Zero Conditional

Think of the zero conditional as the “always true” rulebook of the world, like gravity always pulling you down or water always boiling at 100 degrees.

In the zero conditional, both clauses use the present tense, and “if” and “when” are interchangeable.

If you heat ice,   it melts.
       ^condition  ^result
 
People get thirsty if they don't drink enough water.
^result               ^condition
 
When the sun goes down, it gets dark.
     ^condition         ^result

References

• Zero and first conditional and future time clauses - Test-English