Future tense - Etre

It will be hot = il fera chaud (see lesson 4 chapter 3) why 'fera' and not 'sera'

Above comment relates to lesson 4 chapter 6 not 3 as above

I believe any time you are speaking about the weather you use the verb Faire. So "fera" is the futur simple of "faire" par example Il fait chaud translates to "it is hot" when referring to the weather. Hopefully someone can correct me if I am wrong, but I am pretty sure thats how it is.

Yes Simon has correctly explained it. Although I like to add that not all things are the same like i could say ou se trouve l'hotel (sorry i don't have circumflexes) which means where can i find the hotel and if you think about it usually the translation makes sense. So what i do is look for patterns. Sometimes simple american phrases are said in the longest way possible in french for example.