Which end of water faces a dissolved ion?
You will be able to: Explain and represent hydration using ionic charge and water’s partial charges.
Which end of water faces a dissolved ion?
When a soluble salt dissolves, neighboring water molecules surround its ions. Their orientations are not arbitrary: the oxygen and hydrogen ends carry different partial charges.
A useful starting point: What must a solution particle diagram conserve? →
Words and symbols before equations
- Hydration
- Solvation by water molecules.
- Solvation
- Interactions of solvent particles with a dissolved species.
- Cation
- An ion with positive charge.
- Anion
- An ion with negative charge.
- δ
- The symbol marking partial, rather than whole-ion, charge.
What this picture assumes
Six water molecules show orientation preferences around one ion in three dimensions. This is not a universal hydration number or an equilibrium simulation. Waters exchange and reorient in real solution. O is δ− and H is δ+; the ion retains its full charge. Radii and distances are schematic.
Read the picture in three steps
- Read the species and labels first. Identify what each symbol and line represents. Read the units and fixed conditions before comparing quantities.
- Na⁺ attracts water’s oxygen δ− ends. Six waters in optional 3D are an illustrative shell, not a fixed universal count.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Water is bent and polar. Its oxygen end is partially negative; its hydrogen ends are partially positive. Opposite charge regions tend to approach.
Around a cation, water’s oxygen end points generally inward. Around an anion, hydrogen ends tend to point inward. These are dynamic orientational preferences rather than a motionless cage.
A hydration shell is not evidence that the ion has become a neutral atom or that every ion has the same fixed coordination number. The rotatable model uses six waters only to expose directions clearly.
Whether an ionic substance dissolves depends on the overall balance of interactions and thermodynamic factors. A favorable ion–water attraction alone does not prove every salt is highly soluble.
A worked example, step by step
Orient one water molecule near Na⁺ and another near Cl⁻. Identify the attractive charge pairs.
- Label Na⁺ positive and Cl⁻ negative.
- Label the O end of H₂O δ− and the H ends δ+.
- Place oxygen toward Na⁺ and hydrogen toward Cl⁻.
- Explain these as ion–dipole attractions while retaining the ion’s full charge and water’s partial charges.
Dissolving an ion does not erase its charge. The pictured six-water shell is a schematic, not a universal measured coordination number.
Should oxygen point toward Cl⁻ in the simple favorable orientation?
Compare with an explanation
No. Both are negative charge regions; water’s positive hydrogen ends tend to face the chloride ion.
Predict. Change one thing. Explain.
Switch from Na⁺ to Cl⁻. Predict the orientation change, then use the optional 3D view to inspect waters approaching along different spatial directions.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Na⁺ attracts water’s oxygen δ− ends. Six waters in optional 3D are an illustrative shell, not a fixed universal count.
Six water molecules show orientation preferences around one ion in three dimensions. This is not a universal hydration number or an equilibrium simulation. Waters exchange and reorient in real solution. O is δ− and H is δ+; the ion retains its full charge. Radii and distances are schematic.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using particle interactions, concentration, gas behavior or energy transfer. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionDraw and label water around K⁺ and around Br⁻. Explain each orientation and state one limitation of a static hydration diagram.
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Compare with the answer and four-point rubric
- 1 point: Label water oxygen δ− and hydrogens δ+.
- 1 point: Orient the oxygen end toward K⁺.
- 1 point: Orient hydrogen ends toward Br⁻.
- 1 point: State that waters move and exchange, or that the pictured number/distances are schematic rather than fixed universal values.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What is hydration?
Solvation of a species by water.
RECALL 2Does hydration remove ionic charge?
No.
RECALL 3Why is 3D helpful here?
It shows favorable orientations around an ion from multiple directions.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Which end of water faces a dissolved ion?
- Cation → water O end inward.
- Anion → water H ends inward.
Remember: Dissolving an ion does not erase its charge. The pictured six-water shell is a schematic, not a universal measured coordination number.
Conditions: Six water molecules show orientation preferences around one ion in three dimensions. This is not a universal hydration number or an equilibrium simulation. Waters exchange and reorient in real solution. O is δ− and H is δ+; the ion retains its full charge. Radii and distances are schematic.
Refresh Kid · AP Chemistry Unit 3 · Objectives 3.8.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 3.8, objective 3.8.A. CED effective Fall 2024 and June 2026 clarifications checked September 16, 2026. Unit 3: Properties of Substances and Mixtures, Topics 3.1–3.13. Focused lesson names, examples, models and assessments are original Refresh Kid teaching materials, not additional official topics or official AP questions. Official corrections.
The model states its assumptions beside the diagram. Colligative-property calculations and solution molality/mass-percent/volume-percent calculations are not required here. The optional speed-density model illustrates distributions; it does not require memorizing its mathematical derivation.
Teaching resources: The Organic Chemistry Tutor video titles/descriptions and topic coverage were checked for optional links; no claim is made to have watched every video. No creator scripts, examples, worksheets or artwork were copied. GitHub’s 3D website collection and its Three.js camera-control example informed the idea of controllable spatial inspection. Scientific diagrams, geometry and interactions here are original. The self-hosted Three.js runtime retains its MIT license. Camera rotation changes the view, not the chemistry.
Independent teacher review and observation of students remain pending. Implementation checks do not certify scientific accuracy, accessibility or learning effectiveness. This is a review edition.
Optional official resource: Released AP Chemistry questions and scoring guides. This archive contains questions across units; it is not an assignment of every question to this lesson.
The teaching sequence is informed by the IES learning guide; this exact implementation has not been evaluated with learners.
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