When you observed those dramatic demonstration and performed the
other combination reactions, you would have noted the following signs of reaction and concluded that a
chemical reaction (change) had taken place:
–change
in color and/or
–production
of a solid where there was no solid or a different colored solid from
that already present in the mixture, and/or
–Bubbling
or evolution of a gas that is colored or has an odor, and/or
–Change in energy--either production of energy in the form
of sound energy (explosion) or generating heat energy
(increasing temperature) or using up heat energy (decreasing
temperature).