Why won’t iodine dissolve in water?
In 1.7.4 you met tincture of iodine, a solution of iodine in alcohol. But drop a crystal of iodine into water and shake it as long as you like: the water turns only a pale yellow-brown, and most of the iodine stays at the bottom. Table salt is the opposite. It dissolves easily in water but not at all in gasoline. What decides whether one substance dissolves in another? This lesson answers that with the forces between particles.
The three steps of dissolving
At the particle level, dissolving a solute in a solvent can be split into three steps:
- Separate the solute: the solute particles (ions or molecules) move apart. Overcoming the forces between them absorbs energy.
- Separate the solvent: solvent molecules spread out to make room for the solute. Overcoming the forces between solvent molecules also absorbs energy.
- Solute meets solvent: solvent molecules surround the solute particles and attract them, which releases energy. In water this step is called hydration水合; in general it is solvation溶剂化.
When an ionic compound dissolves in water, the partially negative O end of each water molecule points toward a cation and the partially positive H ends point toward an anion. Each ion becomes a hydrated ion水合离子. This is how the dissociation in 2.7.5 happens. The attraction between an ion and a polar molecule is called an ion–dipole attraction离子–偶极作用.
Step through the three stages for NaCl, then switch to NaOH and NH₄NO₃ and compare the energy diagrams:
Why dissolving can heat up or cool down a solution
Whether dissolving releases or absorbs heat overall depends on whether step ③ releases more energy than steps ① and ② absorb. The table gives the heat for dissolving 1 mol of each solute at 25 °C:
| Solute | Dissolving 1 mol | Temperature of the solution |
|---|---|---|
| NaOH | releases 44.5 kJ | rises noticeably |
| NaCl | absorbs 3.9 kJ | barely changes |
| NH₄NO₃ | absorbs 25.7 kJ | drops noticeably |
| KNO₃ | absorbs 34.9 kJ | drops noticeably |
That explains what you saw in 1.7.4. US textbooks call this heat the enthalpy of solution溶解焓, ΔHsoln. Chapter 2.10 introduces ΔH properly; a negative value means heat is released.
Endothermic dissolving still happens on its own. Ammonium nitrate dissolves and makes the water cold with no help from outside. The reason is that the particles become more disordered when they mix, an idea called entropy that comes later in chemistry.
Like dissolves like
In 2.4.7 you saw the rule “like dissolves like”: polar solutes tend to dissolve in polar solvents, and nonpolar solutes in nonpolar solvents. In terms of the three steps, a solute dissolves easily when step ③ releases enough energy to pay for steps ① and ②:
- Ionic and polar solutes in water: ion–dipole attractions and hydrogen bonds are strong, so step ③ releases a lot of energy and the solute dissolves.
- Nonpolar solutes in nonpolar solvents: all three steps involve only dispersion forces, so the energy needed and the energy released are about equal. Mixing makes the particles more disordered, so the solute dissolves.
- Nonpolar solutes in water: step ② has to break hydrogen bonds between water molecules, which takes a lot of energy, but I₂ and water attract each other only through weak dispersion forces, so step ③ releases little. Iodine barely dissolves.
Hydrogen bonding matters as much as polarity. NH₃ forms hydrogen bonds with water, so it is extremely soluble (2.4.8). Molecular shape matters too: methanol, ethanol, and propanol mix with water in any proportion, but from butanol on, the bigger the oily hydrocarbon part, the less the alcohol dissolves.
Two liquids that mix in any proportion are miscible互溶; liquids that separate into layers are immiscible不互溶. Using a second, immiscible solvent to pull a solute out of the first one is called extraction萃取. School labs often extract iodine from iodine water with hexane.
Pick a solute and add it to water, to a nonpolar solvent, or to a tube with both. Predict first, then shake:
How Chinese textbooks differ
- Chinese textbooks often describe dissolving as two processes: solute particles spreading into the water (absorbs heat) and solute particles combining with water molecules (releases heat). US textbooks split the first process into separating the solute and separating the solvent, and draw a three-step energy diagram.
- Chinese textbooks classify solubility at 20 °C into four levels (very soluble, soluble, slightly soluble, insoluble) with fixed cutoffs (2.7.3). US textbooks use soluble, slightly soluble, and insoluble without a single set of cutoffs.
- To extract iodine, Chinese labs use carbon tetrachloride, which is denser than water, so the purple-red layer is on the bottom. CCl₄ is toxic, so US school labs usually use hexane instead. Hexane is less dense than water, so the purple layer is on top.
Example 1: NH₃ and CH₄
NH₃ is extremely soluble in water, but CH₄ hardly dissolves at all. Explain why, using polarity and intermolecular forces.
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The idea. Dissolving has three steps: ① separating the solute particles and ② pulling some water molecules apart both absorb energy; ③ solute–water attractions forming releases energy. The more energy step ③ releases, the more easily the solute dissolves. So compare how strongly each solute attracts water.
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Polarity. NH₃ is trigonal pyramidal and polar; CH₄ is tetrahedral and nonpolar (2.4.7).
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Interactions with water. NH₃ has a lone pair on N and N–H bonds, so it hydrogen-bonds with water. CH₄ attracts water only through weak dispersion forces.
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Conclusion. NH₃ is “like” water, so step ③ releases plenty of energy and NH₃ dissolves readily. For CH₄ to dissolve, hydrogen bonds between water molecules would have to break (step ②) with nothing to make up for it, so it barely dissolves.
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One more factor. NH₃ also reacts a little with water, which raises its solubility further:
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A common mistake. Stopping at “NH₃ is polar.” A full answer names the intermolecular force: NH₃ can hydrogen-bond with water, and CH₄ can’t.
Example 2: extracting iodine
You add 2 mL of hexane to 10 mL of iodine water, shake, and let it settle. Describe what you see and explain it. Could you use ethanol instead of hexane?
- Layers. Hexane is nonpolar and water is polar, so they are immiscible. Hexane’s density is about 0.66 g/cm³, less than water’s, so it forms the top layer.
- Where the iodine goes. I₂ is nonpolar, so by “like dissolves like” it is far more soluble in hexane than in water. Shaking brings the two liquids into close contact so most of the iodine moves into the hexane; settling lets the layers separate again.
- What you see. Two layers form: the top layer turns purple and the bottom layer becomes paler, almost colorless.
- Separating the layers. In a separatory funnel, drain the bottom (water) layer through the stopcock, then pour the hexane layer out through the top, so the two don’t mix again.
- Ethanol? No. Ethanol is miscible with water, so no separate layer forms and the iodine can’t be separated. An extraction solvent must be immiscible with the first solvent, and the solute must be much more soluble in it.
- A common mistake. Assuming the colored layer is always on top. Density decides: with dichloromethane or CCl₄, which are denser than water, the purple layer is on the bottom.
- potassium chloride, KCl
It is ionic, and water molecules can surround K⁺ and Cl⁻ (ion–dipole attractions).
- octane, C₈H₁₈, the main component of gasoline
Octane is nonpolar, so it attracts a nonpolar solvent through dispersion forces: like dissolves like.
- glucose, C₆H₁₂O₆
Glucose has several –OH groups that hydrogen-bond with water.
- bromine, Br₂
Br₂ is nonpolar and much more soluble in nonpolar solvents than in water, so bromine can be extracted from water just like iodine.
- methanol, CH₃OH
Methanol is small, and its –OH group hydrogen-bonds with water, so it mixes with water in any proportion.
- vegetable oil
Oil is made of nonpolar molecules, so it doesn’t dissolve in water but mixes with nonpolar solvents.
- hydrogen chloride, HCl
HCl is polar and ionizes into H⁺ and Cl⁻ in water, so it is extremely soluble in water.
- ethanol, C₂H₅OH
Its –OH group hydrogen-bonds with water, and its ethyl group attracts nonpolar solvents through dispersion forces, so it mixes with both.
- acetone, CH₃COCH₃
The polar C=O group accepts hydrogen bonds from water, and the two methyl groups let it mix with nonpolar solvents. That is why acetone is such a useful solvent.
Common mistakes
Mistake 1: “Dissolving always releases heat” (or always absorbs it). The overall heat is the sum of three steps: dissolving NaOH releases heat, dissolving NH₄NO₃ absorbs it, and NaCl barely changes the temperature.
Mistake 2: “All ionic compounds dissolve in water.” Step ① has to break apart the ionic crystal. The crystals of AgCl, CaCO₃, and BaSO₄ are held together so strongly that hydration can’t make up for it, so they are insoluble (the solubility rules in 2.7.3).
Mistake 3: “Oil doesn’t dissolve because water repels it.” Water and oil molecules actually attract each other weakly through dispersion forces. Oil doesn’t dissolve because the hydrogen bonds between water molecules are so strong that the energy to break them isn’t paid back.
Quick check
Which of these is the best solvent for extracting iodine from iodine water?
Hexane is immiscible with water, and iodine is far more soluble in it than in water. Ethanol mixes with water, so no separate layer forms.
Which of these makes the water noticeably colder when it dissolves?
For NH₄NO₃, step ③ releases less energy than steps ① and ② absorb, so dissolving absorbs 25.7 kJ/mol. NaOH and concentrated sulfuric acid release heat, and NaCl barely changes the temperature.
Summary
- Dissolving happens in three steps: separate the solute (absorbs energy), separate the solvent (absorbs energy), and let solute and solvent attract (releases energy; in water this is hydration). Their sum decides whether dissolving is exothermic or endothermic.
- In water, the O end of each water molecule faces a cation and the H ends face an anion, forming hydrated ions held by ion–dipole attractions.
- Like dissolves like: attractions between solute and solvent must pay for separating each of them. Molecules that hydrogen-bond with water dissolve in water; nonpolar molecules dissolve in nonpolar solvents. Extraction takes advantage of this difference.
On to AP (for later)
AP topic 3.10 asks you to explain solubility with the intermolecular forces between solute and solvent, often by comparing two solutes in water. Particle-diagram questions in topic 3.8 expect water molecules drawn the right way around ions: O toward cations and H toward anions, and drawing them backward loses points. Topic 7.14 uses entropy and free energy to explain why an endothermic process like dissolving NH₄NO₃ can still happen on its own.
Data sources: heats of solution from the CRC Handbook.