What happens in a reaction
We know that new substances form in a chemical change. But that can come about in three ways.
1 — several become one
Two or more substances combine and a new substance forms.
2 — one becomes several
One substance turns into two or more substances.
3 — rearrangement
The substances present are organised in a different way and new substances arise.
What all three have in common
Whichever way it happens, one and the same thing is going on: the substances you start with turn into the substances you end with. So every reaction has two sides — the side it starts on and the side it finishes on.
No atoms are made or lost
In none of the three are atoms destroyed or created. The same atoms are arranged in a new way — a fact we put to work in section 16.2 when we balance equations.
On the next page we give those two sides their names.