Sometimes if and whether are interchangeable, but sometimes using one or the other will change the meaning of your sentence. Here are some examples and an explanation.
Whether Versus Whether or Not
That last example is also a good lead in to our second topic: When do you need an or not after whether? Why did I say, "Call Squiggly whether or not you are going to arrive on Friday?"
Often, the or not is just extra fluff and should be left off. In my first example, where I said, "Squiggly didn't know whether Aardvark would arrive on Friday," adding an or not wouldn't change the meaning or emphasis. Squiggly didn't know whether or not Aardvark would arrive on Friday means the same thing as Squiggly didn't know whether Aardvark would arrive on Friday. Or not is superfluous, so leave it out.
On the other hand, you need the full phrase whether or not when you mean "regardless of whether." It shows that there is equal emphasis on both options.
Call Squiggly regardless of whether you are going to arrive on Friday.
Call Squiggly whether or not you are going to arrive on Friday.
Finally, a listener wrote to say that her boss was driving her crazy by saying "rather or not" instead of "whether or not." So I'll add that rather or not is incorrect; whether is a conjunction and rather is an adverb, and they are not interchangeable. Whether or not is the correct way to show that there are two possibilities or you mean "regardless of whether."
Bernstein, T.M. Do's Don'ts and Maybes of English Usage, Times Books: New York. 1977, p.237.
Garner, B. Garner's Modern American Usage, third edition.. Oxford: Oxford University Press. 2009, p. 689.
if. American Heritage College Dictionary. Fourth Edition. Houghton Mifflin Company:Boston. 2007, p. 689.
rather. Dictionary.com. Dictionary.com Unabridged (v 1.1). Random House, Inc. http://dictionary.reference.com/browse/rather (accessed: May 23, 2008).
Kilian, C. "Rather? Whether?" Ask the English Teacher. May 26, 2006. http://crofsblogs.typepad.com/english/2006/05/rather_whether.html (accessed: May 23, 2008).
whether. Dictionary.com. Dictionary.com Unabridged (v 1.1). Random House, Inc. http://dictionary.reference.com/browse/whether (accessed: May 23, 2008).