Topic 2.2

Essential idea: Water is the medium of life.


2.2.U1 Water molecules are polar and hydrogen bonds form between them

The oxygen and hydrogen atoms that make up the water molecule have different pulling properties on the electrons that are shared in their covalent bonds. This pulling property is known as electronegativity. The oxygen atom has a negative charge because it pulls the shared pair of electrons closer to its nucleus. Therefore, the hydrogen atom has a positive charge because ithas the shared pair of electrons pulled farther away from its nucleus. The water molecule is polar because of this unequal sharing of electrons between the atoms (known as a polar covalent bond). Figure 1 outlines the polarilty in water.

Water link Water animation

2.2.U2 Hydrogen bonding and dipolarity explain cohesive, adhesive, thermal and solvent properties of water

The thermal and cohesive properties of water (outlined in Figure 2 below) can be attributed to its ability to form hydrogen bonds between water molecules. a hydrogen bond is formed when when an H atom (positive end) of one water molecule is attracted to an O atom (negative end) of another water molecule. Although this is considered a weak bond, the hydrogen is considered important in the chemistry of life. Water's ability to act as a solvent (outlined in Figure 4 below)results from the polarity of its molecules. It can dissolve ionic compounds because the negative end of the molecule is attracted to positive ions while the positive end is attracted to negative ions. Polar molecules, which contain positive and negative charges are dissolved in water because become surrounded by polar water molecules. the figure below outlines these properties and gives an example of each property.


2.2.U3 Substances can be hydrophilic or hydrophobic

Hydrophilic vs Hydrophobic


2.2.A1 Comparison of the thermal properties of water with those of methane

Comparison of thermal properties of water and methane
Correlation between smoking and cancer explained

2.2.A2 Use of water as a coolant in sweat

Correlation between smoking and cancer explained

2.2.A3 Modes of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in relation to their solubility in water

Modes of transport of nutrients in the blood
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