For something that is always true (a fact, a rule that never changes), use the present form in both halves. No will, because it isn't about one future time, it's about every time.
When it's always true
If you heat water to 100 degrees, it boils. (always)
If it rains, the ground gets wet. (every time)
Plants die if they don't get water. (a fact)
Not "If you heat water, it will boil." — say "If you heat water, it boils.". A general fact uses the present in both halves: it boils, not "will boil".
Try it: A science teacher is explaining a fact that is true every single time, for everyone, forever. Which fits a fact that's always true? (If you mix red and blue, you get purple. / If you mix red and blue, you will get purple.) Answer: If you mix red and blue, you get purple. — It's a fact that's true every time, so use the present in both halves: you get purple. The will version points to one future time, but a general truth isn't about a single moment, it's about always.
Try it: Hiroshi states a general fact about water that is true every time. Hiroshi talks about one particular plan for tomorrow if a certain thing happens. Match each one. Which fits each? Answer: Hiroshi states a general fact about water that is true every time. → 1 → If you freeze water, it turns to ice. · 2 → If it snows tomorrow, we will stay in. Hiroshi talks about one particular plan for tomorrow if a certain thing happens. → 1 → If you freeze water, it will turn to ice. · 2 → If it snows tomorrow, it stays cold. — A fact that's always true uses present + present: it turns to ice. One future time uses present + will: if it snows tomorrow, we will stay in. The fact has no will; the single future plan does.