Structure & Properties of Water (College Board AP® Biology): Study Guide

Polarity & hydrogen bonding in water

• The properties of water allow it to sustain living systems; these essential properties include its:

  • polarity

  • high specific heat capacity

  • high heat of vaporization

Polarity

• Water molecules contain an oxygen atom that is covalently bonded to two hydrogen atoms

• The electrons in these covalent bonds 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

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

  • The hydrogen bonds between the water molecules need to be disrupted before their kinetic energy can increase

• This means that the temperature of water changes slowly, even when environmental temperatures fluctuate; water is said to be thermally stable, or to act as a thermal buffer

• This benefits living organisms because:

  • aquatic birds and mammals expend less energy on thermoregulation during homeostasis

  • enzyme activity in the cells of aquatic organisms remains close to optimal, despite external temperature change

High heat of vaporization

• Hydrogen bonds mean that a lot of energy is needed to convert water from a liquid to a gas

  • Energy is required to break these hydrogen bonds before molecules can escape into a gaseous state

• This means that when water evaporates, it absorbs heat energy from its surroundings—such as the skin or another surface—producing a cooling effect

• This property is significant for living organisms because:

  • strategies such as sweating and panting allow heat loss by evaporation in animals

  • transpiration from leaves allows leaves to maintain a temperature that is cooler than the surrounding air

Cohesion, adhesion & surface tension

• 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