Water as a Solvent

Key Concepts

• Water is a polar molecule.
• Hydrogen bonds act between water molecules
• Dissolving a solute in water produces an aqueous solution.
• In general¹, the solute can be:

  - an ionic compound

  - a polar covalent compound

• In general, non-polar compounds do not dissolve in water
• Table summarising the types of intermolecular forces acting between water (the solvent) and different types of solutes in an aqueous solution:

  Solvent	Solute Type	Intermolecular Force	Probable Solubility in Water
  water (polar covalent)	ionic compound	ion-dipole interaction	soluble
  water (polar covalent)	polar covalent molecule	dipole-dipole interaction or hydrogen bonds	soluble
  water (polar covalent)	non-polar covalent molecule	weak intermolecular forces (dispersion forces or London forces)	insoluble

Water Molecule

Water, H₂O, is the hydride of a Group 16 element, oxygen. A water molecule is made up of 2 atoms of hydrogen and 1 atom of oxygen, electrons are shared between the oxygen atom and each hydrogen atom resulting in covalent bonds. However, the electrons involved in the covalent bonds are not shared equally, the negatively charged electrons are more attracted to the oxygen atom than they are to the hydrogen atoms, therefore each O-H bond is polar. The oxygen atom has a partial negative charge, O^δ-, and each hydrogen atom has a partial positive charge, H^δ+.

^δ-O-H^δ+

The shape of a molecule of water is determined by the interaction between the central oxygen atom's lone-pairs of electrons and its bonding-pairs of electrons. Since the lone-pairs (non-bonding pairs) of electrons repel each other to a greater extent than the repulsion between lone-pairs and bonding pairs or between the two bonding pairs of electrons, the O-H bonds are pushed closer together resulting in a bent molecule.

This bent shape is not symmetrical, so the dipoles due to the difference in the electronegativity of oxygen and hydrogen atoms do not cancel out and the water molecule has a permanent net dipole.

Intermolecular forces known as hydrogen bonds are attractive forces that act between water molecules. In the diagram below, covalent bonds between O and H atoms are shown as black solid lines and hydrogen bonds between water molecules are shown as red dotted lines:

Hydrogen bonds are stronger than the attractive forces that act between most other polar molecules (dipole-dipole interactions) and much stronger than the attractive forces that act between non-polar molecules (dispersion forces, or, London forces), which results in water having a higher melting point and boiling point than expected resulting in water being a liquid for most of the temperature and pressure range that supports life on earth, but also allows water to dissolve an enormous number and variety of substances, making water an excellent solvent with which to make aqueous solutions.

Dissolving Ionic Compounds in Water

Sodium chloride, NaCl, is the main constituent of table salt. Cooks aften add table salt to water during cooking. At room temperature and pressure, sodium chloride exists as a crystalline solid. The crystals are made up of sodium ions, Na⁺, and chloride ions, Cl^-, held together in 3-dimensional lattice by electrostatic forces of attraction which Chemists call ionic bonds. In the diagram below, a single layer of the 3-dimensional sodium chloride lattice is shown:

When we drop a teaspoon of sodium chloride into water in a pot, the sodium chloride dissolves in the water.

The solute is solid sodium chloride, NaCl_(s)
The solvent is liquid water, H₂O_(l)
And the result of the solute (NaCl_(s)) dissolving in liquid water (H₂O_(l)) is known as an aqueous solution of sodium chloride (NaCl_(aq)).

The partial positive charge on the hydrogen atoms in water molecules, H^δ+, is attracted to the negatively charged chloride ions in the sodium chloride lattice:

This attraction is referred to as an "ion-dipole interaction", that is, the partial charge on the polar molecule is attracted to an ion with the opposite charge. The "ion-dipole interaction" is shown as a red dotted line between a Cl^- ion and the partial positive charge on a H atom of the water molecule, H^δ+ in the diagram above.

If this ion-dipole attraction is strong enough, an ion can be pulled completely out of the ionic lattice. Because water molecules in liquid water are in constant motion, this dislodged ion will travel further away from the lattice and will end up being completely surrounded by water molecules. Other water molecules will be attracted to the other ions in the lattice, for example, the partial negative charge on an oxygen atom of a water molecule, O^δ-, will be attracted to a sodium ion, Na⁺, in the lattice.

Once again, if the attraction between the positively charged ion and the partial negative charge on the oxygen atom of the water molecule is strong enough, then the ion will be pulled away from the ionic lattice and eventually will be completely surrounded by water molecules.

This process continues until all the ions making up the lattice have been evenly dispersed throughout the liquid water to make an aqueous solution of sodium chloride.

Dissolving Polar Molecules in Water

Note the partial positive charge on the hydrogen atom (δ+) bonded to the oxygen atom, and also note the partial negative charge (δ-) on this oxygen atom.

When ethanol dissolves in water, the partial positive charge on this hydrogen atom is attracted to the partial negative charge on water's oxygen atom as shown in the diagram below:

The red dotted line represents the intermolecular force of attraction between a water molecule and a molecule of ethanol.
Both water and ethanol are polar molecules, and, in both molecules the polarity of the molecule is due to a very electronegative atom (oxygen) being covalently bonded to a hydrogen atom.
Since the difference in the electronegativity of oxygen and hydrogen is relatively large, the difference between the δ+ on the H atom of ethanol and the δ- on the O atom of water is relatively large, so the intermolecular force of attraction is quite strong and is called a hydrogen bond.

Water molecules can pull ethanol molecules away from the other ethanol molecules, which become completely surrounded by water molecules. Since the molecules are in constant motion, the ethanol molecules surrounded by water molecules will become evenly dispersed, resulting in the formation of an aqueous solution of ethanol.

Non-Polar Molecules in Water

Let's consider one of the molecules you might find in crude oil, the saturated hydrocarbon (alkane) known as octane which has the molecular formula C₈H₁₈ and the 2-dimensional structural formula shown below:

There is no permanent dipole present in this molecule, it is a non-polar molecule. The intermolecular forces acting between one octane molecule and another are only very weak (dispersion forces or London forces).

Polar water molecules can interact only very weakly with non-polar octane molecules, that is, the intermolecular force of attraction between an octane molecule and a water molecule is the very weak dispersion force (or London force).
As seen in the discussion above, polar water molecules are much more strongly attracted to each other and form the stronger hydrogen bonds between water molecules.
So, when add octane to water, the water molecules minimise their interaction with the octane, and the reverse is also true, the octane molecules minimise their interactions with the water molecules.
Forming an aqueous solution would maximise the interactions between water and octane molecules, since individual octane molecules would be completely surrounded by water molecules, so an aqueous solution is not formed. Instead, we see that 2 distinct layers form, a layer of octane molecules and a layer of water molecules. The only interaction between water molecules and octane molecule occurs at the interface between the two layers so the interactions between them have been minimised.