Strategies for Effective Conduct of Practical Chemistry Works in Senior
Secondary Schools in Nigeria.
Lawrence Achimugu
Abstract
Practical work in chemistry laboratory has been identified as central to the teaching and
learning of chemistry. The paper highlighted the importance of practical work in chemistry.
It then discussed how to effectively conduct practical chemistry in the laboratory. The paper
pointed out the necessary precautions. Furthermore, the paper identified common errors
committed by candidates during practicals and suggested the way forward.
Introduction
Chemistry as a branch of science can be taught and learned most effectively if chemistry
teaching involves hands-on and minds-on or activity-centered or student-centered, rather than
lecture method or “chalk and talk” method which is teacher-centered, theoretical, boring,
disconnected and artificial. In line with this, the senior secondary school curriculum recommends
the use of discovery or inquiry approach of teaching chemistry which emphasizes practical
works in the laboratory.
Laboratory is a place equipped for experimental study in science. In his own view
Nwaigwe (1997) defined laboratory as a school building equipped with facilities, materials and
apparatus which students use, in carrying out investigation. Urevbu (1990) explained that while
laboratory work can be regarded as practical work performed in laboratory, practical work could
be performed elsewhere outside the laboratory. In other words, chemistry practical activities are
not confined to the laboratory only
A lot has been done to improve chemistry teaching in senior secondary schools in
Nigeria. Inspite of that, students continue to perform poorly in chemistry. Adeyegbe (1997)
traced the poor performance of students in practical chemistry, to teachers’ inability to take the
candidates through practical sessions in the laboratory. Achimugu (1997) noted that teachers
cannot display correct understanding of science processes when he himself cannot organize and
conduct practical classes. In the same vain, Ikeobi (1999) lamented that there are numerous
examples where candidate’s volumetric analysis titres could not be marked because the
supervisor’s titres differed widely from that of the students and in some cases supervisor’s titre
values were so outrageous that they were discarded altogether. The above situation has created
the need for this paper on effective strategies for conducting practical works in chemistry.
Importance of Practical Chemistry
For centuries, chemistry educators believe that practical chemistry is indispensable in chemistry
teaching and learning processes. Specifically, the importance of chemistry practical are a
follows:
1.
It help students develop science process skills such as observing, classifying, predicting,,
measuring, drawing, recording data, hypothesizing, etc.
2.
It promotes the development of scientific attitudes such as objectivity, honesty, curiosity,
patience, open-mindedness, etc.
1
3.
4.
5.
6.
7.
8.
9.
10.
It helps students to understand and appreciate the spirit and methods of science such as
problem solving, analytic minds and methods of science.
It is used to reinforce what is learnt in the theory class and hence encourages the spirit of
experimentation.
It arouses and maintains interest and curiosity in chemistry.
It helps students to develop manipulative skills and proficiency in writing reports.
It enhances students’ better understanding of concepts and principles and by so doing,
significantly contributes to students achievements in chemistry.
It encourages students to be active in the class, in other hand, discourages abstraction,
rote memorization and unattentiveness in the class.
It leads to fundamental and applied research in chemistry at all levels of education.
It helps to verify laws and theories that the students have already learnt.
Effective Conduct of Practical Chemistry
Many Chemistry teachers do not bother how they conduct practical chemistry in the laboratory.
In most schools it is done haphazardly. To conduct an effective practical chemistry, the teacher
should ensure that:1.
He/She try out the experiment to ensure problem free practicals.
2.
He/She has treated the topic in earlier theory lesson or briefly treat the theory before
practical.
3.
He/She teaches the students necessary precautions.
4.
The laboratory environment is safe, clean and conducive for the students to work and
achieved the desired result.
5.
The utility services in the laboratory are in good working condition i.e water, first aid
box, fire extinguishers, etc.
6.
All the apparatus and materials required for the practical are provided.
7.
The reagents required for the practical are prepared in large quantity.
8.
The entrance and exit routes of the laboratory are free.
9.
Identify the number of students that will take part in the practicals. Where the number
of students outshoot the available facilities, they should be grouped.
10.
He/She prepares practical guidelines or instructions to guide the students. This could
be written on the board or printed out and given to each student or group.
11.
He/She works round to assist that students in areas of identified difficulties.
12.
He/She outline the order of reporting practical chemistry i.e. tabular form.
13.
Students and laboratory assistants clean up the tables, wash the apparatus and replace
them to their respective positions after each practical.
14.
Students leave the laboratory in, an orderly manner.
15.
He/She marks the students practical works and makes necessary corrections for the
students.
Precaution During practical Chemistry
On this subheading the precautions to be taken will be discussed under volumetric and
qualitative analysis.
A.
Volumetric Analysis
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1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
Ensure that all the apparatus needed for titration are properly washed, rinsed with
distilled water and dried.
Rinse the burette with the acid solution to be used. This is to avoid diluting the acid
solution with the remains of water earlier used.
Rinse the pipette with the alkali to avoid diluting the alkali.
Do not rinse the conical flask with the base to avoid increase volume of the base i.e
pipette volume.
Do not blow the last drop at the tip of the pipette to avoid using more volume than the
pipette was made to deliver.
Ensure that air bubbles are not trapped into burette and pipette, to avoid reducing the
volumes of the burette and pipette.h
Tighten the burette tap to avoid leakage.
Always remove the funnel from the burette during titration to avoid an increase in the
volume of the solution in the burette due to drains from funnel.
Fix the burette in vertical position to avoid error of parallax i.e. if it is slant, the acid
level will be tilted to one side, thus correct reading become impossible.
Always use two or three drops of indicators. Too much indicators will result to not
having a sharp end-point.
Place the conical flask on a white tile or paper to avoid overshooting the end-point.
The burette jet will not be too far from the conical flask to avoid acid drops from
falling outside the flask. while the flask is swirled.
Swirl the conical flask gently, then vigorously to obtain a homogenous solution.
As you approach the end-point, be careful to add the acid drop by drop. This is to
avoid overshooting the end-point, as one drop is enough to change the colour of the
end-point.
Take your reading with the burette fixed at eye level. You can adjust the burette to
your eye level and face the graduation properly to your face. Don’t climb a laboratory
stool to take readings so as to avoid falling or pouring away the solution.
Record your burette reading as soon as they are taken. Do not record in a rough paper.
Record your reading in two places of decimals.
Titre values to be used for averaging should not differ by more than 0.20cm3 (i.e. +
0.20cm3). The titration should be repeated until consistent titre values are obtained.
Burette readings should be presented in tabular form. State clearly the volume of
pipette and indicator used.
Student should learn to round off their calculation in volumetric analysis to three
significant numbers. Significant figures are all the figures from the first non-zero digit
to the last non-zero digit plus all terminal zeros if this occurs after a decimal place.
Examples 0.0024 or 0.0244 or 0.244 or 2.44 or 24.4 or 244 are all three significant
figures.
3
B. Qualitative Analysis
1.
2.
3.
4.
5.
Use clean apparatus in carrying out qualitative analysis.
Use small quantities of the reagents.
Use boiling tube as the test tube for heating.
Always hold the test tube in a slanting position when heating.
When identifying the odour of the gas, do not inhale the gas directly as some gases
are poisonous.
6.
When heating, do not fill test tube to more than half of its volume.
7.
Use distilled water in diluting or dissolving your samples.
8.
In most cases, delivery tube should be used in passing a gas to a solution.
9.
Follow instructions strictly and addition of reagents should be sequential.
Marking scheme or guide for chemistry practical
A good marking scheme forms chemistry teacher preparation to good practical works of
his/her students. Teachers should develop a good marking scheme for each practical he/she
conducts. Furthermore, he/she should let the students know the areas of their strengths and
weaknesses after marking their practical works. Then he/she should try to remedy the situation in
the subsequent practical classes.
To illustrate how to develop a good marking scheme, I shall use WAEC (2010) i.e.
May/June 2010 WASSCE chief examiner’s report on chemistry. Below are the questions model
answers/marking schemes,
Question 1
A
is a solution containing 12.0g dm-3 of acid NaHSO4
B
is a solution containing NaOH
(a)
Put A into the burette and titrate it against 20.00 cm3 or 25.00 cm3 portions of B using
methyl orange as indicator. Report the titration to obtain consistent titers. Tabulate your
readings and calculate the average volume of A used. The equation for the reaction
involved in the titration is:
NaHSO4(aq) + NaOH(aq)
Na2SO4(aq) + H2O(1)
[H = 1.00, O = 16.0; Na = 23.0; S = 32.010]
[10 marks]
(b)
From your results and the information provided, calculate the
(i)
concentration of A in mol dm-3
(ii)
concentration of B in mol dm-3
(iii)
mass of Na+ formed in solution during the titration
[11 marks]
Model Answer and Making schemes
Volume of the pipette = 25.00cm3
[1mark]
Indicator used = methyl orange
[1 mark]
Burette reading s
Rough 1st Titre
2nd Titre
3rd Titre
Final burette readings (cm3)
24.30
24.10
24.00
24.00
Initial burette readings (cm3) 0.00
0.00
0.00
0.00
3
Volume of Acid used (cm )
24.30
24.10
24.00
24.00
2 marks
2 marks
2 marks
4
Average volume of acid used = 24 .10 + 24.00 +
3
Subtotal = 10 marks
3
= 24.03cm2marks
(i) Concentration of A in mol dm-3
Molar mass of NaHSO4 = 23 + 1 + 32 + (4x16) = 120g mol-1
in mol dm-3 Mass concentration = 12/120 = 0.100 Moldm-3
Molar mass
Subtotal 3 marks
(ii) Concentration of B in mol dm-3 CAVA/CBCB = 1/1
Cn = CAVA/VB
= 0.10 x 24.03/25
[1mark]
Concentration of A
[2marks]
[1mark]
[2marks]
= 0.0960 moldm-3 = a mol dm-3 [3 s.f to score]
[No mark for wrong unit]
Subtotal 4marks
[1Mark]
Alternative Method:
1000cm3 of A contains 0.100 mol
24.03cm3 of A contains 0.100 X 24.03 = 0.00240 mole of A [1Mark]
1000
1 mole of A reacts with 1 mole of B
0.00240 moles of A react with 0.00240 mole of B
25cm3 of B will contain 0.00240 moles
[1mark]
3
1000cm of B will contain 0.00240 x 1000
25cm3
-3
= 0.0960 mol dm (3 s.f to score) [No score for wrong unit] [2marks]
Subtotal = 4marks
iii) Mass of Na+ formed in solution
Amount of NaOH in the solution = 25 x 0.0960 = 0.00240 moles
1000
[1mark]
Mole ratio NaOH: Na2 SO4 = 1:1
Amount of Na2 SO4 = 0.0024
Atomic mass of Na+ = 2 x 0.0024 = 0.0048
[1mark]
Mass of Na+ formed = 23 x 0.0048= 0.110g
[2marks]
Subtotal = 4marks
Grand total 11 marks
Question 2
C and D are inorganic salts. X is a solution of an inorganic compound.
Carry out the following exercises on C, D and X. Record your observation and identify any
gas(es) evolved. State the conclusion draw from the result of each test.
(a) (i) Add about 5cm3 of distilled water to C in a test tube and shake
thoroughly.
Divide
the resulting solution into two portions.
5
(ii) Put about 2cm3 of X into a separated test tube and add the first
portion
of solution in 2(a)(i)
(iii) To the second portion of the resulting solution in 2(a)(i), add
few
drops of Bacl2(aq) followed by excess dill. HCl.
[8marks]
(b) Add about 10cm3 of distilled water to D in a boiling tube and shake thoroughly.
(i) To about 2cm3 of the resulting solution in a test tube, add NaOH(aq) in drops and then in
excess
(ii) To another 2cm3 portion of the resulting solution add few drops of
K2CrO4(aq)
[8marks]
6
Subtotal 8marks
Model answer with Marking Scheme
(a)(i)
TEST
C(s) + distilled water
OBSERVATION
INFERENCE
C dissolved to give a Colourless salt
colourless solution [Accept C
[1mark]
is soluble No mark for wrong
colour]
(ii)
C(aq) + X (aq)
[1mark]
(iii)
C(aq) + BaCl2(aq)
X decolorized colourless
[1mark]
C is reducing
agent
[1mark]
SO42-, SO32-S2- or
CO32[1mark]
White precipitate
[1mark]
+ excess HCl(aq)
(b)
D(s) + distilled water
Precipitate
SO32- confirmed
dissolved/precipitate soluble
[1mark]
[1mark]
D dissolved to give a Colour salt
colourless solution
[1mark]
[1mark]
[Accept D is soluble No mark
for wrong colour]
Subtotal 8marks
(i)
D(aq)+NaOH(aq)
drops
Pb2+ or Ca2+
[1mark]
in
White precipitate
[1mark]
Pb2+
[1mark]
(aq)
+ excess NaOH
(ii)
D(aq) + K2CrO4(aq)
Precipitate
dissolved/precipitate soluble
[1mark]
Yellow precipitate
[1mark]
7
Pb2+ confirmed
[1mark]
Common Errors in Practical Chemistry
The common errors committed by candidates during practical chemistry which normally
lead to loss of marks are as follows:
A.
Volumetric Analysis
No unit or wrong unit
Inaccurate burette reading eg 24.55cm3
Excessive approximation or no approximation at all or lack of knowledge of
significant figures.
4.
Volume of pipette not stated.
5.
Indicator used not stated.
6.
Averaging non-concordant titre values
7.
Poor mathematical/manipulation skills
8.
Frequent mutilation of titre values to agree with those of the supervisor.
9.
Poor knowledge of mole concept.
10.
Putting alkali in the burette instead of acid.
1.
2.
3.
B.
Qualitative Analysis
Spelling mistakes/poor communication skills.
Wrong charges on works eg Ca+ or Pb+
Presentation of wrong test observation and inferences
Inability to observe correctly changes such as partial solubility, colour changes,
precipitates odour, etc.
5.
Wrong use of technical (chemical) terms e.g. particles for precipitate.
6.
The use of self derived abbreviation such as SodOH(aq) for NaOH(aq), NH+3 for NH4+,
CO4 for CO32-, etc.
7.
Carrying out additional test that are not required e.g. when a student is suspecting
Ca2+ instead of using NaOH(aq) specified in the question, he/she goes to carry out
confirmatory test for Ca2+.
8.
Inference not agreeing with the observation or entirely wrong inference.
9.
Inability to separate observation from inference arising from the addition of reagent in
drops and then in excess. For example when sodium hydroxide solution is added in
drops observe that ‘white precipitate is formed’ when the alkali is added in excess,
follow up by observing that the precipitate is either soluble or insoluble in excess
alkali. You can separate the two observation but if you want to join them ensure that
you state it clearly.
10.
Inability to separate the observations and inference arising from precipitating agent
during confirmatory test of anions e.g. specimen A+BaCl2(aq) + dilute HCl; on
addition of BaCl2 solution make your observation and inference then add dilute HCl
and again make your final observation and inference. (Draw examples from the 2010
question treated in this paper).
1.
2.
3.
4.
The Way Forward
Student weaknesses in chemistry were attributed to inadequate exposure to (practical)
laboratory work and poor preparation. WAEC (2010) suggested the following remedies to
overcome the weaknesses
8
1.
2.
3.
4.
5.
6.
Candidates should be exposed to more laboratory practicals.
Candidates should prepare adequately for examinations.
Qualified teachers should be employed to impart the requisite knowledge.
laboratory should be well and adequately equipped to allow for more practical work
by students;
Attendance of WAEC coordination should be compulsory for teachers;
Teachers should be encouraged to attend workshops, refresher courses and
conference such as STAN in order to be exposed to new frontiers of knowledge.
Conclusion
The current trends in chemistry education which places emphasis on inquiry or discovery
approach to teaching and learning makes it a must for chemistry teachers to integrate the
theoretical aspect of the subject with practical works in the laboratory. Therefore, the teaching of
chemistry using practical works in the laboratory should be a rule rather than an option to
chemistry teachers, especially if chemistry is to make its maximum contribution to education and
well–being of the Nigeria society.
Reference
Achimugu, L. (1997). “organizing and conducting Senior Secondary
Certificate
Examination (SSCE) Chemistry Practical”. Proceedings of Kogi
State
third
Annual STAN Workshop, held at FCE
Okene in
October, 13 – 16.
Achimugu, L. (2009). Senior Secondary School Certificate Practical
Chemistry,
HEBN
publishers plc, Ibadan.
Adeyegbe, S.A. (1997). “ A Review of Chief Examiners Reports on SSCE May/June 1994
Chemistry Examination papers” STAN Annual National Chemistry Workshop
Proceedings held at Minna March 24- 28.
Ikeobi, O.I (1996). “Talk back” STAN Bulletin 3, (1), 6 – 7
Merriam, W. (1977). Webster’s New Collegiate Dictionary, G & C Merriam company, USA
Nwaigwe, B.O. (1997). “Organizing and conducting laboratory practical in Secondary School”
Proceeding of Kogi State third Annual STAN Workshop Oct.
13 – 16
Urevbu, A. (1990). Methodology of science teaching in Nigeria. Juland Education
publisher
West African Examination Council (2010) Chief examiners Report on the May/June
West African Senior School Certificate Examination (WASSCE) Chemistry,
Publication
9
WAEC