topic 2.2- water

Question 1

what type of molecule is water?

Answer 1

water is a polar molecule

Question 2

what is a polar molecule?

Answer 2

a molecule with an uneven distribution of charge

Question 3

what is a hydrogen bond?

Answer 3

a weak interaction, or intermolecular force, that forms when a hydrogen atom in one polar molecule is attracted to a slightly negative atom of another polar covalent molecule.

Question 4

state the 4 main properties of water

Answer 4

• adhesive properties
• cohesive properties
• thermal properties (high specific heat capacity/high latent heat of vaporisation)
• solvent properties

Question 5

define cohesion

Answer 5

binding together of two of the same type of molecules

Question 6

why are water molecules cohesive?

Answer 6

• polarity
• hydrogen bonds form
• stick together

Question 7

give two example of a real-life use of cohesion.

Answer 7

water transport in plants, transpiration:
- water sucked/pulled up through xylem vessels at a low pressure.
surface tension:
- allows insects to move on surface

Question 8

why are water molecules adhesive?

Answer 8

• polarity
• hydrogen bonds form
• stick with other molecules

Question 9

define adhesion

Answer 9

the binding of water molecules to other polar molecules

Question 10

give a real-life example of adhesion

Answer 10

• in leaves, water adheres to cellulose molecules in cell walls.
• if water evaporates from the cell wall and is lost from the leaf, adhesive forces cause water to be drawn out of nearest xylem vessel
• this keeps walls moist to absorb CO2 for photosynthesis

Question 11

define specific heat capacity

Answer 11

amount of energy needed to raise the temperature of water by 1’C

Question 12

explain water’s high specific heat capacity

Answer 12

energy needed due to intermolecular H-bonds.

Question 13

give a real-life example of the uses of water’s high specific heat capacity

Answer 13

water is a thermally table habitat for aquatic organisms as its temperature remains relatively stable in comparison to air/land.

Question 14

define latent heat of vaporisation

Answer 14

the heat needed to cause a change of state from a liquid to a gas in water (ie for a water molecule to become a vapour molecule)

Question 15

explain water’s high latent heat of vaporisation

Answer 15

hydrogen bonds need to be broken in order for water to evaporate

Question 16

explain a real-life use of water’s high latent heat of vaporisation

Answer 16

sweat:
- sweat secreted by glands
- heat needed for the evaporation of water in swear is taken from tissues of the skin, reducing their temperature
- blood therefore cooled

Question 17

why does water have such solvent properties?

Answer 17

• polarity of the water means allows other charged particles to dissolve

Question 18

why does liquid water have a high density?

Answer 18

in liquid state, inter molecular hydrogen bonds hold water molecules closer together.

Question 19

what uses does water’s high density have?

Answer 19

it has a support function and aids buoyancy

Question 20

why is water less dense as a solid?

Answer 20

as water solidifies, the intermolecular hydrogen bonds help to create a regular formation

Question 21

why is water being less dense as a solid significant?

Answer 21

floats on liquid water:
- provides a habitat for arctic/antarctic animals
- insulates water below during winter

Question 22

define hydrophobic molecules

Answer 22

molecules that are insoluble in water but dissolve in other solvents (eg propanone)- this happens for non-polar molecules (no negative/positive charges).

Question 23

give an example of hydrophobic molecules

Answer 23

all lipids, fats and oils

Question 24

define hydrophilic substances

Answer 24

substances that are chemically attracted to water (any substance that dissolves in water)

Question 25

give 2 examples of hydrophilic molecules

Answer 25

polar molecules (glucose, Na/Cl ions)
substances water adheres to (cellulose)

Question 26

explain hydrophobic interactions

Answer 26

• hydrogen bonds form between water particles but not with NPMs, so H bond cage formed around them
• the NPMs are forced to join together to form larger groups as there is a slight attraction between them

Question 27

state the main difference between water and methane

Answer 27

water is polar, methane is non polar

Question 28

describe the difference in properties between water and methane

Answer 28

methane has:
- lower latent heat of vaporisation
- lower density
- lower SHC
- lower melting/boiling point

Question 29

explain the usefulness of water’s solvent properties in the blood plasma

Answer 29

• NaCl is dissolved as Na+ and Cl- ions
• glucose associates with water and travels in the plasma
• amino acids are transported in an ionised state
• water-insoluble substances can not travel in solution
• oxygen is non-polar OR insufficient amounts of oxygen can dissolve in plasma
• oxygen binds to haemoglobin OR is transported by red blood cells
• cholesterol and fats are hydrophobic OR insoluble in water
• lipids and fats partner with proteins to form lipoproteins