Pure Chemistry Practical Exam 14 Years Analysis
Chemical Calculations & Key Sources of Error
How to Answer Planning Question in Chemistry Practical
It is the time of the year again. GCE O-Level Examination has started in Singapore for academic year with the science practicals.
Students in Singapore who are taking the O-Level Pure Chemistry syllabus (code: 6092) will be taking their Paper 3 Practical Exam on 2 October 2024.
During the last few lessons in our Sec 4 Pure Chemistry and IP Chemistry tuition classes, I have recapped with my students on the key points to take note of for their upcoming Chemistry practical examination.
Today, I would like to share with you readers on some of the key points you should also take note of.
Format of Paper 3 Practical:
First of all, we must know that Paper 3 Practical is worth a total of 40 marks and this constitute 20% weightage of your overall Chemistry grade. The time allocated to finish your practical paper is 1 hour and 50 minutes.
This practical paper consists of a variable number of compulsory practical questions. Students must attempt all the questions and should not skip anyone of them.
One, or more, of the questions will incorporate assessment of Planning (P) and require candidates to apply and integrate theoretical knowledge and understanding from different sections of the Pure Chemistry syllabus.
The assessment may also include questions on data-analysis which do not require practical equipment and apparatus at all. It requires the treatment of given experimental data in drawing relevant conclusion and analysis of proposed plan. As such, students should revise their theory and know their concepts well before going to their practical exam.
Candidates are not allowed to refer to notebooks, textbooks or any other information during the assessment.
A copy of the Notes for Qualitative Analysis will be given to students for reference, which includes the procedures and observations when carrying out tests to test for (1) Cations, (2) Anions and (3) Gases.
Do note that the assessment of Planning (P) will have a weighting of 15% and the rest will have a weighting of 85%.
Assessment of Skills:
According to Singapore Examination Assessment Board (SEAB), you will be assessed in the following skill areas:
(a) Planning (P)
You should be able to:
- identify key variables for a given question/problem
- outline an experimental procedure to investigate the question/problem
- describe how the data should be used in order to reach a conclusion
- identify the risks of the experiment and state precautions that should be taken to keep risks to a minimum
(b) Manipulation, measurement and observation (MMO)
You should be able to:
- set up apparatus correctly by following written instructions or diagrams
- use common laboratory apparatus and techniques to collect data and make observations
- describe and explain how apparatus and techniques are used correctly
- make and record accurate observations with good details and measurements to an appropriate degree of precision
- make appropriate decisions about measurements or observations
(c) Presentation of data and observations (PDO)
You should be able to:
- present all information in an appropriate form
- manipulate measurements effectively for analysis
- present all quantitative data to an appropriate number of decimal places/significant figures
(d) Analysis, conclusions and evaluation (ACE)
You should be able to:
- analyse and interpret data or observations appropriately in relation to the task
- draw conclusion(s) from the interpretation of experimental data or observations and underlying principles
- make predictions based on their data and conclusions
- identify significant sources of errors and explain how they affect the results
- state and explain how significant errors may be overcome or reduced, as appropriate, including how experimental procedures may be improved
Topics and Skills to be tested:
Before going to the Chemistry practical exam, you must first be very sure of the type of questions (and thus the hands-on practicals) that you will be tested. You should have some experience by now, based on those Chemistry practicals sessions you went through with your school teachers. Check your practical worksheets to revise.
You must be able to use appropriate apparatus/equipment to record a range of measurements such as mass, length, time, volume and temperature.
In addition, you will be expected to handle a range of experimental techniques such as:
1. Titration
- acid-base titration (with suitable indicators such as methyl orange, screened methyl orange, and thymolphthalein)
- other types of titrations may also be required, and where appropriate, sufficient working details will be given
2. Speeds of reaction that may involve measuring of the quantities of products or reactants at regular time intervals
- measuring the volume of a gas produced
- measuring the decrease in mass of a reaction mixture
- measuring the time taken for a precipitate to be formed
- measuring the time taken for a coloured solution to be decolourised
3. Separation techniques
- simple paper chromatography
- crystallisation
- filtration
- distillation
4. Preparation of salts
- titration
- acids + excess insoluble substances
- precipitation
5. Gas collection
- displacement of water
- downward delivery
- upward delivery
6. Drying gases with the use of drying agents such as
- concentrated sulfuric acid
- quicklime
- fused calcium chloride
7. Displacement reactions such as
- metals displacement reaction
- halogen displacement reaction
- displacement of ammonia from ammonium salts
8. Tests for oxidising and reducing agents
- use of acidified potassium manganate (VII) as oxidising agent to test for reducing agent
- use of aqueous potassium iodide as reducing agent to test for oxidising agent
9. Qualitative Analysis
- test for the 8 cations
- test for the 5 anions
- test for the 6 gases
According to SEAB, you would not be required to carry out physical tests involving Pb2+ ions or sulfur dioxide gas.
Note that reactions involving ions not included in the Notes for Qualitative Analysis may be tested. In such cases, you will not be expected to identify the ions but only to draw conclusions of a general nature.
You should not attempt tests, other than those specified, on substances, except when it is appropriate to test for a gas.
10. Test of simple organic reactions
- test-tube reactions indicating the presence of unsaturation (C=C)
- test-tube reactions to test for presence of carboxylic acids
11. Energy changes
- dissolving of a solid in water
- reactions between two aqueous solutions such as acid-alkali neutralisation reaction
Procedure to investigate energy changes involving the dissolving of a solid in water would simply be:
- Measure the temperature of water
- Add the solid into the water and then stir the mixture
- Measure the temperature of the water after the solid has completely dissolved in it
12. Dataloggers
You are expected to be familiar with the use of dataloggers. Planning questions may include the appropriate use of data-loggers.
13. Mole concepts & chemical calculations
You may be required to carry out simple calculations as detailed in the theory syllabus. You will be looking at using formulae listed in the topics of mole calculations and energy changes.
Tips on Practical Techniques:
We should also be aware of the accuracy that is expected in titration and general instructions for qualitative analysis.
(a) You should normally record burette readings to the nearest 0.05 cm3 and they should ensure that they have carried out a sufficient number of titrations, e.g. in an experiment with a good end-point, two titres within 0.20 cm3.
(b) In qualitative analysis, you should use approximately 1 cm depth of a solution (1-2 cm3) for each test and add reagents slowly, ensuring good mixing, until no further change is seen. You should indicate at what stage a change occurs. Answers should include details of colour changes and precipitates formed, and the name and test for any gases evolved.
Common Reagents Available:
It is also good to know what are the common chemical reagents which are usually made available for practical exam.
According to SEAB, they are:
- hydrochloric acid (approximately 1.0 mol / dm3)
- nitric acid (approximately 1.0 mol / dm3)
- sulfuric acid (approximately 0.5 mol / dm3)
- aqueous ammonia (approximately 1.0 mol / dm3)
- aqueous sodium hydroxide (approximately 1.0 mol / dm3)
- aqueous barium nitrate (approximately 0.2 mol / dm3)
- aqueous silver nitrate (approximately 0.05 mol / dm3)
- limewater (a saturated solution of calcium hydroxide)
- aqueous potassium manganate(VII) (approximately 0.02 mol / dm3)
- aqueous potassium iodide (approximately 0.1 mol / dm3)
- aluminium foil
- red litmus paper
- blue litmus paper
- Universal Indicator paper
For GCE O-Level Pure Chemistry Examination (code: 6092), there will still be Paper 2 (Written Paper) with 50% weightage and Paper 1 (MCQ Paper) with 30% weightage after Paper 3 (Practical Paper). As such, it is important that students continue to revise their theories and concepts all the way until the last paper.
O Level Pure Chemistry Final Exam Boosters
| Name of Workshop | Date & Time | Venue |
| A1: Practical Theory | 26 & 27 Sep 2024, 6pm to 10pm | Onsite at SMU or Online |
| A2: The Right Keywords | 8 & 9 Oct 2024, 6pm to 10pm | Onsite at SMU or Online |
| A3: Score 100% O Level Pure Chemistry MCQ | 31 Oct & 1 Nov 2024, 9am to 1pm | Onsite at SMU |
Mr Chua told us to draw out the experiment before doing it which I felt is very useful and I’ll definitely do that in the practical. The planning questions were useful and a good consolidation for me to revise. The common mistakes part was very useful because I learnt what I didn’t really understand or was not aware of, and now I still remember those mistakes so I thought it was very useful
Christen Foong, Anderson Secondary School, E8 to A1
“This programme tells me the topics that I am weak in and it emphasises the importance of getting full marks for Chemistry MCQ. I have learnt to manage my time wisely, which is crucial in doing Paper 1 as I usually could not finish the paper. I learnt more about the strategies of doing MCQ and revise on key concepts. I hope I am able to get full marks and I will recommend this to anyone taking O-Levels.”
Cher Yi Jia, Nan Hua High School
To Your Chemistry Success,
Sean Chua