Chemistry
What this section tests
Chemistry items cover atomic structure, the periodic table, bonding, chemical equations, moles, acids and bases, states of matter, and solutions.
Must-know content
- Atomic number = protons (defines the element). Mass number = protons + neutrons. Isotopes differ in neutrons.
- Periodic table: groups are columns (similar properties), periods are rows. Metals sit left, nonmetals right, noble gases far right.
- Bonding: ionic bonds transfer electrons between metal and nonmetal; covalent bonds share electrons between nonmetals.
- Balancing equations: change only coefficients, never subscripts, so atoms are conserved.
- Moles: one mole equals 6.02 × 10²³ particles; molar mass is grams per mole.
- Acids and bases: pH below 7 is acidic, 7 neutral, above 7 basic. The scale is logarithmic.
- Reaction types: synthesis, decomposition, single replacement, double replacement, combustion.
- Phase changes: melting, freezing, evaporation, condensation, sublimation; endothermic absorbs heat, exothermic releases it.
Traps to expect
- Treating mass number and atomic mass as the same value.
- Forgetting that pH 3 is ten times more acidic than pH 4, not one unit more.
- Balancing by altering a subscript, which changes the compound’s identity.
- Confusing solute (dissolved) with solvent (dissolving, usually the larger amount).
Rule of thumb
Count each type of atom on both sides before and after any coefficient change; the totals must match.
Worked example
Balance H₂ + O₂ → H₂O. Left side has 2 H and 2 O; right has 2 H and 1 O. Put a 2 in front of H₂O: right becomes 4 H and 2 O. Put a 2 in front of H₂: left becomes 4 H and 2 O. Balanced: 2H₂ + O₂ → 2H₂O.