Lesson Written by Carolina Sylvestri
EMSI Workshop – Mercury in the Environment
July 17-20, 2006
Stanford University
This is an example lesson that will use mercury and/or its compounds as examples in the
overall presentation of main chemistry topics. This will culminate in a Ksp lesson with lab,
which will introduce a mini unit on mercury in the environment.
20 Second Story
Two aqueous solutions can react and possibly produce a precipitate, which is an insoluble product. By
using a double replacement reaction we can predict the products. Through experimentation we can
determine if a precipitate forms and through deductive reasoning and on the basis of our lab data we can
generate a general solubility chart.
Pre Assessment (Day One)
Given: we have already learned what dissolving means (solvation), what the physical properties of a solution are,
and how to balance some different types of chemical reactions .
The students are asked to do the following:
1. Balance the following reactions
a. aluminum foil + hydrochloric acid  aluminum chloride + hydrogen gas
b. copper II nitrate solution + potassium hydroxide solution  copper II hydroxide solid
+ potassium nitrate solution
2. Predict the products and then balance the following reactions
a. sulfuric acid + sodium hydroxide  water + ____________
b. silver sulfate solution + magnesium chloride solution  _________ + _________
3. Now, write in the states (s, l, g, or aq) of each substance in the above reactions.
Content Construction (Still Day One)
1. Perform two different lab demos.
a. First, we mix sodium chloride and aluminum nitrate solutions. Both reactants are
colorless and we find upon mixing we retain the colorless characteristic. The question
is did a reaction occur? Using active participation the students are prompted to
talk it over with the student next to them. All students participate.
b. Second, we mix silver nitrate, a colorless solution, with potassium chromate, a yellow
solution, and students immediately see a red sold formed. The question is did a
reaction occur? Using active participation the students are prompted to talk it
over with the student next to them. All students participate.
c. The class discusses that the first demo does not represent a reaction because all four
ions in solution remain as ions in solution and nothing happened. However, the second
demo represented a chemical reaction because two of the four ions in solution ended
up bound together as a solid. We wonder if the other two ions remain as ions in
solution or if they are bound together as a solid as well. Together, we ask this question
and decide to study the reaction.
2. Using the board we look at this reaction
AgNO3Iaq) + K2CrO4(aq)  ? + ?
3.
4.
5.
6.
We go back and look at the pre assessment questions and find the model for the
prediction of products. We find that the technique is simply that the two outside ions go
together and the inside ions go together. This means that the silver and chromate ions
go together and the potassium and nitrate ions go together. We show this on the board.
We formulate silver chromate and potassium nitrate correctly and then balance the
equation. We make the point that at least one of these products has to be a solid. How
will we know if one of the products is a solid? Students are prompted to talk this over
with the student next to them. All students participate.
We discuss that one way is to run an experiment and make the observations.
The students try to predict the products and balance the equation for a reaction
between two aqueous solutions, mercury II nitrate and sodium sulfide. They try this at
their seats and I walk around and assess their work. Each student’s work is looked at,
every single student.
Next, we put the reaction on the board.
Hg(NO3)2(aq) + Na2S(aq)  HgS(s) + 2NaNO3(aq)
7. At this point, we have a short discussion on the solid produced. We talk about how hard
it is to dissolve this mercury compound. The students learn that the common name for
this compound is cinnabar, that it has two forms, and that it plays an important role in
mercury pollution. We talk about how this compound facilitates the formation of methyl
mercury, which is the major pollutant in our environment and is the problem we face in
contaminated fish. We mention we will return to this compound next semester when we
study solution equilibrium.
8. We return to the equation, and discover that due to the toxicity of mercury, we cannot
run an experiment. We ask, “Is there another way to learn whether or not the predicted
products are solids?”
9. Students are given a reading assignment and worksheet to complete. This will give
them practice with reactions between two aqueous solutions.
Discourse with Explicit Rules for Language. (Day Two)
1. We begin with the same equation from the day before. Mercury II nitrate reacts with
sodium sulfide to produce mercury II sulfide (solid) and sodium nitrate. We show on
the board once again how the outside ions go together and the inside ions go
together. We clearly state that this specific rearrangement of ions is called a
DOUBLE REPLACEMENT REACTION. Not all double replacement reactions
produce solids, but we will focus on that subset of double replacement reactions.
2. The teacher introduces the term PRECIPITATE. This is the solid, also called the
insoluble product. These reactions are called PRECIPITATION REACTIONS, a
subset of DOUBLE REPLACEMENT REACTIONS.
3. Next, students learn how to break up a correctly balanced PRECIPITATION
REACTION into its ionic form. This is called a TOTAL IONIC EQUATION. We stay
with the same reaction and do this on the board. We make a very clear point that
the PRECIPITATE cannot be broken down into its component ions because it is not
aqueous.
4. Next, students learn how to simplify a TOTAL IONIC EQUATION its simplest form.
This is called a NET IONIC EQUATION. We stay with the same example and turn
the TOTAL IONIC EQUATION into a NET IONIC EQUATION.
5. We discuss that in simplifying we actually cancelled out or got rid of those two ions
that did not change but remained exactly the same, as ions in solution. We clearly
state that those ions that do not change in any way are called SPECTATOR IONS.
We further this discussion by using the football game analogy. The fans are the
SPECTATOR IONS because they are not taking part in playing the game, however,
the football players on the field are the ions that make the PRECIPITATE because
they take part in the game playing.
6. Teacher passes out a short worksheet for students to practice DOUBLE
REPLACEMENT REACTIONS (PRECIPITATION REACTIONS) by predicting
products and writing the PRECIPITATE, writing the TOTAL IONIC EQUATION, NET
IONIC EQUATION, and picking out the SPECTATOR IONS.
7. The teacher will have students work on this in class and walk around to monitor and
assess every student.
Discourse with Explicit Rules for Language (Day Three)
1. We take any student questions about the worksheet or yesterday’s discussion.
2. We introduce the lab. We explain that because we cannot always directly observe a
PRECIPITATION REACTION because it might involve a toxic substance, as is true in
the case of mercury, we need to be able to look up if a particular product is a
PRECIPITATE or if it is not. We clearly state that if a product is a solid, it is called a
PRECIPITATE OR INSOLUBLE. We clearly state that if a product is not a precipitate,
it is called SOLUBLE. We look this information up on what is called a SOLUBILITY
CHART.
3. The objective of the lab is for the students to gather PRECIPITATION data by reacting
many different aqueous solutions. The students will have five different solutions sets,
and each set contains four or five different solutions. Each solution set will have its own
data table. The student will then analyze each data table and through deductive
reasoning identify each PRECIPITATE. The student will then summarize this data into
a SOLUBILITY CHART. The students will write a NET IONIC EQUATION for each
reaction.
4. The teacher will demonstrate how all of this is done. The students will see a model of a
constructed data table with sample data.
5. The pre lab student assignment is to construct a data table for each reaction set and to
predict the products for each reaction they will observe. They will be prompted to (prior
to the lab) eliminate those products that could not possibly be PRECIPITATES.
Scaffolding Opportunities for Discourse (Day Four and Five)
1. This is FUN!! Student will run the lab and during this two-day lab, the students will be
continually discussing what they are doing by interacting with their peers and teacher.
The students will be using the language over and over in doing the lab. They will have
questions and they will answer questions. They will be doing, observing, thinking, and
writing. They will also be SMILING!! The students will have ample opportunity to
reinforce the EXPLICIT LANGUAGE FOR DISCOUSE.
2. The teacher will be constantly monitoring and assessing. This will provide the teacher
ample opportunity to ask students questions, to answer student’s questions, to guide
students, and to reinforce the EXPLICIT LANGUAGE FOR DISCOURSE.
3. Finally students will turn in a completed lab report several days after the lab is
completed.
4. The labs will be returned in about 3 days and then a final discussion will take place on
results and the use of the solubility chart.