Water – The Universal Solvent
A solution in which water is the solvent is called an aqueous solution. Because
water can dissolve so many different solutes, it is called the universal solvent.
Molecular Compounds
When certain atoms form
compounds, they share
electrons, this is called covalent
bonding. Compounds that
contain covalent bonds are
called molecular compounds,
or molecules.
If a molecule has an even
distribution of electrons, it is
called nonpolar. In a H2
molecule the electrons are
shared equally.
If the electrons are not shared
evenly, it is called polar. In a
water (H2O) molecule the
electrons are not shared equally.
Due to the larger size of the oxygen nucleus, electrons spend more time with the
oxygen atom than the hydrogen atoms. This results in a partial negative charge
around the oxygen atom and a partial positive charge around the hydrogen atoms.
Ionic Bonds
Some atoms do not share
electrons when they form a
bond. Instead, they gain or lose
an electron. When this happens,
the number of protons and
electrons are no longer equal
this results in a positive or
negative charge on the atom.
Atoms with a charge are called
ions. The positive and negative
ions are then attracted to each
other to form an ionic bond, and
the compound that is formed is
called an ionic compound. Salt is
an ionic compound made up of
positive sodium ions and
negative chlorine ions.
How Water Dissolves Ionic Compounds
Water molecules are polar and attract positive
and negative ions. The positive part of a water
molecule (hydrogen atom) is attracted to the
negative ion. The negative part of a water
molecule (oxygen atom) is attracted to the
positive ion. When an ionic compound is
mixed into water, the different ions of the
compound are pulled apart by the water
molecules.
How Water Dissolves Molecular
Compounds
Water can dissolve molecular compounds,
such as sugar, but it doesn’t break the
sugar molecule apart. Instead water
simply moves between the different sugar
molecules. A sugar molecule is polar and
the negative portion of the sugar
molecule is attracted to positive portion
(hydrogen atom) of the water molecule.
The positive portion of the sugar molecule
is attracted to the negative portion
(oxygen atom) of the water molecule.
What will dissolve?
A substance that dissolves in another is said to be soluble in that substance. Ex:
sugar is soluble in water, metal is not soluble in water
Like Dissolves Like
Polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar
solutes.
Ex:
sugar (polar) + water (polar)
->
dissolves
Oil (nonpolar) + water (polar)
->
does not dissolve
How much will dissolve?
Solubility is a measurement that describes how much solute dissolves in a given
amount of solvent. The solubility of a material is the amount of the material that
can dissolve in 100 g of solvent.
63 g of potassium chromate can be dissolved in 100 g of water (25°C)
-> high solubility
0.00025 g of barium sulfate can be dissolved in 100 g of water (25°C)
-> low solubility
Solubility in Liquid-Solid Solutions
The solubility of many solutes changes if you change the temperature of the
solvent. If you heat water, you can dissolve more sugar at a faster rate. However
the solubility of some solutes, such as salt does not change.
Solubility in Liquid-Gas Solutions
An increase in temperature decreases the solubility of a gas in a liquid-gas solution.
Warm coke loses its bubbles faster than cold coke. Carbon dioxide is less soluble in
a warm solution.
Saturated Solutions
A solution that contains all of the solute that it can hold under the given conditions
is called a saturated solution. If you put ten packs of sugar into a glass of water,
only the first few packs will dissolve and the rest will settle to the bottom of the
glass.
A hot solvent usually can hold more solute than a cool solvent can. When a
saturated solution cools, some of the solute usually falls out of the solution.
However, if the solution is cooled slowly, excess solute remains dissolved for a
period of time. Such a solution is said to be supersaturated because it contains
more than the normal amount of solute.
Rate of Dissolving
A solute dissolves faster when the solution is stirred, shaken, the temperature is
increased, or the solute is broken into small pieces. These methods increase the
rate at which the surfaces of the solute come into contact with the solvent. This
increases the dissolving rate.
Molecules are always moving and colliding. Chemical reactions take place as a
result of these collisions. Molecules move faster at higher temperatures than
lower temperatures. Typically chemical reactions have a higher rate of reaction at
higher temperatures and a lower rate of reaction at lower temperatures.
Concentration
The concentration of a solution tells you how much solute is present compared to
the amount of solvent. A concentrated solution has more solute per given amount
of solvent than a dilute solution.
Measuring Concentration
A fruit drink that has 15% fruit juice and 85% water is more concentrated than a
fruit drink with 10% fruit juice and 90% water.
Effects of Solute Particles
All solute particles affect the physical properties of the solvent, such as its boiling
point and freezing point. When water begins to freeze, its molecules arrange
themselves in a particular pattern. When salt is added to water, it changes the
way the molecules arrange themselves and lowers the freezing point.
When salt is added to water, the solute particles interfere with the evaporation of
the solvent particles. More energy is needed for the solvent particles to escape
and increases the boiling point.