A chemical reaction, or chemical change, is one in which a product is produced which has chemical properties that differ from the original reactants. The reaction occurs when atoms are combined, separated, or rearranged.
Although the reaction is causing changes to occur at the atomic level, it is possible at times to observe physical changes in the system which would indicate a chemical change is underway.
Examples of things to look for would be
- if two or more chemicals are combined and heat is either generated or absorbed it would be an indication of a chemical change.
- if there is an appearance of a color where previously there had been none, one might assume a chemical reaction has occurred.
- if there is an evolution of gas from the reaction vessel, one might assume chemical reaction is occurring.
- if a solid precipitate forms where there had been none before
- if the electrical or magnetic properties change (magnetic materials become non-magnetic or conductors of electricity become non-conductors)
- if flames are present
- if light is emitted
There are physical changes which can cause some of these, but especially if more than one of these changes is observed from the same system, it would be a good indication that there was a chemical reaction occurring.