The boiling point of a liquid does not have one fixed universal value but changes mainly as which external condition is varied?

Introduction / Context:
Boiling point is a key concept in thermodynamics and everyday science. We often hear that water boils at 100 degree Celsius, but this value is not universal. It applies only at a specific pressure, namely one standard atmosphere. In real situations such as high altitude locations or pressure cookers, the temperature at which a liquid boils changes. This question asks you to identify the main external factor that determines how the boiling point of a liquid varies, which is crucial for understanding cooking, industrial processes, and phase diagrams.

Given Data / Assumptions:
• We are considering the boiling point of a generic liquid. • The options include pressure, temperature of the surroundings, volume of the liquid, and density. • Standard definitions of boiling point and vapour pressure are assumed. • The liquid is heated until its vapour forms bubbles throughout the bulk of the liquid.

Concept / Approach:
A liquid boils when its vapour pressure becomes equal to the external pressure acting on its surface. Vapour pressure is the pressure exerted by the vapour in equilibrium with the liquid at a given temperature. If the external pressure is higher, the liquid must reach a higher temperature for its vapour pressure to match this value, so the boiling point rises. If the external pressure is lower, as at high altitudes, the vapour pressure equals the external pressure at a lower temperature and the boiling point falls. Volume of liquid and density do not directly determine the boiling point, and the ambient temperature alone is not the fundamental controlling variable; it is the pressure condition that matters most.

Step-by-Step Solution:
Step 1: Recall the definition of boiling point as the temperature at which vapour pressure of the liquid equals external pressure. Step 2: Recognise that external pressure is usually atmospheric pressure for open containers. Step 3: Imagine moving to a high mountain, where atmospheric pressure is lower; the liquid boils at a lower temperature. Step 4: Imagine using a pressure cooker, where pressure is deliberately increased; the liquid then boils at a higher temperature. Step 5: Note that changing the amount of liquid or its density does not fundamentally change the vapour pressure dependence on temperature. Step 6: Conclude that external pressure is the main factor that makes boiling point vary.

Verification / Alternative check:
This explanation is supported by phase diagrams, which plot temperature versus pressure for a substance and show a boiling curve where liquid and vapour phases coexist. At each pressure on this curve, there is a corresponding boiling temperature. Practical experience also confirms this: cooking times can be longer at high altitudes because water boils at a lower temperature, and pressure cookers are used to raise boiling temperatures for faster cooking. Thermodynamic relations such as the Clausius Clapeyron equation further quantify how boiling point changes with pressure. All these sources agree that pressure is the primary variable controlling boiling point.

Why Other Options Are Wrong:
Option B, temperature of the surrounding air, may influence heat transfer rate but does not define the boiling point; the boiling temperature is determined by equality of vapour pressure and external pressure, not by ambient air temperature alone. Option C, volume of the liquid, affects how long it takes to reach boiling but not the temperature at which boiling occurs for a given pressure. Option D, density, changes slightly with temperature and pressure but does not directly set the boiling point. Thus, none of these options correctly captures the fundamental control of boiling point.

Common Pitfalls:
Students often memorise that water boils at 100 degree Celsius without attaching the crucial phrase at one atmosphere pressure. This can lead to confusion when studying high altitude cooking or industrial boilers. Another pitfall is to think that if you apply more heat, the boiling temperature will keep rising, but at fixed pressure, additional heat simply converts more liquid to vapour once boiling has started. Remembering that boiling point is defined via vapour pressure equalling external pressure helps avoid these conceptual errors.

Final Answer:
The correct choice is External pressure acting on the liquid surface, because boiling point varies mainly with pressure, and liquids boil when their vapour pressure equals the external pressure imposed on them.