I. A liquid boils when its saturated vapor pressure is equal to the external pressure II. Dissolved substances…

Step 1: Analyze the question We must decide which of the four statements are true about the boiling point of a liquid, then pick the option that contains exactly the true ones.

Step 2: Check each statement

• I. A liquid boils when its saturated vapour pressure equals the external pressure. TRUE. This is the definition of boiling point.
• II. Dissolved substances in pure water increase the boiling point. TRUE. A non-volatile solute lowers the vapour pressure of the solvent. The liquid must be heated to a higher temperature before its vapour pressure reaches the external pressure. This is called boiling point elevation. For example, salty water boils at a slightly higher temperature than pure water.
• III. Increasing the external pressure increases the boiling point. TRUE. A higher external pressure means the vapour pressure must reach a higher value, which needs a higher temperature. This is how a pressure cooker works.
• IV. Dissolved substances decrease the boiling point. FALSE. It contradicts II. Impurities (non-volatile solutes) raise the boiling point.

Step 3: Combine The true statements are I, II and III only, and IV is false, so options B, C and D (which each include IV) are all wrong.

Answer: A. I, II and III only

Tip: For boiling point, remember "Pressure up, boiling point up. Impurities up, boiling point up." For melting/freezing point, impurities lower it. Many students mix these up in exams, so memorize the contrast: impurities raise the boiling point but lower the freezing point.