Chemical Changes
* Energy is ALWAYS involved. Chemical changes involve the breaking and forming of chemical bonds.
* Endothermic Reactions: These absorb energy from their surroundings. The products have more energy than the reactants. Examples:
* Melting ice (solid to liquid)
* Photosynthesis (light energy is absorbed to create sugars)
* Baking a cake (heat energy is used to cook the ingredients)
* Exothermic Reactions: These release energy to their surroundings. The products have less energy than the reactants. Examples:
* Burning wood (heat and light are released)
* Explosions (rapid release of energy)
* Rusting (slow release of heat)
Key Points about Chemical Changes:
* New substances are formed. The atoms in the reactants are rearranged into different molecules.
* Energy is stored in chemical bonds. Breaking bonds requires energy, while forming bonds releases energy.
* Often irreversible. While some chemical changes can be reversed, many are not easily undone.
Physical Changes
* Energy is sometimes involved. Physical changes involve changes in the state or appearance of a substance, but not its chemical composition.
* No new substances are formed. The atoms within the substance remain the same.
* Can be reversible. Many physical changes can be reversed by changing the conditions.
Examples of Physical Changes:
* Change of state: Melting, freezing, boiling, condensation, sublimation, deposition. These involve energy changes to overcome the forces holding molecules together.
* Changing shape: Cutting, bending, tearing, crushing.
* Dissolving: Salt dissolving in water. The salt molecules are surrounded by water molecules, but the salt itself doesn't change chemically.
Key Points about Physical Changes:
* Energy may be required or released, but it's usually less significant than in chemical changes.
* No new chemical bonds are formed or broken.
* The composition of the substance remains the same.
In Summary
* Chemical changes involve energy changes and result in new substances.
* Physical changes may or may not involve energy changes and don't result in new substances.
Let me know if you have any other questions about this!
