Topic 2.

WATER
Topic Outline AUDIO

 Structure of Water H H
O
 Hydrogen Bonding
–
 Thermal Properties +
H
 Water versus Methane
O
 Cohesive Properties +
H
–
–
 + H
H
 Solvent Properties O
O
 Significance of Water H
H

Water is the medium of life

Water Structure

Water is made up of two hydrogen atoms covalently bonded to an oxygen atom

Oxygen has a high electronegativity and attracts shared electrons more strongly
• This results in electrons orbiting closer to the oxygen atom – creating polarity

δ– Unequal electron
O O distribution results
H H δ+ H H δ+ in a permanent dipole
(O negative, H positive)
Water Structure Water Polarity

Water molecules are polar and hydrogen bonds form between them
Hydrogen Bonding

The dipolarity of a water molecule allows it to form polar

associations with other charged molecules (polar or ionic)
δ–

δ+
Hydrogen bonds are particularly strong polar associations that
form between hydrogen and either fluorine, oxygen or nitrogen H bonds

• This is because H atoms have a very weak electronegativity, δ+

while F,O & N atoms have a very strong electronegativity δ–

Water molecules will collectively form hydrogen bonds (H + O)

Water molecules are polar and hydrogen bonds form between them
Properties of Water

Intermolecular bonding between water molecules give water distinct properties:

Thermal Properties
• Water can absorb significant amounts of heat energy before changing state

Cohesive / Adhesive Properties

• Water ‘sticks’ to water molecules (cohesion) and charged substances (adhesion)

Solvent Properties
• Water dissolves polar / ionic substances (making it an effective transport medium)

Hydrogen bonds and dipolarity explains the cohesive, adhesive, thermal and solvent properties of water
Thermal Properties (Coolant)

Water has the ability to absorb significant amounts of heat before changing state
• This is due to the extensive hydrogen bonding between water molecules –
these all need to be broken (via heat energy) before a change in state occurs

Thus water is a highly effective coolant, making it a principal component of sweat

⇒
⇒

⇒
⇒
⇒

Hot skin produces sweat Heat causes evaporation Heat absorbed, skin cool

Use of water as a coolant in sweat

Thermal Properties (Water vs Methane)

Methane and water have similar structures (size, weight, orbital formations), but
different thermal properties due to the ability for water to form hydrogen bonds

Substance Structure Freezing Boiling Heat Capacity ΔHFus ΔHVap

H
Methane 109.5°
2.20 0.94 8.2
(CH4) C
–182.5°C –161°C (J.g .°C )
–1 –1
(kJ.mol )
–1
(kJ.mol–1)
H H
H

H δ+

Water 104.5°
4.186 6.01 40.7
(H2O) O
0°C 100°C (J.g .°C )
–1 –1
(kJ.mol )
–1
(kJ.mol–1)
H δ–
δ+ δ–

Comparison of the thermal properties of water with those of methane

Cohesive / Adhesive Properties

Water has the ability to form intermolecular associations Adhesion

with other molecules that share common properties

Cohesion: Water interacts with other water molecules

(it can form intermolecular hydrogen Cohesion

bonds)

Adhesion: Water interacts with polar / ionic molecules

(it will not interact with non-polar
Adhesion
molecules)
Solvent Properties

Water is considered the universal solvent – it can dissolve many substances that
contain electronegative atoms (all polar and charged / ionic substances)

This occurs because the polar attraction of large quantities of water molecules can
sufficiently weaken electrostatic forces and result in the dissociation of atoms

+
–
+
+ – +
–
+ –
–
Salt Crystal (NaCl) Hydration Shells (dissolved)
Solvent Properties (Solubility)

Substances that freely associate and readily dissolve in water are hydrophilic
• Hydrophilic substances include all polar molecules and ions

Substances that do not freely associate or dissolve in water are hydrophobic

• Hydrophobic substances include non-polar molecules (such as fats and oils)

Hydrophilic = water-loving Hydrophobic = water-hating (no dissolving)

Substances can be hydrophilic or hydrophobic

Solvent Properties (Transport)

The transport of materials within the blood will depend upon their solubility

Water Soluble Substances:

• Sodium chloride (ionic) and glucose (polar) are freely transported in the blood
• Amino acids are transported in an ionized state (amine / carboxyl group charged)
• Oxygen is soluble in low amounts – generally transported in red blood cells

Water Insoluble Substances:

• Lipids (cholesterol, fat) are packaged with proteins (lipoproteins) for transport

Modes of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood
Other Properties

Water is transparent, allowing light to pass right through it

• Important for photosynthesis to occur in aquatic plants

Water is less dense as a solid (ice) than in its liquid form

• Water adopts a lattice formation (due to hydrogen bonds) Liquid water

• This results in more space existing between water molecules

• Consequently, ice is lighter than water (i.e. it will float)
• This is important for life on earth as floating ice will insulate
bodies of water from freezing air temperatures (>0°C) Solid ice
Topic Review

Can you do the following?

• Describe the molecular structure of water

• Outline the thermal properties of water
• Compare the properties of water and methane
• Distinguish between cohesion and adhesion
• Identify hydrophilic and hydrophobic substances
• Outline the solvent properties of water
• Explain the significance of water to living things