Chemistry Competency Exam PCC.Final Test 2026\2027.
Pure Substance
Sample of matter with both definite and constant composition and distinct chemical properties. To avoid
confusion, a pure substance is often referred to as a "chemical substance."
Examples:
**Chemical elements and compounds. Alloys and other solutions may also be considered pure if they have
a constant composition.
Water
Diamond
Gold
Table salt (sodium chloride)
Examples of substances that aren't pure:
** any heterogeneous mixture is not a pure substance. If you can see differences in the composition of a
material, it's impure, at least as far as chemistry is concerned.
Rocks
Oranges
Wheat
Shoes
Mixture
Consists of two or more different elements and/or compounds physically intermingled,
Can be separated into its components by physical means, and
Often retains many of the properties of its components.
Element
Consists of only one kind of atom,
Cannot be broken down into a simpler type of matter by either physical or chemical means, and
Can exist as either atoms (e.g. argon) or molecules (e.g., nitrogen).
Cannot be broken down by chemical means
Atoms of an element all have the same number of protons, but they can have different numbers of
electrons and neutrons
Compound
consists of atoms of two or more different elements bound together,
Can be broken down into a simpler type of matter (elements) by chemical means (but not by physical
means),
, Chemistry Competency Exam PCC.Final Test 2026\2027.
Has properties that are different from its component elements, and
Always contains the same ratio of its component atoms.
Homogenous
mixture where the components that make up the mixture are uniformly distributed throughout the
mixture
Examples: air, sugar water, rainwater, vodka, vinegar, steel
Heterogenous
mixture where the components of the mixture are not uniform or have localized regions with different
properties
Different samples from the mixture are not identical to each other
Always two or more phases in a heterogeneous mixture, where you can identify a region with properties
that are distinct from those of another region, even if they are the same state of matter (e.g., liquid,
solid)
Examples: cereal in milk, vegetable soup, pizza, gravel, soil
Atoms
Atoms are composed of subatomic particles
Protons (+)
Neutrons
Electrons (-)
Structure
Nucleus --> protons and neutrons
Electrons fly around the nucleus in a small cloud
- Like charges repel each other; unlike charges attract
- In a normal (neutral) atom the number of protons and the number of electrons are equal
- Everything in the universe is made of matter, and matter is made of atoms
Molecule
two or more atoms bonded together
Chemical formula --> way of representing molecules
Diatomic molecule --> any molecule made up of 2 atoms
Molecular compounds --> contains more than 1 type of atom
Covalent Bond
Sharing of electron pairs between atoms
Ion
Pure Substance
Sample of matter with both definite and constant composition and distinct chemical properties. To avoid
confusion, a pure substance is often referred to as a "chemical substance."
Examples:
**Chemical elements and compounds. Alloys and other solutions may also be considered pure if they have
a constant composition.
Water
Diamond
Gold
Table salt (sodium chloride)
Examples of substances that aren't pure:
** any heterogeneous mixture is not a pure substance. If you can see differences in the composition of a
material, it's impure, at least as far as chemistry is concerned.
Rocks
Oranges
Wheat
Shoes
Mixture
Consists of two or more different elements and/or compounds physically intermingled,
Can be separated into its components by physical means, and
Often retains many of the properties of its components.
Element
Consists of only one kind of atom,
Cannot be broken down into a simpler type of matter by either physical or chemical means, and
Can exist as either atoms (e.g. argon) or molecules (e.g., nitrogen).
Cannot be broken down by chemical means
Atoms of an element all have the same number of protons, but they can have different numbers of
electrons and neutrons
Compound
consists of atoms of two or more different elements bound together,
Can be broken down into a simpler type of matter (elements) by chemical means (but not by physical
means),
, Chemistry Competency Exam PCC.Final Test 2026\2027.
Has properties that are different from its component elements, and
Always contains the same ratio of its component atoms.
Homogenous
mixture where the components that make up the mixture are uniformly distributed throughout the
mixture
Examples: air, sugar water, rainwater, vodka, vinegar, steel
Heterogenous
mixture where the components of the mixture are not uniform or have localized regions with different
properties
Different samples from the mixture are not identical to each other
Always two or more phases in a heterogeneous mixture, where you can identify a region with properties
that are distinct from those of another region, even if they are the same state of matter (e.g., liquid,
solid)
Examples: cereal in milk, vegetable soup, pizza, gravel, soil
Atoms
Atoms are composed of subatomic particles
Protons (+)
Neutrons
Electrons (-)
Structure
Nucleus --> protons and neutrons
Electrons fly around the nucleus in a small cloud
- Like charges repel each other; unlike charges attract
- In a normal (neutral) atom the number of protons and the number of electrons are equal
- Everything in the universe is made of matter, and matter is made of atoms
Molecule
two or more atoms bonded together
Chemical formula --> way of representing molecules
Diatomic molecule --> any molecule made up of 2 atoms
Molecular compounds --> contains more than 1 type of atom
Covalent Bond
Sharing of electron pairs between atoms
Ion
