Detailed and comprehensive notes on water (Edexcel biology A). [“A-Level Biology: Edexcel A Year 1 & 2 Complete Revision & Practice” (CGP, ISBN: 2986), “Salters-Nuffield AS/A level Biology Student Book 1” (Pearson, ISBN: 1007) and “Salters-Nuffield A level Biology Student Book 2” (Pears...
Water as a Solvent
• Water has an unevenly distributed charge, making it a polar molecule.
• The hydrogen end is slightly positive and the oxygen ends are slightly negative, since the electrons are more
concentrated at that end.
• Ionic molecules dissolve easily in water.
o The positive ion gets attracted to the negative (oxygen) ends.
o The negative ion gets attracted to the positive (hydrogen) ends.
• The ions as a result become hydrated in aqueous solution (i.e. surrounded by water molecules).
• E.g. the amine group (-NH2) in amino acids dissolves in water so that it can be transported through the blood.
Specific Heat Capacity
• The hydrogen bonds between water molecules are very strong, so large amounts of energy are required to
break them.
• This means water has a high specific heat capacity, meaning it takes a large amount of energy to heat 1g of
water by 1°C.
• Therefore, a large input of energy is equivalent to only a small increase in temperature.
• E.g. this helps organisms to avoid rapid changes in their internal structure, so they can maintain a steady
temperature, even when the surrounding temperature changes dramatically.
Specific Latent Heat of Vapourisation and Specific Latent Heat of Fusion
• (Same reasoning as above)
• Water absorbs a lot of energy before it changes state.
• E.g. sweating / panting is used to cool the body as heat is absorbed by the water.
High Surface Tension
• Water molecules form hydrogen bonds with other water molecules. This gives water a cohesive nature.
• The cohesive forces between water molecules are shared with all neighbouring molecules.
• However, those on the surface have no neighbouring molecules above them, so they form even stronger bonds
with molecules besides them.
• This allows water to better resist an external force, which means it has a high surface tension.
• E.g. transpiration streams are held together by cohesion, such as in xylem vessels.
Hydrolysis Reactions
• Hydrolysis reactions use water to breakdown polymers into monomers.
• They polymer is broken into 2 components
• E.g. the body breaks down complex carbohydrates into monosaccharides via hydrolysis so it is small enough
to get absorbed by cells.
Change in Density with Temperature
• As temperature decreases, water molecules slow down (less kinetic energy) and are able to form more stable
hydrogen bonds, locking them into position.
• Since they aren’t moving, they aren’t able to form as many hydrogen bonds with other molecules.
• This leads to ice water molecules not being as close together as in the case of liquid water, so ice has a lower
density.
• E.g. this allows many organisms to survive below the surface of ice.
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