5. Water

• A covalent molecule.

• Each molecule consists of 1 oxygen atom and 2 hydrogen atoms that are covalently linked to form an inverted V-shaped molecule.

Fig. 25 Structure of a water molecule.

• Water molecule is polar in nature.

• Water molecules are held together by hydrogen bonds.

Key properties of water

1. Water has a high heat capacity.

For every 1 g of sweat that is evaporated from the surface of the skin, 2260 Joules of heat is lost.

2. Water is a good solvent, making it a good transport medium in plant and animal systems.

3. Cohesive forces in the form of hydrogen bonds exist between water molecules.

Fig. 26 Cohesive forces between water molecules known as hydrogen bonds are represented by black dashed lines.

4. Adhesive force refers to the force of attraction between two different types of molecules. Eg adhesive forces between water and xylem vessel allow water column to be drawn up the xylem vessel.

Fig. 27 Adhesive forces between water molecules and walls of xylem vessels are represented by blue dashed lines.

5. Water is denser than ice.

- For frozen ponds and lakes, ice floats over warmer water. The warmer water at the bottom of the ponds and lakes allows survival of aquatic life.

- Ice insulates the warmer water at the bottom of ponds and lakes from further loss of heat.

6. Other properties

- Water maintains the turgidity of plant cells.

- Water is also necessary for the process of photosynthesis in plant cells.

Functions of water

n Forms a large part of the cytoplasm of the cell, tissue fluid as well as the transporting medium – blood.

n Water is a requirement for enzymatic reactions to occur.

n Important component of various body fluids, eg digestive juice.

n Important solvent for metabolic wastes, eg urea.