Bittern, also known as bittern, is a salt solution such as magnesium chloride and calcium sulfate. In the process of making tofu, adding bittern can solidify protein, thus forming tofu. In this process, magnesium ions and calcium ions in bittern react with protein in soybean, which leads to the cross-linking reaction of protein, thus forming tofu.

From the physical point of view, the process of adding bittern did not change the original material form, but solidified tofu through chemical reaction. Therefore, this process is closer to the phase transition in physical change, that is, the process of liquid transforming into solid.

Generally speaking, adding bittern to make tofu has both chemical and physical changes. Chemical change mainly refers to the cross-linking reaction of protein, while physical change mainly refers to the phase change of liquid.