Tr
i al
試
I n q u i ry - b as e d C h e m i s t ry Ex p e ri m e n t s
探究為本化學實驗
Exemplars for Secondary 4-5 Chemistry
中四至中五化學科教材示例
Department of Chemistry
The Chinese University of Hong Kong
香港中文大學 化學系
Science Section
Education Department
教育署 科學組
2002
Ve
行
r si
版
on
Contents
Page
Experiments
1
Chemical Tests for Calcium Carbonate
1
2
Electrolysis of Aqueous Copper(II) Chloride
9
3
Making Sulphur Dioxide
21
4
Identification of White Solids
29
Appendix
71
References
74
Chemical Tests for Calcium Carbonate
Student Handout
Purpose
1.
To design and perform chemical tests for calcium carbonate.
2.
To show the presence of calcium carbonate in a variety of minerals and building materials.
Background
Calcium carbonate is the major constituent of many minerals such as limestone and marble. It
has been widely used as a construction material for buildings and statues.
In this investigation, you will design and perform simple chemical tests for calcium carbonate.
You will also investigate which kinds of rocks and minerals contain calcium carbonate.
Task
1.
Design chemical tests for a solid sample of calcium carbonate. Write down the step-by-step
procedures, the expected observations, and the safety precautions.
2.
Consult your teacher of the feasibility of your proposed tests.
3.
4.
Perform the chemical tests.
Record the observations of the experiment and collect the relevant data if necessary.
5.
Collect two types of minerals / building materials and test for the presence of calcium
carbonate using the above tests.
6.
7.
Record the observations of the tests.
Prepare a report according to your teacher’s instruction.
1
Safety
Safety spectacles and laboratory coat must be worn in the laboratory during experiment. Handle
all chemicals with great care. Avoid direct contact of chemicals with skin. Dispose of chemical
waste, broken glassware and excess materials according to your teacher’s instruction.
Safety information on the chemicals used in the investigation can be found in the Material Safety
Data Sheet (MSDS). Consult your teacher for details.
Pay special attention when handling concentrated acids and naked flames.
Perform the experiment in a well-ventilated laboratory / fume cupboard.
Materials Available
Calcium carbonate
Platinum wire
Concentrated hydrochloric acid
Dilute hydrochloric acid
Bunsen burner
Conical flasks
Boiling tubes / test tubes
Sulphuric acid
Delivery tube
Bicarbonate indicator
Limewater
Mortar and pestle
Glass syringe / PVC tubings
*Special items will be provided upon request
2
Questions for Further Thought
1.
Apart from the usage as building materials, can you suggest one other application of calcium
2.
carbonate in our daily lives?
How can you protect a marble statue from weathering?
3.
Do you think that the above tests are unambiguous for the identification of calcium ion
(Ca2+) and carbonate ion (CO32)?
Are there any other ions which will give similar
observations?
3
Chemical Tests for Calcium Carbonate
Teacher Notes
In this investigation, students will design and carry out simple chemical tests for calcium
carbonate, which is a very common natural material. Through this investigation, students will
learn some of the characteristic chemical properties of calcium ion (Ca2+) and carbonate ion
(CO32), and the basic concept of qualitative analysis.
Curriculum Link
Section 1: Planet Earth
1.3
Rocks and minerals
Section 4: Acids and Alkalis
4.1
Acids
Section 9: Detection and Analysis
9.2
Detection of substances
Pre-Laboratory Talk
1.
Introduce the idea of “Qualitative Analysis” to students.
2.
Suggest students to verify their experimental results with appropriate control experiments.
3.
Remind students to avoid skin contact with acids.
4.
Let students know about the format of their laboratory reports.
4
Time Required
Introductory Session:
Introducing the experiment to students, ask them to write an
Experimental Session:
experimental procedure and submit for approval
Pre-laboratory talk (10 minutes)
Experiment (70 minutes)
Experimental Report:
Hand in a report after one week
Discussion and Presentation: Follow-up discussion and presentation with a teacher’s
(optional)
conclusion at the end (80 minutes)
Grouping
Two to four students in a group.
Some Acceptable Approaches
1.
The calcium ion can be identified by the brick-red colour in flame test.
2.
The carbonate anion can be identified by the reaction with dilute acids or by thermal
decomposition. Both procedures give carbon dioxide, which can be tested with limewater
(which turns milky) or bicarbonate indicator (which changes from red to yellow).
3.
Alternatively, accurate measurement of the volume of carbon dioxide evolved from thermal
decomposition can further confirm the carbonate anion. Note that the reaction with dilute
acids will give less accurate results as some of the carbon dioxide evolved will dissolve into
the solution.
5
Remarks
1.
Special precautions must be exercised when concentrated acids are used.
2.
When the flame test is carried out, make sure that no inflammable materials are placed
nearby.
3.
Make sure that the platinum wire is sufficiently cleaned in the flame test to avoid
interference. For optimum results, dip the platinum wire into concentrated hydrochloric acid
4.
and put it into the flame. Repeat this procedure until the wire gives a non-luminous flame.
The limewater used for testing CO32 should be freshly prepared.
5.
Teacher needs to determine the most appropriate concentration of acid and the amount of
calcium carbonate used in the test.
6.
7.
Be care of the sucking back action of limewater if CO32 is tested by thermal decomposition.
It is better to arrange a discussion and planning session a week before the experimental
session. Students should hand in their plans earlier so that teachers will have enough time to
comment and approve, and laboratory technicians can also prepare the necessary materials
in advance.
Laboratory Report
During the pre-laboratory talk, students should be instructed to include some or all of the
following items in their laboratory reports.
1.
Title
2.
Purpose
3.
4.
Apparatus and Reagents Used
Chemical Reactions Involved
5.
Procedure
6.
Observation
7.
8.
Results
Discussion (including possible sources of error and improvements of the experiment)
6
9.
Conclusion
10. Questions for Further Thought
11. Supplementary Questions
Suggested Solutions to Questions for Further Thought
1. Apart from the usage as building materials, can you suggest one other application of calcium
carbonate in our daily lives?
Answer:
Calcium carbonate can also be used to neutralise soil and lakes, which have high acidity due
to the effect of acid rain. It is also useful in the manufacturing of steel, paper, plastics,
paints, and adhesives, etc.
2. How can you protect a marble statue from weathering?
Answer:
A statue made of calcium carbonate (or marble) can be protected from erosion by applying a
coating of wax or lacquer.
3. Do you think that the above tests are unambiguous for the identification of calcium ion (Ca2+)
and carbonate ion (CO32)? Are there any other ions which will give similar observations?
Answer:
The hydrogencarbonate anion (HCO3), which also gives carbon dioxide when being heated
or treated with acids, will interfere the interpretation of the carbonate test.
7
Supplementary Questions
Teachers may discuss the following question with students or let them work on this question and
write the answers in their laboratory reports.
1.
The erosion of rock that due to chemical substances dissolved in rainwater is called
chemical weathering. Limestone suffers from erosion by the dissolved carbon dioxide in
raindrops. Explain the phenomenon.
Answer:
CaCO3(s) + H2O(l) + CO2(aq)  Ca(HCO3)2(aq)
Carbon dioxide dissolves in raindrops giving carbonic acid, which reacts with calcium
carbonate to give soluble calcium hydrogencarbonate. Therefore, the limestone dissolves
into the carbonic acid formed.
Related Websites
http://www.omya.com/ (accessed on 13/5/2002)
http://www.oase-uk.co.uk/expert/kohlendiox.html (accessed on 13/5/2002)
http://www.bbc.co.uk/schools/gcsebitesize/chemistry/geology/rockcyclerev2.shtml (accessed on
13/5/2002)
8
Electrolysis of Aqueous Copper(II) Chloride Solution
Student Handout
Purpose
1.
2.
To investigate the electrical conductivity of copper(II) chloride.
To study the products in the electrolysis of an aqueous copper(II) chloride solution.
Background
Ionic compounds consist of cations and anions in lattice structures. Their electrical
conductivity depends on their physical states (e.g. solid, liquid, or in aqueous solution). When
electricity is applied to aqueous solutions of these compounds, redox reactions may occur.
This process, known as electrolysis, can be employed for electroplating and generation of
pure gases such as hydrogen and oxygen. In this investigation, you will study the electrical
conductivity of copper(II) chloride (CuCl2). Also, you will study the electrolysis of aqueous
copper(II) chloride solution and the products generated in the process.
Task
1.
2.
3.
4.
5.
6.
7.
Design and draw a simple circuit diagram to illustrate the electrical conductivity of
copper(II) chloride in solid state and in aqueous solution.
Design a setup for the electrolysis of aqueous copper(II) chloride solution. Draw the
corresponding circuit diagram. Suggest simple tests to study the properties of the
products generated.
Draw diagrams or write scripts for the step-by-step sequence of the experiments based
on the circuits designed in steps 1 and 2.
Seek advice from your teacher for your designed procedures and the corresponding
safety precautions.
Carry out your tests designed in step 3.
Record the observations of the experiments.
Prepare a report according to your teacher’s instruction.
9
Safety
Safety spectacles and laboratory coat must be worn in the laboratory during experiment.
Handle all chemicals with great care. Avoid direct contact of chemicals with skin. Dispose of
chemical waste, broken glassware and excess materials according to your teacher’s instruction.
Safety information on the chemicals used in the investigation can be found in the Material
Safety Data Sheet (MSDS). Consult your teacher for details.
Avoid inhaling deeply the gas produced in the experiments when detecting its smell. Perform
the experiment in a well-ventilated laboratory.
Pay attention to the electrical wiring and the power supply to avoid electrical shock. Also be
aware of short-circuit.
Materials Available
Solid CuCl2
Deionised water
Beakers
Four 1.5 V dry batteries with holder
Variable resistor
Ammeter
Light bulb and holder
Wires with crocodile clips
Graphite rods
Plastic rods
Electrode holders
Blue litmus paper / pH paper
Test tubes
Droppers
*Special items will be provided upon request
10
Questions for Further Thought
1.
2.
3.
Is copper(II) chloride an electrolyte? Is it possible for a covalent compound to be an
electrolyte? If so, give an example to illustrate.
How can your setup for electrolysis be converted into one for electroplating of copper on
small metal objects?
Explain why the electrical conductivities of ionic compounds depend on their physical
states.
11
Electrolysis of Aqueous Copper(II) Chloride Solution
Teacher Notes
In this investigation, students will learn that mobile charged particles are responsible for
electrical conductivity. Therefore solid copper(II) chloride (CuCl2) is an electrical insulator
(it is composed of immobile ions), whereas aqueous CuCl2 is an electrical conductor
(copper(II) ion and chloride ion are free to move in solution). Students will also learn the
concept of electrolysis and its applications through the experiment of electrolysis of CuCl2.
Students are requested to design a proper setup for the experiments using the materials
available. Teachers can supply more apparatus and materials not listed in the student handout
so that students have to think carefully and figure out the essential apparatus and materials
needed for their own setup.
Curriculum Link
Section 2: The Microscopic World
2.5 Structures and properties
Section 5: Chemical Cells and Electrolysis
5.5 Electrolysis
Pre-Laboratory Talk
1.
2.
3.
4.
5.
6.
Introduce the concept of “electrical conductivity” to students.
Remind students to record the changes on the electrodes.
Ask students to predict the products of the electrolysis and to suggest methods to identify
them.
Remind students not to inhale deeply any irritating gas (chlorine) produced in the
electrolysis.
Remind students to avoid a direct contact of the two electrodes which will cause a shortcircuit.
Let students know about the format of their laboratory reports.
12
Time Required
Pre-laboratory talk: 10 minutes; Experimental design: 25 minutes;
Performing experiments: 50 minutes; Preparing the report: one week.
Grouping
Four students in a group.
Some Acceptable Approaches
1.
Students may design a setup such as the
one shown in Figure 1 to carry out the
experiment.
They may insert the
electrodes into CuCl2 powder and an
aqueous CuCl2 solution separately. They
Cathode
may also insert the electrodes into Anode
deionised water for control measurement.
Container
Students may prepare a CuCl2 solution by
CuCl 2 solution
dissolving a known amount of solid CuCl2
into deionised water and then measure the
Figure 1
conductivity of the resulting solution. In
this case, students may be asked to
calculate the concentration of the solution.
Another approach is to start with deionised water. A small amount of solid CuCl2 or a
mA
2.
3.
4.
few drops of concentrated CuCl2 solution (~ 1M) are added each time and the electrical
conductivity is monitored continuously. Students should find that the conductivity
increases when more and more CuCl2 is added.
The conductivity can be monitored qualitatively with a light bulb or quantitatively with
an ammeter.
13
Remarks
1.
2.
3.
4.
5.
Make sure that students use only the necessary amount of solid CuCl2 to observe the gas
bubbles. Approximately 0.5M CuCl2 is appropriate for this experiment.
It may take several minutes before the products are observed.
In the electrolysis experiment, students should find a brownish and shiny solid (metallic
copper) deposited on the cathode. Students should also observe gas bubbles appeared at
the anode. A smell of bleaching solution may also be detected. When a wet pH paper or
blue litmus paper is brought near the anode, or put into the test-tube that contains the gas
product, the paper should turn red and then white in colour.
After the experiment, the deposited copper can be removed by sandpaper or by soaking
the graphite electrode in dilute nitric acid for 2-3 hours. The cleaned electrode is
reusable.
It is better to arrange a discussion and planning session a week before the experimental
session. Students should hand in their plans earlier so that teachers will have enough
time to comment and approve, and laboratory technicians can also prepare the necessary
materials in advance.
Laboratory Report
During the pre-laboratory talk, students should be instructed to include some or all of the
following items in their laboratory reports:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Title
Purpose
Apparatus and Reagents Used
Chemical Reactions Involved
Procedure
Observation
Results
Discussion (including possible sources of error and improvements of the experiment)
Conclusion
Questions for Further Thought
Supplementary Questions
14
Suggested Solutions to Questions for Further Thought
1.
Is copper(II) chloride an electrolyte? Is it possible for a covalent compound to be an
electrolyte? If so, give an example to illustrate.
Answer:
Yes, CuCl2 is an electrolyte because it forms mobile Cu2+ and Cl ions when dissolved in
water. Also, it is possible for a covalent compound to be an electrolyte. For example,
gaseous HCl is a polar covalent compound, but it ionises and forms H+ and Cl ions
when dissolved in water. Therefore, HCl is also an electrolyte.
2.
How can your setup for electrolysis be converted into one for electroplating of copper on
small metal objects?
Answer:
Replace the graphite cathode with a small metal object and the graphite anode with a
copper foil.
3.
Explain why the electrical conductivities of ionic compounds depend on their physical
states.
Answer:
In solid state, the ions are immobile and they cannot transfer electrical charges from one
location to another. However, in molten state or in aqueous solution, the ions are free to
move and hence electrical charges can be conducted.
15
Supplementary Questions
Teachers may discuss some of the following questions with students or let them work on these
questions and write the answers in their laboratory reports.
1.
In this electrolysis experiment, you will find that the ions from the ionic compound are
converted to their elemental forms. This is, however, not always the case. For example,
consider the electrolysis of an aqueous solution of NaOH using graphite electrodes:
(a) What is produced at the anode?
(b) Will the Na+ ions be reduced at the cathode? If they are not, what ions are reduced
and what is produced? How can sodium metal be produced by electrolysis?
Answer:
(a) Oxygen gas is formed: 4OH(aq)  O2(g) + 2H2O(l) + 4e
(b) No, the Na+ ions will not be reduced at the cathode since sodium is higher in the
electrochemical series than hydrogen. Instead, the H+ ions (coming from water)
will be reduced to hydrogen gas: 2H+(aq) + 2e  H2(g)
Sodium metal can be produced by electrolysis of a molten sodium salt such as
sodium chloride.
2.
In this experiment, the graphite electrodes act as conductors to complete the circuit and
provide surfaces for oxidation and reduction. However, some types of electrodes may
also participate in the electrolysis. If two copper electrodes are used in the electrolysis of
an aqueous copper(II) sulphate solution, what is the change at each electrode? Write the
half-equations for the changes. What is the application of this setup?
Answer:
At the anode, the copper electrode becomes thinner: Cu(s)  Cu2+(aq) + 2e
At the cathode, the copper electrode becomes thicker: Cu2+(aq) + 2e  Cu(s)
This setup can be used for copper refining.
16
Laboratory Report
(Sample)
Title: Electrolysis of Aqueous Copper(II) Chloride Solution
Purpose:
1.
2.
To investigate the electrical conductivity of copper(II) chloride.
To study the products in the electrolysis of an aqueous copper(II) chloride solution .
Apparatus and Reagents Used:
Solid CuCl2, deionised water, beakers, four 1.5 V alkaline batteries with holder, ammeter,
connecting wires with crocodile clips, graphite rods, electrode holders, blue litmus paper, test
tubes, dropper.
Chemical Reactions Involved:
H 2O
CuCl2(s) 
 Cu2+(aq) + 2Cl(aq)
Cu2+(aq) + 2e  Cu(s)
2Cl(aq)  Cl2(g) + 2e
Procedure:
Part (A): Testing of electrical conductivity of solid CuCl2, deionised water and CuCl2
aqueous solution
1. Solid CuCl2 was put into a beaker.
2. Two graphite electrodes were connected to a battery holder and an ammeter in series.
3. The electrodes were inserted into the CuCl2 powder.
4. Four pieces of 1.5 V alkaline batteries were put into the battery holder.
5. The electrical current was recorded.
6. The electrodes were removed from the solid CuCl2 and rinsed with deionised water.
7. Deionised water was put into another beaker.
8. The electrodes were inserted into the deionised water.
9. The electrical current was recorded.
10. Solid CuCl2 was dissolved into the deionised water to make a 0.5 M solution.
11. The electrical current was recorded.
17
12. The alkaline batteries were removed from the battery holder.
13. The electrodes were removed from the solution and rinsed with deionised water.
Part (B): Electrolysis of CuCl2 and identification of the products
1. 50 cm3 of 0.5 M CuCl2 solution was prepared and transferred to a 100 cm3 beaker.
2. The electrodes were inserted into the solution.
3. The alkaline batteries were put into the battery holder.
4. The electrical current was recorded and the changes on both electrodes were noted.
5. A piece of blue litmus paper was wetted with deionised water. It was then brought near
the anode.
6. The colour changes on the blue litmus paper were recorded.
7. The alkaline batteries were removed from the battery holder.
8. The electrodes were removed from the solution and rinsed with deionised water.
Observation:
1.
2.
3.
4.
The solid CuCl2 and deionised water did not conduct electricity.
The aqueous CuCl2 solution conducted electricity.
During electrolysis, a brown and shiny solid was formed on the cathode.
During electrolysis, colourless bubbles were formed at the anode. The smell of
bleaching solution was also detected. A piece of wet blue litmus paper turned to red and
then white when it was put near the anode.
Results:
1.
2.
3.
Solid CuCl2 did not conduct electricity because copper(II) (Cu2+) and chloride (Cl) ions
in the lattice are not mobile. When solid CuCl2 was dissolved in water, Cu2+ and Cl
ions became mobile in the solution. As a result, the aqueous solution conducted
electricity.
During the electrolysis, a brownish and shiny solid was formed on the cathode. The
solid seemed to be a metal because of its shiny appearance. Since the only metallic
species in the solution is the Cu2+ ion, it is most likely that Cu2+ ions are reduced to
metallic copper on the cathode surface. The brownish colour of the solid is consistent
with this suggestion.
Colourless bubbles were formed at the anode. When a piece of wet blue litmus paper
was put near the anode, the paper turned to red first, suggesting the acidic property of the
gas. The subsequent bleaching of the paper as well as the bleaching smell suggested that
the gas produced was most likely to be chlorine, which could be generated from the
oxidation of Cl ions in the solution.
18
Discussion:
1.
2.
3.
4.
The conductivity of an electrolyte solution varies with its concentration. The higher the
concentration, the higher the conductivity will be. It is because at higher concentrations,
there are more mobile cations and anions bearing electrical charges and moving from
one place to another in the solution.
An ammeter, but not a light bulb can measure the electrical conductivity of the solution
quantitatively. Therefore an ammeter is preferred here.
Judging the nature of the solid deposited on the cathode by its appearance only may lead
to a wrong result. To verify the proposition, the solid can be scratched out for further
investigations such as physical measurements and chemical tests.
The gas produced at the anode can be collected with a test tube for chemical tests of
chlorine for confirmation.
Conclusion:
1.
2.
Solid CuCl2 does not conduct electricity. However, it conducts electricity when
dissolved in water.
During the electrolysis of aqueous CuCl2 solution, metallic copper is formed at the
cathode and chlorine gas is produced at the anode.
19
Making Sulphur Dioxide
Student Handout
Purpose
1.
2.
To prepare sulphur dioxide gas.
To study the chemical properties of sulphur dioxide gas.
Background
Sulphur dioxide (SO2) is commonly used in food industry, paper manufacture and chemical
production. Despite its wide applications, it causes pollution to the Earth. In this
investigation, you will prepare sulphur dioxide gas and perform simple tests to study its
chemical properties.
Task
1.
2.
3.
4.
5.
6.
Design experimental procedures and a setup for generating and collecting sulphur
dioxide gas with the materials provided. Draw a diagram for the setup.
List the chemical properties of sulphur dioxide gas. Suggest tests with detailed
procedure for these properties.
Seek advice from your teacher for your designed setup and procedures, the suggested
tests and the corresponding safety precautions.
Perform the procedures you designed and the tests you suggested.
Record the observations of the experiment.
Prepare a report according to your teacher’s instruction.
Safety
Safety spectacles and laboratory coat must be worn in the laboratory during experiment.
Handle all chemicals with great care. Avoid direct contact of chemicals with skin. Dispose of
chemical waste, broken glassware and excess materials according to your teacher’s instruction.
21
Safety information on the chemicals used in the investigation can be found in the Material
Safety Data Sheet (MSDS). Consult your teacher for details.
Avoid inhaling deeply the sulphur dioxide gas produced in the experiment. Small quantity of
reagents should be used in order to produce adequate amount of sulphur dioxide for
investigation. Perform the experiment in a well-ventilated laboratory / fume cupboard.
Materials Available
For SO2 gas generation and collection:
Solid sodium sulphide (Na2S)
Solid sodium sulphite (Na2SO3)
Solid sodium sulphate (Na2SO4)
Diluted sulphuric acid
Deionised water
Boiling tubes
Delivery tube
Gas syringe
Gas jar
Blue litmus paper / pH paper
Bunsen burner
Droppers
*Special items will be provided upon request
For testing the chemical properties of SO2 gas:
All common glassware, chemicals, and apparatus in chemistry laboratory are available
on request in advance.
Questions for Further Thought
1.
2.
Can we collect sulphur dioxide gas by displacement of water? Why?
Acid rain is a serious environmental problem in the modern world. One of the species
causing acid rain is the sulphur dioxide gas in the atmosphere. How is sulphur dioxide
gas emitted into the atmosphere? How does it form acid rain?
22
Making Sulphur Dioxide
Teacher Notes
In laboratory, there are three common methods to generate sulphur dioxide (SO2) gas, namely
(i) warming acidified sulphite solution, (ii) burning sulphur in air, and (iii) adding copper to
concentrated sulphuric acid. Since solid sulphur, concentrated sulphuric acid and copper are
not provided in this investigation, students are expected to generate sulphur dioxide gas using
the first method. Since the basic setup of the first method is quite simple, teachers can supply
more apparatus and materials not listed in the student handout so that students have to think
carefully and figure out the essential apparatus and materials needed for their own setup.
The chemical properties of sulphur dioxide gas will then be studied. It is an acidic gas,
bleaching agent and reducing agent. Students will suggest and perform tests for these
properties. For example, they may observe its acidic property and bleaching property by
bringing a piece of moist blue litmus paper or pH paper into contact with the gas, and test its
reducing property with an appropriate oxidising agent.
Curriculum Link
Section 6: Products from Important Processes
6.2 Sulphuric acid and sulphur dioxide
Pre-Laboratory Talk
1.
2.
3.
4.
5.
6.
Remind students that if warming is necessary, it should be done gently.
Remind students to use as small quantity of reagents as possible.
Remind students not to inhale deeply the sulphur dioxide gas.
Remind students to clamp the gas syringe gently and do not point its end to anyone if it
is used.
Remind students to record the observations.
Let your students know about the format of their laboratory reports.
23
Time Required
Pre-laboratory talk: 10 minutes; Experimental design: 25 minutes;
Performing experiments: 50 minutes; Preparing the report: one week.
Grouping
Four students in a group.
Some Acceptable Approaches
1.
2.
3.
Students may add solid sodium sulphite
syringe
plastic tube
(Na2SO3) into dilute sulphuric acid. The
solid may dissolve completely but it does
not affect the result. They may design a
stopper
setup such as the one shown in Figure 1, in
boiling tube
which the gas is collected with a gas syringe.
A moist blue litmus paper or pH paper may
dilute sulphuric acid
be used for testing the acidic property and
solid sodium sulphite
bleaching property of SO2 gas. The paper is
expected to turn red and then white.
warming
The reducing property of SO2 gas can be
Figure 1
tested with an oxidising agent such as the
acidified potassium dichromate solution.
Upon reduction, dichromate ion (Cr2O72) is converted to chromium(III) ion (Cr3+) and
the colour of the solution changes from orange to green.
Remarks
1.
Solid sodium sulphide (Na2S) should not be used in this investigation. Upon treatment
with acid, this compound will generate hydrogen sulphide (H2S) instead of SO2 gas.
24
2.
3.
Should the students suggest using Na2S, teachers should let them realise this is an
undesirable reaction.
Some students may confuse sodium sulphite (Na2SO3) with sodium sulphate (Na2SO4),
they may put Na2SO4 into diluted sulphuric acid.
It is better to arrange a discussion and planning session a week before the experimental
session. Students should hand in their plans earlier so that teachers will have enough
time to comment and approve, and laboratory technicians can also prepare the necessary
materials in advance.
Laboratory Report
During the pre-laboratory talk, the students should be instructed to include some or all of the
following items in their laboratory reports:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Title
Purpose
Apparatus and Reagents Used
Chemical Reactions Involved
Procedure
Observation
Results
Discussion (including possible source of errors and improvement of the experiment)
Conclusion
Questions for Further Thought
Supplementary Questions
Suggested Solutions to Questions for Further Thought
1.
Can we collect sulphur dioxide gas by displacement of water? Why?
Answer:
We cannot collect SO2 gas by displacement of water because it will dissolve into the
water.
25
2.
Acid rain is a serious environmental problem in the modern world. One of the species
causing acid rain is the sulphur dioxide gas in the atmosphere. How is sulphur dioxide
gas emitted into the atmosphere? How does it form acid rain?
Answer:
When sulphur-containing fuel is combusted, SO2 gas is formed and emitted into the
atmosphere. It will dissolve into the water in rain to form sulphurous acid, making the
rain acidic.
Supplementary Questions
Teachers may discuss some of the following questions with students or let them work on these
questions and write the answers in their laboratory reports.
1.
In laboratory, there are three common methods to generate SO2 gas:
(i) warming acidified sulphite solution;
(ii) burning sulphur in air;
(iii) adding copper to concentrated sulphuric acid.
(a) For each method, write chemical equations for these reactions involved.
(b) For each sulphur-containing species shown in Part (a), write down the oxidation
number for the sulphur atom. Hence, determine whether the sulphur-containing
species is an oxidising agent or a reducing agent in each reaction.
Answer:
(a) (i) SO32(aq) + 2H+(aq)  SO2(g) + H2O(l)
(ii) S(s) + O2(g)  SO2(g)
(iii) SO42(aq) + Cu(s) + 4H+(aq)  SO2(g) + Cu2+(aq) + 2H2O(l)
(b) (i) SO32(aq): O.N. = +4; SO2(g): O.N. = +4
Sulphite is neither an oxidising agent nor a reducing agent in this reaction.
(ii) S(s): O.N. = 0; SO2(g): O.N. = +4
Sulphur is a reducing agent in this reaction.
26
(iii) SO42(aq): O.N. = +6; SO2(g): O.N. = +4
Concentrated sulphuric acid is an oxidising agent in this reaction.
2.
A bleaching process may proceed by reduction or by oxidation.
(a) Suggest one chemical which bleaches by reduction and one by oxidation.
(b) Although reduction and oxidation are two reverse processes, there is a common
action in these two types of bleaching process. What is it?
Answer:
(a) Chemical which bleaches by reduction: SO2 gas
Chemical which bleaches by oxidation: Cl2 gas
(b) The common action is the conversion (by reduction or by oxidation) of the dye
molecules to colourless species.
3.
When a piece of filter paper wetted with acidified potassium dichromate solution is
brought in contact with SO2 gas, the colour of the paper changes. Write down the
chemical equation occurring and describe the colour changes. What property of SO2 gas
is illustrated in this reaction?
Answer:
3SO2(g) + Cr2O72(aq) + 2H+(aq)  3SO42(aq) + 2Cr3+(aq) + H2O(l)
The paper is expected to change from orange (due to dichromate ion) to green (due to
chromium(III) ion) in colour.
The dichromate ion is reduced to chromium(III) ion. Therefore, this reaction illustrates
the reducing property of SO2 gas.
27
Identification of White Solids (Short Version)
Student Handout
Background
You have learned various chemical tests for the identification of different ions and
compounds. In this investigation, you need to apply these knowledge and skills to identify
three unlabelled white solids, which are known to be citric acid, sodium sulphate and sodium
hydrogencarbonate.
Purpose
To distinguish three known compounds by suitable tests.
Task
1.
2.
3.
4.
Draw a flowchart to show how you can distinguish the three white solids using the
materials provided. Propose additional tests to confirm the chemical nature of the solids.
Seek advice from your teacher for your designed procedures and the corresponding
safety precautions.
Carry out the tests you suggested and record the observations.
Prepare a report according to your teacher’s instruction.
Safety
Safety spectacles and laboratory coat must be worn in the laboratory during experiment.
Handle all chemicals with great care. Avoid direct contact of chemicals with skin. Dispose
of chemical waste, broken glassware and excess materials according to your teacher’s
instruction.
29
Safety information on the chemicals used in this investigation can be found in the Material
Safety Data Sheet (MSDS). Consult your teacher for details.
Pay special attention when handling concentrated acids.
Materials Available
Dilute hydrochloric acid
Dilute sodium hydroxide solution
Concentrated hydrochloric acid
Limewater
Magnesium ribbon
Platinum wire
Bunsen burner
Glass rod
Wooden splint
Matches
Test tubes with rack
Washing bottle
Dropper
Delivery tube
Spatula
*Special items will be provided upon request
Questions for Further Thought
1.
2.
How can you distinguish between sodium hydrogencarbonate and sodium carbonate?
Without using any other reagents, how can you identify sodium sulphate from three
unlabelled solids known to be citric acid, sodium sulphate and sodium
hydrogencarbonate?
30
Identification of White Solids (Long Version)
Student Handout
Background
You have learned various chemical tests for the identification of different ions and
compounds. In this investigation, you need to apply these knowledge and skills to identify
four unlabelled white solids. Three of them are known to be citric acid, sodium sulphate and
sodium hydrogencarbonate, while the remaining one is an unknown compound.
Purpose
1.
2.
To distinguish three known compounds from four unlabelled white solids by suitable
tests.
To identify the remaining unknown compound.
Task
1.
2.
3.
4.
5.
Draw a flowchart to show how you can distinguish the three known compounds using
the materials provided. Propose additional tests to confirm the chemical nature of the
compounds.
Suggest a sequence of tests with detailed procedures to identify the remaining unknown
compound.
Seek advice from your teacher for your designed procedures and the corresponding
safety precautions.
Carry out the tests you suggested and record the observations.
Prepare a report according to your teacher’s instruction.
31
Safety
Safety spectacles and laboratory coat must be worn in the laboratory during experiment.
Handle all chemicals with great care. Avoid direct contact of chemicals with skin. Dispose
of chemical waste, broken glassware and excess materials according to your teacher’s
instruction.
Safety information on the chemicals used in this investigation can be found in the Material
Safety Data Sheet (MSDS). Consult your teacher for details.
Pay special attention when handling concentrated acids and oxidising agents.
Materials Available
Dilute hydrochloric acid
Dilute sodium hydroxide solution
Concentrated hydrochloric acid
Limewater
Silver nitrate solution
Acidified potassium permanganate solution
Magnesium ribbon
Platinum wire
Bunsen burner
Wooden splint
Matches
Test tubes with rack
Washing bottle
Dropper
Delivery tube
Spatula
Glass rod
*Special items will be provided upon request
Questions for Further Thought
1.
2.
How can you distinguish between sodium hydrogencarbonate and sodium carbonate?
Without using any other reagents, how can you identify sodium sulphate from three
unlabelled solids known to be citric acid, sodium sulphate and sodium
hydrogencarbonate?
32
Identification of White Solids
Teacher Notes
Before performing the investigation, students should learn various topics in the curriculum
including the reactivity of metals, acids and alkalis, redox reactions, reactions of chlorine and
sulphuric acid, and flame test, etc.
In the long version, students need to use all those knowledge and laboratory skills to
distinguish the three known compounds, namely citric acid, sodium sulphate and sodium
hydrogencarbonate, out of four unlabelled white solids. They also need to identify the
chemical composition of the remaining unknown white solid, which can be set as potassium
chloride or ammonium chloride.
The short version is a simplified one without the unknown compound. Teachers can adopt any
version for their students.
Curriculum Link
Section 4: Acids and Alkalis
4.1 Acids
4.2 Alkalis
Section 9: Detection and Analysis
9.2 Detection of substances
Pre-Laboratory Talk
1.
2.
3.
4.
Ask students to propose a systematic approach to solve the problem.
Apart from the basic tests to distinguish the three known compounds, ask students to
perform as many chemical tests as possible to confirm the chemical nature of the solids.
Remind students the corrosive nature of concentrated hydrochloric acid and the oxidising
properties of silver nitrate solution and acidified potassium permanganate solution.
Let students know about the format of their laboratory reports.
33
Time Required
Pre-laboratory talk: 10 minutes; Experimental design: 25 minutes;
Performing experiments: 80 minutes; Preparing the report: one week.
Grouping
Four students in a group.
Materials Available
Known white solids: citric acid, sodium hydrogencarbonate, sodium sulphate
Possible unknown white solids for the long version: potassium chloride, ammonium chloride
For other materials, please refer to the corresponding section in the Student Handout.
Some Acceptable Approaches
1.
2.
3.
4.
5.
6.
Students may dissolve the white solids in deionised water and use pH paper to test the
acidity of the solutions. The one that is the most acidic is citric acid. Teacher should be
awared that aqueous ammonium chloride is also weakly acidic. It may give similar
observations as citric acid.
The two known sodium salts can be identified by flame test.
The presence of sulphate ion can be identified by adding BaCl2(aq) to the solutions.
Students may add a dilute acid to the four solids separately. Only sodium
hydrogencarbonate will give a gas which can turn limewater milky.
Students may use sodium hydroxide solution to test the metals ions. The presence of
certain cations such as zinc, calcium and aluminum ions can be eliminated if no
precipitate is observed.
Students may identify the presence of potassium ion in the unknown compound by flame
test.
34
7.
8.
Students may add sodium hydroxide solution and heat the mixture. If the gas evolved
can turn moist red litmus paper blue, this can confirm the presence of ammonium ion in
the unknown.
Students may also test the chloride ion in the unknown by adding silver nitrate solution.
The appearance of a white precipitate can confirm the presence of chloride ions in the
unknown.
Remarks
1.
2.
Teachers should decide how to clue students in their planning stage. It depends on the
ability of students and how far they are from the right track.
It is better to arrange a discussion and planning session a week before the experimental
session. Students should hand in their plans earlier so that teachers will have enough
time to comment and approve, and laboratory technicians can also prepare the necessary
materials in advance.
Laboratory Report
During the pre-laboratory talk, students should be instructed to include some or all of the
following items in their laboratory reports:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Title
Purpose
Apparatus and Reagents Used
Chemical Reactions Involved
Procedure
Observation
Results
Discussion (including possible source of errors and improvement of the experiment)
Conclusion
Questions for Further Thought
Supplementary Questions
35
Suggested Solutions to Questions for Further Thought
1.
How can you distinguish between sodium hydrogencarbonate and sodium carbonate?
Answer:
Both aqueous solutions of sodium hydrogencarbonate and sodium carbonate are basic.
However, the pH of the carbonate solution should be higher than that of the
hydrogencarbonate solution. Alternatively, by adding the same volume of a dilute acid
to equal number of moles of excess sodium hydrogencarbonate and sodium carbonate,
hydrogencarbonate will give more carbon dioxide than carbonate. In a quantitative
approach, we can add excess dilute acid to a known amount of the samples and measure
the amount of carbon dioxide gas produced. For every gram of pure sodium
hydrogencarbonate, 286 cm3 of carbon dioxide will be produced; whereas for every gram
of pure sodium carbonate, 226 cm3 of carbon dioxide will be produced.
2.
Without using any other reagents, how can you identify sodium sulphate from three
unlabelled solids known to be citric acid, sodium sulphate and sodium
hydrogencarbonate?
Answer:
We can first prepare an aqueous solution of any one of the three unknown solids. The
other two solids can then be added to the solution separately. If no gas is evolved for
both solids, the solution is made from sodium sulphate. On the other hand, if a gas is
evolved when one of the solids is added, the remaining solid, which will not generate a
gas, will be sodium sulphate.
36
Supplementary Questions
Teachers may discuss some of the following questions with students or let them work on these
questions and write the answers in their laboratory reports.
1.
In the flame test, different metal ions show different colours. How can you explain these
phenomena?
Answer:
The heat energy from a flame excites the valence electrons of the metal ions from the
ground state to various excited states. Excited states are not stable and immediately
return to lower states with heat loss to surrounding and emission of light. Different
metals emit lights of different wavelengths. Therefore, different colours are observed.
2.
Flame emission spectrometer is a modern instrument for the determination of metals.
Explain the working principle of this instrument based on your understanding of the
flame test.
Answer:
In the flame test, different metal ions will give different colours in the flame. The more
sample we put in the flame, the higher the intensity of the colour we observe. Flame
emission spectrometer is a precise instrument, which detects the wavelength and
intensity of the emission. It uses a prism or grating to disperse the emitted lights and a
photosensitive detector to detect the emission rather than using human eyes in the flame
test. The spectrometer can therefore detect samples, both qualitatively and quantitatively,
with a much lower concentration.
37
Appendix
Report of the try-out of the exemplars in schools
In February and March 2002, two exemplars were tried in
three secondary schools. The try-out included the following
components:
 Planning of the investigation:
Students discussed the investigation in groups and
handed in their preliminary designs of experiments.
 Revision of preliminary design:
Students revised their designs after more discussion,
more information searching and receiving teacher’s
comments.
 Performing experiment:
Students performed experiments and tackled problems that arisen.
 Writing report:
Students handed in their reports a week after the experimental session.
In the try-outs, students’ designs and experimental reports were examined. Students’
performances in the planning sessions and experimental sessions were observed. Furthermore,
students’ attitudes towards inquiry-based approach were studied through class observations
and revealed from written feedbacks after try-outs. Teachers’ comments on the
implementation of inquiry-based experiments were collected through discussion.
The comments from teachers and feedbacks of students are reported here below.
Teachers’ comments
1.
2.
My students showed active participation in the
activities. They have expressed their
enjoyment.
Many students showed higher enthusiasm and
responsibility in doing experiments. They have
shown from their preliminary designs and
revised designs of experiments that they have
71
3.
4.
5.
6.
7.
thought through the meaning of each step of the experiments.
When my students presented their designs in front of the class, they showed logical
reasoning.
Some of my students showed frustrations when they faced problems.
My laboratory technician showed concerns on the safety issues, because students were
designing experiments by themselves.
I feel enthusiastic in this kind of approach. I have modified one of the investigations for
my S4 students.
My students shared to me that they learned a lot in this kind of investigations.
Students’ feedbacks
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
We need to plan the experiment beforehand; our
planning skills improve a lot.
It can train up our creativity, as we have to design
our own experimental procedures.
Our communication skills improve a lot through
discussions.
We can apply what we have learnt in chemistry
lessons.
The experiments are quite funny and challenging,
we feel more relax during these experiments.
We have satisfaction when the experiments are completed successfully.
There may be difficulties in gathering the necessary information or data to perform the
experiments.
We feel nervous when we have no idea on how to perform the experiments.
Time for doing the experiments is not enough; the number of laboratory sessions should
be increased.
Negligent planning may pose dangers in the experiments.
It will be better if the correct procedures are discussed after the experiment.
Though facing many problems, it is still a
good experience.
增加學生對化學科的興趣。
加強對化學實驗的了解。
引起求知慾。
訓練策劃實驗的能力。
加深對內容的印象。
72
18.
19.
20.
21.
22.
同學間的意見有分歧，容易與同學爭執。
對實驗結果感到不肯定。
不清楚實驗的詳細內容，容易出錯。
以前未試過設計實驗，因此開始時感到無助和缺乏信心。
希望有更多類似的經驗。
73
References
1.
V. L. Lechtanski, Inquiry-Based Experiments in Chemistry, American Chemical Society,
Washington, D.C., 2000.
2.
K. Davies, In Search of Solution, Royal Society of Chemistry, Cambridge,
1990.
3.
J. Taylor, In Search of More Solution, Royal Society of Chemistry, Cambridge, 1994.
4.
C. Wood and R. Sleet, Creative Problem Solving in Chemistry - Solving Problems
through Effective Groupwork, Royal Society of Chemistry, Cambridge, 1993.
5.
“Using the Science Writing Heuristic to Move toward an Inquiry-Based Laboratory
Curriculum: An Example from Physical Equilibrium”,
J. A. Rudd II and T. J. Greenbowe, J. Chem. Educ., 2001, 78, 1680.
6.
“Beyond Density: An Inquiry-Based Activity Involving Students Searching for
Relationships”,
S. Demeo, J. Chem. Educ., 2001, 78, 201.
7.
“Modification of Small-Scale One-Pot Reactions to an Inquiry-Based Laboratory
Exercise”,
C. J. Wilcox, J. Chem. Educ., 2001, 78, 62.
8.
“A Guided-Inquiry General Chemistry Course”,
J.J. Farrell, R. S. Moog and J. N. Spencer, J. Chem. Educ., 1999, 79, 570.
74