A solution is a homogeneous mixture where a solute is evenly dissolved in a solvent, so the properties are the same throughout. The main calculation you need is molarity, $M=\frac{\text{moles solute}}{\text{liters solution}}$ , plus the dilution equation $M_1V_1=M_2V_2$ for mixing or diluting stock solutions. For AP Chemistry, use total solution volume, not just solvent volume, in molarity.

Solutions and Mixtures Summary

Solutions are homogeneous mixtures, which means their macroscopic properties do not vary throughout the sample. Heterogeneous mixtures are different because their properties depend on where you sample them. AP Chemistry Topic 3.7 uses that distinction to set up concentration calculations.

The main concentration tool is molarity: moles of solute divided by liters of total solution. The numerator is the solute amount, and the denominator is the total solution volume, not just the solvent volume. You use molarity to calculate solute particles, volume, or concentration.

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Why This Matters for the AP Chemistry Exam

This topic is mostly a calculation skill in AP Chemistry. You select a logical computational pathway, follow dimensional analysis, and pay attention to significant figures to find moles of solute, volume, or molarity. These calculations show up in both multiple-choice and free-response questions, and they are the foundation for later work in titration, equilibrium, and acid-base chemistry where concentration drives everything.

You also need to tell the difference between homogeneous and heterogeneous mixtures at the macroscopic level, since that distinction explains why solution properties stay uniform.

Key Takeaways

A solution is a homogeneous mixture, so macroscopic properties do not change with location in the sample. In a heterogeneous mixture, properties depend on where you look.
Solutions can be solids, liquids, or gases. The solute is dissolved; the solvent does the dissolving.
Molarity is the standard lab concentration unit: M = moles of solute / liters of solution, with units of mol/L.
The numerator counts only the solute in moles. The denominator is the total volume of the solution, not just the solvent.
Use M1V1 = M2V2 to find concentrations or volumes when you dilute or mix, because moles of solute stay constant.
Molality, percent by mass, and percent by volume are not tested on the AP exam, so focus your calculation practice on molarity.

Mixtures: Homogeneous vs Heterogeneous

A mixture combines two or more substances that are not chemically bonded. The key AP distinction is how uniform the mixture is.

Homogeneous mixtures are uniform throughout. The macroscopic properties do not vary from one spot to another. Salt water is a good example; you cannot see the separate parts.
Heterogeneous mixtures are not uniform. The macroscopic properties depend on location in the mixture. Soil is a good example, since you can see different pieces and the composition changes from spot to spot.

Want a closer review of mixtures? Check this Unit 1 guide on mixture composition.

What Are Solutions?

A solution is just another name for a homogeneous mixture. The particles are evenly mixed, so the solute is uniformly distributed in the solvent.

The solute is the substance that is dissolved.
The solvent is the substance that does the dissolving.

Solutions can be solids, liquids, or gases. These everyday examples connect the idea to real life. The specific substances below are examples to build intuition, not required AP content.

Solid solutions

Alloys are solid solutions. They form when two or more elements, at least one a metal, are mixed in liquid form and then cooled.

Steel: solute is carbon, solvent is iron.
Brass: solute is zinc, solvent is copper.

Liquid solutions

These are what most people picture when they hear "solution."

Salt water: solute is salt, solvent is water.
Sugar water: solute is sugar, solvent is water.

Gas solutions

Air around you.
Carbonated water: solute is dissolved carbon dioxide, solvent is water.

In every case, a solution forms when the solute disperses uniformly throughout the solvent.

Solvation and hydration

Solvation is the process of a solvent dissolving a solute. Solvent molecules surround the solute particles, and we say the solute is "solvated." When the solvent is water, this process is called hydration, and the solute becomes surrounded by water molecules.

Molarity: The Concentration You Need

Concentration measures how much solute is dissolved in a given amount of solution. The concentration unit you use most in AP Chemistry is molarity.

Molarity (M) is the number of moles of solute per liter of solution:

EQUATION: M = n_solute / L_solution

The units are mol/L. For example, 24 moles of HCl in 2 L of solution gives 24 / 2 = 12 M HCl.

Note: the numerator counts only the solute. The denominator is the total volume of the solution, which includes both solute and solvent together.

Other concentration units (not on the AP exam)

Percent by mass, percent by volume, and molality are real concentration units, but they are not assessed on the AP exam. If you continue with chemistry later you will see them, but for this course, put your practice time into molarity.

Forming a Solution

Forming a solution can be thought of in steps, and each step involves an energy change:

Separate the solute into individual particles.
Separate the solvent particles to make room (this breaks some intermolecular forces).
Form new attractions between solute and solvent particles.

Depending on the substances involved, the first two steps may have a small effect or be nearly negligible.

Diluting Solutions

Lowering the concentration of a solute is called diluting the solution. You could remove solute or add solvent, but removing a precise amount of dissolved solute is usually impractical. Imagine trying to pull an exact mass of salt back out of salt water. Adding solvent is far easier, so that is what chemists usually do.

Why does adding solvent lower the concentration? Look at the definition: M = n_solute / L_solution. If the volume of solution increases while moles of solute stay the same, molarity goes down.

Use the dilution equation to find exact values:

EQUATION: M1V1 = M2V2

Here M1 and V1 are the molarity and volume of the original solution, and M2 and V2 are the molarity and volume of the diluted solution.

Dilution matters in the lab. Stock solutions are kept very concentrated because it is easier to store less material, then they are diluted to the working concentration when you run an experiment.

Why M1V1 = M2V2 works

Molarity times volume gives moles: (mol/L) times L equals mol. When you dilute, the moles of solute stay constant. So the moles before dilution (M1V1) must equal the moles after dilution (M2V2). The concentration drops only because the volume changed while moles held steady. Solve for whatever variable you need by rearranging the equation.

How to Use This on the AP Chemistry Exam

Problem Solving

Set up molarity as M = n_solute / L_solution. If you are given grams, convert to moles first using molar mass.
Watch your volume units. Molarity uses liters, so convert mL to L before plugging in.
To find moles of solute, rearrange to n = M times V (in liters). To find particles, multiply moles by Avogadro's number.
For dilution and mixing problems, use M1V1 = M2V2 and solve for the unknown.
Track significant figures and units through every step so your final answer is precise.

Free Response

Show the full computational pathway, including the setup and units, not just a final number.
When asked to compare or explain, state clearly whether a mixture is homogeneous or heterogeneous and connect that to uniform versus location-dependent properties.

Common Trap

Do not put the solvent volume in the denominator of molarity. It is the total volume of the solution.

Common Misconceptions

"Solutions are always solid-in-liquid." Solutions can be solid, liquid, or gas. Alloys and air are solutions too.
"Molarity uses the volume of solvent." Molarity uses the total volume of the solution, which includes both solute and solvent.
"Dilution changes the moles of solute." Adding solvent does not change moles of solute. It only increases the volume, which lowers the concentration.
"Homogeneous and heterogeneous are about how things look." The real test is whether macroscopic properties are uniform throughout (homogeneous) or depend on location (heterogeneous).
"I should memorize molality and percent by mass for the exam." Those are not assessed on the AP exam. Spend your time on molarity and dilution instead.

Vocabulary

The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.

Term	Definition
heterogeneous mixture	A mixture in which macroscopic properties vary depending on the location within the mixture.
homogeneous mixture	A mixture with uniform composition and properties throughout the sample.
macroscopic properties	Observable physical and chemical characteristics of a substance that can be measured at the bulk level, such as melting point, boiling point, and vapor pressure.
molarity	A measure of solution concentration expressed as the number of moles of solute dissolved per liter of solution.
solute	The substance being dissolved in a solution, typically present in a smaller amount than the solvent.
solution	A homogeneous mixture in which one or more solutes are uniformly dissolved in a solvent.
solvent	The substance, typically a liquid, in which a solute dissolves to form a solution.

Frequently Asked Questions

What is a solution in AP Chemistry?

A solution is a homogeneous mixture, meaning its macroscopic properties do not vary throughout the sample. Solutions can be solids, liquids, or gases.

What is the difference between homogeneous and heterogeneous mixtures?

A homogeneous mixture is uniform throughout, so its properties do not depend on location. A heterogeneous mixture is not uniform, so its properties vary depending on where you sample it.

What is molarity?

Molarity is moles of solute divided by liters of solution. It is the most common concentration unit used in AP Chemistry laboratory-style calculations.

What goes in the denominator of molarity?

The denominator is liters of total solution, not liters of solvent alone. The numerator is moles of solute.

How do I calculate moles from molarity?

Rearrange molarity equals moles divided by liters of solution. Moles of solute equals molarity times solution volume in liters.

What is the common mistake in solutions and mixtures problems?

The common mistake is using solvent volume instead of total solution volume or forgetting to convert milliliters to liters before using molarity.