Question

Why water is compound not a mixture give reason

Answer 1

A mixture is a combination of two or more ingredients that can be separated by mechanical means, for example a mixture of iron filings and sulphur could be separated with a magnet. A compound is a combination of two or more atoms which can only be separated by chemical means, for example salt, which is made from chlorine and sodium, but can only be split by chemical reaction.
Water is a compound because it can only be separated into hydrogen and oxygen by chemical reaction.

Answer 2

Water is a compound and not a mixture because it has a fixed ratio. Another reason is that when we mix two hydrogen atoms and one oxygen atom we get water, it is a total new compound.

Also it does not have the properties of its constituents