2- HOW DOES WATER DISSOLVE SOLIDS AND LIQUIDS?
The liquid we have to deal with in living system is
water. All life forms are made of it and cannot survive without it.
The many unusual properties of water that make it so important for
living systems are described in your textbook (Moyes & Schulte p. 27-28), so
go and read about it if you want to know more. Right now I am just
going to talk about the property of water as a solvent.
Water dissolves only solids and liquids whose molecules are
electrically charged (e.g. ionic compounds such as salts and
polar molecules such as glucose or glycerol).
How does water dissolve a crystal of salt?
- Not only water molecules move, but they are also polar.
- Because of their electric charges water molecules will be attracted to
the ions, constituent of the salt.
- Water molecules will
surround each ion at the surface of the salt crystal, bind
to them (weak electrostatic binding) and weaken the ionic
bonds holding these ions to the rest of the salt.
- After a while,
the water molecules will isolate these ions and break their ionic
bond with the rest of the salt.
- Each ion surrounded by its shell
of water molecules will then move randomly in the water.
- After a
while, when all the salt has dissolved, no matter where you sample
in the salt mixture the concentration of salt will be identical.
How does water dissolve glucose?
It is very similar to what happens with the salt, except that the
glucose molecule does not split into ions.
- Glucose is a polar molecule.
- The positive and negative poles of the glucose molecule will attract
water molecules.
- The water molecules will then surround each molecule
of glucose and weaken the bonds holding it to the rest of the molecules
of sugar.
- After a while, the water molecules will isolate these glucose
molecules and break their bonds with the rest of the sugar.
- The glucose
molecule surrounded by its shell of water molecules will then move randomly
in the water.
- After a while, when all the sugar has dissolved, no matter
where you sample in the mixture the concentration of glucose will be identical.
You can understand now why water cannot dissolve
compounds that are completely nonpolar: water has no grip
on these molecules to break their bond on each other.