2.1 Essential ideas

2.1.2 Water

Figure 2.1.2a – Water moleculeFigure 2.1.2a – Water molecule
Chemical structure of water: a) ball and stick b) valence electrons c) bond angle

  • Each molecule of water consists of one oxygen atom and two hydrogen atoms oriented at 104.5 degrees. The chemical formula of water is H2O.

Water molecules are polar

  • Oxygen is more electronegative than hydrogen, meaning oxygen pulls electrons more strongly than hydrogen.
  • As a result, there is slightly more negative charge near the oxygen atom in a covalent bond between hydrogen and oxygen. When electrons are not shared equally the result is a polar covalent bond.
  • We represent polar bonds by showing partial charges (δ+ and δ-) at either end of the bond.
  • Some molecules are made of polar bonds but are not polar overall. Let’s compare a molecule of water to a molecule of carbon dioxide.

Figure 2.1.2b – Water molecules are polar.Figure 2.1.2b – Water molecules are polar

  • In both molecules, oxygen is the more electronegative atom, so the bonds are polar.
  • There are unbonded electrons on every oxygen atom. Electrons repel each other.
  • On the water molecule, the electrons on the oxygen atom ‘bend’ the shape of the molecule. As a result, there is a slightly more positive pole and a slightly more negative pole. The molecule has a net dipole, meaning that it is polar overall.
  • On the carbon dioxide molecule, the unbonded electrons pull equally in both directions. The molecule has no net dipole and is non-polar overall.

Hydrogen bonding and dipolarity

  • When polar molecules interact with each other, opposite ends of the dipoles attract. 

Figure 2.1.2c – Hydrogen bonds are intermolecular dipole interactions, not true covalent bonds.Figure 2.1.2c – Hydrogen bonding in water
Hydrogen bonds are intermolecular dipole interactions, not true covalent bonds.

  • Hydrogen bonds occur when hydrogen that is bound to a more electronegative atom is attracted to other nearby electronegative atoms.
  • Hydrogen bonds are intermolecular interactions – they are not as strong as true covalent bonds.

Hydrogen bonding and dipolarity explain the properties of water

  • Although they are weaker than covalent bonds, hydrogen bonds restrict the movement of water molecules. Energy must be supplied to break hydrogen bonds.
  • Living organisms benefit from the properties of water, described below.

Thermal properties

  • High boiling point: Water is liquid over a range of temperatures that correspond to most habitats on Earth, from 0° to 100°C, while most other molecules of the same mass are gaseous over this range.
  • High specific heat: It takes a lot of energy to raise the temperature of water (4.18J/g/°C). This is beneficial for organisms because temperatures in aquatic habitats remain relatively stable, while air temperatures fluctuate.
  • High latent heat of vaporisation: Latent heat of vaporisation refers to the energy required to change the state of water from liquid to gas. A lot of energy is needed to evaporate water. Water is an excellent coolant.

Cohesive properties

  • Cohesion is the intrinsic property of certain molecules to ‘stick’ to other molecules of the same type.
  • This is beneficial to vascular plants – an uninterrupted column of water is pulled against gravity from the roots to leaves by tension caused by evaporation from the leaves. This would not be possible if water was not cohesive.

Figure 2.1.2d – Hydrogen bonds are responsible for cohesion and adhesion.Figure 2.1.2d – Cohesion vs adhesion
Hydrogen bonds are responsible for cohesion and adhesion.

Adhesive properties

  • Adhesion is the property of ‘sticking’ to other types of molecules.
  • Water is able to adhere to many different substances because of hydrogen bonds.
  • This is beneficial because it allows objects to get ‘wet’. For example, breathing surfaces (lungs, gills) need to be moistened to allow the exchange of gases.

Solvent properties

  • Water can dissolve many organic and inorganic substances that have polar regions. The strong polarity of water can interrupt ionic bonds and dissociate salts.

Figure 2.1.2e – Dipolarity explains why water molecules surround dissociated ions and keep them in solution.Figure 2.1.2e – Dipolarity explains why water molecules surround dissociated ions and keep them in solution.

  • Cytoplasm is mostly water. Many important substances are transported in and out of cells in solution with water.

Substances can be hydrophilic or hydrophobic

  • Substances can be classified according to whether they dissolve in water or not. They are either ‘hydrophilic’ (water-loving) or ‘hydrophobic’ (water-fearing).

 

Hydrophilic substances

Hydrophobic substances

Molecule polarity

Polar

Non-polar

Solubility in water

Soluble

Insoluble

Solubility in non-polar solvents (e.g. benzene)

Insoluble

Soluble

Intermolecular interactions

Dipole interactions, including hydrogen bonds

Hydrophobic interactions

Examples

Glucose, most amino acids and proteins, sodium chloride and other ionic compounds

Fatty acids and lipids, cholesterol

H2OFigure 2.1.2f – H2O window
Water sticks to windows because of its adhesive properties. It makes tracks on the window because of its cohesive properties. Hydrogen bonding explains both properties.

Try it!

  • Use ball and stick sets to make models of the following molecules: CH4, CO2, H2O, NH3, CH3OH. Which are polar? Which are non-polar? Which interact to form hydrogen bonds?
  • Fill in a copy of the this table.

panting dogFigure 2.1.2g – Dog panting
Dogs do not perspire but panting takes advantage of the cooling effect of high latent heat.

Concept help

  • Do not be confused by dipole charges. They are partial charges within a molecule that has an overall charge of zero. The only truly charged particles are ions.
  • Hydrogen bonds and hydrophobic interactions are intermolecular forces (between two or more molecules). Covalent bonds are intramolecular (within one molecule).
  • During a state change, the added energy does not result in a change of temperature. The energy is ‘latent’, or hidden.
  • By associating with each other, hydrophobic substances are excluded from dissolving in water. Hydrophobic substances do not ‘fear water’ as much as they ‘love’ each other.
  • There are a few exceptions, but ‘like dissolves like’ is generally true.