Circle only those of the following reactions that are redox reactions:
k2cro4 + bacl2 ? bacro4 + 2 kcl
cu+s ? cus
agno3 + hcl ? hno3 + agcl
2 na + 2 h2o ? 2 naoh + h2
mgbr2 + 2 naf ? mgf2 + 2 nabr
2 co + o2 ? 2 co2
so2 + h2o ? h2so3

options B,D and F

Explanation:

Since redox reactions are those which involves both oxidation and reduction

In B , Cu is oxidized and S gets reduced

D, Na gets oxidized and hydrogen gets reduced

F, carbon gets oxidized and Oxygen gets reduced

In g, there is no change in oxidation no of s in both product and Reactants is same +4

Similarly in the case of Ag and Mg.

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.

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.