International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What temperature indicates pure water based on its boiling point?

1. 0°C

2. 50°C

3. 100°C

4. 150°C

The correct answer is: 100°C

The boiling point of pure water at standard atmospheric pressure (1 atm or 101.3 kPa) is 100°C. This temperature is well-established in scientific literature and serves as a definitive reference for identifying pure water. Boiling point is a critical physical property of a substance, and for pure water, it reflects the temperature at which water transitions from a liquid to a gas. The significance of boiling point in indicating purity lies in the fact that impurities typically alter the boiling point of a substance. For example, if there are dissolved salts or other substances in the water, they can elevate the boiling point above 100°C, whereas the presence of certain other impurities might cause the boiling point to decrease. Therefore, a boiling point of exactly 100°C suggests that the water is indeed pure, as no impurities have influenced its boiling behavior. In contrast, temperatures like 0°C and 50°C correspond to other states or conditions of water, such as its freezing point or part of the temperature range before reaching boiling. Similarly, 150°C is significantly higher than the boiling point of pure water at standard atmospheric pressure, indicating that this temperature would suggest either a different substance or very high-pressure conditions, further confirming that only 100°C is indicative of