1. Formation of New Substances: This is the most fundamental characteristic. A chemical change always results in the formation of new substances with different chemical properties than the original substances.

2. Change in Chemical Composition: The chemical makeup of the substances involved is altered. Atoms rearrange and form new bonds.

3. Difficult or Impossible to Reverse: While some chemical changes can be reversed, many are not. Reversing the change often requires additional chemical reactions.

4. Energy Change: Chemical changes involve either the release (exothermic) or absorption (endothermic) of energy. This energy change can be observed as heat, light, or sound.

5. Often Accompanied by Observable Signs: Chemical changes are frequently accompanied by visual cues like:

* Color Change: A change in color, such as when iron rusts or a piece of paper burns.

* Gas Production: Bubbles forming in a solution or the release of a gas into the air.

* Precipitate Formation: The formation of a solid (precipitate) from a solution.

* Change in Temperature: The reaction may feel hot or cold.

Important Note: Not all of these characteristics are always present in every chemical change. For example, some chemical reactions may not be accompanied by a visible color change.