1. Formation of New Substances: This is the most fundamental change. The atoms in the reactants rearrange to form entirely new molecules with different properties. This is why the products of a reaction often look and behave differently from the starting materials.
2. Energy Change: Chemical reactions either release energy (exothermic reactions) or absorb energy (endothermic reactions). This energy change can be observed as heat, light, or sound.
3. Changes in Physical Properties: The physical properties of the reactants can change dramatically during a reaction. For example:
* Color change: A reaction might produce a colored substance from colorless reactants.
* Formation of a precipitate: An insoluble solid might form from a solution.
* Production of gas: Bubbles of gas might be released.
It's important to note that not all chemical reactions exhibit all three of these changes. Some reactions might involve only a subtle change in energy or a single new product.
