CBSE Class 9 Answered

A compound is a substance made up of two or more elements chemically combined in a fixed proportion by mass.

for example: NaCl cannot be separated into its constituents by physical process and the properties of NaCl is completely different from that of Na and Cl, so NaCl is a compound and not a mixture.

Water is a compound not mixture because -

(i). Water cannot be separated into its constituents, hydrogen and oxygen by physical methods.

(ii). Heat and light are given out when water is prepared by burning hydrogen in oxygen.