1. Observing Chemical Change

2. Properties Review Matter has two kinds of properties:
Physical property: anything that you can see, smell, taste, feel, or hear about a substance. It also includes melting point, boiling point, conductivity, and if it dissolves.
A physical change is a change in the way a substance looks. It is still the same substance after the change.
Ex: melting, dissolving, bending

3. Chemical Properties
Chemical property: describes how a substance changes into another substance.
Reactivity is a chemical property.
A chemical change is a change in matter (reaction) that produces a new substance that wasn’t there before.
Ex: Combustion, oxidation, tarnishing

4. Chemical Bonds + Mg: → MgO
New substances form in a chemical reaction because the chemical bonds between the atoms break.
The atoms form new bonds between different atoms, so the new substances are made up of a different combination of atoms with different properties.
+ Mg: → MgO
Magnesium oxide melts at a temperature greater than 2800◦ C but Mg melts at 650◦ C

6. Evidence for a Chemical Reaction
To see if a chemical reaction took place, look for a change in properties or a change in energy.
A. Change in properties:
Change in color.
Appearance of a precipitate: a solid that forms from a solution during a chemical reaction.
NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq)
Formation of a new gas

7. What is the difference between a physical property and a chemical property?
2. There are two ways to know if a chemical reaction took place. What are they and give one example of each.
3. What is a precipitate?
4. In terms of chemical bonds, what happens during a chemical reaction?
5. Why would a change in temperature indicate that a chemical reaction occurred?

8. More Evidence B. Change in Energy:
Did the substance heat up or cool down after substances were mixed together?
Endothermic reactions are reactions where energy is absorbed.
Exothermic reactions are reactions where energy is released.

9. Quiztime! Physical or Chemical?