Matter and Its Changes
What this domain tests
Matter and Its Changes tests classification of matter, physical versus chemical changes, separation techniques, atomic structure, periodic trends, and phase behavior. Expect items that ask you to classify a substance or change, interpret a heating curve, or use isotope data to calculate average atomic mass.
Classifying matter
- A pure substance has fixed composition: an element (one kind of atom) or a compound (two or more elements chemically combined).
- A mixture has variable composition and its components retain their properties. Homogeneous mixtures (solutions) are uniform; heterogeneous mixtures are not.
- Physical separation exploits differences in properties: filtration for an insoluble solid, distillation for boiling-point differences, chromatography for different affinities for the stationary and mobile phases, and evaporation or magnetism for suitable mixtures.
Physical versus chemical changes
- Physical changes alter state, shape, or size without forming a new substance; chemical changes form new substances by breaking and forming bonds.
- Evidence of chemical change includes gas evolution not caused by boiling, precipitate formation, an unexpected color change, and release of heat or light.
- Intensive properties (density, melting point, conductivity) do not depend on sample size; extensive properties (mass, volume) do.
Atomic structure and periodic trends
- Atomic number Z equals the number of protons; mass number A equals protons plus neutrons. Isotopes have the same Z but different A; ions differ in electron count.
- Average atomic mass = Σ(isotopic mass × fractional abundance).
- Across a period, atomic radius decreases while ionization energy and electronegativity increase. Down a group, radius increases while ionization energy and electronegativity decrease and metallic character increases.
Phase changes
- Melting, vaporization, and sublimation are endothermic; freezing, condensation, and deposition are exothermic.
- Temperature stays constant during a phase change at a given pressure because added energy breaks intermolecular forces rather than increasing average kinetic energy.
Worked example
Chlorine is 75.78% chlorine-35 (34.969 u) and 24.22% chlorine-37 (36.966 u). Average atomic mass = 0.7578(34.969) + 0.2422(36.966) = 26.50 + 8.953 = 35.45 u. The result is a weighted average, not the simple mean of 35 and 37.
Traps that cost marks
- Calling a phase change a chemical change, or claiming temperature rises during boiling.
- Averaging mass numbers instead of using isotopic abundances.
- Confusing isotopes with ions.
- Treating a homogeneous mixture as a compound; compounds require chemical separation.