1.4 Composition of Mixtures

What is composition of mixtures in AP Chemistry?

A mixture contains two or more types of atoms, molecules, or formula units, and their proportions can change from sample to sample, unlike a pure substance. In AP Chemistry, this topic is mostly about using elemental analysis (mass data) to find the relative number of atoms in a sample and to check how pure that sample is.

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Why This Matters for the AP Chemistry Exam

This topic builds the bridge between mass you can measure and the number of atoms you cannot count directly. You will use mass percent and mole conversions to describe what a mixture is made of and to judge a sample's purity. That same reasoning shows up across the exam whenever you connect lab measurements to particle-level claims, justify a conclusion with data, and explain whether a result supports a chemical claim. Expect both multiple-choice problems that ask you to calculate composition and free-response items that ask you to explain or justify with evidence.

Key Takeaways

• A pure substance has only one type of atom, molecule, or formula unit; a mixture has two or more types in proportions that can vary.
• Mixtures are physically combined, so they can usually be separated by physical methods without changing the substances themselves.
• Homogeneous mixtures look uniform throughout; heterogeneous mixtures have visibly different regions.
• Elemental analysis uses mass data to find the relative numbers of atoms in a sample and to determine its purity.
• Separation methods like distillation, filtration, and chromatography rely on differences in physical properties such as boiling point, solubility, and polarity.
• Convert masses to moles before comparing amounts of different substances, since equal masses do not mean equal numbers of particles.

Pure Substances vs. Mixtures

Matter can be sorted by composition into pure substances and mixtures.

A pure substance contains only a single type of atom, molecule, or formula unit. A mixture contains two or more types, and their relative proportions can vary from one sample to the next. That variability is the key difference: a pure compound always has the same composition, but a mixture's makeup can change.

A formula unit is the lowest whole number ratio of atoms used to describe a compound. Pure substances are built from atoms or formula units of a single type, while mixtures combine atoms or formula units of two or more types.

Because mixtures are combined physically, not chemically, you can usually separate them using physical properties.

Types of Mixtures

Mixtures fall into two categories: homogeneous and heterogeneous.

• Homogeneous mixtures are uniform in composition. You cannot see the separate parts.
  • Salt water looks like a single liquid even though it contains water and dissolved salt.
  • Air is a homogeneous mixture of many gases you cannot see.
• Heterogeneous mixtures are not uniform, so the proportions vary from one spot to another, and you can often see the different parts.
  • Rocky road ice cream shows visible chocolate ice cream, nuts, and marshmallows.
  • A salad shows each ingredient you combined.

Homogeneous mixtures tend to be harder to separate because the components are mixed at the particle level. Heterogeneous mixtures are often easier to take apart because the components stay physically distinct.

Elemental Analysis and Purity

Elemental analysis uses mass data to figure out the relative numbers of atoms in a sample and to check whether that sample is pure.

The general approach connects measurable mass to countable particles:

• Start with the mass of each element or the mass percent of each element in the sample.
• Convert each mass to moles using molar mass with the relationship $n = m/M$.
• Compare the moles to find the relative numbers of atoms.

If the relative numbers of atoms match what a pure compound should contain, the sample is consistent with that pure substance. If the measured composition does not match, the differences point to impurities or to a mixture of substances. This is how mass measurements become evidence about what a sample actually contains and how pure it is.

Separating Mixtures

Chemists separate mixtures using differences in physical or chemical properties. Three common methods are distillation, filtration, and chromatography.

Distillation

Distillation separates components of a liquid mixture based on differences in boiling points. The most volatile substance, the one with the lowest boiling point, evaporates first and is collected as vapor.

For a mixture of water and alcohol, alcohol has the lower boiling point, so it evaporates first. The vapor collected contains more alcohol than water.

Filtration

Filtration uses a filter or mesh to separate solids from liquids, and it works for heterogeneous mixtures.

Picture filtering a mixture of salt, water, and sand. Only the sand is caught in the filter paper, because salt dissolves in water and passes through. The filtrate is salt water, so filtration alone does not fully separate every component. To recover the salt, you would then evaporate the water from the salt water.

Thin-Layer Chromatography (TLC)

Chromatography identifies and compares the substances in a mixture based on their differing attractions to a solid and a liquid, which often comes down to differences in polarity.

In TLC, a small sample spot is placed near the bottom of a plate. A solvent then climbs the plate and carries the sample's components different distances, separating them. Marker ink works well as an example because it is usually a mixture of several substances.

Polarity and TLC

Polarity is one way to compare substances and solvents. A substance can be polar or nonpolar, and the rule "like dissolves like" means polar dissolves in polar and nonpolar dissolves in nonpolar. That is why oil (nonpolar) and water (polar) do not mix.

Most TLC plates use polar silica as the stationary phase, while the solvent is the mobile phase. Polar compounds are attracted more strongly to the polar stationary phase, so they move less. Nonpolar compounds travel farther with the solvent.

So if a compound travels farther up the plate in a nonpolar solvent, it is the more nonpolar (less polar) compound.

You can describe how far a component travels using the retention factor:

Rf = (distance traveled by the component) / (distance traveled by the solvent)

Rf is always between 0 and 1. Because Rf compares the component's distance to the solvent's distance, you can compare results even when the solvent fronts ended up at different heights.

AP note: Other types of chromatography exist, but TLC is the version most commonly seen on the AP Chemistry Exam.

How to Use This on the AP Chemistry Exam

Problem Solving

When a question gives you mass percents or masses of elements in a mixture or compound, convert to moles before comparing. Equal masses of two elements do not contain equal numbers of atoms, so use $n = m/M$ first, then look at the mole ratios.

Free Response

When a prompt asks you to draw a reactant or product mixture from given amounts, track how many atoms of each element you have. Build the molecule that uses your most limited element first, then use the leftover atoms for the remaining species. For example, with 8 N atoms and 12 O atoms forming NO and O2, you can draw 8 NO molecules, which uses 8 N and 8 O, leaving 4 O atoms for 2 O2 molecules. Always use the key provided and follow drawing directions exactly; orientation does not matter as long as the species are correct.

Using Sources Effectively

For TLC questions, justify polarity claims with the interaction between the substance and either the solvent or the plate. A dye that travels farther in a nonpolar solvent is the least polar because it is more attracted to the nonpolar solvent and less attracted to the polar plate. To match an unknown to a known dye, compare retention factors or relative distances rather than raw distances, since the solvent fronts may differ. A dye and an unknown that both travel about halfway to the solvent front (Rf near 0.50) are the same substance.

Common Trap

Watch for TLC plates developed to different solvent front heights. You cannot compare raw travel distances directly. Use Rf values or relative positions so the comparison is fair.

Common Misconceptions

• "A mixture always looks like more than one thing." Homogeneous mixtures like salt water and air look uniform, but they are still mixtures because they contain more than one type of substance in variable proportions.
• "Distillation and filtration do the same job." Distillation separates by boiling point and works for liquids that are mixed together. Filtration separates insoluble solids from liquids and works for heterogeneous mixtures.
• "Filtration separates everything." Dissolved substances pass through the filter with the liquid. Filtration removes only insoluble components.
• "Equal masses mean equal numbers of atoms." Because elements have different molar masses, you must convert mass to moles before comparing how many atoms are present.
• "The dye that travels farther is more polar." In a nonpolar solvent, the dye that travels farther is the less polar one, since nonpolar substances are carried more readily by a nonpolar solvent.
• "Pure substances and mixtures both have fixed composition." Only pure substances have a fixed composition; a mixture's proportions can vary from sample to sample.

Related AP Chemistry Guides

• Unit 1 Overview: Atomic Structure and Properties
• 1.1 Moles and Molar Mass
• 1.2 Mass Spectra of Elements
• 1.3 Elemental Composition of Pure Substances
• 1.7 Periodic Trends
• 1.6 Photoelectron Spectroscopy