Solutions and Mixtures — AP Chemistry
1. Classification of Mixtures and Solutions ★☆☆☆☆ ⏱ 3 min
A mixture is a physical combination of two or more pure substances, where no covalent or ionic chemical bonds form between components, though new intermolecular interactions do develop. Mixtures are split into two broad categories for AP Chemistry: heterogeneous mixtures (non-uniform composition with distinct visible phases) and homogeneous mixtures (uniform composition at the molecular level), which are called solutions.
Unlike pure substances, solutions can have variable composition, making concentration calculations a core recurring skill for this topic. This subtopic contributes 3-5% of total AP Chemistry exam score, as part of Unit 3 which makes up 18-22% of the overall exam, appearing in both multiple-choice and free-response sections.
2. Concentration Units and Interconversions ★★☆☆☆ ⏱ 5 min
Concentration describes the relative amount of solute (the minor component of a solution) dissolved in solvent (the major component). AP Chemistry requires mastery of five common concentration units:
- **Mass percent**: Ratio of solute mass to total solution mass, multiplied by 100
- **Mole fraction**: Ratio of moles of a component to total moles of all solution components
- **Molarity ($M$)**: Moles of solute per liter of *total solution*
- **Molality ($m$)**: Moles of solute per kilogram of *pure solvent*
- **ppm/ppb**: Used for very dilute solutions of trace solutes
\% \text{ mass} = \frac{m_{\text{solute}}}{m_{\text{solution}}} \times 100\% = \frac{m_{\text{solute}}}{m_{\text{solute}} + m_{\text{solvent}}} \times 100\%
\chi_A = \frac{n_A}{n_{\text{total}}}
The sum of all mole fractions in any solution equals 1, and mole fraction is temperature independent.
M = \frac{n_{\text{solute}}}{V_{\text{solution (L)}}}
Molarity is temperature dependent because liquid volume expands and contracts with temperature changes.
m = \frac{n_{\text{solute}}}{m_{\text{solvent (kg)}}}
Molality is temperature independent, so it is the preferred unit for colligative property calculations where temperature changes occur.
\text{ppm} = \frac{m_{\text{solute}}}{m_{\text{solution}}} \times 10^6 \\ \text{ppb} = \frac{m_{\text{solute}}}{m_{\text{solution}}} \times 10^9
Exam tip: Always explicitly label whether you are using solvent mass or total solution mass when starting calculations to avoid common errors
3. Energetics of Solution Formation ★★★☆☆ ⏱ 3 min
Solution formation occurs in three distinct steps, each with a characteristic enthalpy change, based on breaking and forming intermolecular attractions:
- Step 1: Separate solute particles from each other, overcoming solute-solute intermolecular attractions: $\Delta H_1 > 0$ (always endothermic, energy is required to separate attracted particles)
- Step 2: Separate solvent particles from each other to make space for solute, overcoming solvent-solvent intermolecular attractions: $\Delta H_2 > 0$ (also always endothermic)
- Step 3: Mix solute and solvent particles, forming new solute-solvent intermolecular attractions: $\Delta H_3 < 0$ (always exothermic, energy is released when new attractions form)
\Delta H_{\text{soln}} = \Delta H_1 + \Delta H_2 + \Delta H_3
If the magnitude of the exothermic $\Delta H_3$ is larger than the sum of $\Delta H_1 + \Delta H_2$, $\Delta H_{\text{soln}}$ is negative (exothermic, releases heat). If $\Delta H_1 + \Delta H_2 > |\Delta H_3|$, $\Delta H_{\text{soln}}$ is positive (endothermic, absorbs heat). The 'like dissolves like' rule comes directly from this relationship.
Exam tip: Breaking any attraction (intermolecular or covalent/ionic) is always endothermic
4. Factors Affecting Solubility and Henry's Law ★★★☆☆ ⏱ 3 min
Solubility is the maximum concentration of a solute that can dissolve in a solvent at a given temperature and pressure. Key factors affecting solubility are pressure, temperature, and intermolecular interactions:
- **Pressure**: Only affects solubility of gaseous solutes in liquid solvents. Higher partial pressure of the gas above the solution increases solubility.
- **Temperature**: For solid solutes in liquids, solubility usually increases with increasing temperature (most dissolution is endothermic, per Le Chatelier's principle). For gaseous solutes in liquids, solubility *always* decreases with increasing temperature.
- **Intermolecular interactions**: Solubility is highest when solute-solvent intermolecular forces are similar in strength to solute-solute and solvent-solvent forces (the 'like dissolves like' rule).
C = kP
This is Henry's Law, where $C$ is the molar solubility of the gas, $k$ is the Henry's Law constant (specific to the gas-solvent pair and temperature), and $P$ is the partial pressure of the gas above the solution.
Exam tip: FRQ require mechanistic explanations for solubility trends, not just rules
Common Pitfalls
Why: Students confuse molality (solvent mass) with molarity (total solution volume) and mass percent (total solution mass), so they plug in the wrong value by default.
Why: Students misremember that 'breaking bonds is exothermic', and incorrectly extend this to breaking intermolecular attractions between solute or solvent particles.
Why: Students associate Henry's Law with solubility, so they use it for any solute when pressure is mentioned.
Why: General rules lead students to assume this is always true, ignoring exceptions where dissolution is exothermic.
Why: For very dilute aqueous solutions, solvent mass ≈ solution mass, so students get away with this for dilute problems, and carry the error over to more concentrated solutions.