Water

Question 1

Structure of water

Answer 1

• dipolar molecule (“di” = 2 & “polar” = charges)
• uneven distribution of charge - O2 slightly negative, H2 slightly positive
• delta symbol (+-): slight positive/negative
• due to charges: hydrogen bonds form between an O2 & H2 of (different) neighbouring molecules

Question 2

Key properties of water and why?

Answer 2

Why? formation of H bonds & dipolar molecule
1. Metabolite (e.g. condensation & hydrolysis reactions)
2. Important solvent in reactions
3. High specific heat capacity - it buffers temperature
4. Large latent heat of vaporisation - providing a cooling effect - w loss of water through evaporation
5. Strong cohesion between water molecules - supports water columns & provides surface tension
- transport medium due to solvent & metabolite

Question 3

Metabolite?

Answer 3

• water involved in many metabolic reactions (metabolism) e.g. photosynthesis, hydrolysis & condensation
• reason why essential that approx 90% of plasma is water & cytoplasm in cells largely composed of water

Question 4

Solvent?

Answer 4

• good/universal solvent so - many substances dissolve in it
• so many essential polar substances dissolve in water - enables them to be transported easily around animals & plants - blood/xylem to cells

Why?
- due to dipolar: polar/charged molecules dissolve readily in it
- slight + charge on H2 atoms attract any - ions in solutes & slight - on O2 atoms attract any + ions in solutes
- polar molecules = hydrophilic (aka attracted to water)
BUT:
- non-polar molecules e.g. lipid can’t dissolve in water = hydrophobic (aka repelled by water)

Question 5

Why high specific heat capacity?

Answer 5

• H bonds between molecules need to be broken to raise temp of water - requires a lot of heat energy…
• temp of water remains relatively stable - even if surround temp fluctuates significantly
• so internal temp of plants & animals remain relatively constant despite outside temps (because large portion of organism is water)
• important for enzyme controlled reactions so: don’t denature/reduce in activity w temp fluctuations
• buffers changes in temp so - thermostable - stable environment for aquatic organisms

Question 6

Large latent heat of vaporisation?

Answer 6

• a lot of energy required to evaporate water (convert liquid to gas)
• due to H bonds - heat energy needed to break between the molecules to turn into a gas
• so: provides cooling effect e.g. sweat releases water onto skin -> large amounts of heat energy from skin transferred to water -> evaporate - removing a lot of heat & cooling organism w little loss of H2O
• e.g. transpiration in plants

Question 7

Strong cohesion?

Answer 7

• H2O molecules sticky tgt by H bonds
• due to this - water moves up xylem in plants (due to transpiration) as continuous column (rather than individual molecules) & due to adhesion (the ability of a substance to stick to a different substance/ the attraction between unlike molecules)
• also: provides surface tension - enables small invertebrates to move & live on surface - habitat away from predators within

Question 8

Electronegativity?

Answer 8

the measure of an atom’s pull on a bonded pair of electrons

Question 9

Condensation reaction?

Answer 9

a chemical reaction which joins 2 molecules tgt w the formation of a chemical bond and involves the elimination of a molecule of water

Question 10

Hydrolysis reaction?

Answer 10

a chemical reaction which breaks a chemical bond between two molecules and involves the use of a water molecule

Question 11

Importance of density & freezing

Answer 11

• as water freezes it expands (water molecules are now held further apart by hydrogen bonds) so - frozen water is less dense than liquid water so - floats
  -frozen ice layer insulates the water below it so - the water below doesn’t freeze (allowing sustained life below)

Question 12

Transparent?

Answer 12

light travels through water, allowing aquatic plants/algae to photosynthesise