Which of the following statements are true ?

(A) Crystallisation is a chemical change.

(B) Photosynthesis is a chemical change.

(C) Ripening of fruit is a physical change.

(D) Formation of curd from milk is a chemical change.

The correct option is (B) and (D).

To determine which statements are true, let us evaluate each statement individually by understanding the difference between physical and chemical changes:

(A) Crystallisation is a chemical change:
Crystallisation is a process where solid crystals precipitate from a solution. Since no new chemical substance is formed and it is a physical technique used for purification, crystallisation is a physical change. Therefore, statement (A) is false.

(B) Photosynthesis is a chemical change:
During photosynthesis, green plants synthesize glucose and oxygen from carbon dioxide and water using sunlight. Since new chemical substances with completely different properties are produced, photosynthesis is a chemical change. Therefore, statement (B) is true.

(C) Ripening of fruit is a physical change:
Ripening involves chemical reactions that alter the fruit's color, aroma, texture, and taste due to changes in organic acids and sugars. Since these changes produce new chemical substances and are irreversible, ripening of fruit is a chemical change. Therefore, statement (C) is false.

(D) Formation of curd from milk is a chemical change:
When milk turns into curd, lactic acid bacteria convert lactose into lactic acid, producing a new substance with different chemical properties that cannot be converted back to milk. Thus, the formation of curd is a chemical change. Therefore, statement (D) is true.

Hence, statements (B) and (D) are true.