also become more concentrated, increasing the potential for precipitation and scale formation.

Calcium and magnesium in hard water can also react with traditional soap, producing an insoluble material commonly known as soap scum. This residue can deposit on bathtubs, sinks, shower walls, glass, and other surfaces. Hard water can also make it more difficult for soap to lather and may require greater amounts of soap or detergent.

Soap scum can remain on the skin and hair after washing, although the effects can vary from person to person. Hard water and soap residues may contribute to a feeling of dryness or irritation in some individuals, but hard water itself should not be described as a proven cause of skin inflammation or itching.

Traditional soaps typically contain salts of fatty acids. Calcium and magnesium can react with these soap molecules to form relatively insoluble calcium and magnesium soaps, producing the familiar soap scum.

The deposits found on cookware, faucets, shower fixtures, and bathroom tiles are generally composed of minerals from the water, particularly calcium and magnesium compounds. Their exact composition depends on the water chemistry and conditions under which the water evaporates or is heated. These deposits should not generally be described as sodium carbonate.

By Michael Chiu, MWS, CI
Previously published in Ming Pao Daily News – Updated and Revised in 2026