Structure & Properties of Water (College Board AP® Biology): Study Guide
Polarity & hydrogen bonding in water
The structure of a biological molecule determines its properties; differences in structure may include:
the components from which a molecule is built
the sequence in which the components occur
Within the structure of water molecules the electrons are shared unequally between oxygen and hydrogen; the oxygen atom has a partial negative charge (𝛿-) and the hydrogen atoms have a partial positive charge (𝛿+)
The unequal distribution of charge means that water is a polar molecule
The polar nature of water allows weak forces of attraction known as hydrogen bonds to form
The hydrogen bonds between water molecules provide water with properties that are essential to living systems
The properties of water
The polar nature of water molecules and the resulting hydrogen bonds provide water with some unusual properties
Examples of properties that are essential for the survival of living organisms include:
cohesion
adhesion
Cohesion
Cohesion can be defined as:
Forces of attraction between molecules of the same type
Cohesion means that water molecules are attracted to each other; examples of the importance of this property in living organisms include:
plant transport
Water molecules are drawn upwards behind other water molecules in the xylem due to cohesion
This provides plants with a water supply and enables the transport of dissolved mineral ions
surface tension
Cohesive forces mean that water molecules at the surface of a body of water are attracted to water molecules below the surface; this creates surface tension
Surface tension allows small organisms to live on the water's surface, e.g. pond skaters are small insects that walk on the surface of ponds and lakes
Adhesion
Adhesion can be defined as:
Attractive forces between molecules of a different type
Adhesion means that water molecules are attracted to other molecules
In plant transport this means that adhesion between water molecules and the xylem walls draws water upwards by capillary action
Other properties of water
Property | Description | Importance for living organisms |
|---|---|---|
High specific heat capacity | Hydrogen bonds reduce the movement of water molecules, meaning that a lot of energy is needed to raise the temperature of water | Water is thermally stable |
High heat of vaporization | Hydrogen bonds mean that a lot of energy is needed to convert water from a liquid to a gas | Sweating and transpiration allow cooling by evaporation |
Low density when frozen | Hydrogen bonds form an open lattice when water freezes, meaning that ice floats on water | Polar ice provides a habitat for some animals, and ice acts as an insulating layer over water |
Solvent | Polar water molecules are attracted to charged particles in a solution, allowing ionic compounds, e.g. NaCl, to dissolve | Water can transport substances in a solution, and provides a medium for chemical reactions |
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