Structure of zero conditional

Structure of the zero conditional
positive negative question
If/when you heat water to 100 degrees, it boils. If/when you don't heat water to 100 degrees, it doesn't boil. What happens if/when you heat water to 100 degrees?

The zero conditional - common mistakes
Common mistakes Correct version Why?
If/when people eat too much, they will get fat.

Water boils when it will reach 100°C.
If/when people eat too much, they get fat.

Water boils when it reaches 100°C.
We use the present simple in both clauses of the zero conditional. We are saying that the condition can be true at any time (it is a fact).
If means the same as when in a zero conditional sentence. We can also use whenever.