Oil spills are devastating — and stubbornly hard to clean up. Why doesn't oil just dissolve in seawater? Dry cleaning removes grease stains that water can't touch — how? Both questions have the same answer: "like dissolves like." The polarity of the solvent and solute must be compatible for dissolution to occur. This single principle explains everything from drug delivery to industrial extraction to why you can't clean a greasy pan with cold water.
Core Content
Dissolving is an energy balance. For a solute to dissolve in a solvent, three things must happen:
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.
| Solute type | Solvent type | Compatible IMFs? | Soluble? | Example |
|---|---|---|---|---|
| Ionic (Na⁺, Cl⁻) | Polar (H₂O) | ✅ Ion-dipole forces | Yes | NaCl dissolves in water |
| Polar molecule | Polar (H₂O) | ✅ H-bonds or dipole-dipole | Yes | Ethanol, sucrose dissolve in water |
| Non-polar molecule | Polar (H₂O) | ❌ Only dispersion vs H-bonds | No | Hexane, oils don't dissolve in water |
| Non-polar molecule | Non-polar (hexane) | ✅ Dispersion–dispersion | Yes | Iodine, grease dissolve in hexane |
| Ionic | Non-polar (hexane) | ❌ Ion needs stronger interaction | No | NaCl doesn't dissolve in hexane |
Water is the most common polar solvent in chemistry and biology. Its exceptional solvent properties come from:
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.
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.
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.
Insert diagram showing soap molecules forming a micelle around a grease droplet: non-polar tails pointing inward (into the grease), polar/ionic heads pointing outward (into water). Label: hydrophilic head (charged/polar), hydrophobic tail (non-polar C chain), micelle, grease particle, water molecules.
Worked Examples
| (a) NaCl — ionic solid NaCl consists of Na⁺ and Cl⁻ ions held in a lattice by strong electrostatic forces. Water (polar): δ− O atoms attract Na⁺; δ+ H atoms attract Cl⁻ → ion-dipole forces form → ions are hydrated and pulled from lattice → dissolves in water. Hexane (non-polar): only dispersion forces available → far too weak to pull ions from the lattice → does NOT dissolve in hexane. Prediction: Soluble in water; insoluble in hexane. | NaCl + water is the classic "like dissolves like" example. The ionic charges require the strong polar interactions only water (or other polar solvents) can provide. This is also why ionic compounds are excellent electrolytes in water. |
| (b) I₂ — non-polar molecular solid I₂ molecules are held together by dispersion forces only (non-polar, symmetric). Water (polar): Disrupting H-bonds in water provides no comparable energy gain from I₂–water interaction (only dispersion–H-bond mismatch) → mostly insoluble in water (I₂ has very low solubility in water, ~0.03 g/100 mL). Hexane (non-polar): Dispersion forces between I₂ and hexane are compatible — both types of dispersion force are disrupted and reformed with comparable energy → dissolves readily in hexane. Prediction: Mostly insoluble in water; soluble in hexane. | A classic demonstration: add iodine to a mixture of water and hexane. The I₂ (brown) concentrates in the hexane layer, leaving the water colourless. This is used in solvent extraction procedures. |
| (c) Ethanol (CH₃CH₂OH) — polar with –OH group Ethanol has an –OH group → can form hydrogen bonds with water (O–H···O). The non-polar CH₃CH₂– chain can interact via dispersion forces with hexane. Water: H-bonds between ethanol's –OH and water's –OH → compatible IMFs → ethanol is fully miscible with water (dissolves in all proportions). Hexane: The –OH group is polar → H-bonding lost but weak dispersion forces with hexane → limited solubility in hexane. Prediction: Fully miscible with water; limited solubility in hexane. | Short-chain alcohols (methanol, ethanol, propanol) are fully miscible with water because the –OH group dominates. Longer-chain alcohols (hexanol, octanol) become less water-soluble as the non-polar chain grows longer than the –OH contribution. |
| Identify the errorThe student's general rule ("ionic compounds dissolve in water") is an oversimplification. It is not universally true — many ionic compounds are insoluble or sparingly soluble in water (BaSO₄, AgCl, CaCO₃, CaF₂ are all classic examples). | Students often memorise "ionic = water soluble" as a rule. It's a tendency, not a rule. The actual determinant is the balance between lattice energy and hydration energy. |
| Correct explanationSolubility of ionic compounds in water depends on the balance between: • Lattice energy: the energy required to separate the ions (input energy) • Hydration energy: the energy released when water molecules surround the ions (released energy) BaSO₄ has a very high lattice energy (Ba²⁺ is large; SO₄²⁻ has 2− charge → strong ionic attraction). The hydration energy released when Ba²⁺ and SO₄²⁻ are hydrated is insufficient to compensate for the high lattice energy → dissolution is thermodynamically unfavourable → BaSO₄ is insoluble. | This is why BaSO₄ is used in gravimetric analysis of sulfate (it's so insoluble it precipitates quantitatively). It's also used as a "barium meal" in medical X-rays — the patient swallows BaSO₄ suspension; it's X-ray opaque but safely insoluble (non-toxic because it doesn't dissolve and release Ba²⁺ ions). |
Activities
1 Predict the solubility of each substance in water and explain in terms of IMFs: (a) KNO₃ (ionic), (b) octane (C₈H₁₈, non-polar), (c) glucose (C₆H₁₂O₆, has multiple –OH groups), (d) AgCl (ionic, very high lattice energy).
2 A student adds iodine (I₂) to a test tube containing both water and hexane. Predict what will be observed and explain the result.
A Student error: "Carbon dioxide (CO₂) is polar because it has two polar C=O bonds, so it should dissolve well in water." Identify the error and give the correct reasoning.
B Student error: "Vegetable oil doesn't dissolve in water because oil has no intermolecular forces." Identify the error and give the correct statement.
C Student error: "Ethanol is fully miscible with water because ethanol has no ionic bonds." Identify the incomplete reasoning and provide the correct mechanistic explanation.
Multiple Choice
1. Which substance would be most soluble in hexane (a non-polar solvent)?
2. Which correctly explains why oil and water are immiscible?
3. The solubility of alcohols in water decreases as the carbon chain length increases (e.g. methanol > ethanol > propanol > butanol). The best explanation is:
4. Soaps clean greasy surfaces by:
5. Which pair of liquids would be expected to be miscible?
Short Answer
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
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
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
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.
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.
1. C — 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.
2. B — The IMF mismatch explanation. Density difference (D) explains why oil floats but not why they don't mix (miscibility ≠ density).
3. D — 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.
4. A — Micelle mechanism: non-polar tails in grease, polar heads in water. No chemical reaction, no change in polarity of grease.
5. C — Ethanol has –OH group → H-bonds with water → miscible. Hexane and water are immiscible (non-polar vs polar). Motor oil is non-polar.
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).
Tick when you've finished all activities and checked your answers.