Unit 3 Properties of Substances and Mixtures

AP Chemistry Solutions

Solute versus solvent, how a solution forms at the particle level, why like dissolves like, and how to read particle drawings of solutions, in four worked examples.

A solution is the most common kind of mixture in chemistry: nearly every reaction in the course happens in one. This page sets up the vocabulary, solute, solvent, and solution, then explains why some substances dissolve and others do not, using the same intermolecular forces that explain boiling points. It ends with the particle drawings and the one concentration unit, molarity, that the exam uses.

Solute vs Solvent and What Makes a Solution

Solution: a homogeneous mixture, with the same composition throughout. See pure substances and mixtures for how it fits into the classification of matter.
Solvent: the substance that does the dissolving, usually the component present in the larger amount.
Solute: the substance that is dissolved, usually the component present in the smaller amount. A solution can have several solutes.
Aqueous (aq): water is the solvent, the case in almost every AP problem.

Solutions are not always liquids. Air is a gaseous solution with nitrogen as the solvent, and brass is a solid solution of zinc in copper. What makes something a solution is that the solute is spread out as individual particles, molecules or ions, which is why a solution looks uniform and passes through filter paper.

How Dissolving Works at the Particle Level

Separate the solute particles by overcoming the attractions holding them together, such as ionic attractions in a salt or hydrogen bonds in sugar.
Make room in the solvent by overcoming some solvent–solvent attractions.
Form new solute–solvent attractions, such as ion–dipole attractions or hydrogen bonds.

Steps 1 and 2 cost energy and step 3 releases it. A solute dissolves well when the new solute–solvent attractions are comparable to, or stronger than, the attractions that had to be broken.

Why Like Dissolves Like

Fig. 1 · Like dissolves like

Substances with similar intermolecular attractions mix. Water, a polar solvent, forms strong attractions to ions and polar molecules, so it dissolves salts and sugars. Hexane, a nonpolar solvent, offers only London dispersion forces, which cannot replace ionic attractions or hydrogen bonds, but match those in nonpolar solutes such as iodine and oil.

Like dissolves like is a guide, not a rule for every ionic compound. Some ionic solids, such as AgCl, barely dissolve in water because their ionic attractions are too strong; the specific cases are in the solubility rules.

Reading Particle Drawings of Solutions

Concentration: compare the number of solute particles per unit of volume, not the total number.
Ions: a dissolved ionic compound appears as separate cations and anions, in the ratio given by its formula, never as intact formula units.
Orientation: water molecules point their oxygen (partial negative) end toward cations and their hydrogen (partial positive) ends toward anions.
Solvent: at low concentration, the drawing shows many more solvent particles than solute particles.

Worked Example 1: Identifying the Solute and Solvent

Identify the solute(s) and solvent in each: (a) salt water, (b) carbonated water, (c) air, (d) brass.

(a) Solute: NaCl. Solvent: water.
(b) Solute: CO₂ gas. Solvent: water. A gas can be the solute in a liquid solution.
(c) Solvent: N₂, the main component. Solutes: O₂, Ar, CO₂, and others.
(d) Solvent: copper, the main component. Solute: zinc.

The solvent is the component present in the larger amount and sets the physical state of the solution; everything dissolved in it is a solute.

Worked Example 2: Predicting Which Solvent Dissolves Each Solute

Predict whether each dissolves better in water or in hexane: (a) KI, (b) I₂, (c) glycerol, C₃H₅(OH)₃.

(a) KI: water. It is ionic, and water forms strong ion–dipole attractions with K⁺ and I⁻.
(b) I₂: hexane. It is nonpolar, so its dispersion forces match hexane’s; it is only slightly soluble in water.
(c) Glycerol: water. Its three O–H groups hydrogen bond with water; hexane cannot replace those hydrogen bonds.

Name the attraction in each case. “Like dissolves like” alone usually earns less than an answer that says which attractions break and which form.

Worked Example 3: Ranking Solutions From a Particle Drawing

A drawing shows three solutions of the same solute. Box A has 4 solute particles in 1.0 L. Box B has 8 solute particles in 2.0 L. Box C has 6 solute particles in 1.0 L. Rank them by concentration.

Particles per liter: A = 4 ÷ 1.0 = 4; B = 8 ÷ 2.0 = 4; C = 6 ÷ 1.0 = 6.
Ranking: C > A = B.

Box B has the most particles but the same concentration as A, because its volume is twice as large. Dividing by volume is exactly what molarity does, and it is the step that turns a drawing question into a particulate diagram calculation.

Worked Example 4: Making a Solution of Known Molarity

How many grams of NaCl are needed to make 250. mL of a 0.100 M NaCl solution?

Moles of solute: n = M × V = 0.100 mol/L × 0.250 L = 0.0250 mol.
Mass: 0.0250 mol × 58.44 g/mol = 1.46 g NaCl.
Procedure: dissolve the 1.46 g in some water in a 250 mL volumetric flask, then add water up to the mark.

Molarity counts liters of solution, not liters of solvent, which is why you fill to the mark instead of adding 250 mL of water. Ion concentrations and mixing problems continue in molarity and dilution.

Five Solution Practice Questions With Answers

In vinegar, a solution of acetic acid in water, which is the solute and which is the solvent?
Show answer
Acetic acid is the solute; water is the solvent.
Will CCl₄ dissolve better in water or in hexane? Explain.
Show answer
Hexane. CCl₄ is nonpolar, so its dispersion forces match hexane’s, and it cannot form the strong attractions water needs to replace its hydrogen bonds.
Why does NaCl dissolve in water but not in hexane?
Show answer
Water forms ion–dipole attractions strong enough to replace the ionic attractions in the crystal; hexane offers only dispersion forces, which are far too weak.
What mass of KCl is needed for 0.500 L of 0.200 M KCl solution?
Show answer
0.200 × 0.500 = 0.100 mol; 0.100 × 74.55 = 7.46 g.
Why can’t filtration separate salt from salt water?
Show answer
The salt is dissolved as individual ions small enough to pass through the filter. Use evaporation or distillation instead.

Common Solution Mistakes

Assuming the solvent is always water. It is the component present in the larger amount; in air it is nitrogen.
Drawing dissolved NaCl as NaCl units. Ionic solutes are drawn as separate ions.
Orienting water backward around ions. Oxygen faces cations; hydrogens face anions.
Using liters of solvent in molarity. Molarity uses liters of solution.
Saying “like dissolves like” without naming forces. Explain which attractions break and which form.
Assuming every ionic compound is soluble in water. Check the solubility rules.

Solution vocabulary and like-dissolves-like reasoning show up throughout Unit 3, the heaviest unit on the exam. To see how stronger Unit 3 answers would move your overall estimate, try the AP Chem Score Calculator.

Frequently Asked Questions

What is the difference between a solute and a solvent?

The solute is the substance that dissolves, usually the one present in the smaller amount. The solvent is the substance it dissolves in, usually the one present in the larger amount. In salt water, salt is the solute and water is the solvent.

What is a solution in chemistry?

A solution is a homogeneous mixture: a solute spread evenly through a solvent at the level of individual particles, so the composition and properties are the same throughout. Solutions can be liquids, gases (air), or solids (brass).

What does like dissolves like mean?

Substances with similar intermolecular attractions tend to dissolve in each other. Polar and ionic solutes dissolve in polar solvents such as water; nonpolar solutes dissolve in nonpolar solvents such as hexane.

Why is water called the universal solvent?

Water is highly polar and can both donate and accept hydrogen bonds, so it dissolves a very wide range of ionic and polar substances. It does not dissolve nonpolar substances such as oil well, so the name is an exaggeration.

Which concentration unit does AP Chemistry use?

Molarity, moles of solute per liter of solution. It is the only solution concentration calculation required on the exam; molality and percent by mass or volume are not tested.

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