Polarity of Water Molecules

• Water molecules are polar. This is because they have two polar O-H bonds and a non-symmetrical v-shaped or bent shape, leading to the water molecules having a net dipole.

• For a molecule of such as low mass (18 g/mol) water has higher melting and boiling points than expected. The melting point of water is 0 degrees Celsius and the boiling point is 100 degrees Celsius.

• There are relatively strong attractive forces between water molecules, this is due to the relatively large electronegativity difference between the hydrogen and oxygen in the polar O-H bonds, and the small size of the water molecule. The relatively strong attractive forces between molecules (intermolecular forces) mean that more energy is needed to overcome these forces of attraction and separate the molecules. This explain the relatively high melting and boiling points. Even though the strength of the intermolecular forces of attraction are quite high they are much less than the strength of covalent or ionic bonds.

• Some substances are able to dissolve in water, this is due to the polarity of water molecules. This will be explained in more detail later.

Video - Water and Hydrogen Bonding

Testing for Polarity in Liquids

• We know that molecules in liquids have mobility, they can move.

• When polar molecules, such as water are placed near an electrically charged object the polar molecules align and attracted to the charged object.

• You can try this at home by charging up an inflated balloon, charging up an acetate rod, or even a plastic ruler, by rubbing it on a woollen jersey/jumper and then placing it near a stream of water from the tap. You will see that the stream of polar water is deflected towards the charged balloon.

• This provides a test to find out which liquids are polar and which are not.

• Polar liquids are deflected towards a charged object. Non polar liquids are not deflected towards the charged object

• The video below shows this.

NCEA Worked example - Solubility of Iodine

Compare the solubilities of iodine, I2 (s), in water, H2O (l) - a polar solvent, and in cyclohexane, C6H12 (l) - a non-polar solvent.

Use your knowledge of structure and bonding to explain the solubility of iodine in these two solvents.

Iodine is a non-polar (covalent) molecular made up of I2 molecules which are held together by weak intermolecular forces of attraction. The iodine molecules are non-polar because there is no difference in electronegativity of the atoms in the molecule. The weak intermolecular forces of attraction between iodine molecules are relatively easy to break. In these examples the iodine molecules are the solute.

Iodine is soluble in cyclohexane, but not in water. This is because like dissolves like. By looking at the diagram of the cyclohexane molecule you can see that the cyclohexane molecule is non-polar. Non-polar solutes e.g. iodine can dissolve in non-polar solvents e.g. cyclohexane. In order to dissolve then the solute-solvent forces of attraction (in this case iodine-cyclohexane attractions) are strong enough to overcome the solute-solute (iodine-iodine) forces of attraction and the solvent-solvent (cyclohexane-cyclohexane) attractions because all the attractive forces are similar (non-polar).

Iodine will not dissolve in water, because iodine is non-polar and water is polar. The iodine-water attractions are not strong enough to overcome the solute-solute (iodine-iodine) forces of attraction and the solvent-solvent (cyclohexane-cyclohexane) attractions.

Explaining What Happens When Magnesium Chloride dissolves In Water.

When the ionic solid magnesium chloride is placed in water, the charged ends of the polar water molecules are attracted to the positive and negative ions in the magnesium chloride crystal lattice. The partially negative (delta negative) oxygen side of the polar water molecule is attracted to the positively charged magnesium ions (magnesium cations) and the partially positive (delta positive) hydrogen side of the polar water molecule is attracted to the negatively charged chloride ions. This attractive force allows the ions to break away from the lattice and become surrounded by water molecules. The hydrated ions can then move independently through the solution.

Video 4b- Comparing Dissolving of Ionic Solids and Covalent Compounds