Water Structure

2.2 Water Water Structure Explain the polarity of water • Water is made up of two 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 ………………………………………………………………………………………………………………………………………………….......... Draw a diagram of two water molecules, showing the intermolecular bonding between them Hydrogen bond O H δ + H H δ + H δ O + δ – δ δ + – Water Properties Distinguish between cohesion and adhesion with relation to water 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 substances …………………………………………………………………………………………………………………………………... Describe the biological significance of the cohesive and adhesive properties of water • The cohesive properties of water results in water having a high surface tension (due to extensive H bonding) ………………………………………………………………………………………………………………………………………………….......... • This is biologically significant as it allows small insects to move along the surface of water (e.g. 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 a transpiration stream in plants (flow of water against gravity) ………………………………………………………………………………………………………………………………………………….......... Explain the thermal properties of water • Water molecules can form extensive hydrogen bonding between molecules, which require energy to break ………………………………………………………………………………………………………………………………………………….......... • This means it takes a lot of thermal energy (heat) to change the temperature (or state) of water ………………………………………………………………………………………………………………………………………………….......... • Hence, water has a high specific heat capacity (and a high heat of vaporisation / fusion) ………………………………………………………………………………………………………………………………………………….......... Describe the biological significance of the thermal properties of water • Because water has a high specific heat capacity, it functions as an excellent biological coolant ………………………………………………………………………………………………………………………………………………….......... • Sweating results in evaporative cooling, as ambient heat is absorbed to evaporate water (break H bonds) ………………………………………………………………………………………………………………………………………………….......... • This cools the air surrounding the skin and also directly draws heat from the skin ………………………………………………………………………………………………………………………………………………….......... Compare water (H2O) and methane (CH4) Similarities: • They have a similar size and molecular weight (water = 18 daltons, methane = 16 daltons) ………………………………………………………………………………………………………………………………………………….......... • They have comparable valence structures (tetrahedral orbitals– although water is bent due to unbonded pairs) ………………………………………………………………………………………………………………………………………………….......... Differences: • Water has a significantly higher boiling point and melting point ………………………………………………………………………………………………………………………………………………….......... • Water has a higher specific heat capacity (plus a higher heat of vaporisation / fusion) ………………………………………………………………………………………………………………………………………………….......... • This is because water is polar and can form hydrogen bonds (methane is non-polar and doesn’t form H bonds) ………………………………………………………………………………………………………………………………………………….......... Distinguish between hydrophilic and hydrophobic Hydrophilic substances are soluble in water (hydrophilic = water ‘loving’) ………………………………………………………………………………………………………………………………………………….......... Hydrophobic substances are insoluble in water (hydrophobic = water ‘hating’) ………………………………………………………………………………………………………………………………………………….......... Describe (with examples) how the solubility of molecules affects their mode of transport within the blood • Ionic compounds (e.g. salt) dissociate in water and are transported within blood plasma in a dissolved state ………………………………………………………………………………………………………………………………………………….......... • Glucose and other monosaccharides are water soluble and hence are transported freely within blood plasma ………………………………………………………………………………………………………………………………………………….......... • Amino acids are zwitterions and can be freely transported within blood plasma in an ionised state ………………………………………………………………………………………………………………………………………………….......... • Oxygen is soluble in water, but only in low amounts (most oxygen is complexed to haemoglobin in red blood cells) ………………………………………………………………………………………………………………………………………………….......... • Lipids (fats and cholesterol) are non-polar and insoluble in water (they are transported in blood as lipoproteins) …………………………………………………………………………………………………………………………………………………..........