2.2 water

Question 1

explain the thermal properties of water

Answer 1

water molecules can form extensive hydrogen bonding between molecules, which require energy to break

this means it takes a lot of thermal energy to change the temperature of water

hence, water has a high specific heat capacity and a high heat of vaporization

Question 2

describe the biological significance of thermal properties of water

Answer 2

because water has a high specific heat capacity it functions as a biological coolant

sweating results in evaporative cooling as heat is absorbed to evaporate water (break H bonds)

this cools the air surrounding the skin and also directly draws heat from the skin

Question 3

compare water H2O and methane CH4

Answer 3

similarities: have a similar size and molecular weight (w= 18 daltons, m= 16 daltons)

they have comparable valence structures

differences: water has a significantly higher boiling point and melting point

water has a higher specific heat capacity and a higher heat of vaporization

this is because water is polar and can form hydrogen bonds whereas methane is non polar and cannot form H bonds

Question 4

distinguish between hydrophilic and hydrophobic

Answer 4

hydrophilic substances are soluble in water

hydrophobic substances are insoluble in water

Question 5

describe with examples how the solubility of molecules affects their mode of transport within blood

Answer 5

ionic compounds (ex:salt) dissociate in water and are transported within blood plasma in a dissolved state Na+ and Cl-

glucose and other monosaccharides are water soluble and hence are transported very freely within blood plasma

amino acids are both negative and positive charges and can be freely transported within blood plasma in an ionized state (dissolved state)

oxygen is soluble in water, but only in low amounts (most oxygen is complexed to hemoglobin in red blood cells for transport)

lipids (fats and cholesterol) are non polar and insoluble in water (they are transported in blood as lipoproteins)

Question 6

explain the polarity of water

Answer 6

water is made up of 2 hydrogen atoms covalently bonded to an oxygen atom

the oxygen atom has a high electronegativity and attracts the shared electrons more strongly

this results in polarity (the O atom is slightly negative, while the H atoms are slightly positive)

due to this polarity, water can form hydrogen bonds between the O of one molecule and the H of another

Question 7

draw a diagram of two water molecules, showing the intermolecular bonding between them

Answer 7

see paper

Question 8

distinguish between cohesion and adhesion with relation to water

Answer 8

cohesion: cohesion is when two identical molecules stick together via intermolecular bonding

water molecules are cohesive (they can stick together via hydrogen bonding)

adhesion: adhesion is when two different molecules stick together via intermolecular bonding

water molecules are adhesive with polar or charged surfaces

Question 9

describe the biological significance of the cohesive and adhesive properties of water

Answer 9

the cohesive properties of water result in water having a high surface tension due to extensive hydrogen bonding

this is biologically significant as it allows small insects to move along the water surface

ex: water striders

the adhesive properties of water result in capillary action when in contact with charged or polar surfaces

this is biologically significant as it allows for transpiration stream in plants (flow of water against gravity)