1. Chemistry Unit - 8th Grade Science
What are Substances?
Chemistry Unit - 8th Grade Science

2. What are “Substances”? Substances can be identified as either…
Elements
Compounds
Mixtures

3. So, what is a substance?
A substance cannot be further broken down or purified by physical means.
Substance – matter of a particular kind.
Each substance has its own characteristics and properties that are different from any other substance.

4. Characteristics of Pure Substances
Fixed Composition
Cannot be separated into simpler substances by physical methods. (Physical Changes)
Can only be changed in identity and properties by chemical methods.
Properties do not vary.

5. What is a Pure Substance?
Compounds
Composed of two or more types of elements that are chemically combined.
Always in a definite ratio
Examples: H2O; CO2
Elements
Cannot be decomposed into simpler substances by chemical changes.
Examples: Hydrogen (H), Oxygen (O)

6. What is a Mixture?
Mixtures are two or more substances that are not chemically combined.
Mixtures do not…
Have constant boiling points.
Have constant melting points.

7. Characteristics of Mixtures
Variable Composition
Components retain their characteristic properties.
May be separated into pure substances by physical methods.
Mixtures of different compositions may have widely different properties.

8. Homogeneous Mixtures
Homogeneous Mixtures – look the same throughout but can be separated by physical means.
Examples: Air, Steel, Clear Salt-Water.

9. What are Solutions?
Solutions – Homogeneous mixtures that do not scatter light and are created when something is dissolved in pure water.
Can be easily separated through distilation or evaporation.
Examples: Sugar Water, Salt Water

10. Heterogeneous Mixtures
Heterogeneous Mixtures – composed of large pieces that are easily separated by physical means. (Density, Polarity, Metallic properties)
Will not have the some composition throughout.
Components are distinguishable.

11. Law of Conservation of Matter Law of Conservation of Mass
“Matter can not be created, nor can it be destroyed.”
Matter can be transformed into something different as long as you start and finish with the same amount.
Example: If I place log into a fire that has the mass of 50 kg, how much mass should the ash have after according to this law?
Answer: 50 kg