Why can a salt dissolve while cooling the solution?
You will be able to: Explain dissolution using competing enthalpy and entropy contributions.
Why can a salt dissolve while cooling the solution?
A cold pack can cool as a dissolved substance absorbs heat. Dissolving is not automatically exothermic; the total free-energy balance includes entropy as well.
A useful starting point: Can a positive ΔG° still allow forward change? →
Words and symbols before equations
- Lattice separation
- Separating ions from the ionic solid, requiring energy.
- Solvent reorganization
- Changing solvent–solvent interactions to accommodate solute.
- Hydration
- Water molecules interact with dissolved ions.
- ΔHsolution
- Net enthalpy of the full dissolution process, not one separated step.
What this picture assumes
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Hypothetical standard dissolution at 300 K. Net enthalpy = positive separation cost minus hydration-release magnitude. Supplied total entropy already includes solvent effects. This is not an actual solubility calculator.
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.
- Supplied standard dissolution at 300 K: net ΔH°=10 kJ/mol and total ΔS°=50 J/(mol·K), giving ΔG°=-5 kJ/mol. The sign uses all contributions, including supplied solvent entropy; actual solubility is not computed.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Separating an ionic lattice and reorganizing solvent can cost energy. Forming ion–water interactions releases energy. Their balance determines the net dissolution enthalpy.
Dispersing solute can increase accessible arrangements, but ordering water near ions can oppose that gain. The total entropy change cannot always be guessed from ion count alone.
An endothermic dissolution can be favored when its entropy contribution sufficiently lowers free energy at the stated temperature and composition. Conversely, a favorable-looking single contribution does not decide the total.
The explorer uses supplied thermodynamic contributions. It does not claim that a generic ion picture can predict actual solubility without data; saturation and activities still matter.
A worked example, step by step
A supplied standard dissolution has lattice/solvent separation cost +30 kJ/mol, hydration contribution −20 kJ/mol and ΔS°=+50 J/(mol·K). Evaluate ΔG° at 300 K.
- Net ΔH°=+30−20=+10 kJ/mol.
- Convert ΔS° to +0.050 kJ/(mol·K).
- TΔS°=300×0.050=15 kJ/mol.
- ΔG°=10−15=−5 kJ/mol: entropy outweighs the endothermic cost under standard conditions.
Neither dissolving nor increasing the number of dissolved ions guarantees a positive total entropy change or a favorable overall process.
Can stronger hydration both release energy and restrict water arrangements?
Compare with an explanation
Yes. Enthalpy and entropy contributions must be considered separately.
Predict. Change one thing. Explain.
Vary the supplied separation and hydration energies one at a time. Keep entropy and temperature fixed, then explain which balance changes the free-energy sign.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Supplied standard dissolution at 300 K: net ΔH°=10 kJ/mol and total ΔS°=50 J/(mol·K), giving ΔG°=-5 kJ/mol. The sign uses all contributions, including supplied solvent entropy; actual solubility is not computed.
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Hypothetical standard dissolution at 300 K. Net enthalpy = positive separation cost minus hydration-release magnitude. Supplied total entropy already includes solvent effects. This is not an actual solubility calculator.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using energy and entropy contributions, electron and ion bookkeeping, or the stated cell reaction. 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 questionFor supplied ΔH°=+8 kJ/mol and ΔS°=+40 J/(mol·K), evaluate standard dissolution favorability at 300 K and explain why an ion picture alone is insufficient.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: Convert ΔS° to +0.040 kJ/(mol·K).
- 1 point: TΔS°=12 kJ/mol and ΔG°=8−12=−4 kJ/mol.
- 1 point: The entropy contribution outweighs the positive enthalpy.
- 1 point: Solvent reorganization and opposing contributions require data; a schematic alone cannot establish total free energy or actual solubility.
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 releases energy during hydration?
Formation of favorable ion–water interactions.
RECALL 2What can oppose solute dispersal entropy?
Restrictions on solvent arrangements near ions.
RECALL 3Does a cool solution imply dissolution is impossible?
No.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Why can a salt dissolve while cooling the solution?
- ΔHsolution combines separation and solvation contributions.
- Use the total ΔH and ΔS, with stated conditions, to assess free energy.
Remember: Neither dissolving nor increasing the number of dissolved ions guarantees a positive total entropy change or a favorable overall process.
Conditions: Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Hypothetical standard dissolution at 300 K. Net enthalpy = positive separation cost minus hydration-release magnitude. Supplied total entropy already includes solvent effects. This is not an actual solubility calculator.
Refresh Kid · AP Chemistry Unit 9 · Objectives 9.6.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 9.6, objective 9.6.A. CED effective Fall 2024 and June 2026 clarifications checked September 17, 2026. Unit 9: Thermodynamics and Electrochemistry, Topics 9.1–9.11. 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. Numerical thermodynamic examples state standard conditions, temperature, reaction scaling and unit conventions. Supplied data and schematic geometry are teaching models. Standard ΔG° describes standard-state favorability and relates to K; actual direction depends on composition. Thermodynamic favorability does not predict rate. Nonstandard cell potential is taught through Q, distance from equilibrium and qualitative Nernst reasoning; algorithmic substitution alone does not demonstrate the assessed understanding. Electrode positive/negative labeling is excluded from assessed scope. Oxidation at the anode and reduction at the cathode remain essential. Faraday calculations assume the stated current efficiency and electron stoichiometry. Rotatable particle models are schematic inventories, not measured molecular trajectories. Virtual models do not replace required supervised laboratory work.
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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