Question

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

Answer 1

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:

`CO_2(s) rarrCO_2(g)`

`CH_4(g) + 2O_2(g) rarr CO_2(g) +2H_2O(l)+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.