Question

How gas is changed back into gas when it exits the gas barrel?

Answer 1

We can heat the barrel that is by increasing temperature & decreasing pressure A liquid can be turned back to a gas.

The same way, A gas which is turned as a liquid can be again converted into gas by Increasing temperature.

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Answer 2

Usually gas is compressed to make liquid. So when the compressed gas is inside the barrel, it is in it's liquid form.But when we pass it through a pipe or something else it turns in to gas again. Because in atmospheric pressure a gas is always a gas.