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In this screencast we will talk about the idea of solubility which is the maximum amount
of a substance that can be dissolved to a specified amount of liquid at equilibrium.
Let's say that we have a block of potassium nitrate salt and we drop that block of salt
into a beaker of water. The salt will fully dissolve until it reaches a point where no
more salt will dissolve. This is what describes a saturated solution. A saturated solution
is a solution which contains as much of a dissolved species as it can at equilibrium.
If the solution were not saturated we could keep adding more salt until eventually no more
would dissolve and at that point it would be saturated. What happens if we reach saturation
but then decrease the temperature? Generally when we decrease the temperature solubility
will decrease. Let's look at the solubility of potassium nitrate. Say we cool this solution
quickly. We will be coming down from some saturation point on this line and as we decrease
temperature the solubility of KNO3 will decrease. If we started with a solution that was saturated
at 80 degrees C and we decreased the temperature to 40 degrees C the amount of KNO3 in solution
will be greater than the solubility at 40 degrees C. This constitutes a super saturated
solution. A super saturated solution is when more solute is dissolved than the solution
can hold. Generally this is done as described here where we dissolve to the saturation point
and then decrease the temperature. A common example is if you were to dissolve sugar in
water and then heat that water such that you could dissolve a large amount of sugar. Then
later we can cool that water with some form of substrate in the mixture and in this case
form rock candy as the sugar will come out of solution slowly from its super saturated
state. What is important about this system is that we were able to reach a saturated
solution quickly by heating the water and adding sugar. When we cool the liquid the
solution stays super saturated. The solubility of the solution does not tell us how long
it takes for the system to reach equilibrium. Slowly over time the rock candy will form
but it will not happen immediately. Typically as engineers we will use systems were the
system is saturated with the solute but not super saturated. If our system is saturated
we could use our solubility chart to determine solubility based on temperature. Going back
to our KNO3 example we might say we have saturated water at 50 degrees and we can use that temperature
to look on our chart at the KNO3 salt and see that at 50 degrees we have a solubility
of around 85 grams of KNO3 per 100 grams of H2O. It is important to remember when using
these charts that this is a ratio of how much salt is dissolved in the liquid not a mass
percent. There is not 85% KNO3. To figure out what the mass percent of KNO3 is in the
liquid we would need to use another equation and say that "x" KNO3 equals 85 grams divided
by 85 grams of KNO3 plus 100 grams of water. This is going to give us a mass fraction for
KNO3 of 0.46. This give use the mass fraction of KNO3 once saturation is reached but it
tells us nothing about how much time it takes to reach that equilibrium. When dealing with
any of these solubility systems remember that our charts tell us what the system will look
like at equilibrium but says nothing about how long it will take to get there.