Water

How are the electrons distributed in the covalent bond between oxygen and hydrogen?

How are the electrons distributed in the covalent bond between oxygen and hydrogen?

The oxygen atom is more electronegative so it attracts the electrons more giving it a partial negative charge (δ-), leaving the hydrogen atoms with a partial positive charge (δ+).

The partial charges in a water molecule mean that it is what?

A polar molecule

Hydrogen bond

a weak electrostatic attraction between the δ- on an oxygen atom of 1 water molecule and the δ+ on a hydrogen atom of another water molecule.

How many hydrogen bonds can each water molecule form?

4

Strength of hydrogen bonds

Individually, hydrogen bonds are weak, however collectively they are strong.

Liquid at room temperature: Description

Water is a liquid at room temperature and atmospheric pressure.

Liquid at room temperature: explanation

Water molecules constantly move around breaking and re-forming hydrogen bonds. It takes energy to break the hydrogen bonds to form a gas. Despite hydrogen bonds, water has low viscosity and flows easily.

Liquid at room temperature: Biological importance

• This provides a habitat for living organisms in rivers, lakes and oceans.

• Water is a major component of cells and is a medium for metabolic reactions.

• Water is also an important transport medium.

Density: Description

Water is densest at 4°c. Solid water (ice) is less dense so floats on the surface of liquid water.

Density: explanation

When water freezes the water molecules form a regular lattice and are held in place by hydrogen bonds, so the molecules are more spread out than in a liquid.

Density: Biological importance

• The density of water allows large aquatic organisms such as whales to float.

• Water freezes from the surface down. Ice insulates the surface of water so large bodies of water do not freeze solid allowing aquatic organisms to survive the winter underneath the ice.

• Ice is a habitat for some organisms such as polar bears (require ice to hunt from).

• Warmer water is less dense and rises. Changes in density with temperature, is important in circulating nutrients in oceans.

Polar solvent: Description

Water is a polar molecule and dissolves other polar molecules and ions.

Polar solvent: Explanation

Water is a polar molecule and is attracted to other polar or changed molecules and ions. Water surrounds the particles forming a hydration shell and separates them, so they dissolve.

Polar solvent: Biological importance

• Most metabolic reactions take place in aqueous solution.

• Substances are transported in aqueous solution e.g. glucose in blood plasma.

Cohesion and adhesion: Description

Water is viscous as its molecules cohere to each other and adhere to other molecules.

Cohesion and adhesion: Explanation

Water molecules form hydrogen bonds with each other (cohesion) so tend to stick to each other. Water molecules also form hydrogen bonds with other polar molecules (adhesion).

Cohesion and adhesion: Biological importance

• Cohesion allows water to form continuous columns of water, which is important for transport of water in xylem vessels in plants. This helped by adhesion to lignin in the xylem vessel walls.

• Surface tension allows insects such as pond skaters to move across the surface of water.

High specific heat of capacity: Description

A large amount of energy is required to make bodies of water change temperature.

High specific heat capacity: Explanation

Hydrogen bonds restrict the movement of water molecules. Energy is required to break the hydrogen bonds and raise the temperatures of water. Energy is also released when hydrogen bonds form.

High specific heat capacity: Biological importance

• Internal temperature changes are minimised which helps organisms (contain 70%-90% water) to maintain a stable body temperature for enzyme activity.

• Temperature changes of large bodies of water are minimised, so they provide a thermally stable environment for aquatic organisms.

High latent heat of vaporisation: Description

A large amount of heat energy is required to separate a water molecule from other from other water molecules in a liquid to become a vapour.

High latent heat of vaporisation: Explanation

A lot of energy is required to break the many hydrogen bonds between the water molecules so water molecules can separate to become a vapour.

High latent heat of vaporisation: Biological importance

• Evaporation of water cools organisms (e.g. sweating and panting in animals/evaporation from the mesophyll cells as part of transpiration in plants).