WaterWater

Lesson objectives 2) Explain the importance of water as a solvent in transport including its dipole nature.

Friday 21 April 2023

Starter: Write down any properties of water you can think of.

Our Water PlanetOur Water Planet

~1,260,000,000,000,000,000,000 litres~70% on the surface

Polar MoleculePolar Molecule

• Made from 2H and O covalently bonded together• Electrons are shared unevenly – O attracts

electrons away from H – causes polarity• It is dipolar in nature which gives water many

special properties

Hydrogen bondingHydrogen bonding

• Different polar charges attract• δ+ H of one water molecule forms an attraction with δ- O

of another water molecule

Hydrogen bond – weak interaction between slightly –ve charged atom and a slightly +ve charged hydrogen

Properties of waterProperties of water

• Going to look at each of the follow properties of water:– water as a solvent– heat capacity– latent heat of vaporisation– density– Cohesion & surface tension

Water as a solventWater as a solvent• Water is an excellent solvent for ionic (e.g. salt)

and some covalent substances (e.g. glucose)

• Hδ+ attracts the –ve anion (Cl-)

• O δ- attracts the +ve cation (Na+)

• As the substance gets spread out in the water it dissolves

Dissolving ionic substance e.g. salt

Water as a solventWater as a solvent

• Glucose doesn’t come apart like salt.

• -OH group is a dipole• Hδ+ of water is attracted to

O δ- of glucose’s –OH group• O δ- of water is attracted to

Hδ+ of glucose’s –OH group• This separates glucose

molecules from each other so they disappear

Dissolving covalent substance e.g. glucose

Importance of water as a solventImportance of water as a solvent

• Metabolic reactions normally take place in solution – molecules & ions need to be free to move.

• Enables transportation of substances e.g. blood plasma is mainly water

• Enables osmotic effect

Thermal PropertiesThermal Properties• Individual hydrogen bonds are weak but

collectively they make water very stable.

Specific Heat CapacitySpecific Heat Capacity• The amount of heat required to raise the

temperature of 1Kg of liquid by 1oC.• This is high in water• Also means a lot of energy can be lost before

the temperature drops.• Specific heat capacity of water is 4.2KJ Kg-1 oC-1

Why do you think this is useful in a biological sense?

• Water provides very constant temperatures in aquatic environments.

Latent heat of vaporisationLatent heat of vaporisation• Amount of heat energy required to vaporise a

liquid (liquid gas)• Very high in water due to hydrogen bonding.

Why do you think this is useful in a biological sense?

• A lot of energy can be lost during sweating or panting with minimal water loss.

Water DensityWater Density

• Water is at its densest at 4oC.• Below this temperature the density of water decreases.• Therefore water below 4oC will rise to the top – ice floats

Why do you think this is useful in a biological sense?

Water densityWater density

• Surface ice acts as an insulator, allowing aquatic organisms to live in the water below

• The temperature gradient within the water column causes circulation of nutrients (e.g. O2)

Cohesion & surface tensionCohesion & surface tension• Cohesion is a force where molecules stick together.• As a result of cohesion at the surface of a liquid a force

called surface tension exists.• Water has the highest surface tension of any liquid

Why do you think this is useful in a biological sense?

• This allows – the translocation of water in the xylem– Small organisms can skate over water.

And finally...And finally...• A water molecule is ___________ because it has

a positive and a negative pole as a result of the uneven distribution of ___________ within it. This creates attractive forces called ______________ between water molecules, causing them to stick together. This stickiness, otherwise called _________, of water means that its molecules are pulled inward at its surface. This force is called _____________. Water is the raw material for the process of _____________ in green plants and acts as a __________ in the synovial joints of animals.

dipolar

electrons

hydrogen bonds

cohesion

surface tensionphotosynthesis

lubricant

HomeworkHomework

• Using today’s notes and further research, write an essay on ‘the biological importance of water’.– There are other things which I haven’t included!