The reversible arrow
The arrow in the reaction of iron with steam is unlike any we have seen so far — it has two heads.
3Fe + 4H2O ⇌ Fe3O4 + 4H2
For extra knowledge
Turn your attention to the ⇌ sign in the reaction above. It says that the reaction is a reversible reaction — that is, that just as products can form from the reactants, reactants can form from the products.
The two arrows compared
→ — one way
The reaction runs from reactants to products only. Burning magnesium is like this — burnt magnesium is not magnesium again.
⇌ — both ways
The reaction can run either way. Which direction dominates depends on the conditions.
This is still a chemical change
Do not conclude that because it is reversible it is a physical change. New substances form, so it is chemical — and we saw back in section 16.1 that reversibility is not the deciding question.
Remember
Where the ⇌ sign appears the reaction is reversible: products can form from reactants and reactants from products.