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Pure Substances
Elements vs. Compounds
Today’s Focus
Recall
Matter
Physical substance
Occupies space
Properties
Mass
Volume
Density
● 3 states of matter
○ Solid
○ Liquid
○ Gas
Atoms & Molecules
Atoms
Extremely small
Made of: electrons (-), protons
(+), and neutrons
Molecules
Made up of atoms
Two or more atoms
Ex. H2O, O2, CO2
What makes a substance “Pure”?
Homogenous
Uniform composition throughout
MANY identical particles (elements or compounds)
● Properties are constant throughout the whole mixture
i.e When you divide water in half both halves are identical
Constant chemical composition
i.e When you divide water in half it will be H2O in both halves NOT H2
in one half and O in the other
A Pure Substance can be:
1. Element
ONE type of atom
Can be two or more of the SAME
atom
Impossible to separate into other
substances
e.x O2, C, H2, K, Na
2. Compound
Substance CAN’T be separated by
mechanical or physical means
Composed of two or more
DIFFERENT atoms
Identical molecules make up the
compound
e.x CO2, H2O, NaCl (salt)
Element (Oxygen)
Water = Compound
● Composed of H2O
molecules
● The molecule is made up
of elements
○ H= Hydrogen
○ O= Oxygen
Compound
Molecule
…. For example WATER
Element
(Hydrogen)
Compound vs. Molecule
**All Compounds are molecules BUT not
all molecules are compounds**
That means…
O2 IS a molecule but is NOT a compound, but
why??????
...because…
it only contains one type of element, Oxygen
Take Away!
Pure Substances
Made of many identical
molecules
Smallest particle of matter