# What gas's temperature increases as it decompresses?

• noblethewhite
In summary, the principle behind a gas's reaction to pressure is that if the pressure is increased past the inversion temperature of the gas, there is an increase in temperature.
noblethewhite
I'm not sure if this is where this question should go, but it can't really follow the template in the homework section. I've looked on search engines, phrasing this question differently but can't seem to find the answer. It would be the exception to Charles Law, I believe.

What makes you think that such a single gas exists? Or is a reaction involved?

See: "free expansion, and/or Joule-Thomson (also, Thompson), or Joule-Kelvin, or Kelvin-Joule coefficient/effect."

Ah, I was considering

$$\left(\frac{\partial T}{\partial V}\right)_{S,N}$$

which is never positive for a gas (a positive value corresponds to a gas heating during decompression). However, in the Joule-Thomson expansion it looks like we're considering

$$\left(\frac{\partial T}{\partial P}\right)_{H,N}$$

which can be negative at high temperatures (and a negative value corresponds to a gas heating during decompression). That's interesting, Bystander, I hadn't heard of the Joule-Thomson inversion temperature before, thanks.

I looked into this a little more because it's not intuitive (to me, at least) how the temperature of a gas could increase during "decompression." We could use that word to describe a Joule-Thomson experiment (constant and unequal pressures on either side of a porous plug), but it seems more accurate to say that we are pushing the gas irreversibly through the plug, and we are definitely doing work on the system. This, of course, has a different connotation than a free decompression, in which no positive work enters the system (and the gas may end up doing work on the environment, decreasing the temperature of the gas).

In the Joule-Thomson arrangement, we do work on the gas before it enters the plug, and recover that energy as work when the gas expands on the other side. In the case of an ideal gas, the amounts are equal and the temperature doesn't change. In the case of a real gas, atomic/molecular repulsion increases at high pressures and temperatures, and we must do more work on the upstream side to obtain a given pressure. This excess work ends up heating the gas.

Mapes said:
I looked into this a little more because it's not intuitive (to me, at least) how the temperature of a gas could increase during "decompression." We could use that word to describe a Joule-Thomson experiment (constant and unequal pressures on either side of a porous plug), but it seems more accurate to say that we are pushing the gas irreversibly through the plug, and we are definitely doing work on the system. This, of course, has a different connotation than a free decompression, in which no positive work enters the system (and the gas may end up doing work on the environment, decreasing the temperature of the gas).

In the Joule-Thomson arrangement, we do work on the gas before it enters the plug, and recover that energy as work when the gas expands on the other side. In the case of an ideal gas, the amounts are equal and the temperature doesn't change. In the case of a real gas, atomic/molecular repulsion increases at high pressures and temperatures, and we must do more work on the upstream side to obtain a given pressure. This excess work ends up heating the gas.

I'm in a process technology course and the instructor asked this question as one of his bonus questions. There's suppose to be an actual gas that does this. I'm not really that far in my knowledge of physics yet but I'm guessing that would be the principle behind this gas's reaction to pressure.

Last edited:
noblethewhite said:
I'm in a process technology course and the instructor asked this question as one of his bonus questions. There's suppose to be an actual gas that does this. I'm not really that far in my knowledge of physics yet but I'm guessing that would be the principle behind this gas's reaction to pressure.

Lots of gases --- long's they're above their J-T inversion temperatures. Couple with J-T inversion Ts well below room T are technically "difficult" to liquify. Hint enough for you?

Well found out the answer. That is the principle behind it, but the answer he was looking for is hydrogen. Apparently in plants its a big worry if any pipe or container containing hydrogen leaks, b/c if it does the rapid drop in pressure will cause the hydrogen to heat up and ignite. So you can imgaine the problems that would cause.

## 1. What is the ideal gas law?

The ideal gas law, also known as the universal gas law, describes the relationship between the pressure, volume, temperature, and number of moles of a gas. It is expressed as PV = nRT, where P is the pressure, V is the volume, n is the number of moles, R is the gas constant, and T is the temperature in Kelvin.

## 2. How does temperature affect the behavior of a gas?

As stated in the ideal gas law, temperature is directly proportional to the pressure of a gas. This means that as the temperature of a gas increases, its pressure also increases. Additionally, as the temperature increases, the gas molecules move faster and collide more frequently with the walls of the container, resulting in an increase in pressure.

## 3. Why does a gas decompress when the temperature increases?

When the temperature of a gas increases, the molecules gain more kinetic energy and move faster. This causes the molecules to collide with each other and the walls of the container more frequently, resulting in an increase in pressure. In order to maintain a constant pressure, the volume of the gas must increase, leading to a decrease in density and decompression.

## 4. Which gas's temperature increases the most when it decompresses?

The temperature increase when a gas decompresses depends on its initial temperature and the amount of pressure it is under. In general, gases that are at higher initial temperatures and under higher pressure will experience a larger temperature increase when decompressing.

## 5. What factors can affect the temperature increase when a gas decompresses?

The temperature increase when a gas decompresses is affected by several factors, including the initial temperature and pressure of the gas, the type of gas, and the amount of gas present. Other factors such as external temperature and pressure, as well as the nature of the container and any surrounding materials, can also influence the temperature change during decompression.

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