Why does a kettle develop a white crust?
Heating hard water can cause dissolved substances to form solid calcium carbonate deposits. Water can look completely clear before those mineral deposits appear.
- The scientific idea
- Hard water
- Fields of science
- Chemistry
How it works
Clear water can leave a solid record behind. When hard water is heated, calcium carbonate deposits can form on a kettle or water heater. The minerals were already present in dissolved form, even though the water looked transparent.
Look closer
At the sink, hardness has another effect: calcium and magnesium can react with soap to form poorly soluble compounds. Some of the soap becomes the residue often called soap scum. A kettle's mineral scale and a bathtub's soap residue are therefore related clues, but different materials. Synthetic detergents can behave differently from traditional soap in hard water. Understanding the dissolved ingredients helps explain why washing can change from one water supply to another.