Solubility: Study Guide (Chemistry 3.10)

Soluble, insoluble and miscible

Stir salt into water and it disappears into a clear solution. Stir sand into water and it sinks to the bottom. Pour cooking oil onto water and it floats as a separate layer. This topic explains these everyday results using the intermolecular forces from Topic 3.1.

A few words first:

• A substance is soluble in a solvent if a noticeable amount dissolves to form a solution. Solubility is a matter of degree: “insoluble” in practice means that only a tiny amount dissolves.
• Two liquids are miscible if they mix in all proportions to form a single phase. Ethanol and water are miscible.
• Liquids that separate into two layers are immiscible. Hexane and water are immiscible; hexane is less dense, so it forms the top layer.

Dissolving at the particle level

When a solute dissolves, three things happen to the attractions between particles:

1. Solute–solute attractions are overcome, as the solute particles are separated from each other.
2. Solvent–solvent attractions are partly overcome, as solvent particles move apart to make room.
3. Solute–solvent attractions form, as each solute particle becomes surrounded by solvent particles.

Steps 1 and 2 cost energy. Step 3 releases energy. A substance dissolves readily when the solute–solvent attractions in step 3 are comparable in strength to the attractions lost in steps 1 and 2. That happens when the solute and the solvent have similar kinds of intermolecular interactions. This is the idea behind the rule of thumb “like dissolves like”.

Background: mixing also spreads the particles out, which on its own favours dissolving. This is why two liquids with almost identical forces, such as two alkanes, mix freely. A later unit gives this tendency a name (entropy).

Solute	Solvent	Interactions involved	Result
Ethanol, C₂H₅OH	Water	Both form hydrogen bonds; ethanol–water hydrogen bonds replace those lost	Miscible
Iodine, I₂	Hexane	Both nonpolar; dispersion forces replaced by similar dispersion forces	Dissolves well
Hexane	Water	Water–water hydrogen bonds would be lost; only weak dispersion attractions between hexane and water	Immiscible: only about 0.01 g dissolves per litre
Sodium chloride	Water	Strong ion–ion attractions replaced by strong ion–dipole attractions	Soluble
Sodium chloride	Hexane	Strong ion–ion attractions; hexane offers only weak dispersion attractions	Insoluble
Iodine, I₂	Water	Water’s hydrogen bonds disrupted; I₂ is nonpolar	Only slightly soluble

Notice the pattern in the last four rows. A nonpolar substance struggles to dissolve in water not because water repels it, but because water molecules attract each other so strongly through hydrogen bonds. Pushing a nonpolar molecule between them breaks those hydrogen bonds and offers nothing as strong in return.

Ionic compounds in water: ion–dipole attractions

Water is a polar molecule. Its oxygen end carries a partial negative charge (δ−) and its hydrogen ends carry partial positive charges (δ+). When an ionic solid dissolves, each ion becomes surrounded by water molecules held by ion–dipole attractions, which are stronger than hydrogen bonds between water molecules. The orientation of the water molecules follows from Coulomb’s law: opposite charges face each other.

When you draw a particulate model of an ionic solution (as in Topic 3.8), check three things: the ions are separate from each other, the water molecules face each ion the right way round, and the numbers of cations and anions match the formula.

Not every ionic compound dissolves

Ion–dipole attractions are strong, but so are the attractions between ions in the solid. When the ions attract each other very strongly, the water molecules may not be able to pull them apart to any useful extent. Calcium carbonate (limestone and chalk) and silver chloride are ionic, yet only tiny amounts dissolve in water. So “ionic means soluble in water” is not a safe rule. For this course you should know that all sodium, potassium, ammonium and nitrate salts are soluble in water; you will use this when writing net ionic equations in Unit 4. How much of a sparingly soluble salt dissolves is treated with numbers in Unit 7.

Molecules with a polar end and a nonpolar end

Many molecules have both a polar part and a nonpolar part. The straight-chain alcohols are the classic example. Each has one O–H group, which can hydrogen bond with water, attached to a hydrocarbon chain, which has only dispersion forces.

Worked example 1: water or hexane?

Question. For each substance, predict whether it dissolves better in water or in hexane, C₆H₁₄. Justify each answer with the intermolecular interactions involved. (a) Magnesium sulfate, MgSO₄. (b) Paraffin wax, a mixture of long-chain alkanes such as C₂₅H₅₂. (c) Glucose, C₆H₁₂O₆, which has five O–H groups. (d) Iodine, I₂.

(a) Water. MgSO₄ is ionic. Water molecules form strong ion–dipole attractions with Mg²⁺ (oxygen ends inward) and SO₄²⁻ (hydrogen ends inward), which can compensate for the ion–ion attractions lost. Hexane offers only weak dispersion attractions, nowhere near enough.

(b) Hexane. The wax molecules are nonpolar, held together by dispersion forces. Hexane molecules interact with them through the same kind of force, so dissolving swaps like for like. In water, the wax would break up water’s hydrogen bonds and offer only dispersion attractions in return.

(c) Water. Each glucose molecule can form several hydrogen bonds with water through its O–H groups, replacing the glucose–glucose and water–water hydrogen bonds that are lost. Hexane cannot hydrogen bond.

(d) Hexane. I₂ is nonpolar, with a large electron cloud (106 electrons) and strong dispersion forces. Hexane molecules attract it by dispersion forces too. Only a small amount dissolves in water, for the same reason that hexane does not.

Check. Each answer names the forces in the solute, in the solvent and between the two. An answer such as “like dissolves like” with nothing else would not earn credit.

Worked example 2: why chain length matters

Question. Use Figure 2. (a) Describe the trend in solubility in water as the chain gets longer. (b) Explain the trend at the particle level. (c) Predict how the solubility in water of 1-octanol (8 carbon atoms) compares with that of 1-hexanol, and which of 1-butanol and 1-hexanol has the stronger attractions to hexane molecules.

(a) The first three alcohols are miscible with water. From 1-butanol on, solubility falls steeply: 73 g/L, then 22 g/L, then 5.9 g/L. Each extra CH₂ group cuts the solubility by a factor of roughly 3 to 4.

(b)

1. Every alcohol has one O–H group, which forms hydrogen bonds with water. This part of the molecule is “like” water.
2. The hydrocarbon chain interacts with water only through weak dispersion attractions. Water molecules next to the chain lose hydrogen bonds with each other and gain little in return.
3. As the chain grows, the nonpolar part becomes a larger share of the molecule while the single O–H group stays the same. The molecule becomes less “like” water, so less of it dissolves.

(c) 1-Octanol has an even longer nonpolar chain, so it should be less soluble in water than 1-hexanol (below 5.9 g/L). In hexane the trend reverses: a longer chain gives stronger dispersion attractions with hexane and a smaller polar fraction, so 1-hexanol has the stronger attractions to hexane and is the more “like” hexane of the two. This is why longer-chain molecules of this kind become more soluble in nonpolar solvents.

Worked example 3: how good is “like dissolves like”?

Question. A student writes: “Like dissolves like: polar and ionic solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents.” Explain the degree to which this model describes the solubility of (a) hexane and of sodium chloride in water, (b) 1-butanol in water, and (c) calcium carbonate in water.

(a) The model works well. Hexane is nonpolar and water is polar, so the model predicts that hexane will not dissolve; it is immiscible. NaCl is ionic and water is polar, so the model predicts that it dissolves; it does. The model captures the key particle-level idea: dissolving works when the solute–solvent attractions resemble those being replaced.

(b) The model only partly works. 1-Butanol has a polar O–H group and a nonpolar four-carbon chain, so it is partly “like” water and partly not. The model cannot say how much dissolves. It is a qualitative model: the data show 73 g/L, which is neither miscible nor insoluble.

(c) The model fails. CaCO₃ is ionic, so the model predicts that it dissolves, but only a tiny amount does. The model looks only at types of interaction, not their strengths. The attractions between Ca²⁺ and CO₃²⁻ ions in the solid are strong enough that ion–dipole attractions to water cannot replace them to any great extent. To explain this, you need to compare the strengths of the attractions lost and gained.

Summary. “Like dissolves like” is a good first guess for which solvent to choose. It does not predict amounts, and it can fail when one set of attractions is much stronger than the model assumes.

Common misconceptions

• “Water repels nonpolar molecules.” There are weak attractions between water and hexane. Hexane stays out because water molecules attract each other much more strongly.
• “All ionic compounds dissolve in water.” Many do not, such as calcium carbonate and silver chloride.
• “When sugar dissolves, its molecules break apart into atoms.” Dissolving separates whole molecules from each other. The covalent bonds inside glucose stay intact.
• “Dissolving and melting are the same.” Melting needs heat and involves one substance. Dissolving involves a solvent, and the solute particles end up surrounded by solvent particles.
• “Insoluble means none dissolves.” Even hexane dissolves in water very slightly. “Insoluble” means the amount is negligible for most purposes.
• “Bigger molecules are always less soluble.” Glucose is larger than 1-hexanol but far more soluble in water, because of its many O–H groups. Compare the balance of polar and nonpolar parts, not size alone.
• “Oil and water do not mix because oil is less dense.” Density only decides which layer is on top. Intermolecular forces decide whether the liquids mix.
• Drawing water the wrong way round. Around a cation, oxygen faces in; around an anion, hydrogen faces in.

Where this leads

Solubility ties together the intermolecular forces of this unit. You will meet it again when you decide which ionic compounds form precipitates and write net ionic equations (Unit 4), when you look at the energy changes of dissolving (Unit 6), and when you calculate how much of a sparingly soluble salt dissolves (Unit 7). Next comes Topic 3.11, Spectroscopy and the Electromagnetic Spectrum. If you want to revisit separation methods first, see Topic 3.9. Try the practice questions now, then use the revision notes and the checklist to consolidate.