Solubility and Choice of Solvents
Quick Notes
- Solubility refers to how well a substance can dissolve in a solvent.
- A substance is likely to dissolve if:
- It can form similar intermolecular forces to those present in the solvent
- The strength of solute–solvent forces is comparable to solute–solute and solvent–solvent forces
- Water is a polar solvent capable of forming hydrogen bonds:
- Dissolves ionic compounds (by hydration of ions)
- Dissolves small polar molecules (e.g. alcohols)
- It is a poor solvent for non-polar or large polar molecules that cannot form hydrogen bonds
- Non-aqueous solvents (e.g. hexane, ethanol) are used for dissolving non-polar substances (e.g. iodine, oils)
Full Notes
Solubility is the ability of a substance (solute) to dissolve in a solvent to form a homogeneous solution.
It depends on:
- The types of particles involved (ions, molecules)
- The interactions between solute and solvent particles
- The types of intermolecular forces present
The general rule is: “Like dissolves like” – substances with similar types of bonding or intermolecular forces are likely to be soluble in each other.
Water as a Solvent
Water is a highly polar solvent and can form hydrogen bonds. This makes it very effective at dissolving ionic compounds and small polar molecules.
Dissolving Ionic Compounds
When ionic compounds dissolve, water molecules surround and separate the ions.
- Positive ions are attracted to the δ⁻ oxygen in water
- Negative ions are attracted to the δ⁺ hydrogen
This process is called hydration. If the hydration energy is greater than the lattice energy (energy required to break apart the ionic solid), the compound dissolves.
Dissolving Small Polar Molecules
Water also dissolves small molecules that can form hydrogen bonds with water, such as:
- Alcohols (e.g. ethanol)
- Ammonia
- Simple carboxylic acids
Whether a substance dissolves in water depends on the relative strength of the interactions between solute and water molecules compared to the interactions within the solute itself and within water. For example, ethanol is soluble in water because the intermolecular forces between ethanol and water molecules are similar in strength to the forces within ethanol and within water. Hexanol is not soluble because water–water hydrogen bonds are stronger than water–hexanol interactions, so it is preferable for water molecules to bond with each other.
Water as a Poor Solvent
Water is a poor solvent for large polar molecules that cannot form hydrogen bonds (e.g. halogenoalkanes) and non-polar molecules (e.g. iodine, hydrocarbons).
These substances cannot interact favourably with water’s hydrogen bonding network. The hydrogen bonding between water molecules is stronger than solute–water interactions, so water molecules bond with each other rather than mix with the solute.
Non-Aqueous Solvents
For substances that don’t dissolve in water, non-aqueous solvents can be used. These solvents dissolve substances through London forces or dipole–dipole interactions instead of hydrogen bonding.
Examples
- Hexane: non-polar solvent, dissolves oils and iodine
- Ethanol: polar, can dissolve both polar and some non-polar compounds
- Diethyl ether, chloroform: used in organic chemistry for extracting non-polar solutes
Solubility in these solvents depends on matching the type of intermolecular force:
- Non-polar solutes dissolve in non-polar solvents (e.g. iodine in hexane)
- Polar solutes dissolve in polar solvents that can interact with similar strength attractions
Summary
- Solubility depends on similarity and strength of intermolecular forces between solute and solvent.
- Water dissolves ionic compounds and small polar molecules by hydrogen bonding or ion–dipole interactions.
- Non-polar and large polar molecules that cannot hydrogen bond are insoluble in water.
- Non-aqueous solvents like hexane or ethanol are used to dissolve non-polar or mixed polarity substances.