The correct option is **A. I, II and III**. Let's break down each statement to understand why this is the case and clarify the concepts involved. ### Statement Analysis **I. A liquid boils when its saturated vapour pressure is equal to the external pressure.** - **Explanation:** This statement is true. Boiling occurs when the vapor pressure of a liquid equals the external pressure acting on the liquid. At this point, bubbles of vapor can form within the liquid and rise to the surface, leading to boiling. This is a fundamental principle of phase changes and is crucial in understanding boiling point behavior. **II. Dissolved substances in pure water lead to an increase in the boiling point.** - **Explanation:** This statement is also true. When a non-volatile solute (like salt or sugar) is dissolved in water, it raises the boiling point of the solution compared to pure water. This phenomenon is known as boiling point elevation, which occurs because the presence of solute particles disrupts the formation of vapor bubbles, requiring a higher temperature to achieve the vapor pressure necessary for boiling. **III. When the external pressure is increased, the boiling point increases.** - **Explanation:** This statement is true as well. Increasing the external pressure raises the boiling point of a liquid. This is because a higher pressure requires the liquid to reach a higher temperature for its vapor pressure to equal the external pressure. This principle is why pressure cookers can cook food faster; they increase the pressure, which raises the boiling point of water. **IV. Dissolved substances in pure water decrease the boiling point.** - **Explanation:** This statement is false. As mentioned in the explanation for statement II, dissolved substances actually increase the boiling point of the solution. This is a common misconception; the presence of solute particles interferes with the ability of the liquid to vaporize, thus requiring a higher temperature to boil. ### Summary of Why Other Options Are Incorrect - **Option B (I, II, III, and IV):** This option is incorrect because it includes statement IV, which is false. - **Option C (I, II, and IV):** This option is incorrect because it includes statement IV, which is false. - **Option D (II, III, and IV):** This option is incorrect because it includes statement IV, which is false, and it omits statement I, which is true. ### Common Pitfalls - Confusing boiling point elevation with boiling point depression. Remember that adding solutes raises the boiling point, while removing solutes (like in a vacuum) can lower it. - Misunderstanding the relationship between pressure and boiling point. Higher external pressure means a higher boiling point, which is counterintuitive for some students. ### Revision Summary - Boiling occurs when vapor pressure equals external pressure (Statement I). - Dissolved substances increase the boiling point of a liquid (Statement II). - Increased external pressure raises the boiling point (Statement III). - Remember that dissolved substances do not decrease the boiling point (Statement IV is false). By understanding these principles, you can better grasp the behavior of liquids under different conditions, which is essential for many applications in physics and chemistry.