Articles are weird
It's said that you can't use "a" for things you can't count.
But there are many instances where it's acceptable.
a gas (Can gas be grabbed and counted one by one? It doesn't make sense!)
How did articles originate in English language?
They just make me irritable.
there are many gases
for example: noble gases
The natives could probably help me extend my answer but the only moments I can remember hearing and saying a gas is in chemistry. There are many gases, hydrogen, oxygen, argon, etc. so in that context you may count them. I can think of a few examples:
When sodium mixes with water, a gas is produced.
You have a gas, test it to see which gas it is.
Other examples less related to chemistry:
The Germans used a lethal gas in WWI
But if gas is used to mean gasoline/petrol I don't think you can use A, but a native who uses English in daily life will probably help you in other daily-life contexts.
<<How did articles originate in English language?
Definite articles originated from unstressed demonstratives ["that" one].
Indefinites from unstressed *one (old English "aan")
A gas means a kind of a gas. So the article a is correctly introduced.
"When a gas expands, the kinetic energy..."
The gas is defined to be a system of itneracting entities, as according to the laws of statistical mechanics.
If you can use "a" to anything you can count, then why can't you use it to things like water?
There can be different types of water:
clear water, dirty water, icy water, hot water
You can certainly "count" those different waters, can't you?
So then why can't you say "a water"?
Why do you want to say "a water"? Give me a sentence where you would use it.
I know this is stretching it a bit, but how does this sound? "You'll find a very muddy water that you will find near the banks of the Mississippi."
"You'll find a very muddy water near the banks of the Mississippi."
So much for that
But what do you mean "a water" in that sentence? If you mean a body of water, it's usually better to specify the type of body of water it is if you know it (e.g., stream, pond, whatever). Or if you don't mean a specific body of water, you should say "You'll find some muddy water..." or "The water is muddy...", depending on what exactly you mean.
Of course you can say those things and they are by far the way most people would put it, but sometimes people say "a water" when talking about some property of water. A fisherman, for example, could say something like "It was a muddy water we had to deal with but we got our work done." It's definitely not standard usage but it exists as I have heard it used.
We have the expression "still waters run deep" which puts water in the plural, so putting it in the singular with "a" in front of it is not a huge linguistic leap.