Choosing Solvents
A concise revision guide to how intermolecular forces decide the choice of solvent: hydration of ions, alcohols dissolving in water by hydrogen bonding, why water is a poor solvent for halogenoalkanes, and like-dissolves-like for non-aqueous solvents, for AQA A Level Chemistry.
What Decides Whether Something Dissolves
A substance dissolves in a solvent when the attractions formed between solute and solvent particles are strong enough to compensate for the attractions broken within the solute and within the solvent. Dissolving is therefore a balance of intermolecular forces, and the rule of thumb “like dissolves like” simply means that the new solute-solvent attractions must be of a similar kind and strength to the ones being lost.
Three questions decide the outcome: what holds the solute together (ionic lattice, hydrogen bonds, permanent dipoles or induced dipole-dipole forces), what holds the solvent together, and what the two can form with each other.
Key idea: Energy is needed to break solute-solute and solvent-solvent attractions; energy is released when solute-solvent attractions form. Dissolving happens when the two roughly balance.
Water Dissolving Ionic Compounds: Hydration of Ions
Water is polar: its oxygen is δ- and its hydrogens δ+. When an ionic solid such as sodium chloride is placed in water, the δ- oxygen atoms of water molecules are attracted to the Na⁺ ions and the δ+ hydrogen atoms to the Cl⁻ ions. Each ion that leaves the lattice becomes surrounded by a shell of correctly oriented water molecules; this is hydration, and the attractions formed are called ion-dipole attractions.
The energy released by hydrating the ions is comparable to the energy needed to break up the lattice, so sodium chloride dissolves. Ionic compounds with very strongly bonded lattices, such as magnesium oxide, do not dissolve appreciably because the hydration energy cannot match the lattice energy.
Hydration: water molecules orient their δ- oxygens towards cations and their δ+ hydrogens towards anions, and the attractions formed pay for breaking the lattice.
Exam sentence: Sodium chloride dissolves in water because the δ- oxygen atoms of water molecules attract the Na⁺ ions and the δ+ hydrogen atoms attract the Cl⁻ ions; the energy released when the ions are hydrated compensates for the energy needed to break the ionic lattice.
Water Dissolving Alcohols: Hydrogen Bonding
Methanol, ethanol and propan-1-ol mix with water in all proportions because the O-H group of the alcohol forms hydrogen bonds with water molecules. The hydrogen bonds formed between alcohol and water are similar in strength to the hydrogen bonds broken within the water and within the alcohol, so there is no barrier to mixing.
As the carbon chain grows the alcohol becomes less soluble: butan-1-ol dissolves only to about 7 g per 100 g of water and hexan-1-ol barely at all. The long non-polar chain can only form induced dipole-dipole forces with water, and to make room for it water must break hydrogen bonds that the chain cannot replace. The hydrocarbon part outweighs the single O-H group.
Ethanol dissolves because its O-H group replaces the water hydrogen bonds it disrupts; chloroethane cannot form hydrogen bonds with water, so it does not dissolve.
Key idea: Small alcohols are miscible with water because they hydrogen bond with it; solubility falls as the non-polar chain lengthens.
Why Water Is a Poor Solvent for Halogenoalkanes
Halogenoalkanes such as chloroethane are polar molecules: the C-Cl bond has a permanent dipole. Yet they are almost insoluble in water. The reason is that a halogenoalkane cannot form hydrogen bonds with water: it has no hydrogen bonded to N, O or F, and the halogen atom is a poor acceptor of hydrogen bonds. To dissolve, the halogenoalkane would have to break hydrogen bonds between water molecules to make space for itself, and the weak permanent dipole-dipole forces it could form with water would not pay back that energy.
The same argument applies to many polar molecules without an O-H or N-H group, and it explains why “polar” does not automatically mean “soluble in water”. The question is always whether the solute can form hydrogen bonds with water, or at least attractions comparable to the ones it disrupts.
| Solute | Polar? | Can hydrogen bond with water? | Soluble in water? |
|---|---|---|---|
| Ethanol, C₂H₅OH | yes | yes, O-H | miscible |
| Propanone, CH₃COCH₃ | yes | yes, C=O accepts from water | miscible |
| Chloroethane, C₂H₅Cl | yes | no | almost insoluble |
| Hexane, C₆H₁₄ | no | no | insoluble |
| Sodium chloride | ionic | ions are hydrated | soluble |
Exam sentence: Chloroethane does not dissolve in water because chloroethane molecules cannot form hydrogen bonds with water molecules; the hydrogen bonds between water molecules that would have to be broken are not replaced by attractions of similar strength.
Non-Aqueous Solvents: Like Dissolves Like
Substances that water cannot dissolve usually dissolve in a solvent whose intermolecular forces match their own. A non-polar solute dissolves in a non-polar solvent because the induced dipole-dipole forces broken between solute molecules and between solvent molecules are replaced by induced dipole-dipole forces of similar strength between solute and solvent. Iodine dissolves readily in hexane or cyclohexane but only slightly in water; grease and candle wax dissolve in hexane; halogenoalkanes dissolve in hexane and in ethanol.
| Solute | Forces within the solute | Good solvent | Poor solvent |
|---|---|---|---|
| Iodine, I₂ | induced dipole-dipole forces | hexane, cyclohexane | water |
| Candle wax (long alkanes) | induced dipole-dipole forces | hexane, white spirit | water |
| Chloroethane | permanent dipole-dipole forces and induced dipole-dipole forces | hexane, ethanol | water |
| Sodium chloride | ionic lattice | water | hexane |
| Sugar (sucrose) | hydrogen bonds (many O-H) | water | hexane |
The choice of solvent matters in practical chemistry: an organic product is extracted from an aqueous mixture with a non-polar solvent in a separating funnel because the product dissolves in the organic layer and the ionic impurities stay in the water.
Exam focus: When asked to choose or justify a solvent, name the strongest force in the solute, name the strongest force in the candidate solvent, and say whether the solute-solvent attractions formed can replace those broken.
Common Exam Points
Explain why sodium chloride dissolves in water
δ- oxygen of water attracts Na⁺, δ+ hydrogen attracts Cl⁻; hydration energy compensates for lattice energy.
Explain why ethanol mixes with water but hexane does not
Ethanol forms hydrogen bonds with water; hexane can only form induced dipole-dipole forces, which cannot replace the hydrogen bonds broken in water.
Explain why a polar halogenoalkane is insoluble in water
It cannot hydrogen bond with water; the attractions it forms do not compensate for the water hydrogen bonds broken.
Suggest a solvent for iodine
A non-polar solvent such as hexane, because both are held by induced dipole-dipole forces only.
Check Your Understanding
Predict solubility for substances that are not in the tables above.
FAQs
Use these quick answers to check how intermolecular forces decide what dissolves in what.
Why does sodium chloride dissolve in water?
The δ- oxygen atoms of water molecules are attracted to the Na⁺ ions and the δ+ hydrogen atoms to the Cl⁻ ions, so the ions become hydrated. The energy released by hydration compensates for the energy needed to break up the ionic lattice.
Why do small alcohols mix with water?
The O-H group of the alcohol forms hydrogen bonds with water molecules that are similar in strength to the ones being broken, so there is no barrier to mixing. Solubility falls as the carbon chain lengthens.
Why does a polar halogenoalkane not dissolve in water?
It has no hydrogen bonded to N, O or F, so it cannot form hydrogen bonds with water. The water-water hydrogen bonds that must be broken to make room for it are not replaced by attractions of similar strength.
What does “like dissolves like” actually mean?
It means the solute-solvent attractions formed must be of the same kind and strength as the solute-solute and solvent-solvent attractions broken. Iodine dissolves in hexane because both are held only by induced dipole-dipole forces.
Does polar always mean water-soluble?
No. Chloroethane is polar but almost insoluble in water, because its dipole is not enough to replace the hydrogen bonds it disrupts. The question to ask is whether the solute can hydrogen bond with water.
Copyright and author footprint: This OLS revision page was written for Online Learning System by Dr. Mohammed Al-Fatah. It is designed for A Level Chemistry revision and should not be copied or redistributed without permission.
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