Water gas is a mixture of hydrogen and carbon monoxide.

Why are we using the indefinite article here and not the definite article? We know that water gas is the only mixture of hydrogen and carbon monoxide, and no other mixture can make water gas. Then why is the definite article not being used here?


This is an odd case where either works, but the meanings are slightly different.

If we think of “water gas” as a general idea of what happens when you mix two gases, then it’s “a” mixture, whereas the contents of a specific container with those gases would be “the” mixture.

If we think of it as a specific mixture as opposed to other mixtures, then it is “the” mixture.

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