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Way # 72 on how to pass the chemistry regents. The boiling point and freezing point of water
changes when something gets dissolved in it, forming a solution.
A popular concept appearing on 7 out of 8 exams.
Take a look at these 3 questions from various regents exams, which are all asking about
the boiling point and freezing point of a solution, as compared to plain old water.
After looking at the answers, we can safely say that, for a solution, the boiling point
goes up and the freezing point goes down, no matter what is getting dissolved and they'll
always say "at standard pressure" since a change in pressure may have a different effect.
This even holds true for the short answer type questions like these 2 from the June
2012 and 2010 regents exams. Although these questions only care about the
boiling point of a solution, at standard pressure, of course, this fact still holds true: the
boiling point of the solution will be higher than the boiling point of water.
You could even say that water boils at a lower temperature.
Before we show you the last 2 questions, there is some additional information we need to
tell you: the higher the concentration of IONS, meaning a solute consisting of a metal/nonmetal
combination, the greater this effect will be.
In this first question, they want the highest boiling point, at standard pressure.
We'll need to dissect every solute to see how many ions they truly have, with the red
boxes being the metal ions and the blue boxes being the nonmetal ions.
1 K and 1 Cl means 2 ions for the 1st solute. 2 K's and 1 SO4 means 3 ions for the 2nd solute.
3 K's and 1 PO4 means 4 ions for the 3rd solute. And 1 K and 1 NO3 means 2 ions for the 4th
solute. Since they are all at the same concentration,
the solution with the most ions will have the highest boiling point.
The last question works very similarly except here, the concentrations are not all the same.
Dissecting each solute the same way, 1 Ba and 2 Cl's means 3 ions for solution A.
1 Na and 1 NO3 makes 2 ions for solution B. C6H12O6 consists of nonmetals only so we treat
it as if it's one ion for solution C. And 2 K's and 1 SO3 makes 3 ions for solution
D. To factor in the effect of concentration,
we need to multiply the number of ions by the concentration.
The solution with the most ions will have the highest boiling point and the solution
with the fewest ions will have the lowest boiling point, making the order from highest
to lowest, A, B, D, C. Compared to plain water, a solution's boiling
point will be higher and its freezing point will be lower and the more ions there are,
the greater the effect.