SNGS P3 Practical Notes

1 Notes for Practical Assessment 6092 Chemistry 2025 ________________________________________________________________________ Paper: 3 Weighting: 20% Duration:1 hr 50 min Marks: 40 This paper consists of a variable number of compulsory practical questions. Candidates will be assessed in the following skill areas: (a) Planning (P) • 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) • 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 pf precision • make appropriate decisions about measurements or observations (c) Presentation of data and observations (PDO) • present all information in an appropriate form • manipulate measurements effectively for anaysis • present all qualitative data to an appropriate number of decimal places/significant figures (d) Analysis, conclusions and evaluation (ACE) • 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 principals • make predictions based on their data and conclusions • identify significant sources of error and explain how they affect the results • state and explain how significant errors may be overcome or reduce, as appropriate, including how experimental procedure may be improved One, or more, of the questions may incorporate some assessment of Planning (P) and require candidates to apply and integrate knowledge and understanding from different sections of the syllabus. It may also require the treatment of given experimental data in drawing relevant conclusion and analysis of proposed plan.

2 The assessment of PDO and ACE may include questions on data-analysis which do not require practical equipment and apparatus. Candidates should be able to use appropriate apparatus/equipment to record a range of measurements such as mass, length, time, volume and temperature. In addition, candidates are expected to have been exposed to a range of experimental techniques in the following areas: 1 Titration, e.g. acid-base titration (with suitable indicator such as methyl orange and screened methyl orange and thymolphthalein). Other types of titration may also be required, and where appropriate, sufficient working details will be given. 2 Speeds of reaction that may involve measuring of quantities, e.g. temperature, volume, length, mass or time measurements. 3 Experiments involving separation techniques such as simple paper chromatography, filtration and distillation 4 Salt preparation 5 Gas collection 6 Qualitative inorganic analysis involving an element, a compound or a mixture, including displacement reactions and tests for oxidising ad reducing agents. Candidates should be familiar with the reactions of cations, reactions of anions and tests for gases as detailed in the Notes for Qualitative Analysis. Candidates would not be required to carry out tests involving Pb2+ ions or sulphur dioxide gas. Reactions involving ions not included in the Notes for Qualitative Analysis may be tested: in such cases, candidates will not be expected to identify the ions but only to draw conclusions of a general nature. Candidates should not attempt tests, other than those specified, on substances, except when it is appropriate to test for a gas. 7 Qualitative organic analysis requiring a knowledge of simple organic reactions as outlined in Section VI (of the syllabus) e.g. test-tube reactions indicating presence of unsaturated (C=C) may be set, but this would be for the testing of observation skills and drawing general conclusions only. This is not intended to be an exhaustive list. Candidates are expected to be familiar with the use of data-loggers. Assessment of skill Planning (P) may include the appropriate use of data-loggers. Candidates are not allowed to refer to notebooks, textbooks or any other information during assessment. A copy of the Notes for Qualitative Analysis will be printed as part of the Paper. The above text is extracted from Singapore-Cambridge general Certificate of Education Ordinary Level (2019) (Syllabus 6092)

3 Part I Notes for QA tests I Tests for cations: An unknown solution may contain one or more of these cations: NH4+ Ca2+ Zn2+ Al3+ Cu2+ Fe2+ Fe3+ Reagents used: aqueous sodium hydroxide and aqueous ammonia Observations: ppt.; colour of ppt.; solubility of ppt. in excess alkali Techniques: 1. Volume of unknown solution to start with is normally about 1 – 2 cm3. Starting with too much unknown solution means you would not have sufficient space for adding excess reagents to check the solubility of the ppt. formed. 2. In a case when a ppt. dissolves in excess of a reagent, the resulting colour of the solution formed must be recorded. 3. The technique of adding the reagent: add a few drops of the reagent, without shaking, observe if any ppt. is formed, before proceeding to add excess of the reagent. This is because some cations like Zn2+ forms ppt. which dissolves too readily in excess of the reagents. So adding too much of the reagents too fast in the beginning would lead to immediate disappearance of the ppt. formed initially. 4. To test for the solubility of the ppt. formed in excess reagent, add the reagent generously till the test tube is at least ¾ full. Mix the content well by pouring it back and forth between two test-tubes. 5. If no ppt. is formed with aqueous NaOH, heat the solution to check for presence of ammonia gas to confirm the presence/absence of ammonium ion. In this case the reagent used can only be NaOH(aq). 6. Do not test for ammonia gas if aqueous ammonia is used as reagent, because the reagent alone gives off ammonia gas. 7. Reverse application of cation test: use the cations to test for the presence of an alkali.

4 II Tests for anions: An unknown solution may contain one or more of these anions: NO3- CO32- Cl- I- SO42- To a large extent, the reagent used gives away clues regarding which anion might be present. Reagent Observations/deductions Dilute acid (alone) CO2 evolved; CO32- present OR H2 evolved; a reactive metal present Aqueous silver nitrate; Dil. nitric acid White ppt. formed; Cl- present OR Yellow ppt. formed; I- formed Aqueous barium nitrate/chloride; Dil. nitric acid White ppt. formed; SO42- present Aqueous sodium hydroxide, Al foil and warm NH3 evolved NO3- present Note: 1. Dil. Nitric acid may be added before or after the other reagent in the test for Cl- / I- / SO42-. 2. In the test for nitrate, it is advisable to add excess sodium hydroxide in proportion to the unknown solution. This is because many unknown solutions are acidic by nature. In addition, the cations in the unknown may use up some sodium hydroxide to form ppt. first. So only the excess sodium hydroxide gets to react with nitrate ion to liberate ammonia gas. 3. Reverse application: similar to cations test, the anion could be used as a reagent to test for presence of some ions present in a reverse manner. E.g. Cl- may be used to test for the presence of Ag+.

5 III Tests for oxidising agents and reducing agen