Unique properties of Water.
Melting point
Surface tension
Density
Hydrogen
Bonding in Nature
Due
to hydrogen bonding, water has some unique and very important properties.
Definition: Hydrogen bonds are formed by
electronegative elements with a non-bonding pair of electrons available to
bond with a hydrogen attached to and electronegative element.
Molecules in water and other liquids that undergo hydrogen bonding (eg
ethanol) clump together. More energy is needed to separate these particles
so they have higher boiling points than expected
What are the consequences of Hydrogen bonding in water?
3 main unique features
1. Melting point
2. Surface tension
3. Density
The melting point and boiling point of water is unusually high for a
small molecule.
|
|
Relative
molecular mass
|
Melting
point /°C
|
Boiling
point /°C
|
|
CH4
|
16
|
-182
|
-164
|
|
H2O
|
18
|
0
|
100
|
- Living cells are largely water, so without hydrogen bonding, life
would probably not exist (or at least not as we know it).
The high melting point and boiling point is due to the clumping effect
that occurs in liquids that have hydrogen bonding.
- Water has a high surface tension due to hydrogen bonding.
- Leads to insects being able supported on the surface layer
- Leads to a higher viscosity as it doesn't flow as easily (that clumping
effect again)
- Ice is less dense than water
- Crystal of ice is based on open hydrogen bonded structure
- Ice floats on water
- When it melts, the open crystal lattice breaks up allowing the
molecules to get closer together.
This has enormous implications.
1. Ice in lakes and rivers freezes from the top down allowing insulation
of the occupants from the cold (This wouldn't happen if it were a normal
liquid).
2. Pipes/Car engines need anti-freeze to prevent cracking and damage in
cold weather as expansion occurs.
3. Much weathering of rock is caused the expansion / contraction of freeze
thaw.
Most of the essential compounds of life eg DNA / starch / cellulose owe
some of their properties to hydrogen bonding.
1. Cellulose owes its strength to hydrogen bonding.
2. If paper becomes wet, then some strength is lost due to water hydrogen
bonding with cellulose.
3. Cotton and most natural fibres owe their strength to hydrogen bonding.
4. Proteins rely on intra-molecular hydrogen bonding for their shape.
5. Hydrogen bonds are weak and lose their strength when heated. The
cooking of an egg is an example of this. Mammals have evolved to prevent
this occurring - careful control of internal temperature.
6. Two DNA strands are joined by precise hydrogen bonding between the base
pairs adenine and thiamine and between cytosine and guanine.
|