i) When a protein in its native is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are distrubed, the 2° and 3° structures change and the protein loses its biological activity. This is called denaturation of the protein.
ii) 1) When egg is boiled, it becomes hard because the soluble globular proteins change to in-soluble brous proteins.
2) Curdling of milk which is caused due to the formation of lactic acid by the bacteria present in milk.