Solubility and Like-Dissolves-Like
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Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
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.
Key facts
- The principle "like dissolves like"
- How polarity determines solubility
- Definitions of miscible, immiscible, hydrophilic, hydrophobic
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
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
Dissolving is an energy balance. For a solute to dissolve in a solvent, three things must happen:
- Solute–solute bonds/IMFs must be broken (energy input required)
- Solvent–solvent IMFs must be disrupted (energy input required)
- 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.
| 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 |
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.
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.
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
- Water molecules approach the NaCl lattice surface
- δ− O atoms orient toward Na⁺ ions; δ+ H atoms orient toward Cl⁻ ions
- Ion-dipole forces overcome the interionic (ionic lattice) attraction at the surface
- Na⁺ and Cl⁻ ions are pulled from the lattice, becoming surrounded by a hydration shell
- Hydrated ions disperse throughout the solution
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.
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?
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.
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.
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
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
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.
Worked examples · reveal as you go
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).
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.
Common errors · the 3 traps that cost marks
Misconception to fix
Wrong: All ionic compounds dissolve in water because water is polar.
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.
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.
Quick-fire practice · 5 reps +2 XP per reveal
State the "like dissolves like" rule in one sentence.
Will hexane (C₆H₁₄) dissolve well in water? Explain in one line.
Explain why NaCl dissolves readily in water.
Predict the better solvent for iodine (I₂): water or cyclohexane. Justify.
Ethanol mixes with both water and oil. What structural feature explains this dual behaviour?
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Pick your answer, then rate your confidence — that tells the system what to drill next.
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.
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.
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.
📖 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
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.
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).
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