Through the process of saponification, fats (like tallow, pig, and bone fats) or vegetable oils react with sodium hydroxide to form the sodium salts of fatty acids and glycerin. The resulting mixture is known as soft soap, which serves as a precursor for hard soap production. After adding sodium chloride (a process known as salting out), the soap nucleus rises and separates. The water-soluble glycerin and unwanted fat residues remain in the solution (see also soap).