At standard atmospheric pressure (about 760 mmHg), what is the freezing point of pure water when expressed on the Kelvin scale?

Introduction / Context:
Temperature scales such as Celsius and Kelvin are used throughout physics and chemistry. Knowing key reference points, such as the freezing and boiling points of water, on both scales is essential for converting temperatures and understanding thermodynamic data. This question asks specifically for the freezing point of water at standard atmospheric pressure when expressed in Kelvin.

Given Data / Assumptions:

• Standard atmospheric pressure is approximately 760 mmHg (1 atm).
• Pure water freezes at 0 degrees Celsius at this pressure.
• The Kelvin scale starts at absolute zero, with 0°C corresponding to 273 K approximately.
• The question requires the freezing point in Kelvin, not in degrees Celsius.

Concept / Approach:
The Celsius and Kelvin scales are related by a simple offset. The relationship is T(K) = T(°C) + 273 (more precisely, 273.15). The freezing point of water at 1 atm is defined as 0°C on the Celsius scale. Adding 273 gives the equivalent temperature in Kelvin. Therefore, the freezing point is about 273 K. In most school level chemistry, 273 K is accepted as the reference value for the freezing point of water.

Step-by-Step Solution:
Step 1: Recall the standard freezing point of pure water: 0°C at 1 atm (760 mmHg).Step 2: Use the conversion formula between Celsius and Kelvin: T(K) = T(°C) + 273.Step 3: Substitute T(°C) = 0 into the formula.Step 4: T(K) = 0 + 273 = 273 K.Step 5: Check the options and identify 273 K as the correct value.

Verification / Alternative check:
Similarly, the boiling point of water at 1 atm is 100°C, which corresponds to about 373 K. The difference between freezing and boiling points is 100 degrees on both scales, because Kelvin and Celsius have the same step size, only a different zero point. This consistency further confirms that 0°C must correspond to 273 K and not to any of the other values given in the options.

Why Other Options Are Wrong:
Option a, 0 K, is absolute zero, the lowest possible temperature, not the freezing point of water. Option c, 273 C, confuses the scale and implies an extremely high temperature, hotter than boiling water. Option d, minus 273 C, is extremely cold and corresponds roughly to 0 K, again not the freezing point. Option e, 100 K, is much colder than water's freezing point and would correspond to about minus 173°C, which is not correct for the freezing of water at standard pressure.

Common Pitfalls:
Students sometimes forget whether they should add or subtract 273 when converting between Celsius and Kelvin. Another mistake is to think that 0°C equals 0 K, which is incorrect because the Kelvin scale is shifted downwards by 273 degrees. To avoid these mistakes, remember that Kelvin is always greater than Celsius by about 273 and that absolute zero is at 0 K, which corresponds to about minus 273°C.

Final Answer:
At 760 mmHg, the freezing point of pure water on the Kelvin scale is 273 K.