2.1 Questions and Answers - Chemteach - University of Canterbury - New Zealand

2.1 Questions and Answers Archive

  1. Excellence requirements (comment)
  2. Acidfying barium chloride and silver nitrate
  3. Achieving at both achieve and merit in Task 3
  4. Credit for writing equations
  5. Students that get Task 1 wrong miss completely
  6. Identifying precipitates in Task 3
  7. Omitting OH- in equations
  8. Inability to write equations
  9. Identifying unknown in Task 7
  10. Identifying unknown in Task 7 (clarified)
  11. Leaving out OH-
  12. Complex ion equations
  13. Use of brackets in Pb(Cl)2
  14. Complex ions
  15. Calculated average titres
  16. Crediting students with Task 1 if they can identify precipitates in Task 3
  17. Problems with BaCl2
  18. If ions aren't correctly identified, why is there no follow on error for writing the equation?
  19. Credits for 2.1
  20. Marking schedule for 2.1
  21. Repeat assessments
  22. Using chloride as the anion to identify in Question 2
  23. Use of square brackets

Q1. Comment

There have been a number of queries about the requirement to get 4 out of 4 unknowns correct to receive Excellence. This 100% requirement is not required anywhere else in the chemistry standards as experience has indicated that even the best students will make the occasional slip up and should not be unduly penalised for this.

In this case however, the requirement is not a matter of 100% but a requirement for sufficiency purposes – particularly as some of the ion identifications are fairly trivial.

If the assessor is able to provide more opportunities for ion identification (e.g. 6 instead of only 4) then a requirement such as 5 correct out of 6 would be perfectly acceptable.

The only reason this was not done in the original task was for reasons of time limitations if the entire task is to reasonably carried out in a single period.

Most teachers are aware of the potential difficulties that arise when particular combinations of ions are used together in the unknown solutions. This severely restricts the range of unknowns that can be used in this format and this limited range can be a problem with authenticity in larger schools. However it is perfectly acceptable (and may be advisable) to restructure the task so that for each solution only the cation or anion needs to be identified. Eg: 2 (or 3) sodium salts and two nitrate salts could easily be provided as unknowns without the possibility of complicating reactions occurring. Back to top.

Q2. A number of the teachers asked why not acidify the barium chloride and silver nitrate, as they think it makes the identifications clearer. Also a number would like to have seen the [FeSCN]2+ ion included as a possible complex ion. Many use this as a definitive test for the Fe3+ ion rather than the "orange" precipitate. Presumably they will have to wait till the end of the year and put this is their submissions about the standards.

A. Correct and good suggestions. Back to top.

Q3. The fact that Task 3 can be used for evidence both at achieved and merit (the same for task 5 at merit and excellence) is causing problems because people who marked level 1 externals were clearly instructed that one task could not be credited twice.

A. A task may be designed to show a merit performance but if the student cannot perform at that level it may provide evidence of performance at the lower achievement level. Back to top.

Q4. A student who has task 3 correct say 2/3 and then is unable to write any equations in the practical still can be merit when they have more equations wrong than right over all. This student wrote equations for task 6 when they were not required and got them all wrong too. We would like to change the grid to be more like that of 2.3 and make it so the majority of equations were correct for pptn reactions. The same should apply to complex ion equations.

A. In SC/Bursary we tend to look for correct answers and often sideline incorrect components of a response as being "neutral".

At the end of the day perhaps some professional judgement might need to be applied where the assessors believes that when viewed holistically the student cannot write equations. But i think it opens up can of worms to try and build this formally into the assessment scheme. Back to top.

Q5. At present it appears a student who gets task 1 wrong has missed completely.

A. Correct and this may be an issue with the standard itself which could be looked at when itis revised next year. Back to top.

Q6. We felt that if they can identify the ppts in task 3 correctly they give appropriate evidence, however it should not allow them to then go on to an excellence.

A. But if they can't apply the basic concept of solubility rules to predict a precipitate then I personally don't feel they deserve any recognition for this standard when that is the core of all of the topic. Back to top.

Q7. If the students give the equation in the form

Cu(OH)2(s) + 4NH3(aq) → [Cu(NH3)4]2+ + 2OH-(aq)

And leave out the 2OH- does it give credit for writing complex ion equations?

My feeling is no even though it is a more difficult form than the alternative because the fundamental principle of balance is not there. What is the answer, please - I know it will come up in the workshops.

A. Agree that this is not good enough for excellence. Back to top.

Q8. I have several students who technically gain a grade but because of the number of opportunities to write equations in our assessment they clearly demonstrate they can't write equations - some wrote equations when they were asked to distinguish between the 2 ions in task 6 and got them completely wrong too. I intend putting an extra box called sufficiency which means more correct than incorrect. I know the holistic sheet gets round this but our students are used to checking off each requirement to see where they missed. I think there is an exemplar that goes over a similar problem.

A. Not 100% sure of the intent of your question but will give various interpreted responses.

The majority of the evidence must support the overall decision. The actual no of required correct equations will depend on the particular unknown/pairs of solutions chosen. Majority means more than 50% of the possible equations that could be written for the particular anions/cations chosen.

In the first few (theory) tasks it is only possible to obtain evidence at achievement level as they are only being required to complete equations. If they just missed out here but were able to consistently write correct balanced equations in the later (prac based tasks) this supplementary evidence can be used to gain them credit - but as their intent is to provide evidence vfor merit and higher they should not be counted as part of the total no of opportunities on which to base the majority calculation on. Otherwise weak students who could only complete would be unfairly penalised.

When it comes to decisions at merit etc the early questions can't be included as they do not provide opportunities to "write' equations. Back to top.

Q9. Student 4 is now excellence but the comments as regard to not excluding Zn2+ are still valid - why did we change? I know we discussed the ppt with hydroxide being sufficient for Cu 2+ but why is it now sufficient for Zn2+?

A. I assume you are referring to student 4 Task Six where they respond white ppt forms then disappears, therefore Zn2+.

If it was an ""open" unknown then this would not be sufficient as it could obviously also be Al3+ but in this particular section they are told the unknown is one of only two choices. In this case Al3+ is not an option (it is either Zn2+ or Mg2+) so there is no need to carry on any further. Back to top.

Q10. Sorry for not being clear. No it is student 4,task 7 the unknown where the student identifies Al3+ but has the observations and tests in the same column. They test with NaOH and ppt disappears then add NH 3 and get a white ppt. no mention is made of whether the ppt disappears or remains with excess. the conclusion is Al3+ but has Zn2+ been excluded? Or is it assumed that as they just say a white ppt it must mean it did not disappear?

A. While it would be better to state explicitly that excess had been added in this case there is sufficient consistency to give the student benefit of the doubt. ie that because they have chosen Al3+ they have made a conscious acknowledgement that the ppt has not redissolved in excess so thereefore they almost certainly did that but didn't get around to noting it. i.e your interpretation below is correct. Back to top.

Q11. A student doing task 4 of the written part of the standard in Ironic ions filled in the gaps for the equation as Cu(OH)2(s) + 4 NH3(aq) and did not add the 2OH- to the product side as there were no gaps available. The question was Is this sufficient for merit as the student has filled in the reactants showing an understanding of the chemistry they observed?

A. Not a sufficient answer as clearly it is not balanced. If they use Cu(OH)2 format they are expected to realise that they would need to add in the extra product. Back to top.

Q12. In task 7 we used copper nitrate and zinc sulfate. The copper nitrate and zinc sulfate give 3 possible complex ions. Would students have to get 2 complex ions equations correct to get 7b(E) (so that they have the majority of complex ion equations correct)? "

On p51 of the level 2 training manual, it says in part 6 of Assessment issues - "a majority of the possible equations (both precipitation and complex ion formation) are required." However in many other instances, students will get excellence on just one equation.

Your thoughts please.

A. Your interpretation is correct. This sort of variation/anomaly will arise occassionally as a result of the 50% requirement for sufficiency at merit and excellence levels. i.e if only 2 opportunities are possible, then 1/2 is acceptable, whereas if 3 opportunities are available 2/3 is minimum rquirement - yet if 4 opportunities were available. 2/4 would still be be the minimum requirement for sufficiency purposes.

Asking an even no of questions obviously favours the students here, but in many cases (because of available opportunities or time constraints) this may not be possible. The only real concern is that where there are only 2 opportunities possible an assessor probably has limited confidence in their judgement decision where there only needs to be one piece of positive evidence. Clearly it would be advisable to avoid these situations wherever possible. Back to top.

Q13. At Year 12 is Pb(Cl)2 acceptable, i.e. the use of brackets where it is not appropriate? Our feeling is it is not, and that was what was said at Wellington 's Jumbo Day. What is your view?

A. The use of the bracket when not required is not so much wrong as superfluous, therefore, it should be treated as neutral evidence and ignored. i.e. they shouldn't be penalised for this bit of carelessness (particularly as many able and careful students at this level may commonly include brackets around polyatomic ions such as nitrate when not required either). Back to top.

Q14. Complex ions. If task 4 is "complete complex ion equations", two out of 3 correct is merit, and then task 5 is "write complex ion equations", two correct out of three is Excellence, what do we do with a student who gets only one of task 4 correct (completes one complex ion equation, and two correct from Task 5). Our feeling is we use one of the task 5 equations to award the student Merit but she does not get Ex because we have should not use the same evidence twice. Holistically the student has 3 ticks out of a possible 6 which does not seem to us to be Ex. At Wellington 's Jumbo Day we were encouraged to look holistically when this sort of problem arose, and we were told we cannot use the same evidence twice.

A. Agree that overall holistic judgements are preferable but note that for merit and excellence 50% is the minimum requirement (unlike achievement where a majority of the evidence i.e. >50% has to be positive) and as in your quoted case this requirement has been met overall (3/6) they should be awarded excellence.

In general though this holistic view is preferable to the assessment grids which were designed to simplify the assessment process for teachers when starting out until they developed confidence in their own ability to make holistic judgements consistent with the national standard.

Even when quite experienced teachers mat still continue to use these grids to process the data efficiently but need to remember to do a final holistic check that the final grade awarded is consistent with the overall performance of the student. Back to top.

Below is a very important comment that has come through from NAP members which should significantly help with the problem of non-achievement in C2.1.

Currently, Achievement Standard C2.1 requires that the student demonstrate an ability to apply solubility rules and if they cannot do this then they cannot attain any achievement in the standard as a whole. In the exemplar task this requirement is currently expected to be met through the response to Task One.

However, if the student does not meet the sufficiency requirement there, supplementary evidence can be obtained from their responses to Task Three. Ie: if they can successfully complete Task Three then they must have also been able to correctly predict the presence or lack of a precipitate.

Q15. Would you please advise me on the following:

A student has carried out all the calculations perfectly, however there is a problem with her calculated average titre.

Her values are: 9.90, 9.76, 9.54, 9.70, 9.62, 10.58(!!!) mL. The correct answer should have been 9.50 mL She has 3 titre values within 0.2 mL of the correct one. However she chose to average 9.76 and 9.70 values only, thus having an average titre outside acceptable range. I have given the student Merit, but wonder should she receive Excellence?

A. The quality of the student practical work (as opposed to their processing of results) is judged by appropriate assessor processing of the raw data. In this case 2 different sets of 3 titre values can be determined within ranges of 0.2 mL. i.e. 9.62, 9.70 and 9.76 or 9.54, 9.62 and 9.70.

The average titres are 9.69 and 9.62 respectively - the first option is probably the better selection as its range is slightly lower but both selections give averages thatare within the acceptable range of the correct answer to meet excellence requirements. Therefore excellence should be awarded for the quality fo the practical work itself.

A couple of other points to note - as an assessor you would need to be confident that some of the results weren't made up as 6 titrations is a reasonably high number of attempts for presumably a single period. This is easily confirmed in practice if the quantity of stock solution taken by students is monitored. Secondly, it would be worth looking at the source data for the last titration as it looks as if the student has made a careless error in either recording one reading or in the subtraction process i.e. a value of 9.58 instead of the stated 10.58 would be very consistent with the other recorded values and would strengthen the case for this student being at excellence level for the practical component. Back to top.

Q16. We have some students who have been unsuccessful at Task 1 and also unsuccessful on Task 3. However, the reason for being unsuccessful on Task 3 is not through an inability to identify precipitates but through an inability to write balanced equations for the formation of those precipitates. Does yourcomment on 2.1 NCEA Update #4 mean that if a student can identify the precipitates in Task 3 but not in Task 1 then we can give them Task 1? For one of our students this would make the difference between Excellence and Not Achieved! Please advise.

A. Yes that is my understanding of the decision by the moderators. Back to top.

Q17. Secondly we had a problem with one of the solutions to be identified in Task7.

There were 2 problems with solution (BaCl2)

i) When students added ammonia to the precipitate that formed with silver nitrate it disappeared as it should do but then a lesser precipitate slowly appeared (I tried it myself). This caused many students to conclude that the anion was iodide instead of chloride. I suspect that the precipitate that appeared with ammonia was barium hydroxide. Consequently students who put I - for the X anion got it marked right provided the observations matched this. This meant that the mark scheme had to be amended for 7(b) E to reflect the fact there is no complex ion formed if the anion is I - . Accordingly students who got all 4 precipitation equations right for Task 7 got E.

We would not have had this problem if the flow chart referred to the precipitate colour with silver nitrate as the white of the choride is quite clearly distinguishable from the cream or pale yellow of the iodide. It would be even better if the flow chart used lead nitrate to distinguish between chloride and iodide. I shall take this up at the NCEA day.

ii) A number of students thought the X cation was NH4+. I suspect that their red litmus became contaminated with NaOH when they were testing for ammonia, which gave then a false positive result.

I would appreciate your comments on how we handled this.

A. Re practical difficulties with solution X.

We have previoulsy advised about the potential problems with unexpected contaminating effects from the other ion and the need to personally test your unknowns before using.

Certainly this sort of effect is quite common. In the future to allow greater choice of cations and anions we intend to restructure the task so that we use 4 unknown solutions (sodium or nitrat salts) and only ask for one ion from each.

Of course you can modify the flow diagram in any way you see fit to meet your purpose. Personally I have come across at least 6 different versions in the last year - all with some merits.

I have also previously commented on the common occurrence of students getting a false reading of ammonia because they actually touch the test tube with the litmus paper instead of holding it above the mouth. Obviously traces of the the previously added sodium hydroxide on the lip of the test tube will give the positive test. This is very common for many students and they need to be advised of this problem when being taught the technique.

Re your response to the problem:

I find it difficult to accept your decision to reward students who chose iodide if you are not also prepared to penalise the students who chose chloride under the same set of conditions. I could accept your decision if all students were treated equally but if indeed there is a precipitate formed (in this case as you say presumably from the magnesium hydroxide)then surely the other students who chose chloride should not be correct as this would not match the actual observation. i.e. you can't have your cake and eat it to.

Personally I would offer a reassesssment opportunity of just the 4 unknown ions (we did this to a group of students who were given a "dud" solution in our C2.1 assessment.

Secondly, as the students have not written any equations for complex ions they can't be given credit for this despite their inability to do so not being their fault. Again you would have to provide a further opportunity for this but this would occur anyway with the reassessment opportunity as referred to above. Back to top.

Q18. In 2.1, why is there no follow on error in the last part, if the ion is incorrectly identified and yet the correct precipitation equation is given for that (wrong) ion. Are we supposed to punish students twice for the same mistake? (Assuming a precipitate did form.)

A. These equations are supplementary evidence for the equation writing done in the theory -the reason follow on errors are not acceptable here is that a student could just learn 4 equations by rote and parrot them back irrespective of their relevance. Back to top.

Q19.  I am presently working on the 2.1 Qualitative Analysis assessments for this year and have a query which I am hoping you can clarify please.

In 2005 we carried out a 30 minute theory paper and 45 minute practical paper similar to the 2003 'Secret Reagent' reference assessment.  This was 2 credits for the theory section and 2 credits for the practical section.  However, the 2005 2.1 reference assessment off the TKI site has a 1 hour 3 credit paper.  With no material on solubility rules, ionic equations, etc.  Is this all we need to assess for our girls to achieve 2.1? (I noted on the 2006 matrix it is 3 credits)  I hope we are not a year late and assessed incorrectly last year.  Your thoughts would be greatly appreciated.  Thank you for your time.

A. All Level 2 Chem standards were reviewed and changed for use from 2005 onwards. A summary of the changes can be found at http://www.minedu.govt.nz/web/downloadable/dl7016_v1/summary-of-changes-for-web---chemistry.doc

Specifically for 2.1 the following comments apply.

The credit value has been increased from 2 to 3 credits to better reflect the content of the standard and the learning time required.

The two criteria have been reduced to one with the previous criteria integrated into the single criterion to facilitate the making of holistic judgements and provide clearer differentiation of the skills for each of the achievement grades. 

  • EN 2 has been added to include a statement about following safe procedures.
  • The solubility rules will not be provided as they are implicit in the given procedures (such as a flow chart).
  • EN 6, 7 and 8 clarify the requirements for each achievement grade and define the terms used in the criteria – determine and justify.
  • The list of complex ions in EN 9 for achievement with excellence has changed from will include to may include.   Both FeSCN2+ and Pb(OH)42- have been added to the list of complex ions.

Whenever standards are reviewed and changed to a newer version, the previous version also remains active and available for an interim period of time (usually a year) to avoid difficulties arising from situations such as where a standard is offered as part of a two year teaching programme.  So in your situation it was still valid to use the assessment task associated with the original version 1 of the standard provided you associate the assessment data (and any requests for  moderation samples etc) with version 1 of the standard rather than the later version.

Obviously, for this year (2006) you will need to use the newer version 2 and use a different integrated single task (as per the new sample on tki) for the assessment. You may also need to modify your teaching approach a little as a result of the modifications to the standard itself. Back to top.

Q20. You may not be the right person to help me here, but if you could I'd be really grateful.

 I have taught Y12 chem for the past 4 years but did not teach last year which was when the stds were updated and changes made. I'm now doing the AS2.1 and trying to figure out the marking schedule. The only one that TKI has, which is a generic, is a bit confusing.  I take from it that the students must do the following:

  • Ach - identify 5 ions, support this by observations etc, name or give formula for the ppts formed by those 5 ions
  • Merit = Ach + correctly balance ionic equations for formation of the 5 correct ions making ppts
  • Excellence = merit + identifies 6 ions and writes correctly balanced complex ion formation equations for each of those 6.

Problem:- this means that Task 6 which is a theoretical question has no purpose as it does not contribute towards getting Achieved/Merit or Excellence.  Or does it substitute for incorrect formulas etc in the previous tasks? It was a very simple task to mark before the changes were made - but I'm guessing the changes were made to make getting excellence harder?.

Could you please let me know if I'm correct.

For Chem 2.1:

Achieved and Merit = 5/6 ions identified

Excellence = 6/6 ions identified

Merit = most of ionic equations correct eg 4/6

Excellence = most of complex equations correct eg 2/3

A. For Ach the majority of the ions must be correctly identified with matching observations and evidence that they are aware of the identities of any precipitates formed. For Ach this identification could be either a correct formula or the correct name i.e. a correct balanced equation is not required.

If you were only looking at a total of 6 unknown ions then 4 correct would be sufficient - however in the generic example on tki, a total of 8 unknown ions are given (including the 2 from the "theoretical" task 6) so the minimum requirement for Ach would be 5 correct.

Note that it is not acceptable to count correct IDs separately from the correct observations etc i.e. each correct answer must contain all required facets of ID, observation and matching name/formula of precipitate. 

For Merit the same majority requirement applies but each correct unknown must include correctly balanced equations for each precipitate formed. If full equations are given states must be included for each species. Ionic equations do not require states to be included.

For Excellence at least one of the unknowns correctly identified must be a complex ion with matching observations and balanced equations given for both precipitate and complex formation. Depending on the selection of ions provided, this often means that the sufficiency requirement increases to 6 out of 8 unknowns correctly identified. As before, states are not required for ionic equations. Note that a correct equation for the dissolving of zinc hydroxide in excess hydroxide could be either Zn(OH)2+ 2OH- → [Zn(OH)4]2- or just the balanced equation starting with Zn2+. Note also that an equation such as Zn(OH)2 + 4NH3 → [Zn(NH3)4]2+ + 2OH- is not balanced if the 2OH- is left off the product side. Back to top.

Q21. Repeat Assessments. I have a girl in Year 12 who has attempted, and failed, the “Identifying Ions in Solutions” assessment for the Qualitative Chemistry Achievement Standard 90305. I want her to sit another assessment, but don't know if there are any others available.

A. That particular task is fairly generic but can be reused either for a repeat assessment (or for different classes) simply by changing the particular combination of unknowns. We actually have 3 or 4 different versions which we randomize between our 8 Yr 12 classes who all sit it 2 classes at a time over 4 periods on the same day without any problem.

Also, try and get a similar level of difficulty with the specific observations and remember to give enough opportunities for writing equations to show complex ion formation.Also, just a reminder that sometimes a particular anion can unintentionally interfere with testing for the cation and vice versa so pick the combinations carefully. It also pays to check out the unknowns yourself first as sometimes the concentration (particularly wrt redissolving or not) can be critical. Back to top.

Q22. On a similar vein ….

I've made a change to my AS 2.1 which I'm not so certain about. Question 2 identification involves formation of a complex ion. From the exampler, I can see that we normally don't give complex formations at least for questions 1-4. Will it be fair to put chloride ion as the anion to be identified. I have decided to use NaCl. I do realize the problems we have with chloride ions which was why I didn't use it in task 5 or 6 because task 5 requires identification of both the anion and cation.

A. The particular anions and cations used in AS2.1 can be juggled around (to meet security requirements etc) provided that over the entire task the overall level of difficulty remains similar to the generic example on the website and that sufficient opportunities are provided for a student to write equations describing complex ion formation. Also you need to be careful when selecting specific unknowns that the cation does not interfere with the anion identification and vice versa.

Therefore there is no problem with having examples of coloured ions (such as Cu2+) to identify as the students still have to carry out the procedure, note the actual observation and write the appropriate equations to justify their choice. However, somewhere in the task examples of identifications that are more demanding such as Zn2+ , Al3+ , Cl-, I - etc are expected. Where in the task this happens is largely irrelevant. This allows flexibility so that sufficient “unique” sets of unknowns can be prepared for different classes when necessary. Back to top.

Q23. Use of square brackets in AS 2.1 I have a student who has identified all her precipitates using square brackets. Obviously she does not understand the difference between a complex ion and a ppt. However, all her answers are correct except for the square bracket. She wrote things like [BaSO4](s) and the same thing for complex ions except that she wrote (aq) for the state. the complex ion equations were correct. I'm just not sure what to do about the precipitate ones - is it acceptable at the merit level if the equation is balanced.

A. I would tend to ignore the square bracket as after all we do accept the use of brackets when they are not required e.g.Na2(SO4). Also we use [ ] for complex ions as well as to indicate the concentrations of species in an equilibrium constant expression.

The more important issue for ppt reactions is that they recognize the physical state that the substance is in. Back to top.

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