In what type of reaction do two or more substances combine to form a new compound?
The type of reaction where two or more substances combine to form a new compound is a synthesis reaction that is one of the types of chemical reactions.
The types of chemical reactions are found here:
A simple synthesis reaction is shown by the balanced equation:
In this example a mixture of powdered sulfur and iron filings can be still separated with a magnet. But after the mixture is ignited, it will erupt into a synthesis reaction resulting in heat
The definition of a compound is here:
The definition of a molecule is here: