Solutions form through a process which involves a rearrangement of bonds.
The bonds between particles of a solute and the bonds between particles of a solvent are broken.
New bonds between the solute and the solvent are formed.
Breaking bonds require energy, and forming bonds release energy. It follows that, for a solution to form, the old bonds and the new bonds need to be of similar strength.
- Polar and ionic solutes will dissolve in polar solvents
- Non-polar solutes will dissolve in non-polar solvents
- The solubility of a polar solute, and an ionic compound, decreases as the polarity of the solvent decreases
- The solubility of a non-polar solute increases as the polarity of the solvent decreases
Mixing polar solutes with polar solvents
Methanol, CH3OH, is a polar substance, with hydrogen bonds holding together its molecules.
Water is polar solvent, its molecules are also hydrogen bonded.
- The bonds that need to be broken are the hydrogen bonds in both the methanol and water.
- The bonds that would be formed are also hydrogen bonds.
Thus, it would make sense for methanol to be soluble in water, since similar types of bonds are being formed , and it is.
Propanone (acetone) is a polar liquid with only dipole-dipole forces between its particles.
It, too, is soluble in water.
- The bonds that need to be broken are dipole-dipole bonds between propanone, and hydrogen bonds between water.
- The bonds that would be formed are hydrogen bonds between the propanone and water molecules.
Because the dipole-dipole forces are weaker and the hydrogen bonds broken between water molecules are of similar strength to the new bonds being formed, it forms a solution.
Some substances contain a polar group, but the rest of the molecule is non-polar. These molecules are usually organic molecules with long chains of carbon. Consider butan-1-ol, CH3CH2CH2CH2OH. Although the OH group is polar, the rest of the chain is not.
When the chain is short, the hydrocarbons have little effect, and it is soluble. However, when the chain lengthens, these molecules would push water molecules apart to make space, meaning that more hydrogen bonds would need to be broken, leaving only dispersion forces between the hydrocarbon and water. Thus, molecules with long chains of hydrocarbons are expected to less soluble than those with shorter chains.
Mixing non-polar solutes with polar solvents
Tetrachloromethane is a non-polar liquid, and is insoluble in water.
- The bonds broken are dispersion forces between the CCl4 and water’s hydrogen bonds
- The new bonds are dispersion forces between the water and CCl4 molecules
Although CCl4‘s bonds are of similar strength to the new bonds, water’s hydrogen bonds are much stronger than the new ones, so it is unlikely that a solution will be formed.
Mixing an non-polar solute in a non-polar solvent
Tetrachloromethane would be expected to dissolve in hexane, a non-polar liquid, and it does.
- The bonds to be broken are dispersion forces in both CCl4 and hexane
- The bonds to be formed are also dispersion forces between the molecules
Since the new and old bonds are of similar strengths, they are miscible.
Note that long chains of hydrocarbons increase in solubility in hexane as it lengthens, because they become less polar, contain more electrons and thus the dispersion forces between the hexane and the chain are larger.