IB Biology(HL)--- Water

• Enzymes –biological molecules that increase the rate of chemical reactions– need to be dissolved in water to work

• Water provides a chemically stable medium for all processes of life to function

• An H20 molecule possesses an unequally distributed electrical charge(regardless of being electrically neutral) because of the net negative charge on its oxygen atom and net positive charge on its hydrogen atoms

• The attraction between the positively charged hydrogen atoms of one water molecule and the negatively charged oxygen atom of another water molecule is responsible for hydrogen bonding. Although this weak intermolecular force is not as strong as covalent bonding, its attraction is strong enough to hold water molecules together and attract water molecules to charged particles and other charged surfaces

• Covalent bonding acts within a water molecule while (hydrogen bonds) are formed between water molecules

• Water molecules are held together by hydrogen bonding and are constantly breaking and reforming with surrounding water molecules

• Cohesive forces allow water molecules to be drawn up xylem vessels in plants by the evaporative loss of water from the leaves–water has extremely strong cohesive properties that prevent it from “breaking” under tension

• Single hydrogen bonds are naturally weak, only strong because there are many hydrogen bonds in water allowing the collective to exert large cohesive forces

  • Water’s strong surface tension allows it to form almost spherical droplets because the outermost water molecules form hydrogen bonds with the water molecules beneath them–causing outermost water molecules to therefore exhibit stronger attractive forces to neighboring forces on and below the surface

    • Hydrogen bonding on the surface of water forms a hexagonal lattice that provides water a high surface tension

• The surface tension of water is exploited by insects that ‘surface skate’ —b/c they possess waxy cuticle to prevent their body from getting wet and the mass of the insect is not great enough to break the surface thension

  • Even though the surface may be depressed as the pond skater/insect ‘skates’ across it the hydrogen bond hold it together

• Evaporating Cooling: Important because high temepraturs can be very dangerous to living things as they cause proteins to denature(change shape)

• E.g. Sweating, Dogs pant when they overheat, plants increase transpiration rates when in danger of overheating

Physical Properties of Water

• Density: Refers to the ratio of mass and volume of a substance

• Denser objects are more compact and likely to sink while less dense objects are likely to float

• Higher the Temp = Less Dense(vice versa)

• The crystalline structure formed by water molecules at 0 degrees celsius(more spaced out) make solid water(ice) less dense than liquid water and therefore float

• Buoyancy: The tendency of an object to float in a fluid—Objects float in water when their density is less dense than water and sink when they’re denser

• Specific Heat Capacity/Temperature: The quantity of heat required to raise the temperature of one gram of a substance by one celsius degree

  • Water has a high specific heat capacity because the amount of energy required to raise the temperature of water is high due to the fact a lot of energy is needed to break the hydrogen bonds restricting the movement of water molecules

    • Consequently, aquatic environments are very slow to changes in temperature when the surrounding air of temperature changes—has relatively more stable temperatures than terrestrial environments

• Due to this property of water, it is also able to provide thermal cushioning to organisms(seeing as we are primarily made up of water), protecting cells and organisms from large fluctuations in temperature

• Viscosity: the resistance to flow.

  • Water has more viscosity than air–causing birds like the black-throated loon to have lighter feathers in order to glide with minimum friction

• Interactions between water molecules at its surface create surface tension–however, below the surface, water molecules slip and slide past each other quite easily–low viscosity

Water Crash Course & Videos

• Water is the only substance that occurs naturally in solid, liquid and gas forms

• Water molecules are polar meaning they have a slightly negative charge around its oxygen atom(more greedy for electrons) and a slightly positive charge around its hydrogen atoms, resulting in hydrogen bonding between molecules

• Water has high cohesion, leading to high surface tension(as a non-metallic substance), which allows some insects and animals to walk on water

  • For example, if water was to spill across a countertop, the reason it spreads out instead of “bundling up” is because the adhesive forces because the molecules and the countertop surcace are stronger than the cohesive forces between the individual water molecules in the bead of water

• Capillary Action: Due to adhesion, H20 molecules are attracted to the molecules in the straw and as the water molecules adhere to the straw, other molecules are draw in by cohesion–the surface tension created here causes the water to climb

  • Up the straw until eventually gravity pulling down on the weight of the straw overpowers the surface tension

• Water is an excellent solvent due to its polarity, able to dissolve more substances than any other liquid

• Hydrophillic substances, dissolve in water because their polarity is stronger that the cohesive forces of water causing the water molecules to hydrogen bond around these substances

• Hydrophobic substances, which lack charged poles(non-polar), are pushed out of water due to its cohesive forces

• The charactersistc that solid water(ice) is less dense than liquid water is crucial for sustaining life, as it prevents ice from sinking and disrupting aquatic ecosystems

  • At 0 degrees celsius water molecules start to solidify and the hydrogen bonds in those molecules form crystalline structure that space molecules apart more evenly making frozen water less dense

  • Water’s high heat capacity helps regulate earth’s temperature, making some regions more temperament than others

    • Water has high heat capacity–important so that its difficult to heat up and cool down oceans significantly —> oceans become giant heat sinks that regulate the temperature and climate of our planet

  • The evaporation of water from the skin is an effective cooling mechanism, making sweating an important part of life