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

Options

A. Curdling of milk
B. Formation of crystals by process of crystallisation
C. Fermentation of grapes
D. Digestion of food

Answer

To determine in which of the following processes the identity of the initial substance remains unchanged, we will analyze each option provided:

1. **Curdling of Milk**:
- In this process, milk is transformed into curd. The curd has different properties compared to milk, indicating that a chemical change has occurred. Therefore, the identity of the initial substance (milk) does not remain unchanged.

2. **Formation of Crystals by Crystallization**:
- Crystallization is a physical process where a solute (like CuSO4 in an aqueous solution) is transformed into its solid crystal form. The chemical composition of the substance remains the same; it is still CuSO4. Thus, the identity of the initial substance remains unchanged in this process.

3. **Fermentation of Grapes**:
- Fermentation involves the conversion of sugars in grapes into alcohol and carbon dioxide. This process changes the chemical properties and identity of the grapes, so the initial substance does not remain unchanged.

4. **Digestion of Food**:
- Digestion is a complex biochemical process that breaks down food into smaller molecules. This process alters the chemical structure and properties of the food, indicating that the identity of the initial substance does not remain unchanged.

Based on this analysis, the only process where the identity of the initial substance remains unchanged is:

**Answer: Formation of crystals by the process of crystallization.**