# The collision of molecules during thermal expansion

• B
LHeilua6000
TL;DR Summary
I am confused about why the collision of molecules will be more frequent in thermal expansion even though there are more spaces between molecules.
Greetings everyone.
I learned that the distance between molecules in liquid increases while the temperature increases. Hence, its density is decreased. The process is thermal expansion. At the same time, the collision between molecules would be more frequent when the temperature increases.
My Confusion: Since the distance between molecules increases, there are more spaces between molecules. Thus, they are less likely to collide with their neighboring molecules during their random motion. Therefore, I am confused about why the collision will be more frequent in thermal expansion?

Homework Helper
How do you know that the collisions will be more frequent? What parameter measures this "frequency"?

Homework Helper
TL;DR Summary: I am confused about why the collision of molecules will be more frequent in thermal expansion even though there are more spaces between molecules.

Greetings everyone.
I learned that the distance between molecules in liquid increases while the temperature increases. Hence, its density is decreased. The process is thermal expansion. At the same time, the collision between molecules would be more frequent when the temperature increases.
My Confusion: Since the distance between molecules increases, there are more spaces between molecules. Thus, they are less likely to collide with their neighboring molecules during their random motion. Therefore, I am confused about why the collision will be more frequent in thermal expansion?
You understand that there is a difference between a liquid and a gas consisting of small particles, right?

Details matter. We need to agree on a scenario.

Let us restrict our attention to the case of a more or less ideal gas consisting of a bunch of small particles in a large space bouncing elastically from one another and from the container walls. Liquids are more complicated than that.

If you increase temperature (increasing the average energy of the particles) then you will find that in order to hold pressure constant you will have to allow the gas to expand. It will become less dense. The distance between particles will increase.

Is this the scenario you wish to discuss? Let us assume so.

So now we have this less dense, higher temperature, same pressure gas. We can ask: "is the collision rate of particles now more frequent, less or the same"?

It should be less frequent. Consider a container wall. It is subject, by the properties of the scenario, to the same pressure as before. But each time a particle hits the wall, that particle imparts greater momentum than before. It follows that the rate of collisions must be less than before.

On the other hand, we could be talking about a different scenario. We have a sealed container. We light a flame under it, increasing the temperature of the enclosed gas.

Is this the scenario you wish to discuss? Let us assume so.

Now we have this equally dense, higher temperature, higher pressure gas. We can ask: "is the collision rate of molecules now more frequent, less or same"? The answer should be more frequent this time.

On yet another hand, we could be talking about a different scenario. We have a container with a piston restrained by a spring. We light a flame under the container. The gas heats up and expands, causing the piston to move outward against the spring.

Is this the scenario you with to discuss? Let us assume so.

Now we have this less dense, higher temperature, somewhat higher pressure gas. We can ask: "is the collision rate of particles now more frequent, less or same"? The answer should be that it depends on the spring constant. A very loose spring and it is scenario 1: Less frequent. A very stiff spring and it is scenario 2: More frequent.

• vanhees71, nasu and gmax137
LHeilua6000
You understand that there is a difference between a liquid and a gas consisting of small particles, right?

Details matter. We need to agree on a scenario.

Let us restrict our attention to the case of a more or less ideal gas consisting of a bunch of small particles in a large space bouncing elastically from one another and from the container walls. Liquids are more complicated than that.

If you increase temperature (increasing the average energy of the particles) then you will find that in order to hold pressure constant you will have to allow the gas to expand. It will become less dense. The distance between particles will increase.

Is this the scenario you wish to discuss? Let us assume so.

So now we have this less dense, higher temperature, same pressure gas. We can ask: "is the collision rate of particles now more frequent, less or the same"?

It should be less frequent. Consider a container wall. It is subject, by the properties of the scenario, to the same pressure as before. But each time a particle hits the wall, that particle imparts greater momentum than before. It follows that the rate of collisions must be less than before.

On the other hand, we could be talking about a different scenario. We have a sealed container. We light a flame under it, increasing the temperature of the enclosed gas.

Is this the scenario you wish to discuss? Let us assume so.

Now we have this equally dense, higher temperature, higher pressure gas. We can ask: "is the collision rate of molecules now more frequent, less or same"? The answer should be more frequent this time.

On yet another hand, we could be talking about a different scenario. We have a container with a piston restrained by a spring. We light a flame under the container. The gas heats up and expands, causing the piston to move outward against the spring.

Is this the scenario you with to discuss? Let us assume so.

Now we have this less dense, higher temperature, somewhat higher pressure gas. We can ask: "is the collision rate of particles now more frequent, less or same"? The answer should be that it depends on the spring constant. A very loose spring and it is scenario 1: Less frequent. A very stiff spring and it is scenario 2: More frequent.
Thanks for your explanation! In fact, my confusion occurs from an explanation in a textbook. The textbook state that when the temperature of Liquids increases, the average kinetic energy particles incease. Hence, the collision of molecules will be more frequent.

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