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Solubility
The solubility
of a solute is the maximum quantity of solute that can dissolve in a certain
quantity of solvent or quantity of solution at a specified temperature.
The main factors that have an effect on solubility are:
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nature of the solute and solvent
-- While only 1 gram
of lead (II) chloride can be dissolved in 100 grams of water at room
temperature, 200 grams of zinc chloride can be dissolved.
The great difference in the solubilities of the of these two
substances is the the result of differences in their natures.
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| temperature --
Generally, an increase in the temperature of the solution increases
the solubility of a solid solute. A
few solid solutes, however, are less soluble in warmer solutions. For all gases, solubility decreases as the
temperature of the solution rises.
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| pressure --
For solids and liquid solutes, changes in pressure have practically
no effect on solubility. For
gaseous solutes, an increase in pressure increases solubility and a
decrease in pressure decreases solubility.
(When the cap on a
bottle of soda pop is removed, pressure is released, and the gaseous
solute bubbles out of solution. This escape of a gas from solution is called effervescence.)
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The rate of solution
is a measure of how fast a substance dissolves.
Some of the factors determining the rate of solution are:
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of the particles
-- When a solute
dissolves, the action takes place only at the surface of each particle.
When the total surface area of the solute particles is increased,
the solute dissolves more rapidly. Breaking
a solute into smaller pieces increases its surface area and hence its rate
of solution. (Sample problem:
a cube with sides 1.0 cm long is cut in
half, producing two pieces with dimensions of 1.0 cm x 1.0 cm x
0.50 cm. How much greater
than the surface area of the original cube is the combined surface areas
of the two pieces? 2.0 cm2
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| stirring --
With liquid and solid solutes, stirring brings fresh portions of
the solvent in contact with the solute, thereby increasing the rate of
solution.
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| amount
of solute already dissolved
-- When there is
little solute already in solution, dissolving takes place relatively
rapidly. As the solution
approaches the point where no solute can be dissolved, dissolving takes
place more slowly.
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| temperature --
For liquids and solid solutes, increasing the temperature not only
increases the amount of solute that will dissolve but also increases the
rate at which the solute will dissolve.
For gases, the reverse is true.
An increase in temperature decreases both solubility and rate of
solution.
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Solubility and the nature of a solvent and a solute:
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order for a solvent to dissolve a solute, the particles of the solvent
must be able to separate the particles of the solute and occupy the
intervening spaces. Polar
solvent molecules can effectively separate the molecules of other polar
substances. This happens when
the positive end of a solvent molecule approaches the negative end of a
solute molecule. A force of
attraction then exists between the two molecules.
The solute molecule is pulled into solution when the force
overcomes the attractive force between the solute molecule and its
neighboring solute molecule. Ethyl alcohol and water are examples of polar substances
that readily dissolve in each other.
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| Ammonia,
water, and other polar substances do not dissolve in solvent whose
molecules are nonpolar. The
nonpolar molecules have no attraction for polar molecules and exert no
force that can separate them. However,
nonpolar substance such as fat will dissolve in nonpolar solvents.
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| Polar
solvents can generally dissolve solutes that are ionic.
The negative ion of the substance being dissolved is attracted to
the positive end of a neighboring solvent molecules.
The positive ion of the solute is attracted to the negative end of
the solvent molecule. Dissolving
takes place when the solvent is able to pull ions out of their crystal
lattice or structure. The
separation of ions by the action of a solvent is called dissociation.
When you sprinkle table salt (NaCl) in water and stir, the grains
of salt disappear. From what you have just read (on solubility), you have a
model to explain what actually happens to the salt.
Sodium chloride, an ionic compound, is made of sodium ions and
chloride ions. The slightly
charged ends of water molecules attract these ions.
As a result the ions are dissociated, or separated by the
water molecules and spread evenly throughout the solution.
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See
Solubility Rules
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