Question

Which is an example of the kinetic-molecular theory not holding true for a gas?
The average kinetic energy of particles depends only on the temperature.
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 particles in a gas make up a small portion of the total volume of the gas.

Answer 1

The kinetic molecular theory is based on the following assumptions :

1) Gases are composed of large number of particles that behave like hard spherical objects in a state of constant random motion.

2) The gas particles move in a straight line until they collide with one another or the walls of the container.

3) These particles are smaller than the distance between particles hence most of the volume of a gas is empty space.

4) There is no force of attraction between the gas particles or with the walls of the container.

5) The collision between particles or the particles and the walls of the container is perfectly elastic.

6) The average kinetic energy of gas particles depends on the temperature of the gas and nothing else.

From the points given above the statement that is false is :

The attraction between particles causes a gas to condense at low temperatures.

This is because there is no force of attraction between gas particles.

Answer 2

Explanation:

The kinetic molecular theory states the following.

• Composition of gas involves large number of particles known as molecules which are randomly in continuous motion.
• There is large difference in the size of gas molecules and distance between gas molecules. Therefore, gas molecules have negligible volume.
• Gases have weak intermolecular forces of attraction or repulsion. Therefore, intermolecular forces in gases are negligible.
• Gas molecules randomly collide on the walls of a container without any change in the average kinetic energy of the molecules.  
• The average kinetic energy of any gas depends only on the temperature as at a particular temperature all gas molecules will have same average kinetic energy.

It is known that particles of a gas can never travel in a straight line as they rapidly move in all the directions.

Therefore, the particles in a gas travel in straight lines in a continuous, random motion is an example of the kinetic-molecular theory not holding true for a gas.