The change in which the shape , size, appearance or state of a subtance may alter but its chemical composition remains the same is called a physical change . In a physical change, no new substance is formed. Any change that involves the formation of a new substance and leads to a transformation of chemical identity is called a chemical change.
Chemical changes are usually accompanied with heat exchanges. Chemical changes are permanent changes which are usually irreversible.
Which of the following is always true when a substance undergoes a physical change ?

Options

A. It changes colour.
B. A new substance is formed
C. It boils
D. Its composition remains the same.

Explanation

To determine which statement is always true when a substance undergoes a physical change, we can analyze the characteristics of physical changes step by step.

### Step-by-Step Solution:

1. **Definition of Physical Change**:
- A physical change is defined as a change in which the shape, size, appearance, or state of a substance may alter, but its chemical composition remains the same.

2. **Characteristics of Physical Change**:
- In a physical change, no new substance is formed.
- The molecular composition of the substance remains constant despite changes in state or appearance.

3. **Examples of Physical Change**:
- When ice melts to form water, the state changes from solid to liquid, but the chemical composition (H₂O) remains unchanged.
- When water evaporates to form steam, the state changes from liquid to gas, but the molecular composition (H₂O) is still the same.

4. **Conclusion**:
- The key point is that during a physical change, the molecular composition of the substance does not change. Therefore, the statement that is always true when a substance undergoes a physical change is that its chemical composition remains the same.

### Final Answer:
The correct statement is: "The chemical composition of the substance remains the same."

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