Chemical Reactions 8.5E Investigate how evidence of chemical reactions indicates that new substances with different properties are formed.
How is a chemical change different from a physical change? Matter can be changed in many ways, but all of these changes can be classified as either a chemical change or physical change. Physical Changes Chemical Changes A change that DOES NOT result in a new substance A change that DOES result in a new substance Can be reversed or undone Can’t be reversed or undone Change in size or shape More than a change in size or shape A change in the state of matter (solid, liquid, gas) More than a change in the state of matter Tear paper Burn paper Chop wood Burn wood
What is a chemical reaction? A change in matter that results in a new substance with different chemical and physical properties than the original substance. Identify the chemical changes: Glass breaking Rusting metal Burning toast Cutting grass Fireworks exploding Melting butter Digesting food ✓ ✓ ✓ ✓
What are the signs that a chemical reaction has occurred? The best way to identify that a chemical reaction has occurred is the formation of a new substance with different properties. Here are some clues: Color Change Gas Production Change in Energy Precipitate formation
When does color change indicate that a chemical reaction has occurred? When a color change is unexpected. For example, you would expect to get a green liquid when you mix a blue and yellow liquid. But getting a green liquid from combining two clear liquids is unexpected. Examples of chemical reactions that result in color change: Bleaching clothes Dying hair Burning toast
What are some examples of gas production during a chemical reaction? Bubbles, fizzing, foaming, releasing a vapor (not boiling) Burning wood – the smoke is a new product made of carbon dioxide and water vapor. Combining vinegar and baking soda – bubbles and fizzing releasing carbon dioxide gas
What does “a change in energy mean?” A change in energy in a chemical reaction means that energy is either absorbed or released during a chemical reaction. It is very common for energy in the form of heat (thermal energy) to be absorbed or released during a chemical reaction. This results in either a temperature increase or decrease. Sometimes the change in energy is light energy being absorbed or released. Photosynthesis is an example of a reaction that absorbs light energy. A glow stick is an example of a chemical reaction releasing light energy.
What is a precipitate? A precipitate is a solid that forms during a chemical reaction between two liquids.
Most Importantly…. Always ask “Is it a new substance with different properties?”