The absolute zero on the Kelvin scale is a theoretical temperature at which, in an ideal gas, what would happen?

Introduction / Context:
Absolute zero is a fundamental concept in thermodynamics and temperature measurement. It represents the lowest possible temperature on the Kelvin scale, where the thermal motion of particles is at a minimum. This question focuses on what theoretically happens to molecular motion in a gas at absolute zero, according to the ideal gas model taught in school physics and chemistry.

Given Data / Assumptions:

• Absolute zero corresponds to 0 K, which is approximately minus 273.15 degrees Celsius.
• We consider the behaviour of an ideal gas as temperature approaches absolute zero.
• In the kinetic theory of gases, temperature is directly related to the average kinetic energy of gas molecules.
• The wording is theoretical, as absolute zero cannot be reached exactly in practice.

Concept / Approach:
According to kinetic theory, the temperature of a gas is proportional to the average kinetic energy of its molecules. As temperature decreases, this average kinetic energy decreases, and molecular motion slows down. Extrapolating this idea, at absolute zero the average kinetic energy of an ideal gas would become zero, meaning that the random thermal motion of molecules would cease. While quantum mechanics refines this picture, at the school level we say that at absolute zero molecular motion stops, which is the concept the question is testing.

Step-by-Step Solution:
Step 1: Recall the definition of absolute zero as the lowest limit of the thermodynamic temperature scale, 0 K. Step 2: From kinetic theory, understand that temperature is a measure of the average kinetic energy and motion of gas molecules. Step 3: As temperature decreases toward 0 K, the kinetic energy and speed of gas molecules decrease. Step 4: In the idealised model, at exactly 0 K, the average kinetic energy becomes zero and random thermal motion ceases. Step 5: Select the option that states that molecular motion in a gas would cease, which matches the standard theoretical description.

Verification / Alternative check:
Graphical plots of pressure versus temperature for fixed volumes of gas show straight lines that, when extended, intersect the temperature axis at about minus 273 degrees Celsius. This suggests that pressure, which depends on molecules striking container walls, would fall to zero at this temperature because motion stops. While modern physics explains that some residual quantum motion remains, school textbooks still phrase the behaviour in terms of classical kinetic theory for simplicity. Therefore the answer remains that in an ideal gas, molecular motion would cease at absolute zero.

Why Other Options Are Wrong:
- Water would freeze to ice under all conditions: Water can freeze at 0 degree Celsius under suitable pressure; absolute zero is far lower and the statement is too vague and not the standard definition.
- All gases would become liquids at once: Different gases liquefy at different temperatures and pressures; not all liquefy exactly at absolute zero.
- All gases would instantly become solids: Many gases solidify at low temperatures, but their specific freezing points vary and this is not the precise theoretical definition of absolute zero.
- The colour of all substances would become the same: Colour depends on electronic transitions and structure, not directly on approaching absolute zero; this is not part of the definition.

Common Pitfalls:
Students sometimes confuse absolute zero with the freezing point of water or the temperatures at which individual gases liquefy or solidify. Another confusion is to think of absolute zero as just a very low number rather than a theoretical limit where thermal motion essentially stops. Remembering that kinetic energy of molecular motion goes to zero for an ideal gas at this temperature helps keep the concept clear and connects temperature directly with molecular motion.

Final Answer:
At absolute zero, in the ideal gas model, Molecular motion in a gas would cease completely.