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Module 1 · L12 of 21 30 min ⚡ +50 XP in Learn · +25 to complete Year 11 · Module 1 · IQ4

Solubility and Like-Dissolves-Like

Today's hook, Oil and water flatly refuse to mix, yet a squirt of detergent forces them together. Nothing was heated and nothing reacted.
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Warm up and recall

Warm up first

Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.

Worksheets

Practise this lesson

Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application practice Master Mastery challenge Exam Exam-style questions
01
Recall, your gut answer first
+5 XP warm-up

Salt (NaCl) dissolves easily in water but not in vegetable oil. Wax dissolves easily in oil but not in water. What do the solute and solvent have in common when dissolution occurs? Use your knowledge of polarity to explain.

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What you'll master, and the words for it

03
What you'll master
Know

Key facts

  • The principle "like dissolves like"
  • How polarity determines solubility
  • Definitions of miscible, immiscible, hydrophilic, hydrophobic
Understand

Concepts

  • Why ionic compounds and polar molecules dissolve in polar solvents
  • Why non-polar substances dissolve in non-polar solvents
  • How IMFs between solute and solvent determine solubility
Can do

Skills

  • Predict solubility of a substance in water or organic solvents
  • Explain solubility in terms of IMF compatibility
  • Analyse data to identify solubility patterns and anomalies
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Why Do Things Dissolve? An IMF Perspective

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Why Do Things Dissolve? An IMF Perspective
core concept

Dissolving is an energy balance. For a solute to dissolve in a solvent, three things must happen:

  1. Solute–solute bonds/IMFs must be broken (energy input required)
  2. Solvent–solvent IMFs must be disrupted (energy input required)
  3. Solute–solvent IMFs must form (energy released)

For dissolution to be favourable, the energy released in step 3 must compensate for the energy input in steps 1 and 2. This happens when the solute and solvent IMFs are compatible in type.

The key insight: Water has very strong H-bonds. For water to dissolve a solute, it must break its own H-bond network to accommodate the solute. If the solute can form comparable H-bonds or ion-dipole forces with water (hydrophilic), energy is released that compensates. If the solute is non-polar (hydrophobic), no energy is released, the water network is disrupted for no gain → dissolution doesn't occur.
Solute typeSolvent typeCompatible IMFs?Soluble?Example
Ionic (Na⁺, Cl⁻)Polar (H₂O)✅ Ion-dipole forcesYesNaCl dissolves in water
Polar moleculePolar (H₂O)✅ H-bonds or dipole-dipoleYesEthanol, sucrose dissolve in water
Non-polar moleculePolar (H₂O)❌ Only dispersion vs H-bondsNoHexane, oils don't dissolve in water
Non-polar moleculeNon-polar (hexane)✅ Dispersion–dispersionYesIodine, grease dissolve in hexane
IonicNon-polar (hexane)❌ Ion needs stronger interactionNoNaCl doesn't dissolve in hexane

Dissolving = energy balance: (1) break solute–solute IMFs; (2) break solvent–solvent IMFs; (3) form new solute–solvent IMFs. Dissolution is favourable when solute–solvent IMFs are compatible in type with the IMFs being broken. "Like dissolves like": polar solutes dissolve in polar solvents; non-polar in non-polar; ionic compounds dissolve in polar solvents via ion–dipole forces. Treat "like dissolves like" as a useful first-pass heuristic, not a law: the actual result depends on the full balance of solute–solute, solvent–solvent and solute–solvent interactions and on entropy, which is why exceptions occur (some "like" pairs mix only partly, and some ionic solids stay insoluble).

Pause, copy the highlighted "like dissolves like" rule into your book before moving on.

Write it out: in one or two sentences, explain why oil does NOT dissolve in water using the "like dissolves like" rule.

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Water as a Polar Solvent: Why It's Exceptional

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Water as a Polar Solvent: Why It's Exceptional
core concept

Water is the most common polar solvent in chemistry and biology. Its exceptional solvent properties come from:

  • High polarity: O is highly electronegative (χ = 3.5) → large δ+ on H, large δ− on O → strong permanent dipole → strong dipole-dipole interactions with other polar molecules
  • Hydrogen bonding network: Each water molecule can form up to 4 H-bonds → very cohesive liquid → takes significant energy to disrupt; compensated only by strong solute-water interactions
  • Ion-dipole forces: Water's dipoles align with dissolved ions (δ− O points toward cations; δ+ H points toward anions) → hydration shell surrounds each ion → stabilises the separated ions in solution

The hydration of NaCl, step by step

  1. Water molecules approach the NaCl lattice surface
  2. δ− O atoms orient toward Na⁺ ions; δ+ H atoms orient toward Cl⁻ ions
  3. Ion-dipole forces overcome the interionic (ionic lattice) attraction at the surface
  4. Na⁺ and Cl⁻ ions are pulled from the lattice, becoming surrounded by a hydration shell
  5. Hydrated ions disperse throughout the solution
Not all ionic compounds dissolve: Some ionic compounds have such high lattice energies (e.g. BaSO₄, AgCl, CaCO₃) that the ion-dipole energy gained on hydration is insufficient to compensate. These are described as "insoluble" or "sparingly soluble" in water.

We just saw the "like dissolves like" principle and the energy balance of dissolving. That raises a question: why is water so remarkably effective as a solvent for ionic compounds when other polar solvents are not? This card answers it → water's geometry and high electronegativity of O enable it to form strong ion–dipole interactions that pull ions out of lattices.

Water is an exceptional polar solvent: O is highly electronegative (large δ+/δ−), each H₂O can form up to 4 H-bonds, and dipoles align with dissolved ions. Hydration of NaCl: δ−O of water aligns with Na⁺; δ+H aligns with Cl⁻ → ion–dipole forces pull ions out of the lattice into a hydration shell. For an ionic compound to dissolve, hydration energy must compensate for lattice energy broken. This lattice-versus-hydration comparison is an enthalpy-only simplification: strictly, dissolution is favourable when the free-energy change is negative, so entropy matters too (breaking up the ordered lattice usually increases entropy and helps dissolving).

Add the highlighted hydration explanation to your notes before the check below.

Quick check: which interaction is responsible for stabilising Na⁺ ions in water when NaCl dissolves?

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Non-Polar Solvents and Hydrophobic Substances

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Non-Polar Solvents and Hydrophobic Substances
core concept

Non-polar solvents (hexane, diethyl ether, chloroform, dry cleaning solvents) work by dispersion forces alone. They dissolve non-polar substances because dispersion forces in the solute are compatible with dispersion forces in the solvent, both types are disrupted and reformed with comparable energy. No unfavourable disruption of stronger IMFs occurs.

The "hydrophobic effect" explained

When a non-polar molecule is forced into water, it cannot form H-bonds with water. Water molecules around it must reorganise to preserve their H-bond network, forming a cage-like structure around the non-polar molecule. This highly ordered arrangement has lower entropy and is thermodynamically unfavourable. The result: non-polar molecules are "squeezed out" of water and cluster together. This is why oil droplets coalesce in water.

Amphiphilic molecules

Some molecules have both a polar (hydrophilic) head and a non-polar (hydrophobic) tail. These are called amphiphilic or amphipathic. Examples: soaps, detergents, phospholipids. Soaps work by surrounding grease with their non-polar tails (dissolving in the grease) while the polar heads interact with water, emulsifying grease into water-dispersible micelles.

Hydrophilic heads face the water and hydrophobic tails hide inside, letting non-polar grease dissolve.

We just saw why water dissolves ionic compounds via hydration. That raises a question: how do non-polar substances dissolve, and how do soaps allow oil (non-polar) and water (polar) to mix? This card answers it → non-polar solvents work via dispersion forces; amphiphilic soap molecules bridge polar and non-polar environments through micelle formation.

Non-polar solvents (hexane, diethyl ether) dissolve non-polar solutes via dispersion forces (e.g. oils, grease, I₂). Amphiphilic molecules (soaps, detergents) have a polar head + non-polar tail → form micelles surrounding grease, with polar heads facing water, making grease water-dispersible. BaSO₄ and AgCl are insoluble because their lattice energies (from doubly or highly charged ions) are too high for hydration energy to overcome.

Pause, write the highlighted micelle explanation into your book.

Lab safety, non-polar solvents. Hexane, diethyl ether and similar non-polar solvents are volatile and highly flammable. The activities in this lesson are paper-based, but if you ever handle these solvents in the lab, work in a fume cupboard or a well-ventilated area, wear eye protection, and keep all naked flames away (heat only with a water bath or electric heating, never a Bunsen). Where possible, use a teacher demonstration or secondary data instead.

Match: connect each substance to the solvent in which it dissolves best.

  • Iodine, I₂ (non-polar)
  • NaCl (ionic)
  • Candle wax (long non-polar chains)
  • Sucrose (many,OH groups)
  • Water, extensive H-bonding with,OH groups
  • Hexane, matching dispersion forces
  • Water, ion-dipole interactions hydrate Na⁺ and Cl⁻
  • Hexane, non-polar solvent matches non-polar I₂
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Short Answer Questions

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Short Answer Questions
core concept

6. A chemist has a mixture of iodine (I₂) and sodium chloride (NaCl) dissolved in water. They want to extract the iodine using hexane. Explain why this extraction works, with reference to the principle of "like dissolves like" and the IMFs involved. 4 MARKS

✏️ Answer in your book

7. The amino acid glycine has the formula H₂N–CH₂–COOH. It has both an amino group (–NH₂) and a carboxylic acid group (–COOH). Predict whether glycine would be more soluble in water or hexane, and justify your prediction using IMF reasoning. 3 MARKS

✏️ Answer in your book

8. Explain, using the concept of IMF compatibility, why BaSO₄ is insoluble in water despite being an ionic compound. In your answer, refer to lattice energy and hydration energy. 3 MARKS

✏️ Answer in your book

We just saw how soaps work and why some ionic compounds are insoluble. That raises a question: how do you systematically predict and explain solubility in exam answers? This card answers it → follow the three-step approach: identify both IMF types, judge compatibility, conclude and justify.

Solubility prediction method: identify IMFs of solute → identify IMFs of solvent → judge compatibility (type and strength). For exam answers on a specific solubility question: state the IMF type of both solute and solvent, explain whether they are compatible, then conclude soluble or insoluble. Always cite the specific IMF (ion–dipole, H-bonding, dispersion) rather than just "similar polarity".

Pause, copy the highlighted prediction method into your book before moving on.

Write it out: in one or two sentences, explain why BaSO₄ is insoluble in water even though it is an ionic compound.

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Worked examples

Worked examples · reveal as you go

Worked example +5 XP on full reveal

Predict whether each of the following would be more soluble in water or in hexane (a non-polar solvent), and explain in terms of IMFs: (a) sodium chloride (NaCl, ionic), (b) iodine (I₂, non-polar), (c) ethanol (CH₃CH₂OH, has,OH group).

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(a) NaCl, Classify the compound
NaCl is an ionic compound consisting of Na⁺ and Cl⁻ ions held in a lattice by strong electrostatic forces.
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Assess IMF compatibility with water
Water is polar with strong δ− on O and δ+ on H. The δ− O atoms attract Na⁺ ions; δ+ H atoms attract Cl⁻ ions → ion-dipole forces form. These forces are strong enough to pull ions from the lattice and surround them with a hydration shell.
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Assess IMF compatibility with hexane
Hexane is non-polar and can only form weak dispersion forces. These are far too weak to overcome the strong electrostatic attraction holding the ions in the lattice → NaCl does not dissolve in hexane.
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(b) I₂, Identify IMF type
I₂ is a non-polar molecule with only dispersion forces between molecules.
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Apply "like dissolves like"
Hexane is non-polar with only dispersion forces. The dispersion interactions between I₂ and hexane are compatible with the dispersion forces being broken → I₂ dissolves in hexane. Water's strong H-bond network cannot be disrupted by weak dispersion interactions → I₂ is insoluble in water.
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Worked example +5 XP on full reveal

A student states: "BaSO₄ is an ionic compound, and ionic compounds dissolve in water, so BaSO₄ must dissolve in water." Identify the error in this reasoning and provide a correct explanation.

1
Identify the overgeneralisation
The student's claim that "all ionic compounds dissolve in water" is an oversimplification. While many ionic compounds are soluble in water, some are insoluble or sparingly soluble (BaSO₄, AgCl, CaCO₃ are classic examples).
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State what determines solubility
Solubility of an ionic compound depends on the balance between two competing energies: (1) lattice energy, the energy required to separate the ions in the solid, and (2) hydration energy, the energy released when water molecules surround the separated ions.
3
Analyse BaSO₄ specifically
BaSO₄ has a very high lattice energy because both Ba²⁺ and SO₄²⁻ are doubly charged (Ba²⁺ has a 2+ charge, SO₄²⁻ has a 2− charge). This creates very strong electrostatic attractions in the crystal lattice.
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Compare hydration energy to lattice energy
When Ba²⁺ and SO₄²⁻ ions are separated and enter water, the hydration energy released (from ion-dipole interactions with water) is insufficient to compensate for the large lattice energy that must be overcome. The energy balance is unfavourable for dissolution.
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State the correct rule
The correct statement is: "Ionic compounds tend to dissolve in water if and only if their hydration energy exceeds their lattice energy." For BaSO₄, lattice energy > hydration energy → BaSO₄ is insoluble in water, despite being ionic.
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Misconception to fix

Common errors · the 3 traps that cost marks

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Misconception to fix

Wrong: All ionic compounds dissolve in water because water is polar.

2

Misconception to fix

Right: Water dissolves many ionic compounds through ion-dipole interactions, but not all. Solubility depends on the balance between lattice energy and hydration energy. Compounds with very high lattice energy (e.g., AgCl, BaSO₄) are insoluble despite water's polarity.

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Saying "polar molecules attract non-polar molecules"

A common slip: "water dissolves oil because polar attracts non-polar". The energy gain from a polar–non-polar interaction is small (only dispersion), it can't compensate for the strong H-bonds broken in water. Polar and non-polar substances are not attracted enough to mix.

Fix: Frame solubility as IMF compatibility, not "attraction", like dissolves like because the IMFs broken and formed are of similar strength.

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Drill, then revisit

Quick-fire practice · 5 reps +2 XP per reveal

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State the "like dissolves like" rule in one sentence.

2

Will hexane (C₆H₁₄) dissolve well in water? Explain in one line.

3

Explain why NaCl dissolves readily in water.

4

Predict the better solvent for iodine (I₂): water or cyclohexane. Justify.

5

Ethanol is fully miscible with water, yet it also dissolves in some less-polar organic solvents. What structural feature explains this, and why is its solubility in non-polar oils only limited?

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Revisit your thinking

Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?

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Interactive Tool, Solubility Curve Explorer Open fullscreen ↗
True or false? On a solubility curve, a point that sits below the line represents an unsaturated solution, which can still dissolve more solute at that temperature.
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Multiple choice

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Multiple choice
+5 XP per correct · +25 XP all-correct

Pick your answer, then rate your confidence. That tells the system what to drill next.

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Short answer
ApplyApply4 MARKS

Q1. 6. A chemist has a mixture of iodine (I₂) and sodium chloride (NaCl) dissolved in water. They want to extract the iodine using hexane. Explain why this extraction works, with reference to the principle of "like dissolves like" and the IMFs involved.

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ApplyApply3 MARKS

Q2. 7. The amino acid glycine has the formula H₂N–CH₂–COOH. It has both an amino group (–NH₂) and a carboxylic acid group (–COOH). Predict whether glycine would be more soluble in water or hexane, and justify your prediction using IMF reasoning.

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ApplyApply3 MARKS

Q3. 8. Explain, using the concept of IMF compatibility, why BaSO₄ is insoluble in water despite being an ionic compound. In your answer, refer to lattice energy and hydration energy.

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📖 Comprehensive answers (click to reveal)

Activity 1

1. KNO₃: dissolves in water, K⁺ and NO₃⁻ ions are stabilised by ion-dipole forces with polar water molecules; hydration energy exceeds lattice energy. Octane: insoluble in water, non-polar molecule, only dispersion forces; water's H-bond network is disrupted for no compensating gain; instead dissolves in non-polar solvents (like hexane). Glucose: dissolves in water, multiple,OH groups form H-bonds with water; hydrophilic character dominates. AgCl: insoluble in water, very high lattice energy (Ag⁺ and Cl⁻ are strongly attracted); hydration energy insufficient to overcome lattice energy, even though AgCl is ionic.

2. Observation: iodine (brown/purple) concentrates in the upper hexane layer; the lower water layer becomes essentially colourless. Explanation: I₂ is non-polar, it has only dispersion forces. It is compatible with the dispersion-force-only hexane layer but incompatible with water's H-bond network. The hexane layer therefore dissolves I₂ preferentially. Water and hexane are immiscible (non-polar vs polar) so two distinct layers form.

️ Activity 2

A: Error: the student correctly identified the polar bonds but incorrectly concluded the molecule is polar overall. CO₂ has a linear, symmetric geometry (O=C=O); the two C=O dipoles point in opposite directions and cancel exactly → net dipole = 0 → CO₂ is non-polar despite having polar bonds. As a non-polar molecule, CO₂ has only dispersion forces and is only sparingly soluble in water.

B: Error: "oil has no intermolecular forces" is factually incorrect. All molecules have dispersion forces, oil molecules (hydrocarbons) have dispersion forces between them. The correct explanation: oil is non-polar and has only dispersion forces, which are incompatible with water's strong H-bond network. Dissolving oil in water would require breaking H-bonds in water, which releases no compensating energy from oil–water dispersion interactions → thermodynamically unfavourable → immiscible.

C: Error: "has no ionic bonds" is irrelevant, solubility is determined by IMF compatibility, not the presence/absence of ionic bonds. The correct explanation: ethanol (CH₃CH₂OH) has an,OH group that can form hydrogen bonds with water (O–H···O). These H-bonds between ethanol and water are comparable in strength to the H-bonds within pure water and within pure ethanol, so minimal net energy change occurs when they mix. This IMF compatibility makes ethanol fully miscible with water.

❓ Multiple Choice

Octane is non-polar → dissolves in non-polar hexane (like dissolves like). NaCl is ionic (incompatible with non-polar solvent). Ethanol and glucose have,OH groups (hydrophilic) → more water-soluble.

The IMF mismatch explanation. Density difference (D) explains why oil floats but not why they don't mix (miscibility ≠ density).

As carbon chain grows, the hydrophobic portion dominates. The,OH group is always present but its contribution relative to the entire molecule decreases with chain length.

Micelle mechanism: non-polar tails in grease, polar heads in water. No chemical reaction, no change in polarity of grease.

Ethanol has,OH group → H-bonds with water → miscible. Hexane and water are immiscible (non-polar vs polar). Motor oil is non-polar.

Short Answer Model Answers

Q6 (4 marks): I₂ is non-polar, it has only dispersion forces between molecules (1 mark). Hexane is a non-polar solvent with only dispersion forces; the IMFs between I₂ and hexane are compatible (both dispersion) → I₂ preferentially dissolves in hexane (1 mark). NaCl is ionic, Na⁺ and Cl⁻ ions require ion-dipole interactions to dissolve; water provides these through its polar O–H bonds (1 mark). Hexane cannot provide ion-dipole forces → NaCl remains in the water layer. Adding hexane creates two immiscible layers; I₂ concentrates in hexane, NaCl remains in water → effective separation (1 mark).

Q7 (3 marks): Glycine is more soluble in water (1 mark). The,NH₂ group can act as a H-bond donor/acceptor with water (N–H···O and N···H–O interactions) (1 mark). The,COOH group can also H-bond with water (O–H···O and C=O···H interactions). Both functional groups are hydrophilic and compatible with water's H-bond network. In hexane (non-polar), these polar groups cannot form compatible IMFs → glycine is insoluble in hexane (1 mark).

Q8 (3 marks): Although BaSO₄ is ionic, solubility requires the hydration energy of its ions to exceed the lattice energy (1 mark). BaSO₄ has a high lattice energy due to the large charges involved (Ba²⁺ with 2+ charge and SO₄²⁻ with 2− charge → strong electrostatic attraction) (1 mark). The energy released when water molecules surround Ba²⁺ and SO₄²⁻ (hydration energy) is insufficient to overcome this high lattice energy → dissolution is thermodynamically unfavourable → BaSO₄ remains insoluble (1 mark).