|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?
When to use
We use the zero conditional to express that a particular condition always has the same result. We are saying that something is always true or is a fact:
|If we don’t drink, . . .
||we get thirsty.
|If you heat water, . . .
|they will get
||If people eat too much, they get fat.
||We use the present simple in both clauses of a zero conditional. We are saying that the condition is always true.
|when it will reach
||Water boils when it reaches 100°C.
||If/when you don’t exercise, you get fat.
||If and when have the same meaning here. We can also use whenever.