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'Chemical changes' are the changes in a substance through chemical reactions. The chemical reactants form a new product with equal mass.
The following evidence can indicate that a chemical change took place, although this evidence is not conclusive:

★ Change of color (e.g., rusting of iron causes a change in color from silver to reddish-brown).

★ Change in temperature or energy, such as the production (exothermic) or loss (endothermic) of heat.

★ Change of form (burning paper) (this change is difficult to reverse).

★ An 'unexpected' change in color

★ Light, heat, or sound is given off.

★ Gases formed, often appearing as bubbles.

★ Formation of precipitate (chunks).
For example, placing a pot of water on a hot stove element causes a change in temperature and gas to be released (water vapor) but a chemical change did not take place.
A chemical reaction produces new substances by changing the way in which atoms are arranged. In a chemical reaction old bonds are broken and new bonds are formed between different atoms. This breaking and forming of bonds takes place when particles of the original materials collide with one another.
Compare: physical change