[Unit 2.4] | Water And Its Functions

Question 1

What percentage of the mass of a cell is water?

Answer 1

70-95%

Question 2

Describe the structure of water.

Answer 2

• 2 hydrogen atoms covalently bonded to an oxygen atom.

• The oxygen is more electronegative so attracts the shared pairs of electrons from the covalent bonds.

• Oxygen becomes partially negative and the hydrogens become partially positive.
• It has 2 charged regions and is therefore a dipolar molecule.

Question 3

Name & describe the bond between water molecules.

Answer 3

• Hydrogen bonds

• Partial negative charge on the oxygen atom attracts a partially positive hydrogen atom from another molecule.

Question 4

What key properties of water are contributed to by hydrogen bonding? (5)

Answer 4

• Excellent solvent
• Relatively high Specific Heat Capacity (SHC)
• Relatively high Latent Heat of Vaporisation (LHV)
• Less dense as a solid
• High surface tension & cohesion

Question 5

Explain the significance of the high surface tension of water.

Answer 5

• Surface molecules are not effected by molecules above them so can pull together more tightly & strongly.

• Habitats can therefore exist on the surface of water.

Question 6

Explain the significance of the high cohesion of water.

Answer 6

• Hydrogen bonding allows for strong cohesion of water molecules.

• Allows columns of water to move through the xylem of plants and blood vessels in animals.

Question 7

Explain the significance of the adhesion property of water.

Answer 7

• Water can hydrogen bond to other molecules like glucose

• Allows water to move up the xylem due to transpiration

Question 8

Explain the significance of the high LHV of water.

Answer 8

• To change state, a large amount of thermal energy must be absorbed to break the bonds between molecules.

• This provides a cooling effect as water is removed from the body.

Question 9

Explain the significance of water’s high SHC.

Answer 9

• Water can absorb a large amount of water before raising in temperature a significant amount.

• The temperature does not fluctuate greatly and is therefore stable for habitats.

Question 10

Explain the significance of water’s density

Answer 10

• Ice (being less dense) floats on top of water

• Forms a layer of insulation for aquatic life

Question 11

Explain the significance of water’s colour.

Answer 11

• Colourless and therefore transparent to light

• Means that light can reach plants in water for photosynthesis.

Question 12

Explain the significance of water as a solvent.

Answer 12

• Polar so other polar substances will dissolve in it

• Allows chemical reactions to occur with the free moving cells (as they’re more reactive when free)

• Metabolites can be transported efficiently