It involves the formation of a new, irreversible substance.
Explanation
Changes in matter are classified into physical and chemical changes. A physical change involves a change in physical properties (such as state, shape, or size) without altering the chemical composition. A chemical change (or chemical reaction) involves the transformation of the original substance into a new substance with different chemical properties, and it is generally irreversible.
Detailed Analysis:
- It involves a change in the state of matter. is Incorrect: A change in the state of matter (e.g., liquid to solid) can occur in physical changes as well, such as water freezing into ice. Therefore, a change in state alone is not the definitive basis for classifying a process as a chemical change.
- It involves the formation of a new, irreversible substance. is Correct: The setting of curd is a chemical change because it involves the fermentation of milk sugar (lactose) by Lactobacillus bacteria to form lactic acid. This acid causes the coagulation of milk protein (casein), resulting in a new substance (curd) with different taste and chemical properties. This process is irreversible; curd cannot be converted back into milk.
- It is an endothermic physical process. is Incorrect: The setting of curd is a chemical process, not a physical one. Furthermore, fermentation is typically an exothermic process (releases heat), not endothermic.
- It results in a decrease in total volume. is Incorrect: While density changes may occur, a decrease in total volume is not the fundamental criterion used to define a chemical change.
Key Takeaway:
A process is classified as a chemical change if it results in the formation of a new substance with distinct chemical properties and is irreversible under normal conditions.