Explain why water is a compound and not a mixture ?

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.