When writing equations the mass on the left of the arrow must equal the mass on the right of the arrow.

When writing equation the mass on left side of equation must be equal to the mass on right side. True

Explanation:

The chemical reactions always follow the law of conservation of mass.

Law of conservation of mass:

According to the law of conservation mass, mass can neither be created nor destroyed in a chemical equation.

Explanation:

This law was given by french chemist  Antoine Lavoisier in 1789. According to this law mass of reactant and mass of product must be equal, because masses are not created or destroyed in a chemical reaction.

For example:

In photosynthesis reaction:

6CO₂ + 6H₂O + energy → C₆H₁₂O₆ + 6O₂

there are six carbon atoms, eighteen oxygen atoms and twelve hydrogen atoms on the both side of equation so this reaction followed the law of conservation of mass because total mass is equal on both side of equation.

a corroding iron, if that's one of your choices