Lesson 8: Solubility and the
Properties of Water
Key Points:
1. Learn the terminology related to solubility.
2. Understand what is meant by the term polar
molecule.
3. Understand that because it is a polar molecule
water has many important and unique properties
that make life on earth possible.
Solubility and the Polar Nature of Water
Some substances dissolve easily, others do not. This is solubility,
(see chart in text for a list of ions and their solubility). A substance is
said to be insoluble if it will not dissolve in another substance. If a
substance remains insoluble in a solid form it is called a precipitate.
Solution- Any mixture
containing a solute dissolved
in a solvent. Also known as a
Homogeneous mixture.
Heterogeneous mixture –
two substances mixed
together that do not
dissolve, also known as a
mechanical mixture. E.g.
Oil and water or iron filings
in sand.
Solute- A solute is the substance being dissolved. As an example in
salt water salt is the solute because it dissolves in water.
Solvent- any substance that will dissolve another substance. E.g. In
salt water, water is the solvent because it dissolves the salt.
Precipitate- any solid which is produced in a reaction and will not
dissolve into solution.
Movie
Solubility- the degree to which a substance will dissolve in another
substance i.e. how much solute can go into solution before the
solvent becomes saturated. Can be affected by the properties of
the substance as well as temperature.
Solubility and the Polar Nature of Water
We discussed earlier in the unit how atoms of different elements can
form compounds, and that if electrons are gained or lost the
compound has ionic bonds and is thus an ionic compound. When
two nonmetals collide and react however, they tend to share
electrons in a covalent bond.
Solubility and the Polar Nature of Water
This sharing is not always equal. Some elements are more
electronegative than others and thus attract the shared electrons
more than other elements. Covalent bonds in which the electrons
are unequally shared, and thus distributed in a non-symmetrical
manner are known as Polar Covalent Bonds.
Solubility and the Polar Nature of Water
The water molecule, shaped something like a right angle is a polar
molecule, meaning that the unequal sharing of electrons gives it a
slightly positive end (hydrogen atoms) and a slight negative end
(oxygen atom).
Solubility and the Polar Nature of Water
The anomalous properties of water (like expanding when it freezes)
arise from attraction among these polar molecules as the slightly
“charged” ends attract the oppositely “charged” ends of other water
molecules, as a result weak bonds called hydrogen bonds are formed
between the water molecules. The extra-ordinary qualities of water
are emergent properties resulting from the hydrogen bonding that
orders molecules into a higher level of structural organization.
Solubility and
the Polar
Nature of Water
Although these
bonds break easily
they also reform
easily and
collectively the
hydrogen bonds
hold the substance
together, a
phenomenon called
cohesion.
Cohesion due to
hydrogen bonding
contributes to the
transport of water
against gravity in
plants.
Solubility and the Polar Nature of Water
Hydrogen bonds cause water leaving the veins in a leaf to tug on
molecules farther down in the vessel, and the upward pull is
transmitted along the vessel, all the way to the root. Adhesion
(clinging of a substance to another) also plays a role as water
adheres to the wall of the vessels preventing it from falling from the
pull of gravity. Also related to this is surface tension.
Solubility and the Polar Nature of Water
At the interface between water and air is an ordered arrangement of
water molecules, hydrogen bonded to one another and to the water
below. This makes the water behave as though it were coated with
an invisible film this allows us to skip rocks and for some organisms
to stand, walk or run on water without breaking the surface.