A mixture is when two or more substances combine physically together.
However, in water, two hydrogen atoms combine with one oxygen atom chemically, forming a new substance that has properties different from hydrogen alone or oxygen alone.
For example, if you combine iron powder and sulfur powder physically (just mixing them together without applying heat), you can find that the mixture retains the properties of the original components, i.e. you can still use a magnet to attract the iron inside the mixture.
