Matter and Its Properties
What the exam expects
You must classify matter and predict properties from structure. Key concepts: pure substances (elements, compounds) vs. mixtures (homogeneous, heterogeneous); physical vs. chemical properties and changes; intensive (density, melting point - independent of amount) vs. extensive (mass, volume) properties; the periodic trends that drive properties; and separation techniques (filtration, distillation, chromatography, evaporation) matched to mixture type.
Traps to watch for
- Chemical vs. physical change. A change of state, dissolving, or crushing is physical; rusting, burning, or reacting with acid is chemical. Dissolving sugar is physical even though it "disappears."
- Homogeneous vs. heterogeneous. Salt water is homogeneous (one phase); sand in water is heterogeneous.
- Intensive vs. extensive - density does not change when you cut a sample in half; mass does.
- Phase diagrams and heating curves - flat plateaus occur at melting and boiling because added energy breaks intermolecular forces, not raises temperature.
- Density calculations and unit consistency (g/cm³ vs. g/mL).
Rule of thumb
A property is intensive if it can identify the substance regardless of sample size. A change is chemical if a new substance with a new formula forms.
Worked example
A 25.0 g metal sample has a volume of 3.00 cm³. Density = 25.0 g / 3.00 cm³ ≈ 8.33 g/cm³. Density is intensive, so a 50 g piece of the same metal has the same density but twice the volume. If the sample were cut in half, density would remain 8.33 g/cm³.