Some reactions are reversible. This is indicated by the symbol ⇌
For example:
Related Posts:
Key Reactions quiz 7.08 understand how to balance nuclear equations in terms of mass and charge 7.17 know that nuclear reactions, including fission, fusion and radioactive decay, can be a source… 
Q&A slides - Key Reactions 1:25 write word equations and balanced chemical equations (including state symbols): for reactions…