water molecules are…
polar and hydrogen bonds form between them
water is made of…
Water is made up of two hydrogen atoms covalently bonded to an oxygen atom (molecular formula = H2O)
water bond
While this covalent bonding involves the sharing of electrons, they are not shared equally between the atoms
electronegativity within water
- Oxygen (due to having a higher electronegativity) attracts the electrons more strongly
- The shared electrons orbit closer to the oxygen atom than the hydrogen atoms resulting in polarity
why is water polar?
water is described as being polar because it has a slight charge difference across the different poles of the molecule
- The oxygen atom is slightly negative (δ–) while the hydrogen atoms are slightly positive (δ+)
what associations does water form?
- This charge difference across the molecule (dipole) allows water to form weak associations with other polar molecules
- The slightly negative poles (δ–) will attract the slightly positive poles (δ+) of other molecules, and vice versa
hydrogen bonds
- When a δ+ hydrogen atom is attracted to a δ– fluorine, oxygen or nitrogen atom of another molecule, it forms a hydrogen bond
- Hydrogen bonds are relatively stronger than other polar associations due to the high electronegativity of F, O and N
dipolarity of water molecules
- The dipolarity of a water molecule enables it to form polar associations with other charged molecules (polar or ionic)
- Water can form hydrogen bonds with other water molecules (between a δ+ hydrogen and a δ– oxygen of two molecules)
properties of water
This intermolecular bonding between water molecules gives water distinct properties not seen in other substances:
- Thermal properties – Water can absorb much heat before changing state (requires breaking of hydrogen bonds)
- Cohesive / adhesive properties – Water will ‘stick’ to other water molecules (cohesion) and charged substances (adhesion)
- Solvent properties – Water dissolves polar and ionic substances (forms competing polar associations to draw materials apart)
thermal properties of water
Water has the capacity to absorb significant amounts of heat before changing state
- This is due to the extensive hydrogen bonding between water molecules – the H-bonds need to be broken before a change in state can occur and this requires the absorption of energy (heat)
- Consequently, water is an excellent medium for living organisms as it is relatively slow to change temperature and thus supports the maintenance of constant conditions (internal and external)
thermal properties of water vs. methane
Methane (CH4) provides a good basis for comparison with water due to the many similarities between their structures:
- Comparable size and weight (H2O = 18 dalton ; CH4 = 16 dalton)
- Comparable valence structures (both have tetrahedral orbital formations, but water is bent due to unbonded electron pairs)
differences between water and methane
The differences in thermal properties between water and methane arise from differences in polarity between the molecules:
- Water is polar and can form intermolecular hydrogen bonds (due to high electronegativity of oxygen atom)
- Methane is non-polar and can only form weak dispersion forces between its molecules (carbon has a lower electronegativity)
since water absorbs more heat before changing state…
- Water has a significantly higher melting and boiling point
- Water has a higher specific heat capacity (energy required to raise the temperature of 1 g of substance by 1ºC)
- Water has a higher heat of vaporisation (energy - absorbed per gram as it changes from a liquid to a gas / vapour)
- Water as a higher heat of fusion (energy required to be lost to change 1 g of liquid to 1 g of solid at 0ºC)
evaporation of water
- The change of water from liquid to vapour (evaporation) requires an input of energy
- This energy comes from the surface of the skin when it is hot, therefore when the sweat evaporates the skin is cooled
- Because water has a high specific heat capacity, it -absorbs a lot of thermal energy before it evaporates
- Thus water functions as a highly effective coolant, making it the principal component of sweat
cohesive properties of water
- Cohesion is the ability of like molecules to stick together
- Water is strongly cohesive (it will form hydrogen bonds)
adhesive properties of water
- Adhesion is the ability of dissimilar molecules to stick together
- Water will form intermolecular associations with polar and charged molecules
Cohesive properties explain…
The cohesive properties of water explain its surface tension
- The hydrogen bonding between water molecules allows the liquid to resist low levels of external force (surface tension)
- The high surface tension of water makes it sufficiently dense for certain smaller organisms to move along its surface
Adhesive properties explain…
The adhesive properties of water explain its capillary action
- Attraction to charged or polar surfaces (e.g. glass) allows water to flow in opposition of gravitational forces (capillary action)
- This capillary action is necessary to allow water to be transported up plant stems via a transpiration stream
Solvent properties
- Water is commonly referred to as the universal solvent due to its capacity to dissolve a large number of substances
- Water can dissolve any substance that contains charged particles (ions) or electronegative atoms (polarity)
- This occurs because the polar attraction of large quantities of water molecules can sufficiently weaken intramolecular forces (such as ionic bonds) and result in the dissociation of the atoms
- The slightly charged regions of the water molecule surround atoms of opposing charge, forming dispersive hydration shells
Hydrophilic
Substances that freely associate and readily dissolve in water are characterised as hydrophilic (‘water loving’)
- Hydrophilic substances include all polar molecules and ions
Hydrophobic
Substances that do not freely associate or dissolve in water are characterised as hydrophobic (‘water-hating’)
- Hydrophobic substances include large, non-polar molecules (such as fats and oils)
Modes of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in relation to their solubility in water
The transport of essential molecules within the bloodstream will depend on their solubility in water
- Water soluble substances will usually be able to travel freely in the blood plasma, whereas water insoluble substances cannot
Water Soluble Substances
- Sodium chloride (NaCl) is an ionic compound and its components (Na+ and Cl–) may be freely transported within the blood
- Oxygen is soluble in water but in low amounts – most oxygen is transported by haemoglobin within red blood cells
- Glucose contains many hydroxyl groups (–OH) which may associate with water and thus can freely travel within the blood
- Amino acids will be transported in the blood in an ionized state (either the amine and/or carboxyl groups may be charged)
Water Insoluble Substances
- Lipids (fats and cholesterol) are non-polar and hydrophobic and hence will not dissolve in water
- They form complexes with proteins (lipoproteins) in order to move through the bloodstream
- Hydrophilic portions of proteins, cholesterol and phospholipids will face outwards and shield internal hydrophobic components
