States of Matter
What to know
Expect questions on the kinetic-molecular theory, gas laws, phase changes, and the properties of solids, liquids, and solutions. Master the ideal gas law PV = nRT, and its special cases: Boyle (P–V inverse at constant T), Charles (V–T direct), Gay-Lussac (P–T direct), and Avogadro (V–n direct). Remember temperature must be in kelvin. Dalton's law covers partial pressures; Graham's law compares effusion rates as inverse square roots of molar mass.
For phases, know heating curves: temperature is constant during a phase change while energy goes into breaking intermolecular forces (heat of fusion/vaporization). A phase diagram shows triple point and critical point. Vapor pressure rises with temperature and with weaker intermolecular forces. For solutions, distinguish electrolyte from nonelectrolyte, and know colligative properties-vapor pressure lowering, boiling point elevation, freezing point depression, osmotic pressure-depend on particle count, not identity.
Traps
- Using Celsius in gas-law calculations.
- Forgetting to double particle count for a strong electrolyte like NaCl (i ≈ 2) in colligative-property problems.
- Assuming a gas behaves ideally at high pressure or low temperature (it does not).
- Confusing heat of vaporization with the total heat needed to warm then boil a sample.
Worked example
A 2.0 mol sample of ideal gas occupies 10 L at 300 K. Find pressure. P = nRT/V = (2.0)(0.0821)(300)/10 ≈ 4.9 atm. Rule of thumb: convert to kelvin and liters first, then plug in.