Question

which is an example of the kinetic-molecular theory not holding true for a gas?

• the particles in a gas travel in straight lines in a continuous, random motion.
• the attraction between particles causes a gas to condense at low temperatures.
• the average kinetic energy of particles depends only on the temperature.
• the particles in a gas make up a small portion of the total volume of the gas.

Explanation:

Identify the core principles of Kinetic Molecular Theory

Using the Kinetic Molecular Theory knowledge point

• Gas particles move in continuous, random, straight-line motion.
• The volume of individual gas particles is negligible compared to the total container volume.
• There are no attractive or repulsive intermolecular forces between gas particles.
• The average kinetic energy of gas particles is directly proportional to the absolute temperature.

Analyze the given options against the theory

Using the Real Gas Deviations knowledge point

• Option 1: "The particles in a gas travel in straight lines in a continuous, random motion." This is a core postulate of Kinetic Molecular Theory (KMT).
• Option 2: "The attraction between particles causes a gas to condense at low temperatures." KMT assumes no intermolecular attractions. In real gases, attractions exist and cause condensation, representing a deviation where KMT does not hold true.
• Option 3: "The average kinetic energy of particles depends only on the temperature." This is a core postulate of KMT.
• Option 4: "The particles in a gas make up a small portion of the total volume of the gas." This aligns with the KMT assumption of negligible particle volume.

Determine the correct choice

Using the Real Gas Deviations knowledge point

• Option 2 describes a real-world behavior (condensation due to intermolecular forces) that directly violates the KMT assumption of zero attractive forces.

Answer:

• (A) The particles in a gas travel in straight lines in a continuous, random motion.
• (B) The attraction between particles causes a gas to condense at low temperatures. (Correct answer)
• (C) The average kinetic energy of particles depends only on the temperature.
• (D) The particles in a gas make up a small portion of the total volume of the gas.