Unit 2: Classifying Matter

What is a pure substance?

Matter with a fixed composition and distinct properties; cannot be separated by physical means.
Examples: Water (H₂O), gold (Au), sodium chloride (NaCl).

What are the two types of pure substances?

Elements and compounds.

• Element: Made of one type of atom (e.g., O₂, Cu).

• Compound: Two or more elements chemically combined (e.g., H₂O, CO₂).

What is a mixture?

A physical combination of two or more substances; can be separated by physical means.
Example: Air, salt water, trail mix.

What are the two types of mixtures?

Homogeneous and heterogeneous mixtures.

• Homogeneous: Uniform composition throughout (solution).

• Heterogeneous: Visible differences; not uniform.

What is a homogeneous mixture?

Also called a solution — uniform throughout; components evenly distributed.
Examples: Salt water, air, vinegar.

What is a heterogeneous mixture?

Mixture that is not uniform; different parts can be seen or separated easily.
Examples: Salad, sand and water, oil and water.

How can mixtures be separated?

By physical methods such as filtration, distillation, evaporation, decantation, or chromatography — based on differences in physical properties.

What are the two types of pure substances?

Elements and compounds.

What is an element?

A pure substance made of only one type of atom. It cannot be broken down into simpler substances by chemical means.
Examples: Copper (Cu), Oxygen (O₂), Hydrogen (H₂).

How are elements classified?

Metals, nonmetals, and metalloids.

• Metals: Shiny, conduct electricity, malleable (e.g., Fe, Cu).

• Nonmetals: Dull, poor conductors (e.g., O₂, S).

• Metalloids: Have properties of both (e.g., Si, B).

What is a compound?

A pure substance made of two or more elements chemically combined in a fixed ratio.
Example: Water (H₂O) is 2 hydrogen atoms + 1 oxygen atom.

What are the two main types of compounds?

Ionic compounds and molecular (covalent) compounds.

What is an ionic compound?

Formed when metals transfer electrons to nonmetals, creating positive and negative ions.
Held together by electrostatic attraction.
Example: Sodium chloride (NaCl).

What is a molecular (covalent) compound?

Formed when nonmetals share electrons.
Held together by covalent bonds.
Example: Water (H₂O), Carbon dioxide (CO₂).

What are acids and bases considered as?

They are special types of compounds:

• Acids: Release hydrogen ions (H⁺) in water (e.g., HCl).

• Bases: Release hydroxide ions (OH⁻) in water (e.g., NaOH).

What is a mixture?

A physical combination of two or more substances; can be separated by physical means.
Examples: Air, salad, salt water.

What are the two types of mixtures?

Homogeneous (uniform, called solutions) and heterogeneous (non-uniform).