Solvents and solubilityEdexcel International A Level Chemistry: Revision notes
Section 1
What happens when a substance dissolves
A solute dissolves when the energy released by forming new attractions between solute and solvent is enough to compensate for the energy needed to break the existing attractions.
- Attractions in the solute must be overcome (for example the ionic lattice, or intermolecular forces).
- Attractions between solvent molecules must be overcome (for example hydrogen bonds in water).
- New attractions form between solute and solvent.
A substance is soluble when the new attractions are similar in strength to those broken. When they are much weaker, the substance is insoluble. The solvent is chosen to match the intermolecular forces in the solute.
Section 2
Water as a solvent for ionic compounds
Water molecules are polar: oxygen is δ− and hydrogen is δ+.
When an ionic solid such as NaCl is added to water:
- The δ− oxygen atoms are attracted to the positive ions (Na⁺).
- The δ+ hydrogen atoms are attracted to the negative ions (Cl⁻).
- The lattice breaks up and each ion is surrounded by water molecules. This is hydration.
The energy released on hydration compensates for the energy needed to break the ionic lattice.
Water does not form ionic bonds or covalent bonds with the ions. Name the attraction between δ− O and the cation, and δ+ H and the anion.
Section 3
Water as a solvent for simple alcohols
Simple alcohols such as methanol and ethanol dissolve in water because they form hydrogen bonds with water molecules.
- The δ+ hydrogen of the O–H group in the alcohol is attracted to a lone pair on the oxygen of water.
- The δ+ hydrogen of water is attracted to a lone pair on the oxygen of the alcohol.
These hydrogen bonds are similar in strength to the hydrogen bonds between water molecules that are broken, so ethanol mixes completely with water.
The O–H group is the key feature. A molecule with hydrogen only bonded to carbon cannot be an H-bond donor.
Mention hydrogen bonds between the alcohol and water, then say they match the strength of those broken.
Section 4
Compounds that are poor solvents in water
Halogenoalkanes such as 1-chlorobutane have polar C–Hal bonds but cannot form hydrogen bonds with water, because no hydrogen is bonded to N, O or F.
For a halogenoalkane to dissolve, hydrogen bonds between water molecules must be broken. The attractions that form between the halogenoalkane and water (permanent dipole and London forces) are weaker than those hydrogen bonds. Not enough energy is released, so it is only slightly soluble.
Non-polar molecules such as alkanes behave in the same way and are also insoluble in water.
A halogenoalkane is polar, so do not say it is insoluble because it is non-polar. The reason is that it cannot form hydrogen bonds.
Section 5
Non-aqueous solvents
A non-aqueous solvent is a solvent other than water, for example hexane or ethanol.
A solute dissolves best in a solvent with similar intermolecular forces, because the new attractions formed are similar to those broken.
- Non-polar compounds (alkanes, halogens, many halogenoalkanes) dissolve in non-polar solvents such as hexane. Both have London forces.
- Ionic compounds are not soluble in hexane, because hexane cannot attract the ions.
Example: octane dissolves in hexane. The London forces formed between octane and hexane are similar to those broken in each liquid. Octane does not dissolve in water.
Choose a solvent by asking: what forces are in the solute, and which solvent has forces of a similar type and strength?
'Like dissolves like' is only a summary. State the forces in the solute and the solvent to score the marks.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Solvents and solubility
- Sodium chloride is a white ionic solid. It dissolves readily in water but is insoluble in hexane, C₆H₁₄, which is a non-polar liquid.Explain how water dissolves sodium chloride.2 marks
- Ethanol, CH₃CH₂OH, mixes completely with water. 1-Chlorobutane, CH₃CH₂CH₂CH₂Cl, is a polar liquid but is only very slightly soluble in water.Explain, in terms of intermolecular forces, why ethanol mixes completely with water.2 marks
- A technician is choosing solvents. Water is a polar liquid whose molecules are held together by hydrogen bonds. Hexane is a non-polar liquid whose molecules are held together only by London forces. 1-Bromobutane, CH₃CH₂CH₂CH₂Br, is a polar molecule that cannot form hydrogen bonds.Explain why 1-bromobutane is only slightly soluble in water.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).