Deborah Casavant, Christopher Hyun, and Hallie Kleinfeld-Hayes,
Montgomery Middle School, Montgomery Township, NJ
Robert J. Cava
Princeton University, Princeton, NJ
Lesson 1
1. Many questions will come up during this lesson and those that follow. Some
of those questions will be addressed in later lessons, but some will not, and
are appropriate to addressed during the lesson in progress. We suggest that
the teacher or class begin right at the outset to keep a list of questions that
they would like to see answered in later lessons as the term progresses.
2. This particular lesson takes a substantial amount of time for the teacher
to set up. We suggest that you should begin to prepare several days beforehand.
3. As this is the first inquiry that the students do, the teachers found that
they really remember this one at the end of the term.
4. This lesson introduces the students to the laboratory equipment, and we suggest
that time can be taken to do that thoroughly. This will be helpful later on.
Teachers may find it valuable to describe some vocabulary at this point, for
example the use of words other than “stuff” and more descriptive
words instead of “pretty” or “ugly” or “disgusting”
to describe the results of experiments – more specific, descriptive words
such as “hot”, “cold”, “brown”, “bubbling”…
5. This survey lesson is not really designed to transmit information to the
students. Rather we found it helpful in allowing us to assess the background
knowledge of the students entering the class.
6. For inquiry 1.1, the hot water was changing the volume of the bottles during
the course of the school day!
7. We suggest that the pure substance section (inquiry 1.4) should be omitted
in this lesson. The students at this point are not well prepared to define such
a concept, and this is done later in the curriculum. The inquiry worked without
this.
This concept of defining pure substances is a problem in several lessons. Whether
something appears to be a pure substance depends on how carefully or closely
you look at it. For example, pond water, even if it looks clear, is not necessarily
a pure substance. This could be illustrated more fully at this point if for
example you present something that looks uniform by eye and then show it to
the students under a microscope so they see that it is not uniform.
8. For inquiry 1.5 – after the potassium permanganate is dissolved in
the water, the solutions (or some of them) could be left out by the teacher
to evaporate, and you will grow crystals. Otherwise, you can start this several
days before, and show the students the crystals while they are doing this inquiry.
9. Inquiry 1.7 involves issues of buoyancy that are not addressed anywhere in
this curriculum. You may want to think of another way to do this. We couldn’t
think of one.
An electronic balance to mass the objects used would be helpful.
10. In inquiry 1.8, the experiment might best be done over a sink if possible.
In this inquiry, as well as in others that use alka seltzer, we suggest that
teachers should be careful about the distribution of the alka seltzer - as it
presents a great temptation for the students to use in other ways after they
leave the class.
11. The students don’t all understand that bubbles generated in a liquid
such as is done in inquiry 1.8 are caused by gas evolution. We suggest that
inquiry 1.8 might be better done in a test tube and that the escaping gas be
captured in a balloon stretched over the mouth of the test tube to emphasize
the evolution of the gas.
12. This lesson took 3-4 days. Some of us thought that was an appropriate length
of time to devote to it but some of us thought it could be shortened by omitting
some parts.
Lesson 2
1. Some students may have trouble measuring the volumes in inquiry 2.1. The
result will be a distribution of densities for water – each student will
measure something different, and some will be quite different. This confuses
students. We have several suggestions:
A. The students should use the real volume they got when measuring out the water
rather than the “target” volumes in the table to calculate the densities.
Better yet, table 1 should be expanded to include larger volumes like 75 and
100 ml. This will illustrate that the larger volume and mass measurements lead
to smaller errors and one of the basic scientific principles of measurement
– that it is easier to be accurate when you are measuring large quantities.
A column should be added to table 1, just before the last column. That column
should contain the mass of the water to be used in the density formula. If the
“true” rather than “target” volume is used then a column
should be included for that too.
B. This is one of the places in the curriculum where you could record all the
measurements (of density in this case) the students in your class made and find
the average of all the values. You could do this for example for both the 25
and 100 mL measurements. This will illustrate that the more measurements you
make of a quantity the more accurate the resulting measurement. The average
value should come out closer to the true density than the individual students’
results. You could do this with other classes and teachers to get a large number
of measurements. You could then make a graph of the results: number of measurements
within a particular range in density (say 0.05 gm/cc) vs. the measured density
(a bar graph for example) to show the way the measurements are distributed –
some measurements will be larger than the average and some will be lower, and
the result should be the famous “bell shaped curve” of statistics.
This illustrates another scientific principle about the statistics of measurements.
C. If you use 25,50, 75 and 100 mL measurements as we suggest, then they can
graph mass vs. volume for the entries in table 1, and then take the slope to
calculate the density. They can get from this graph the units of density (change
in mass over change in volume). They can then graph the result between mass
and density and see that it is a constant.
2. The students had trouble understanding what the questions 3-6 were asking.
You may want to think about paraphrasing them before the lesson. Question 4
is particularly confusing.
3. In Inquiry 2.2, the wax had an irregular shape, making the measurement of
the dimensions unreliable. This can be turned to an opportunity for instruction
by asking the student to predict whether their measured density will be too
large or too small based on the shape of the wax block.
4. The more “creative” of the students carved the wax. You use the
same wax blocks again in lesson 3! So watch for this.
Lesson 3
1. Lesson 3 works pretty well. This gets the students beyond the idea that something
might float if it is just smaller than something else.
2. For table 1, some students get bored, you may want to add things like grapes
or pieces of apple to the list to help engage them.
3. For table 2 some students might benefit from having extra columns such as
“mass of graduated cylinder”, and “mass of graduated cylinder
plus liquid” before the column labeled “mass”.
4. For inquiry 3.1, making the density columns can make a mess. We suggest you
use glass-graduated cylinders because they are easier to clean than the plastic
ones.
5. For inquiry 3.1 you may want to try other objects in the density column.
One possibility is an ice cube (at the end of the inquiry).
6. Question 7 is confusing. The corn syrup will mix with the water forming a
uniform liquid eventually if sufficiently agitated.
7. Many students had trouble with question 4 in the homework. You might have
to explain it to them by giving an example. An example might be separating glass
bottles from plastic in recycling.
Lesson 4
1. Believe it or not, this experiment works.
2. The procedure is set up to have the students figure out how to do the experiment.
This seems to be of limited value. It helps focus more on the real lesson for
you to tell them how to do it.
3. Some students had trouble operating the equipment to get the vacuum so you
may have to help them.
4. The students should keep pumping until it is difficult to operate the pump,
but if they pump for too long they will have trouble breaking the seal.
5. Some students may have an irresistible urge to use the vacuum pump on parts
of their bodies.
6. The syringe apparatus (figure 4.3, teachers guide) can be broken by students
who push on both syringes at the same time.
7. We note that the experiment with the syringes is a good illustration of how
the brakes work in your car, except that the force is transmitted from one place
to another by a liquid in a pipe rather than a gas.
8. The bottle is marked with a volume, but that is not the volume that the students
need to use in the experiment! The volume needs to be measured by the students,
because the volume of the stopper must be accommodated.
But you really need dry bottles to do this experiment, because residual water
in the bottles will add a lot of error. So one bottle + student should be used
to measure the volume of a bottle by filling it to the appropriate place with
water and then pouring the water into a graduated cylinder. The best method
is to fill it all the way and then put in the stopper that will displace some
of the water, then pour the water into a graduated cylinder.
9. This is another case where you could average all students’ results
to see if you get a more accurate number.
Lesson 5
1. This is a tough lesson. One of us got frustrated enough with inquiries 5.1
and 5.2 to suggest they shouldn’t be done. Others of us thought they were
interesting enough that they should be done with modification.
2. It is difficult to know from the directions to know how to set up inquiry
5.1. In our case we had someone to show us how to do it.
3. A change in the procedure helped us a lot:
It takes a long time for the apparatus to get to room temperature after being
heated or cooled. Therefore the “room temperature” position should
be taken as the position of the water in the narrow tube directly after the
apparatus is put together. So the room temperature position is the first one
to be measured. Then it is important to put the homemade thermometer in the
cold bath first. The hot bath should be done second.
4. The calibration of the homemade thermometer was done by using a real thermometer
to measure the temperature of the hot bath and cold bath. This brought up the
issue of “calibration” which took some time to explain. Then the
students were asked to determine the value of “room temperature”
from the mark on their tube by extrapolating between the hot bath and cold bath
positions of the water. The temperatures obtained were compared to a thermometer
on the classroom wall. The results were not generally accurate.
5. There were some issues with the markers used to mark the tubes. Grease pencils
did work well, and it was difficult to remove the felt tip marker marks from
the tubes so that they could be used in the next class. (Alcohol might work
to remove the magic-marker marks, or maybe non-permanent transparency markers
could be used). We couldn’t mark the tubes with the temperature intervals
as suggested in the lesson. We think that a strip of paper or a 3x5 card taped
to the side of the tube would be better used for recording the temperatures
and for the calibration in general.
6. Inquiry 5.2: We suggest that this should be done as a demonstration by the
teacher. This seems to happen too fast for the students to be able to calibrate
it themselves. Though the point is to use baths of the same temperature used
in inquiry 5.1, when the air thermometer is put into the hot bath the water
plug shoots out the end of the tube if you follow figure 5.2 in the teacher’s
guide. Perhaps not-so-hot hot water could be used, or a half-water half-air
thermometer.
7. Inquiry 5.3 works. The students liked it.
8. We actually had the apparatus shown in Student sheet 5 question 1. The students
really enjoyed participating in the demonstration. We used an alcohol burner
rather than a Bunsen burner.
9. Inquiry 5.1 was done on one day and 5.2 and 5.3 were done on a second day.
5.1 is a push to do in one day. As long as the students have all the positions
marked on the tube and have recorded the calibration temperatures they can complete
the calibration and the determination of room temperature at home.
Lesson 6
1. This lesson heats many substances to high temperatures and releases a lot
of fumes into the classroom. Most are harmless but heating sulfur in normal
air results in the formation of H2S – which both smells bad (rotten eggs)
and is not so good for you to breathe for the whole day (one of us got a headache
from it). We recommend that the heating of sulfur should be done in a hood.
2. Also, the sulfur coats the tubes and really sticks. It is difficult to get
it off. Therefore for that part we used test tubes that we could throw out.
If you wash the sulfur tubes with water, then you will make a significant amount
of H2S.
3. We thought that the differences between physical and chemical changes described
in this lesson were confusing. The chemical changes were described essentially
as changes in matter that could not be reversed (and some chemical reactions
can be reversed).
4. The students had trouble coming up with their own vocabulary to describe
their observations of the substances that are treated in the experiment. We
found that this worked better if we presented a set of words for the students
to consider. Some were, for example:
Crystalline, granular, powder, solid, liquid, gas, luster, transparent, crackles,
needle-like, boiling, shiny, higher up tube, ring around, turns color, bubbles.
5. The copper (II) sulfate becomes a powder when heated, and is not blue any
more. This can be reversed by adding water again. The material turns blue, and
heats up as the chemical reaction occurs to form a hydrate.
6. Do the students associate the bubbles formed on heating these things with
the loss of matter? In the case of the copper sulfate hydrate in particular
it may be possible to weigh the material before and after heating (including
the test tube) to show that mass was lost.
7. The students didn’t necessarily understand where the hottest part of
the flame was to do the heating in this lesson. We recommend you show them.
Also, these heatings release gasses and result in a substantial amount of sputtering
in some cases. So we recommend that you remind the students to hold the mouth
of their test tube facing away from them and others. We also recommend that
the test tube should be held at an angle instead of straight up and down. (This
prevents the clamp they use to hold the test tube from getting too hot). We
recommend they place the clamp on the test tube near the top, and not in the
middle or at the bottom.
8. One up side of this lesson is that it gives the students a lot of experience
handling lab equipment. In our case we used alcohol burners, which worked well.
This is the first time that the students are shown to use an “arrow”
symbol to show the progress of a chemical reaction. Another interesting aspect
of the lesson is that the students see that doing an experiment on a substance,
such as heating it up in this case, can be used to help figure out what it is.
Lesson 7
1. For this lesson we recommend that you introduce the difference between heat
and temperature and get the students to see the difference before starting.
2. This is an interesting experiment that basically works. We used hot plates
to do this experiment. The hot plates need to be heated before your first class
because they take time to warm up the first time. We had two different kinds
of hot plates in our supplies. They heated by different amounts, so it was helpful
to figure out which heating setting worked best on each one.
3. When the water is being heated towards the boiling point, the students have
to be encouraged to keep taking readings until there is no temperature change,
otherwise they will miss the temperature plateau at the boiling point, and miss
part of the point of the lesson.
4. The experiment suggests that you start with a mixture of ice and water. That
results in the fact that you essentially miss the first temperature plateau
where the ice melts. We fixed this by freezing the thermometer in a chunk of
water in a flexible cup before starting. The ice made in a freezer is below
freezing (we got – 20 degrees), so when it starts from that low temperature
you can clearly see the melting plateau. By the way, the students were surprised
that ice could be colder than 0 degrees! The chunk of ice can be put directly
in the apparatus for doing the experiment with the thermometer stuck in. If
you have too many students to prepare the cold ice for everyone you could do
this as a demo.
5. The students have a difficult time understanding that melting/freezing and
boiling/condensing occur at the same temperature. A particularly difficult question
is to ask them what phase of water is present at 0 degrees or 100 degrees. One
of us did an extra demo by putting very hot water into the plastic bottle from
lesson 4 and showing that it was not boiling. If the bottle was then pumped
on with the vacuum pump used in that lesson, the water does boil, showing that
the boiling point depended on pressure (the pumping reduced the gas pressure
over the hot water).
6. Most of the students will not get a graphic plot like the one shown in fig
7.32 of the teacher’s guide. In the post lab discussions you will have
to show them that they have created a portion of the curve. Show them how the
part they got fits with the curve presented in the figure.
7. The teacher must explain independent and dependent variables to make the
graph required, and the convention that the independent variable is shown on
the x axis and the dependent variable on y axis.
8. We didn’t discuss the basic thermodynamic ideas of heat of fusion,
heat of vaporization, and heat capacity, though they are central to this lesson.
We were, however, asked why the temperature did not change at the phase change
between liquid and vapor. Our answer was to discuss the idea that it takes energy
to break the “bonds” holding the water molecules to each other and
so the heat was used for that instead of raising the temperature. (So you may
have to explain chemical bonds if this comes up and you want to answer.)
9. One of us did this lesson on a low-pressure day, and we found that the boiling
point of water was not 100 C. This surprised the students. Thus the pressure
dependence of the boiling point was a point of discussion.
10. We suggest that you should also remember to ask/point out to them that the
gas that is escaping at the boiling point of water is water vapor- the same
substance as a gas. You may also be able to get them to see that vapor takes
up more volume than a liquid at this point.
11. For homework at the end of lesson 7 we told them to come in with a clean
_ liter plastic bottle (we showed them an example of one) to use for lesson
8
Lesson 8
1. Inquiry 8.1 is designed to show that melting occurs without a change in mass.
But the students did often get changes in mass. This was confusing. We have
several suggestions to attempt to remedy this.
A. It is important to dry off the bottle as when they add the ice they get crushed
ice partly melted everywhere.
B. It might be helpful to weigh the bottles before the experiment and add a
column to the table 8.1 to record the weight.
Then the students could calculate the percent change in mass for each experiment.
Then the different percent changes can be compared directly. This is better
than comparing the absolute change in mass, as in the current version of the
lesson, as each student will have a different mass for their experimental apparatus.
2. Some students will see large weight changes. The students can be encouraged
to think about what the sources of error might be in those cases. You could
assemble all the weight changes (in percent) in your class and make a bar graph
that shows number of students getting a particular % weight change vs. weight
change (in bins – like 0 - .5%, .5-1.0%, 1.0-1.5% etc.), similarly to
our suggestion for an earlier lesson. You will get a distribution that spreads
in both the plus and minus direction from zero unless there’s a common
systematic error in the measurement. This is one of the cases in this lesson
series that you can discuss the statistics of errors in scientific measurements.
This kind of activity supplements questions 6 and 7 on the student sheet.
Lesson 9
1. We found some problems with this assessment.
2. The graduated cylinders were not all tall enough to submerge the long steel
nails. The students happily made their measurements with the nails sticking
out of the liquid!
We made sure we had the right size cylinders for those students measuring the
nails after that.
3. The students should be reminded to be as precise as they possibly can when
they measure the volumes. If they are off by 1 mL in the volume they might get
the wrong answer. In some cases there were bubbles stuck to the objects when
they were in the water. This messes up the volume measurement. We suggest you
might add a small drop of liquid soap to avoid that.
4. There is an error in table 9.1 in the teacher’s guide. The volume of
the steel nail is about 6.0 mL.
5. In the assessment question 5, if the students have never seen the low temperature
part of this figure before (the initial temperature rise and the first plateau)
none of them will understand what this curve is. There are two problems: the
low temperature part is new and the substance is not water, making this especially
confusing.
Lesson 10
1. Some of us thought that this was not a very good project. For example, when
students had to figure out what the source of rubber was the answer was simply
“trees”. Many students thought they could do the project based on
the knowledge they already had. Many students had trouble finding information
on their objects or included information that they didn’t understand.
2. We believe that showing the students a finished project that illustrates
the level of performance expected of them is essential to making this lesson
work, otherwise their research will be superficial and they will state the obvious.
If they don’t do a sophisticated job this can be something like an elementary
school project.
3. As part of this lesson, students can be given a list of science terms covered
to date and be asked to put them into their projects when possible. We collected
terms from the lessons and had about 60 words on our list.
4. The cube design for the presentation worked well but we think it would be
better for it to be larger. Strong paper should be used, otherwise the cubes
get crushed in some cases.
5. One of us gave the students the option of doing a PowerPoint presentation.
Some groups did this and did very well.
6. As part of this activity we had the students get some training in Internet
searches and the use of PowerPoint from the teachers in charge of that curriculum
in our school.
7. The students needed help understanding how to do research of this type. Many
students did not have sufficient referencing for the information they presented.
We think it would be best to tell them beforehand that they should keep track
of where all the information they use comes from while they are acquiring it.
This warning will be especially helpful for students using the web extensively.
8. If students picked objects outside of the suggested list we had to be careful
to keep them reigned in- because they would pick something too complicated –
for example a Nintendo game boy would be too difficult for them to figure out
in the context of this project.
9. One of us suggested that an alternative way of approaching this would be
to propose a problem and have the students do research to figure it out. For
example, the students could address this type of question:
A. What would be your design of a vehicle to be used for transportation on the
moon? What would it be made of? etc.? How about on Jupiter?
B. Same questions for an under-the-sea vehicle.
C. Another idea would be to have the students pick a fictitious object to perform
certain function. The students would describe the materials that the object
was made of, and how they made it work.
D. Another idea might be for the students to select a particular element, “aluminum”
for example, and then describe how it is used in manufacturing. Students should
describe in their project what properties of the element make it useful for
that application.
10. This lesson addresses really what the heart of materials engineering is:
how to select materials to perform functions in real world applications. One
of us had a working chemical engineer come into the class at this point to describe
to the students what kinds of problems they were working on.
11. In the same vein, one of us gave discussed with students that this was a
good example of doing research as a professional scientist or engineer: working
with others, doing research, keeping detailed notes, and then making a presentation
of your results that others can understand, within the constraints of a deadline.
This is a good taste of what being a working scientist or engineer is really
like.
Lesson 11
1. We didn’t like this lesson all that much. We suggest it might be good
to do an abbreviated version of this lesson and move on to the next lesson.
2. This lesson intends to have students determine the difference between pure
substances and mixtures by looking at them. However, many of the substances
cannot be distinguished as pure or mixtures just by eye. In effect, the lesson
shows them that you must worry about the scale at which you investigate something
to determine whether it is a pure substance of a mixture. Air is a mixture of
molecules, for example, and they can’t see that.
The shaving cream, for example, is confusing: it looks all white, but is it
really one pure substance ? (of course not)
3. We thought the sand provided was not a very good example of a mixture, it
didn’t have much “second substance” in it. We thought it might
better be modified by adding something to it like pepper to show a mixture more
clearly.
4. For the sugar and zinc oxide example, with fine grained sugar it’s
hard to tell it’s a mixture until you do the dissolving experiment. It
might be better to use coarse grained sugar and zinc oxide.
5. This would be a good lesson to introduce to the discussion elements, atoms,
molecules, and compounds, if that has not come up before.
6. One of us used this lesson to introduce the idea that mixtures of different
things can have different properties than the pure compounds themselves. For
example to remove water in your gas tank you add alcohol to the gas. This forms
a solution with the water, which then burns, removing the water, whereas water
itself won’t burn in your engine.
Lesson 12
1. We found that students need to shake the test tubes vigorously for at least
30 seconds vs. ten times as the book suggests to make this experiment work.
2. Also we recommend that the students should be told not to push too hard on
the stoppers that they are pushing into the test tubes, because they can break
the test tubes.
3. “Electrolytes” are introduced in the teacher’s guide in
this lesson. We didn’t think this was a good place to discuss this topic
since the students have no ideas yet about ions at all. We introduced this after
lesson 14.
4. Carbon dioxide dissolved in water in soda is one example of a mixture of
gas and liquid that students are familiar with.
5. We suggest you use only a few crystals (less than 10) of the potassium permanganate
in the demonstration where you take that stuff and you put it in a Petri dish
and then put the Petri dish on an overhead projector to show that it dissolves.
If you put too much in the Petri dish then they wont be able to see through
it. It might also help to put something under the Petri dish so they can see
through it.
6. We recommend that this lesson may also best involve a discussion of or introduction
of “molecules”. For example the teacher text often uses the word
“particles” when it is talking about “molecules”. Our
students understood molecules well enough to use that concept.
Lesson 13
1. Again students should be encouraged to shake vigorously after each additional
amount of sodium chloride is added, otherwise they will not be able to tell
when the solution is saturated.
2. With the materials we used, the solution stayed cloudy the whole time! So
it was difficult to tell when the solution was saturated! The students had previously
determined by looking at the copper sulfate demonstration that one of the characteristics
of a solution is transparency, so this was confusing! We think the problem comes
from an insoluble drying agent (silica) that is always present in table salt.
So you should try to get higher purity salt from a chemical supply company to
really make this work. (It is not really very high purity, just technical grade
should do. It’s just that table salt always has drying agents included.)
3. There was a very wide variation in the solubilities determined by the students.
This was confusing, as usual.
To fix this problem we suggest that you tell them how to do this inquiry using
a titration technique: The students should first add salt, shaking vigorously,
until they see the crystals at the bottom. Then after the crystals appear, they
carefully add small amounts of more water until the crystals disappear. They
should keep track of the volume of the added water. This will get them a closer
answer.
4. The teacher guide talks about the solution process being either endothermic
or exothermic. Some of us mentioned this in passing, but we did not discuss
it much. There are extra demonstrations that can be done here if you can get
the materials. One is the dissolution of barium chloride hexahydrate in water,
which is a highly endothermic reaction.
Lesson 14
1. The main concept here was for students to observe that mass is conserved
during dissolving but the combined volume of the solute and the solvent may
change. The students were fascinated about the loss of volume when water was
added to alcohol. Mass was conserved and easily observable. This was a good
inquiry.
2. We thought of another way to illustrate this with common materials. We showed
the students a beaker filled with rocks and asked them whether anything else
could fit into the beaker. They said no. Then we can added water to the rocks
in the beaker and filled in the spaces. This is a very good way to explain to
them what happens when the alcohol is added to the water, though that happens
on the scale of “molecules”.
3. We recommend that in inquiry 14.1 the students carefully put exactly 50mL
of water and alcohol into the starting beakers. Be sure they are measuring the
meniscus. Since when the two liquids are added the volume they will get is only
a few mL short of 100, initial measurement mistakes in the volumes of the pure
water and pure alcohol will result in the students not seeing the expected effect.
4. For 14.2 the students were supposed to design their own procedure for dissolving
a solid and measuring mass. The students had a lot of trouble designing a reliable
procedure: material was lost, for example, and the test tubes have mass, that
sometimes was not considered. Some of this might best be pointed out to them
beforehand. None of them came up with the procedure described in the teacher’s
manual.
5. We had to do a demonstration of 14.2 the next day in class and discuss the
sources of error to help them. We thought this was o.k. and provided a learning
opportunity.
Lesson 15
1. The students did relatively well with this lesson with relatively little
help from the teacher. This would be a good place to show the students what
the filter paper is. They should look at it under a microscope.
2. The rock salt that was provided with the kit did not include much impurity
and so only a few specks of impurity remained on the filter paper. We suggest
that some insoluble impurity be added to the rock salt to make this more obvious.
Such as pepper or the white sand provided by the kit.
3. To grow the salt crystals, an extra activity that can be easily added is
to have some students boil off the water (carefully!!!) to get precipitation,
and have others leave their dishes out in the air overnight to have the water
evaporate slowly. The difference in the crystals was dramatic: slower evaporation
yielded larger crystals. The small cubes of salt were easily visible by eye
in the slowly evaporating case. They are also easily seen in a magnifying glass.
You might ask them why they think the crystals have that shape. Maybe they can
answer this as a supplementary homework project.