In AP Chemistry, a mixture is a physical combination of two or more substances in which each component keeps its own chemical identity, the proportions can vary, and the components can be separated by physical means like filtration or distillation (Topic 1.4, EK 1.4.A.1).
A mixture is what you get when two or more substances are physically combined without reacting. Each substance keeps its own chemical identity, which means you can pull the components apart using physical methods (filtration, distillation, chromatography) without breaking or forming any chemical bonds. Salt water is still salt plus water. Brass is still copper plus zinc.
The CED draws the key contrast in EK 1.4.A.1. Pure substances contain atoms, molecules, or formula units of a single type, while mixtures contain two or more types whose relative proportions can vary. That last part is the dead giveaway on the exam. Water is always 11.2% hydrogen by mass because it's a compound with a fixed formula. Salt water can be 1% salt or 20% salt and it's still salt water. If the composition can change, you're looking at a mixture.
Mixture lives in Unit 1 (Atomic Structure and Properties), Topic 1.4, supporting learning objective 1.4.A, which asks you to explain the quantitative relationship between elemental composition by mass and the composition of substances in a mixture. Translation: AP Chem doesn't just want you to define a mixture, it wants you to do math with one. EK 1.4.A.2 adds that elemental analysis can determine the relative numbers of atoms in a substance and check its purity. So when a problem gives you a 5.00 g mixture of two carbonates and a precipitate mass, it's testing whether you can use mole relationships to figure out how much of each component is in there. Mixtures also set up the entire lab-skills side of the course, since separation techniques like distillation and chromatography only make sense once you understand that mixture components keep their individual properties.
Keep studying AP Chemistry Unit 1
Homogeneous vs. Heterogeneous Mixtures (Unit 1)
Every mixture falls into one of these two buckets. Homogeneous mixtures (solutions) are uniform throughout, like salt water, while heterogeneous mixtures have visibly distinct regions, like sand in water. The classification matters because it determines which separation technique works.
Solutions and Solubility (Units 1 and 3)
A solution is just a homogeneous mixture, and AP Chem leans on this hard. Molarity, dilution, and particulate diagrams of dissolved ions all assume you understand that the solute and solvent keep their identities and can exist in varying proportions.
Distillation and Chromatography (Units 1 and 3)
Separation techniques are the practical payoff of the mixture definition. Distillation exploits boiling point differences, and chromatography exploits how strongly each component sticks to a stationary phase versus a mobile phase. Both work only because mixture components keep their own physical properties.
Empirical Formula and Mass Percent (Unit 1)
Topic 1.3's composition-by-mass skills feed directly into mixture problems. The twist is that a compound's mass percents are fixed by its formula, while a mixture's percents are whatever you mixed in. Exam problems often make you use the fixed compound percents to back out the variable mixture percents.
Mixture questions in AP Chem are almost always quantitative. The classic setup gives you the total mass of a two-component mixture plus one extra clue (a precipitate mass, a combined metal mass, a product mass) and asks you to find the mass or mass percent of one component. For example, a 5.00 g mixture of Na₂CO₃ and K₂CO₃ that produces 4.93 g of BaCO₃ precipitate, or a 100.0 g brass sample that yields 124.8 g of sulfates. These usually require setting up a system with moles. Conceptual MCQs also test the pure-substance-versus-mixture distinction through physical evidence, like a sample that boils over a 5°C range instead of at one constant temperature (that's a mixture, since pure substances boil at a single temperature at a given pressure). On FRQs, mixtures show up constantly as context. The 2018 exam, for instance, had a student prepare an equimolar mixture of NO(g) and NO₂(g), so you need to be comfortable applying gas laws and stoichiometry to mixed samples.
Both involve more than one element, but a compound is chemically bonded with a fixed ratio (H₂O is always 2:1 hydrogen to oxygen), while a mixture is physically combined with variable proportions. You need a chemical reaction to break a compound apart, but only physical methods like distillation or filtration to separate a mixture. On the exam, 'variable composition' or 'separable by physical means' signals mixture; 'fixed formula' signals compound.
A mixture contains two or more substances that each keep their own chemical identity and can be separated by physical means.
Per EK 1.4.A.1, mixtures have variable proportions while pure substances have a single type of particle with fixed composition, and that distinction is the most commonly tested idea in Topic 1.4.
A pure substance boils at one constant temperature, so a sample that boils over a temperature range is evidence you have a mixture.
Mixture math on the exam means using mole ratios and molar masses to find the mass or mass percent of one component from a total mass plus one extra measurement.
Elemental analysis can determine a substance's atomic composition and check its purity, which is how chemists tell a pure compound from a contaminated sample (EK 1.4.A.2).
Separation techniques like distillation and chromatography only work because mixture components retain their individual physical properties, such as boiling point and polarity.
A mixture is a physical combination of two or more substances where each keeps its own chemical identity and the proportions can vary. It maps to Topic 1.4 (Composition of Mixtures) and learning objective 1.4.A, which focuses on calculating mass composition.
Salt water is a mixture, specifically a homogeneous mixture (a solution). The NaCl and H₂O keep their identities, the salt concentration can be anything, and you can separate them by evaporation or distillation without any chemical reaction.
A compound has a fixed ratio of chemically bonded elements (CO₂ is always one carbon to two oxygens), while a mixture has variable proportions of physically combined substances. Compounds require chemical reactions to break apart; mixtures separate with physical methods like filtration or distillation.
Generally no, and that's a tested clue. Pure substances boil at a single constant temperature at a given pressure, while mixtures typically boil over a range. A sample boiling across a 5°C range is exam-speak for 'this is a mixture.'
Set up variables for the masses of each component, then use stoichiometry on whatever extra data the problem gives (like a precipitate mass or total product mass) to solve the system. For example, in a brass problem you'd convert the 124.8 g of CuSO₄ and ZnSO₄ back through mole ratios to find the original zinc mass, then divide by 100.0 g and multiply by 100.