What happens when you combine 2 or more substances 2 create 1 or more new substances?

Answer 1

The short answer is a chemical reaction.

To make this relevant to you though, you need to make sure that you really understand what the question is asking you, which boils down to an ChemGeekSpeak to English translation. You see, scientists take their frustrations out on the rest of the world by using the English language slightly differently than the rest of us, and they don't tell anyone, but they expect you to know exactly what they want specific words to mean. In this case, 'Substance' has a very specific meaning: Element or Compound.

In plain English, you might think the word 'Substance' means some stuff - something physical, but in Chemistry, it means something more specific. A substance has to be either an element [all one type of atom] or a compound [combination of 2 or more elements with a fixed composition by mass]. By definition, a substance is pure - its all one particular thing:

Pure gold [Element]

Pure water [Compound]

It also means it cant be a mixture - like Salt dissolved in water, or pebbles mixed into concrete.

The only way you can take 2 pure substances [either elements or compounds] and have them combine and end up with one pure substance [it must be a compound - since there are at least 2 elements involved] is for them to combine by chemical reaction [since just having them mix would not give you a pure substance].