5.4 Physical and Chemical Changes Learning Goals: Explain a physical change Explain a chemical change Recognize the evidence for each type of change

Discussion: What’s New? Physical or chemical change? What is true about the kinds of matter present before and after a physical change? How is matter different? What substances were formed when the paper burned? What is true about the types of matter present before and after the matter undergoes a chemical change Would it be possible to change these types of matter back into a sheet of paper?

Physical Changes Change of Appearance A physical change is a change that affects one or more physical properties of a substance. The appearance, shape, or size of a substance may be altered during a physical change. Physical changes, such as changes in state, do not change the chemical identity of a substance.

Change of Appearance Physical Changes

Chemical Changes Change of Appearance A chemical change is the process when one or more substances change into entirely new substances with different properties. A substance’s identity changes because the chemical makeup changes. Bonds get rearranged and/or new bonds form. Influenced by temperature: At higher temperatures, chemical reactions happen more quickly

Change of Appearance

Look for the Signs How can you tell a chemical change has happened? There are five key signs that a chemical reaction has occurred. Observing two or more of these signs during a change means you are likely observing a chemical change.

How can you tell a chemical change has happened? Energy that changes from one form to another can be evidence of a chemical change. -Energy can appear as a spike or drop in temperature - Energy can appear in the visible form of light.

How can you tell a chemical change has happened? Colour changes When a substance changes to a new colour, a chemical change has likely occurred

How can you tell a chemical change has happened? Odours can be produced during a chemical change.

How can you tell a chemical change has happened? The production of gas is often evidence of a chemical change. Fizzing and foaming may mean gases are being produced. Careful! Boiling also can produce gas bubbles, but boiling is a physical change.

How can you tell a chemical change has happened? A precipitate is a solid formed from two clear liquids being mixed together. The formation of a precipitate can indicate a chemical change.