Classification of Matter

A pure substance has constant composition. Element = one type of atom; compound = 2+ elements chemically combined in fixed proportions.

Diatomic molecules (O₂, N₂, H₂) are still elements: all atoms are the same type. A compound contains more than one element.

A mixture is a physical combination, separable by physical means. Homogeneous = uniform composition; heterogeneous = distinct regions/phases. Some colloids look uniform but are heterogeneous.

Stainless steel (a uniform alloy) = homogeneous. Sparkling water with visible CO₂ bubbles = heterogeneous (liquid + gas phases). Milk (a colloid) = heterogeneous, though it looks uniform.

Solid: definite shape and volume. Liquid: definite volume, takes the container's shape. Gas: takes the container's shape and volume, readily compressible. All states have mass.

At 25 °C and 1 atm, mercury (Hg) and bromine (Br₂) are the only elements that are liquids.

Physical change/property: observed without making a new substance (melting, freezing, dissolving, dilution, color, temperature). Chemical: forms new substances (combustion, rusting, acidity, caloric content).

Extensive: scales with amount and is additive (mass, volume, amount of substance, internal energy). Intensive: independent of amount (temperature, color, density, boiling point, melting point).

Solubility is intensive: stated per a specified amount of solvent/solution at set conditions, so it doesn't change with sample size. Don't confuse it with the total amount that dissolves (which depends on how much solvent there is).

Weight (not mass) changes with location because weight = mass × local gravitational acceleration. Mass is a fixed quantity independent of location.