The temperature at which pure water boils is not fixed at one value everywhere. In practical thermodynamics, this boiling temperature depends mainly on which external factor?

Introduction / Context:
Boiling point is a key concept in thermodynamics and everyday science, often introduced with the example that water boils at 100 degrees Celsius. However, this famous value is true only under specific conditions. The boiling temperature of water changes with external pressure and is therefore different at high altitudes, in pressure cookers, and under vacuum. This question checks whether you know the primary factor that determines the boiling temperature of water, which is essential for understanding real life cooking, industrial processes, and phase diagrams.

Given Data / Assumptions:
• The substance considered is water. • We are talking about the temperature at which water boils. • The options mention air pressure, specific heat, density, and relative humidity. • We assume standard definitions of boiling point and atmospheric pressure.

Concept / Approach:
By definition, a liquid boils when its vapour pressure becomes equal to the external pressure acting on its surface. For water at sea level, this external pressure is approximately one standard atmosphere, leading to a boiling point of about 100 degrees Celsius. If the external pressure decreases, as it does at high altitudes, water boils at a lower temperature. In a pressure cooker, the internal pressure is higher than atmospheric, so water boils at a higher temperature. Specific heat describes how much energy is needed to raise the temperature of a unit mass of water by one degree, but it does not set the boiling temperature directly. Density changes slightly with temperature but is not the main controlling factor for boiling point. Relative humidity describes moisture in the air and has only a minor indirect effect compared to air pressure.

Step-by-Step Solution:
Step 1: Recall that boiling occurs when vapour pressure of the liquid equals the external pressure. Step 2: Recognise that external pressure is usually atmospheric pressure acting on the liquid surface. Step 3: Observe that at lower air pressure, the equality condition is reached at a lower temperature. Step 4: Observe that at higher pressure, such as in a pressure cooker, water must reach a higher temperature before boiling. Step 5: Conclude that the boiling temperature is mainly controlled by the value of external or air pressure. Step 6: Therefore, among the options given, air pressure is the correct controlling factor.

Verification / Alternative check:
Practical observations confirm this explanation. At high mountain locations, cooks notice that water boils at a lower temperature and food may take longer to cook. Pressure cookers are used to raise the boiling temperature to cook food faster. Phase diagrams in thermodynamics plot temperature against pressure and show a boiling curve where phase change occurs. These diagrams clearly indicate that boiling point is a function of pressure. Specific heat and density values may be used in energy calculations but do not determine the boiling temperature by themselves. Relative humidity mainly affects evaporation and comfort, not the precise boiling point under controlled conditions.

Why Other Options Are Wrong:
Option B, specific heat, affects how much energy is required to heat water but not the temperature at which boiling starts for a given pressure. Option C, density, is a result of temperature and pressure changes and does not independently set the boiling point. Option D, relative humidity, influences evaporation and condensation in the atmosphere but is not the primary factor in determining boiling point in a pot or boiler. Only external air pressure directly enters the condition vapour pressure equals external pressure, which defines boiling.

Common Pitfalls:
A common mistake is to memorise that water boils at 100 degrees Celsius without understanding the pressure dependence. Students may also confuse boiling with evaporation and believe that humidity strongly controls boiling, when in reality humidity mainly affects evaporation into air. Another pitfall is to mix up heating rate with boiling temperature, assuming that if you add more heat, the boiling point will change significantly. In reality, at fixed pressure, extra heat simply turns more liquid into vapour once boiling starts. Remember that changing pressure is the main way to change boiling temperature.

Final Answer:
The correct choice is The surrounding air pressure above the water surface, because boiling point is defined at the temperature where the vapour pressure of water equals the external pressure, which is mainly set by air pressure in normal conditions.