Which of the following statements is/are correct?

Options

A. First law of thermodynamics can be seen as law of conervation of energy. So according to first law, total energy of a system always remains constant.
B. `pH` is an extensive property.
C. Third law of thermodynamics state that entropy of a chemically pure perfect crystal is zero at `0^(@)C`
D. Entropy of an isolated system always increases in an irreversible process.

Explanation

To determine which of the provided statements are correct, we will analyze each statement one by one based on the principles of thermodynamics.

### Step-by-Step Solution:

1. **Statement A: The first law of thermodynamics can be seen as the law of conservation of energy.**
- The first law of thermodynamics states that the total energy of an isolated system is constant. Energy can neither be created nor destroyed; it can only be transformed from one form to another. Therefore, this statement is **correct**.

2. **Statement B: pH is an extensive property.**
- Extensive properties depend on the amount of substance present, such as mass or volume. pH, however, is an intensive property, which means it does not depend on the amount of substance but rather on the nature of the solution. Therefore, this statement is **incorrect**.

3. **Statement C: The third law of thermodynamics states that the entropy of a chemically pure perfect crystal is zero at zero degree centigrade.**
- The third law of thermodynamics actually states that the entropy of a perfect crystal approaches zero as the temperature approaches absolute zero (0 Kelvin, which is -273.15 degrees Celsius). Since the statement incorrectly mentions "zero degree centigrade," it is **incorrect**.

4. **Statement D: Entropy of an isolated system always increases in an irreversible process.**
- According to the second law of thermodynamics, the entropy of an isolated system tends to increase over time, particularly during irreversible processes. This statement is **correct**.

### Conclusion:
- The correct statements are:
- **Statement A**: Correct
- **Statement B**: Incorrect
- **Statement C**: Incorrect
- **Statement D**: Correct

Thus, the correct statements are A and D.

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