The ardor determines the result of a reaction.
[formula A]+[formula B]—> [formula C]
Here, the arrow signifies that when adding formula A and B, you get formula C. An arrow can also go both directions.
[formulaA]+[form.B]<—>[formulaC]+[form.D]
Here, the double sided arrows means that adding formulas A and B will give you C and D, and adding formulas C and D will give you A and B.