(1) The setting of curd is a chemical change because a new substance(lactic acid) is formed.
(2) Once the curd is formed, milk cannot be re-obtained from it. It is an irreversible change.
(3) Also, both milk and curd have different properties. The new substance, i.e. curd is different from the milk in taste, smell, and chemical properties.
(4) Since these are the properties of a chemical change, hence, the setting of curd is a chemical change.