**Correct Option: A. the temperature at which its vapour pressure equals the atmospheric pressure** ### Detailed Explanation: 1. **Understanding Boiling Point**: - The boiling point of a liquid is a critical concept in chemistry. It is the temperature at which a liquid changes into a gas (vapor). This transition occurs when the vapor pressure of the liquid equals the external atmospheric pressure. 2. **Vapor Pressure**: - Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid (or solid) phase at a given temperature. As the temperature of a liquid increases, its molecules gain kinetic energy, leading to an increase in vapor pressure. 3. **Normal Boiling Point**: - The term "normal boiling point" specifically refers to the boiling point of a liquid at 1 atmosphere (atm) of pressure, which is the standard atmospheric pressure at sea level. At this point, the vapor pressure of the liquid equals the atmospheric pressure, allowing bubbles of vapor to form throughout the liquid, leading to boiling. 4. **Why Option A is Correct**: - Option A states that the normal boiling point is "the temperature at which its vapour pressure equals the atmospheric pressure." This is the precise definition of the normal boiling point, making it the correct answer. ### Analysis of Other Options: - **Option B: the temperature at which bubbles begin to form**: - This option is misleading. While bubbles do form when a liquid boils, they begin to form when the vapor pressure of the liquid is equal to the atmospheric pressure, not at a specific temperature. Therefore, this definition is incomplete and does not accurately describe the normal boiling point. - **Option C: the temperature at which the vapour pressure equals 1 temperature**: - This option is incorrect because it is vague and does not provide a clear definition. The phrase "1 temperature" is ambiguous and does not specify the pressure condition (1 atm). The boiling point is defined in relation to pressure, not just temperature. - **Option D: the temperature at which the rate of condensation of vapour equals the rate of vaporisation of the liquid**: - While this statement is true in a general sense (at equilibrium, the rate of condensation equals the rate of vaporization), it does not specifically define the normal boiling point. This condition can occur at various temperatures and pressures, not just at the boiling point. Therefore, it is not a precise definition. ### Summary of Key Points: - The normal boiling point is defined as the temperature at which the vapor pressure of a liquid equals the atmospheric pressure (1 atm). - Vapor pressure increases with temperature; boiling occurs when this pressure matches the external pressure. - Option A is the only option that accurately defines the normal boiling point. - Understanding the relationship between vapor pressure and boiling point is crucial for grasping concepts in physical chemistry. ### Revision Summary: - The normal boiling point is when vapor pressure equals atmospheric pressure (1 atm). - Boiling occurs when vapor pressure matches external pressure, allowing bubbles to form. - Option A is correct; other options are either vague or incorrect. - Familiarize yourself with the concepts of vapor pressure and boiling to avoid common pitfalls in definitions.