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GIna Garraffo

on 3 September 2013

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Transcript of Solubility

So... if you add 36 grams of NaCl to 100g of water at 25 Degrees C, all of the salt dissolves. But if you add one more gram of salt and stir, only .2g of that one gram will dissolve. WHY?

According to the KMT, molecules are in constant random motion. SO, the water molecules are still moving to dissolve the new salt, but there is also reforming happening.

A state of dynamic equilibrium exists
between the solution and the excess solute. The rate of dissolving
equals the rate of crystallization, so the total amount of dissolved solute remains constant.
What would happen if you added more solute?
Particles come out of solution and are deposited as a solid.
What does all this mean?
Well, solubility is the amount of solute that dissolves in a given quantity of a solvent AT A SPECIFIED TEMPERATURE AND PRESSURE. This produces a saturated solution. Solubility is often expressed in grams of solute per 100 grams of solvent.

Sometimes the solubility of a gas is expressed in grams per liter (g/L).
A solution that contains less solute than a saturated solution at a given temperature and pressure is an
unsaturated solution

A saturated solution
contains the maximum amount of solute for a given quantity of solvent at a constant temperature and pressure.
What do you think is a supersaturated solution?

Take a minute to illustrate in your notebooks a saturated and a supersaturated solution.
Some liquids are soluble in each other. For example, water and ethanol. Pairs like these are said to be miscible. Miscible means that two liquids can dissolve each other in all proportions.
Liquids that are insoluble in each other are immiscible. Examples are water and oil, and water and vinegar.
Lets review...
How is solubility expressed? (think units!)
Now that we know that solubility is the mass of solute that dissolves in a given mass of a solvent at a specific temperature,
some factors that would affect solubility.
The solubility of most SOLID substances increases as the temperature of the solvent increases.
For some substances, the solubility decreases with increasing temperatures.
As a solution cools, it cannot contain the same concentration of minerals as it did at a higher temperature, so some of the minerals precipitate.
We can also see precipitation in the form of crystallization. If you have a supersaturated solution, adding something called a seed crystal (a very small crystal) of solute is added. Look:
The effect of temperature on the solubility of gases in liquid solvents is opposite than that of solids. The solubilities of most gases are greater in cold water than hot.

Just think of oxygen! The warmer bodies of water get, the less soluable the water becomes. Those poor fish and other aquatic life! There is less dissolved oxygen for them to consume!
Liquids and solids exhibit practically no change of solubility with changes in pressure!
Gases though, increase in solubility as the pressure increases.

If the pressure is increased, the gas molecules are "forced" into the
solution since this will best relieve the pressure that has been applied.
There is a law to explain all of this!
It is Henry's Law! Henry's Law states that: The solubility of a gas in a liquid is directly proportional to the pressure of that gas above the surface of the solution.
Carbonated beverages provide the best example of this phenomena. All carbonated beverages are bottled under pressure to increase the carbon dioxide dissolved in solution. When the bottle is opened, the pressure above the solution decreases. As a result, the solution effervesces and some of the carbon dioxide bubbles off!
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