In temperature scales, absolute zero is defined as which of the following physical conditions?

Introduction / Context:
Absolute zero is a fundamental concept in thermodynamics and temperature measurement. It forms the lower limit of the Kelvin scale and is closely related to the behaviour of gases and the kinetic theory of matter. Many exam questions ask for the correct physical definition or interpretation of absolute zero, because understanding it helps explain why there is a natural lowest possible temperature and how particle motion relates to temperature.

Given Data / Assumptions:

• Temperature is related to the average kinetic energy of the particles in a substance.
• Kelvin is an absolute temperature scale where 0 K is defined as absolute zero.
• Real gases deviate from ideal behaviour at very low temperatures, but the definition is based on an idealised model.
• Absolute zero corresponds approximately to minus 273.15 degrees Celsius.

Concept / Approach:
According to kinetic theory, the temperature of a substance is proportional to the average kinetic energy of its particles. As a substance is cooled, this energy decreases. Absolute zero is defined as the temperature at which the average kinetic energy of the particles would reach a minimum and classical translational motion would effectively cease. In reality, quantum effects remain, but for basic chemistry and physics, absolute zero is described as the point where molecular motion stops. It is also the point where an ideal gas would extrapolate to zero volume when plotted using Charles law, though real gases liquefy before this point is reached.

Step-by-Step Solution:
Step 1: Recall that the Kelvin temperature scale starts at absolute zero, which corresponds to 0 K. Step 2: Connect temperature with molecular motion: higher temperature means faster average particle motion, lower temperature means slower motion. Step 3: Understand that as temperature approaches absolute zero, the average kinetic energy of particles approaches a minimum value. Step 4: Recognise that for simple explanations, absolute zero is defined as the point where molecular motion would cease. Step 5: Relate this to gas laws: extrapolating the volume of an ideal gas versus temperature graph to zero volume gives a temperature near minus 273.15 degrees Celsius, which is defined as absolute zero.

Verification / Alternative check:
In many textbooks, absolute zero is introduced via Charles law, which states that for a fixed amount of gas at constant pressure, volume is proportional to temperature on the Kelvin scale. Plotting volume versus Celsius temperature and extending the line to where volume would be zero gives a value of about minus 273 degrees Celsius, which is defined as 0 K. The explanation given is that at this temperature, particle motion would stop, and no further cooling would be possible. This supports the definition in terms of cessation of molecular motion, rather than boiling of water or helium.

Why Other Options Are Wrong:
The temperature at which water boils at 298 K is nonsensical, because 298 K is about 25 degrees Celsius, where water does not boil at standard pressure. The boiling point of liquid helium depends on pressure and is not used to define absolute zero. The statement that gas volume becomes zero in practice is not accurate, because real gases liquefy and solidify before reaching absolute zero; zero volume is an ideal extrapolation, not an experimental fact. The temperature at which ice and steam coexist in equilibrium relates to the triple point or phase diagram, not to absolute zero. These options misrepresent the true definition.

Common Pitfalls:
Some students mix up absolute zero with the freezing point of water, the boiling point of water, or the triple point. Others may think that absolute zero is simply where volume is zero, without understanding that this result comes from extrapolation of an ideal gas law. To avoid confusion, remember that absolute zero is fundamentally about the cessation of thermal motion and the lowest possible value of internal energy on the Kelvin scale, not about a specific substance boiling or freezing.

Final Answer:
Absolute zero is defined as the temperature at which, in theory, all molecular motion ceases.