Which alkali is commonly known as lye and used in soap making?

Sodium hydroxide, also called lye or caustic soda, is a strong base used to make soap from fats and oils. It reacts with fats in a process called saponification.

When lye combines with oils or animal fats, it breaks down the fat molecules and creates soap plus glycerin. This chemical reaction has been used for thousands of years.

Lye is extremely caustic and can burn skin, so soap makers must handle it carefully. Modern soap production still uses this same basic chemical process.