Which of the following statements is/are correct?
I. In neutralisation reaction, heat is always given out.
II. Phenolphthalein remains colourless in a window cleaner solution.
III. Tooth decay is caused by the presence of calcium salts.
IV. To dilute an acid, we should always add water to the acid.

Options

A. I and Ill only
B. III only
C. I only
D. I, II, III and IV

Explanation

To determine which of the statements is/are correct, let's analyze each statement one by one.

### Step 1: Analyze Statement I
**Statement I:** In a neutralization reaction, heat is always given out.

- Neutralization reactions occur between acids and bases to produce salt and water.
- These reactions are typically exothermic, meaning they release heat.
- Therefore, **Statement I is correct.**

### Step 2: Analyze Statement II
**Statement II:** Phenolphthalein remains colorless in a window cleaner solution.

- Phenolphthalein is an acid-base indicator that is colorless in acidic solutions and turns pink in basic solutions.
- Most window cleaners are basic in nature, which means phenolphthalein would turn pink.
- Therefore, **Statement II is incorrect.**

### Step 3: Analyze Statement III
**Statement III:** Tooth decay is caused by the presence of calcium salts.

- Tooth decay is primarily caused by acids produced by bacteria that feed on sugars, leading to the demineralization of tooth enamel.
- Calcium salts are actually important for tooth structure and health, not the cause of decay.
- Therefore, **Statement III is incorrect.**

### Step 4: Analyze Statement IV
**Statement IV:** To dilute an acid, we should always add water to the acid.

- The correct procedure for diluting an acid is to add acid to water, not the other way around. This is to prevent the acid from splattering due to the exothermic reaction that occurs when water is added to concentrated acid.
- Therefore, **Statement IV is incorrect.**

### Conclusion
After analyzing all statements:
- **Statement I is correct.**
- **Statements II, III, and IV are incorrect.**

Thus, the correct answer is **only Statement I is correct.**