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?

If/when you eat too much, you get fat.
Water boils when it reaches 100°C.

We use the present simple in both clauses and it means that the condition can be true at any time
(it is a fact).
If means the same as when in a zero conditional sentence.

If I see him, I'll (I will) tell him.     

If you don't hurry, you'll miss the bus.

What will you do if there is a problem?

If you go to England, you will improve your English.

We use present simple in the if-clause.

If I find his address, I will send him the letter.

The main clause has will, which expresses certainty in the future.

I'll tell him as soon as I see him.

We use present simple in the if-clause.

If I had more time, I'd (I would) travel more.   

I wouldn't (would not) refuse if you offered me $10,000.

What would you say if you met Queen Elizabeth?

If I had enough money, I would buy a new computer.

We use the past simple (I had) in the if-clause when we are talking about something which is unlikely to happen or an imaginary situation.

If you didn't hurry so much, you would feel more relaxed.

The main clause has would + infinitive.

If I'd (I had) known you were coming, I would've (would have) waited for you.

If she hadn't been ill, she would have gone to the cinema.

Would you have done it if you'd (you had) known earlier?

If I had asked him, he would have helped me.

In the if-clause we use the past perfect (had + past participle).

If you had spoken to my mother, she would have told you where I was.

The main clause has
would + have + past participle.