For which of the following mixtures will ag2so4(s) precipitate?
A. Pb(NO3)2 and KCl
B. HgCl2 and KBr
C. BaSO4 and NaOH
D. Cu(OH)2 and NaHCO3
C. BaSO4 and NaOH is the correct answer. BaSO4 is soluble in water, but the other two are not.
The following is a list of some easy ways to get from point A to point B, for those of us who want to go around the block or
don’t quite have the time.
-If you’re driving a car, there are many different routes you can take and still arrive at your destination (i.e., home).
-If you’re walking, there’s probably more than one way to traverse a certain amount of blocks and arrive at your destination (i.e., home).
-If you’re biking, it may be possible to bike from point A directly to point B without any stops in between.
-If you’re on a train and have to change trains, you’ll be able to get the same amount of blocks from point A to point B
after changing trains as if you just changed trains directly at point A.
-If you’re swimming, there are many different ways of getting from one place to another (swimming pools).
Many people have trouble with the last example. They think that going from point A to B is the same as going from Point A to any other location before going to B. In fact, it is not. If they were biking there would be stops in between Point A and B. There are no stops between A and B if they are biking directly to B from point A. In general, if there is an “A” then there is a finite number of other locations before point B. If there aren’t any other locations, then you can go straight from A to B.
How does this relate to Chemistry? The answer is in equilibria. There are many types of equilibria (e.g., precipitation), but the important one for this problem was in the HgCl2 question above. In that question, we had a solid (HgSO4) and a liquid (HgCl2) which were separated by a phase boundary (solid – liquid). This relationship was called the globule rule. There are other types of equilibria in which we have a solid (e.g., PbSO4), and a liquid (e.g., AgNO3) or a gas (HCl/NH3). In general, if there is an “A” then there is another location after B from which you can get to A. If there aren’t any other locations then we just go directly from A to B.
The mixtures being tested are the same as in the given illustration. In the illustration, the ball and glass are separated by a phase boundary (the different colors inbetween). There is only one “A” (purple above & below) but there are other possibilities after B. Therefore, it would be possible to get from A to B after changing trains, but not directly from A to B.
In general if there are two locations and one of them is “A”, then we can go from point A to point B by going directly there or by going from one of the other locations. The two locations may be separated in time or space (or both).
