This is a normal phenomenon. Water contains impurities such as calcium and magnesium, which precipitate at the bottom of the pot after heating, so there are some small spots similar to mildew spots. With the increase of boiling times, this phenomenon becomes more and more serious until a thick layer of "scale" appears. Brushing more and more is an illusion, but it is becoming more and more obvious.

In order to remove scale, vinegar can usually be used. Because scale is carbonate, it can be dissolved by acidic substances. Just wait in the kettle with some acetic acid. If you want to avoid scale, you can use pure water if possible. It is suggested to boil the water with vinegar first, then pour it out for 15 minutes, and then use the pot again. If there are black spots, wash them with citric acid, which is available in general chemical stores.

Proportion:

Scale, commonly known as "water rust, water alkali", refers to the white block or powdery substance that the minerals contained in hard water attach to the container after boiling.

The main components are calcium carbonate, magnesium hydroxide, magnesium carbonate, calcium sulfate, magnesium sulfate, calcium chloride and magnesium chloride. Scale has poor thermal conductivity. If the scale formed in the boiler is too thick, it will reduce the efficiency of the boiler, and even cause the boiler to burst and cause boiler accidents.