The tea with milk and lemon curdles due to the acidic nature of lemon.
Explanation:
Milk is made up of a number of proteins like casein. The proteins in milk are denatured by the addition of lemon juice. When lemon juice is added to tea or milk, the proteins such as the negatively charged casein become neutralized. Now, as no negative charge is present on them so they start to crumple together rather then pushing each other apart. Hence, the clumping causes the milk to curdle in tea.
