# How does a student know that a chemical reaction has occurred?

Jun 12, 2017

How else but by observation.........?

#### Explanation:

And typically chemical change is accompanied by a colour change, or by the formation of an insoluble precipitate or by the evolution of a gas. Some chemical reactions are more obvious than others.

You won't be asked to interrogate a process to which you haven't been introduced. Most of the time, designation of a reaction as chemical or physical is pretty clear:

$C {O}_{2} \left(s\right) \rightarrow C {O}_{2} \left(g\right)$

$C {H}_{4} \left(g\right) + 2 {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right) + 2 {H}_{2} O \left(l\right) + \Delta$

In general chemical change is characterized by the formation of new substances and the making and breaking of strong chemical bonds. Physical changes are largely changes of state.