Which of the following can be classified as a mixture?

a
pure air
b
pure water
c
pure nitrogen
d
pure gold

A pure substance is either an element (e.g. carbon, oxygen, nitrogen, sodium, lead etc.) or a compound (e.g. H2O, CO2, NH3, NaCl etc.). Air comprises elements such as N2, O2, argon and also compounds such as CO2, H2O(water vapour) etc. ... Hence, air is amixture and not a pure substance.

a  solution  is always transparent, light passes through with no scattering from solute particles which are molecule in size. the  solution  is homogeneous and does not settle out. a colloid is intermediate between asolution  and a  suspension. while asuspension  will separate out a colloid will not.

answer:

technetium is the most reactive. this is followed by silver. the least reactive out of these three elements is strontium.