Unit 4 Solubility and Solutions
4.1.1—4.1.3
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Assignment #1- finding definitions
4.1.2 General Guidelines Concerning Solubility
If you add oil to water, what happens? Not much - the two don't mix, even if shaken. Why can you clean
some paints with water, but others require a much different type of cleaning solvent? Why do some
substances readily form solutions but others don't?
There is a general saying that "like dissolves like“. This refers to the type of bonding between molecules, if the
bonding between molecules of the substance to be dissolved is similar to the bonding between solvent
molecules, there is a good chance that the substance will dissolve.
Time for a quick review of chemical bonding.
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Intramolecular Bonding
Intramolecular bonding refers to the chemical bonding that holds atoms together within a compound.
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Covalent bonding involves the sharing of electrons between two non-metal atoms. For example,
hydrogen and oxygen atoms form a water molecule, H 2O, using covalent bonding.
Ionic bonding involves the transfer of electrons from a metal atom to a non-metal. One atom loses
electrons, forming a positive cation, while the other atom gains electrons to form a negative anion. An
example is sodium chloride, NaCl
Intermolecular Bonding
Intermolecular bonding, is what holds two or more separate molecules together in the solid and liquid
phases. What type of intermolecular bonding is involved largely depends on two main factors:
whether the bonds within a single molecular are polar or not, and
the overall shape of the molecule
A polar molecule will have one end of the molecule bearing a partial positive charge while another end
carries a partial negative charge. Polar molecules must contain polar bonds.
Like Dissolves Like
Often, nonpolar substances dissolve best in nonpolar solvents, while polar substances dissolve best in polar
solvents.
4.1.3 Factors Affecting Solubility
Imagine pouring salt into a glass of water. Eventually no more salt will dissolve, and any more you add will
simply sit on the bottom. Whenever a solute is dissolved in a liquid, a point is reached in which no more
solute will dissolve. This is the point of saturation.
Saturation is an equilibrium system, which you'll remember is a dynamic state in which both the forward and
reverse reactions continue but at equal rates. We can write a general equation that illustrates this:
solute (s, l, g)
solute (aq)
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Because a saturated solution is at equilibrium, the same factors that affect a system at equilibrium will affect
our saturated system. The two key factors to consider are:
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the effect of temperature and
the effect of pressure on the solubility of gases.
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The Effect of Temperature on Solubility
If you want to get more salt to dissolve in our glass of water, what would you do? If you heated the water you
would be able to dissolve more salt.
Generally, increasing the temperature will increase solubility of solids and liquids. But increasing temperature
will lower the solubility of gases
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We can explain this using Le Châtelier's Principle. The dissolving process in the solid and liquid form is an
endothermic process:
solid or liquid + heat
aqueous
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Adding heat will favor the endothermic direction; in this case favoring the product (or aqueous) side of the
reaction.
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However, the dissolving process of gases is exothermic:
Gas
aqueous + heat
Adding heat to this system will again favor the endothermic reaction, which in this case is the reverse
reaction.
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The Effect of Pressure on the Solubility of Gases
The solubility of gases increases when the pressure above the gas is increased; i.e more gas will dissolve when
pressure is increased.
This can be explained by Le Châtelier's Principle. As our example we will examine a can of coke
Carbonated beverages contain carbon dioxide which is dissolved under pressure. Our equilibrium system is:
CO2 (g)
CO2 (aq)
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CO2 (g)
CO2 (aq)
On the reactant side is one mole of gas; on the product side are zero moles of gas. Increasing pressure will
favor the side with the fewest moles of gas
When you open a can of pop, you increase the volume and decrease the pressure. According to Le Châtelier's
Principle, this will favor the side with the most moles of gas. And that's what happens - when you open a can
of pop it begins to fizz - this is the CO2 coming out of solution. The "fizz" are the gas bubbles forming and
popping
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Assignment 4.1.3
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