In which of the following, the identity of initial substance remains unchanged?

According to the NCERT text, whenever a chemical change occurs, a chemical reaction takes place, and the nature and identity of the initial substance change. Curdling of milk, fermentation of grapes, and digestion of food are all chemical changes where new substances are formed, changing the identity of the starting materials. In contrast, the formation of crystals by the process of crystallisation is a physical change. During crystallisation, only the physical state and arrangement of particles change to form pure crystals, while the chemical identity of the substance remains completely unchanged.