Assessment Issues for 3.1 - Chemistry 3.1 Investigations - Chemteach - University of Canterbury - New Zealand

Chemistry 3.1: Practical Investigations in Chemistry

In this section

Assessment Issues for 3.1

Introduction

Essentially this standard has two Achievement Criteria

  • develop and carry out a plan
  • process, interpret and report the results

Overall, success in the standard is determined by the lowest performance in either of these criteria. However, as with all investigations this standard should be assessed holistically.

It is often useful to initially identify whether there is sufficient evidence of achievement in a number of key characteristics that each of the criteria can be broken down into. These sub outcomes are used in the Assessment Schedule Template for Achievement Standard 3.1.

Rather than have a whole series of check boxes corresponding to each of these sub outcomes it is better to make an overall decision on each of the criteria as indicated by the single check box at each level of achievement for each of the criteria.

Clarification of the intent of each of the sub outcomes follows. Back to top.

Achievement

Purpose statement – to meet the needs of the intent of the standard to investigate “variations in the amount of a chemical substance” this must involve the concentration of more than one sample. This can be achieved by measuring amounts/concentrations of the chemical as a function of time, location, source, treatment (eg: exposure to heat or light) etc. This means an investigation to simply measure the amount of Vit C present in orange juice would not be acceptable. Rather, comparing the Vit C present in a number of different orange juices, or investigating the effect of exposure to light on the amount of Vit C present in orange juice, would be.

Standard analytical method described – this may be as simple as a photocopied page from a lab manual

Limited range of independent variable – investigations should normally include as wide a range of the variable as possible (subject to availability, time available etc) as this is more likely to allow a valid conclusion to be drawn. For example, if the purpose was to identify if boiling vegetables reduced their Vit C content, a before and after measurement would be the minimum expectation. To achieve at a higher level a more appropriate purpose would be to measure the effect of boiling time on the Vit C concentration of vegetables – this would require measurements over an appropriate range (as indicated by initial tests) of boiling times.

Actions and data recorded in log book –actions and their corresponding outcomes must be chronologically recorded in a log book. For authenticity reasons this logbook should be checked occasionally as part of the overall assessment procedure.

Sufficient data/repeats with minimal consistency to allow a conclusion to be drawn - this depends on the particular purpose for each investigation as described above. (At this level there are fairly generous levels of expectation for the quality of data obtained but it is expected that they should be realistic for a student at this level). At this level duplication of data points is generally expected. Note that for spectrophotometric data, simply reinserting a cuvette filled with the same solution does not constitute a duplicated data point (although it does provide some information on the consistency of the data for discussion at a higher level). However, at this level it is not expected that data points on the calibration curve need to be duplicated as the nature of the graphing process provides its own internal consistency check. In this case rather than expecting repeats it is the interval and range of points on the calibration curve that is appropriate.

Data processed using appropriate methods– at this level minor mathematical errors can be ignored and only one MAJOR ERROR is allowable. A major error involves either an incorrect logic process or misinterpretation/application of the chemistry involved. Examples include incorrect application of a mole ratio, using an incorrect volume, ignoring dilution factors or incorrect unit conversion.

A report of the investigation is presented that includes the purpose, method and results - the purpose and method are presented in the report but the level of achievement for these is assessed through the first of the Achievement Criteria.

Conclusion links data to purpose – the processed data must be related back to the original purpose e.g boiled cabbage contains less Vit C than raw cabbage or Brand A has more milk than Brand B. Back to top.

Achievement with Merit - includes the requirements for Achievement plus

Appropriate range of independent variable – this depends on the complexity of the particular analytical method. For investigations involving simple titrations, 4 to 6 samples would be a minimum expectation at this level whereas a more complicated method such as determination of iron in different foods may only require 2 to 3 samples (plus a calibration curve).

Significant variables controlled – examples of this requirement include temperature at which the reaction was carried out (in the determination of alcohol by back titration using dichromate) and pH (for EDTA titrations for Mg, Ca or Al).

Method is written in own words and sufficiently detailed to allow duplication of the results – it is not sufficient to have a hand written “transcript” of a standard procedure but there is an expectation of personal input to convert the method into a format describing what they actually did. This description must include any alterations made to the original method and should also note any safety requirements.

Data recorded in logbook enables independent retrieval – all recorded data must be readily accessible to an independent observer so that checks can be made on authenticity and the data processing requirement.

Quality and quantity of data is sufficient to allow valid conclusion – See appropriate range comment above. All standard solutions must be prepared to a specific concentration or be standardised against another standard solution. It is unacceptable at this level to take some bench reagent nominally labelled as 1.0 mol L-1, dilute it 10 times and claim that it is exactly 0.1 mol L-1 without undergoing a standardisation procedure. Where solutions have been made up by teachers or technicians to precisely known values (e.g. 0.960 mol L-1) it is acceptable to use these concentrations at face value. Unless the analytical procedure is particularly complicated or time consuming a minimum of three independent determinations of each sample would normally be expected.

For titrations, concordant titres must be available for averaging.

Individual measurements repeated to check on reliability – at this level concordant results are expected within the limits of accuracy expected from the particular analytical technique used.

Data is processed correctly using a clearly described method – No logic process or misinterpretation/application of the chemistry involved is allowed at this level. A few minor mathematical errors are still allowed.

The method of processing the data can be described as a series of steps or one fully worked example of the calculation can be presented in detail.

If the investigation involves more than one type of calculation a worked example of each different type of calculation must be clearly shown

Data is summarised using appropriate tables/graphs - the report includes a summary of relevant data such as titre volumes (it is not necessary to include all initial and final volumes). It is not expected or intended that students show details of every calculation (although this may well be included in the logbook). Instead the results of each calculation should be summarised in an appropriate form such as a table. Where constants such as concentrations of standard solutions vary between sets of data this must be clearly indicated in the summary. Any graphs used must be of the correct type i.e. bar graphs for discrete variables (such as brand type) and line graphs for continuous variables (such as time).

Conclusion has clear summary of findings related to purpose – this should have a quantitative basis e.g. Boiled cabbage boiled for 5 minutes contains 30% less Vit C than raw cabbage and boiling for a further 10 minutes reduces the Vit C content by a further 10% or Brand A with 140 mg Ca per 100mL of milk provides more calcium than either Brand B with 110 mg Ca per 100mL of milk or Brand C with 105 mg per 100 mL of milk.

Bibliography or acknowledgement of sources – these should be presented in a recognisable or standard format. Back to top.

Achievement with Excellence - includes the requirements for Achievement with Merit plus

Method is adjusted where necessary to ensure an appropriate range of data values – titrations that are either too low to be valid in terms of accuracy (less than a few mL), or are too large to be practical (more than 50 mL), are unacceptable for Excellence as this should have been corrected by appropriate modification to the method following the initial trials. Similarly, if the range chosen is too narrow or in the wrong area, initial trials should have determined this and more appropriate ranges selected for the investigation.

Any modifications made to the overall investigation or method used are justified – this usually arises as a result of initial trials requiring modifications to amounts (or concentrations) of either the unknown sample or the standard solutions and reagents used in the analytical technique.

Description of method shows clear understanding of overall technique – The general principles (such as balanced equation providing a mole ratio between known and unknown and how the end point is recognised) behind even a simple acid-base titration must be articulated.

With spectrophotometric techniques the relationship between concentration and colour intensity and the nature and purpose of a calibration curve is expected. The description may be in the form of a flow diagram or similar representing all the key steps involved in the overall technique. This is particularly important in more complex procedures such as those involving back titrations.

Quality of data within expected limits of method/equipment used – the level of accuracy of results from titrations should at least be of a standard consistent with the standard of Achievement with Excellence for achievement standards 2.2 (acid-base titrations) and 3.2 (redox titrations). Because of the nature of back titrations and EDTA titrations, expectations with regard to concordancy of titres are reduced. Similarly with spectrophotometric methods, the expectation should be in line with the nature and quality of equipment used but in general should be broadly in line with the accuracy that the assessor themselves could be expected to achieve under similar circumstances.

Quantity of data allows a comprehensive conclusion – the expectation here is significantly higher than that required for merit and should be reflected in the number of data points over a comprehensive range (e.g measuring the % of alcohol present daily during the fermentation process rather than just 3 or 4 times over a 2- 3 week period).

Sufficient duplication of data to allow checks on reliability and validity – as with quantity and quality of data, the number of repetitions required to determine reliability and validity of data depends on the particular experimental method.

Also, for at least one data point the entire analytical process must be repeated from scratch to enable conclusions about the validity of the technique to be made. i.e. it is not sufficient to use portions of the same stock solution for repeated titration or spectrophotometric measurements. A second sample must be taken and the entire procedure replicated from start to finish. This is particularly important for samples where there is expected to be some natural variation (e.g Vit C in fresh fruit, iron in meat, dissolved oxygen in water samples etc) as it provides information about the size of these variations.

Appropriate use of units/significant figures – at this level the expectation is 3 sig figs where possible and appropriate but obviously this is dependent on the nature and quality of the data.

Identification/discussion of sources of error or reliability of data – inane comments such as “I should have tried harder” are not acceptable at this level. Similarly focussing on trivial sources of error such as “there may have been some parallax error if my eye wasn't directly in line with the meniscus” at the expense of potential major sources of error are also insufficient.

Conclusion includes comments on validity of results – this is expected to include a comparison with any values stated by the manufacture or compared with typical values quoted in the literature.

Overall investigation is evaluated – this should include suggestions for improvements to either the method or the investigation as a whole, as well as comments on the significance of the investigation e.g relating the amount of calcium present in different milks back to the recommended daily requirement of calcium.

Back to top.

  • Department of Chemistry
    chemistry@canterbury.ac.nz
    Phone +64 3 364 2413
  • Chemteach
    University of Canterbury
    Private Bag 4800, Christchurch
    New Zealand
  • Follow us
    FacebookYoutubetwitterLinked In