Question

sodium chloride is a substance not a mixture ?give reason

Answer 1

in a mixture 2 substances are put together mixed
in a mixture the substances do not combine with each other
in a sodium chloride the sodium combines with chlorine to form sodium chloride
                                     na+cl           nacl
example
a mixture of sand and gravel

Answer 2

hii,
nacl is a substance not a mixture because
-- it cannot be seperated into its constituents by physical methods. 
--heat and light are involved in the creation /seperation of it. 
--also,it has a fixed composition.

i hope it helped !!☺☺