Learn core ideas with visual explanations, guided examples, and practice designed for high school courses. Optional advanced challenges help you prepare for AP Chemistry and college.
High school level: foundational algebra and step-by-step supportNext step: explore college-level chemistry when you’re ready
Describe materials by their properties and learn how chemists measure them.
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Chemistry studies matter and the changes it undergoes. Measurements include a number and a unit; units help show what a number means and can be converted with ratios equal to one.
Worked example: A sample has a mass of 24.0 g and a volume of 8.0 mL. Density = mass ÷ volume = 24.0 g ÷ 8.0 mL = 3.0 g/mL.
Learning goals
Classify matter as an element, compound, or mixture
A 54 g metal sample occupies 20 cm³. What is its density?
UNIT 02
Atomic Structure
Connect protons, neutrons, and electrons to the identity and charge of atoms.
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An atom’s element is determined by its number of protons. Neutrons change the isotope, while gaining or losing electrons changes the charge and makes an ion.
Worked example: A neutral atom with 11 protons has 11 electrons. If it loses one electron, it becomes a 1+ ion.
What change turns a neutral atom into a positive ion?
UNIT 03
The Periodic Table
Use element position to recognize patterns in structure and chemical behavior.
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Elements are arranged by increasing atomic number. Elements in the same group have similar outer-electron patterns, which helps explain similarities in their chemical behavior.
Worked example: Sodium and potassium are both in Group 1, so each commonly forms a +1 ion.
Explore how atoms form ionic and covalent bonds and how molecular shape affects properties.
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Atoms form bonds through electrostatic attractions and shared electrons. A formula records atom ratios; a molecular shape describes the three-dimensional arrangement of atoms.
Worked example: In water, oxygen has two O–H bonds and two lone pairs. The molecule is bent, even though the four electron groups have a tetrahedral arrangement.
Read, balance, and interpret equations as atoms rearrange into new substances.
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A chemical equation represents a rearrangement of atoms. The number of each atom must be the same on both sides, so balancing changes coefficients—not subscripts.
Worked example: 2 H₂ + O₂ → 2 H₂O has four hydrogen atoms and two oxygen atoms on each side.
How many moles of H₂O can form from 3 mol O₂ if H₂ is in excess?
UNIT 07
States of Matter
Use particle motion and attractions to explain solids, liquids, gases, and phase changes.
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Particles in solids vibrate around fixed positions, particles in liquids move past each other, and gas particles move freely through available space. During a phase change, energy changes particle spacing and attractions.
Worked example: When liquid water freezes, its molecules form an open structure that makes ordinary ice less dense than liquid water.
Learning goals
Compare particle arrangement in each state
Describe melting, freezing, vaporization, and condensation
Connect temperature to average particle kinetic energy
Which statement best describes particles in a liquid?
UNIT 08
Gases and Gas Laws
Relate pressure, volume, temperature, and amount of gas using particle motion.
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Gas pressure comes from particles colliding with container walls. When the amount and temperature are constant, decreasing a gas’s volume increases its pressure.
Worked example: At constant temperature, Boyle’s law is P₁V₁ = P₂V₂. If the volume is halved, the pressure doubles.
What is the molarity of 0.20 mol solute in 0.50 L solution?
UNIT 10
Acids and Bases
Compare acids and bases and connect pH to hydrogen-ion concentration.
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In water, acids increase hydronium concentration and bases reduce it or increase hydroxide concentration. The pH scale is logarithmic, so a change of one pH unit represents a tenfold change in hydronium concentration.
Worked example: A solution at pH 3 has ten times the hydronium concentration of a solution at pH 4.
A reaction releases heat to its surroundings. It is:
UNIT 12
Introduction to Chemical Equilibrium
Explore reversible reactions and how systems respond when conditions change.
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At equilibrium, forward and reverse reactions continue at equal rates. Reactant and product concentrations stay constant, though they are not necessarily equal.
Worked example: If more reactant is added to a system at equilibrium, the system responds by consuming some of the added reactant as it shifts toward products.
