Question

In what type of reaction do two or more substances combine to form a new compound?

Answer 1

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.

Explanation:

Chemical compounds by definition are the result of a synthesis reaction between two or more elements or possibly molecules that are themselves these result of a chemical reaction.

The types of chemical reactions are found here:
https://www.thoughtco.com/types-of-chemical-reactions-604038

A simple synthesis reaction is shown by the balanced equation:
`8Fe_((s)) + S_8((s)) + (ignition)→ 8FeS_((s)) + (heat)uarr`

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 `uarr` and the chemical compound, iron sulfide, which cannot be separated into iron or sulfur by physical means.

The definition of a compound is here:
https://en.wikipedia.org/wiki/Chemical_compound

The definition of a molecule is here:
https://en.wikipedia.org/wiki/Molecule