Representations of Solutions
Construct charge-balanced solution windows with a stated icon scale, represented volume and hydration orientation.
AP Chemistry Topic 3.8 · CED printed p. 67 / PDF p. 74. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Molarity and complete dissociation
- Ion–dipole orientations
A solution diagram needs a count scale
A particle picture can show concentration only if the volume basis and symbol scale are clear. In equal-volume windows, twice as many of a solute’s icons represents twice the concentration when each icon represents the same amount. If a window volume doubles along with count, the concentration is unchanged. A small diagram is a representative model, not a literal view of just six molecules in a laboratory beaker.
For a completely dissociated electrolyte, draw separated hydrated ions in the formula ratio and ensure total charge balance. CaCl2 needs two Cl− for each Ca2+. Dissolved sulfate remains SO42−; it is not disassembled into free S and O atoms. For an intact molecular solute, draw whole molecules distributed among solvent molecules. A partial-ionization model must show both undissociated molecules and ions if that behavior is specified.
Interactions explain what the arrangement means
Solvent molecules are not passive dots. In water, Oδ− sides orient preferentially toward cations and Hδ+ sides toward anions. Thermal motion continually changes the hydration environment; fixed orientations in a sketch indicate favorable interactions, not frozen cages. Use dashed ion–dipole contacts and solid covalent bonds so the drawing does not imply new covalent salt–water bonds.
A dissolved molecular solute can interact with water through dispersion, dipoles or hydrogen bonding while remaining molecular. Mobile ions support electrolyte conduction; intact uncharged molecules usually do not provide comparable charge transport. A well-designed picture thus links counts to concentration and orientation to interactions. Colligative-property and solution molality/mass-percent/volume-percent calculations are outside this AP requirement.
Let's work through it
Work it through
Equal-volume windows A and B represent completely dissociated CaCl2. A contains three Ca2+ icons and six Cl− icons; B has six Ca2+ and twelve Cl−, with identical icon scale.
- Both satisfy the 1:2 cation:anion ratio and net zero charge: A has +6 and −6; B has +12 and −12.
- Because the sampled volumes and icon amounts match, B has twice the analytical salt concentration and twice each ion concentration.
- Add water orientation: O ends face Ca2+, H ends face chloride. Draw separated ions, not intact CaCl2 molecules.
- If B instead represented twice A’s volume, its doubled counts would correspond to the same concentrations. The volume basis is indispensable.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
A window has twice the solute icons but three times the volume of another. What is the concentration ratio?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
A window has twice the solute icons but three times the volume of another. What is the concentration ratio?
Worked answer and reason
2/3, if each icon represents the same solute amount.
Concentration is count per volume, not count alone.
Retrieval 2
What is wrong with a dissolved Na2SO4 diagram showing free S atoms and O2− ions?
Worked answer and reason
It splits the sulfate polyatomic ion. A complete-dissociation model has Na+ and intact SO42−.
Dissociation separates ions that were already constituents of the salt; it does not necessarily break internal covalent ion structure.
Retrieval 3
What does a hydration drawing’s fixed water orientation mean?
Worked answer and reason
It depicts a favorable charge orientation in a dynamic environment, not a permanent frozen cage.
The picture highlights interactions while omitting time-dependent motion.
Catch the tempting mistake
- Twice as many icons always means twice the concentration.
- Only if the represented volume and icon scale are equal.
- Every solute icon should be bonded to a water icon.
- Hydration contacts are noncovalent interactions, not automatically new covalent bonds.
- Dissolving a salt removes its ions’ charges.
- The ions retain their charges and must balance overall.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 3 problem 19 · ap application · particle
- Unit 3 problem 20 · ap synthesis · multiple
Scientific calculator workspace
Use this workspace for arithmetic, logarithms and elementary functions. Keep intermediate precision and report final values with appropriate units and significant figures. Graphing and general equation solving need a suitable approved tool.
Scientific calculator
This calculator requires JavaScript. Use the physics formula sheets and show your working manually.
+ − * / ^ ! · pi · e · sqrt · sin, cos, tan · asin, acos, atan · atan2(y,x) · ln, log · abs · exp · hypot(x,y). Use * between factors. Keep units in your written working.
For the teacher
- Have students state icon scale and volume basis before comparing diagrams.
- Look for intact polyatomic groups and correct water partial-charge orientation.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 3.8.A
- Using particulate models for mixtures: i. Represent interactions between components. ii. Represent concentrations of components.
- 3.8.A.1
- Particulate representations of solutions communicate the structure and properties of solutions, by illustration of the relative concentrations of the components in the solution and/or drawings that show interactions among the components.
- Colligative properties will not be assessed on the AP Exam.
- Calculations of molality, percent by mass, and percent by volume for solutions will not be assessed on the AP Exam.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 (supplied source) — Exact topic-numbered learning objectives, essential knowledge, science practices and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §11.2, Electrolytes — Dissociation versus intact molecular dissolution; charge mobility and electrolyte particle evidence.
Glossary and deeper reading · 11 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
-
Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
-
Ion–dipole force
An ion–dipole force is an interaction between an ion and a polar molecule. The molecule orients its oppositely charged region toward the ion. For example, water's oxygen end points toward a dissolved sodium cation.
-
Mixture
A mixture contains two or more substances whose proportions can vary. Its components retain their chemical identities until a chemical reaction changes them. For example, water and ethanol can form mixtures with many different compositions.
-
Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
-
Solute
A solute is a component dispersed in a solvent to form a solution. It need not be a solid, and a solution need not be liquid. For example, carbon dioxide is a gaseous solute in carbonated water.
-
Electrolyte
An electrolyte forms mobile ions when dissolved and can allow a solution to conduct electric current. Strong and weak electrolytes differ in the extent to which dissolved species form ions under the stated conditions. For example, dissolved sodium chloride supplies sodium and chloride ions to water.
-
Solvent
A solvent is the component used as the dissolving medium in a solution. It is often present in the greater amount, though the designation also depends on the mixture's context. For example, water is the solvent in a dilute aqueous sugar solution.
-
Cation
A cation is a positively charged ion. Its total positive nuclear charge exceeds its total electron charge. For example, calcium commonly forms a cation with charge +2 by losing two electrons.
-
Anion
An anion is a negatively charged ion. It has more electrons than needed to balance its positive nuclear charge. For example, a chlorine atom gaining one electron forms a chloride anion.
-
Hydrogen bond
Hydrogen bonding is a directional interaction involving hydrogen bonded to a sufficiently electronegative atom and an appropriate electron-rich acceptor. In the introductory model, N, O, and F commonly provide the relevant partners. For example, water molecules hydrogen bond when one molecule's O–H group interacts with another molecule's oxygen lone pair.
-
Colligative property
Colligative properties depend chiefly on the number of dispersed solute particles in an appropriate dilute-solution model. Familiar examples include boiling-point elevation and freezing-point depression. For example, an ideally dissociated salt can affect freezing temperature more than the same molal amount of a nondissociating solute, though real concentrated solutions require additional care.