Exemplar for internal assessment resource Chemistry for Achievement Standard 91162
Exemplar for Internal Achievement Standard
Chemistry Level 2
This exemplar supports assessment against:
Achievement Standard 91162
Carry out procedures to identify ions present in solution
An annotated exemplar is an extract of student evidence, with a commentary, to explain
key aspects of the standard. It assists teachers to make assessment judgements at the
grade boundaries.
New Zealand Qualifications Authority
To support internal assessment
© NZQA 2014
Exemplar for internal assessment resource Chemistry for Achievement Standard 91162
Grade Boundary: Low Excellence
1.
For Excellence, the student needs to carry out procedures to comprehensively
justify the identification of ions present in solution.
This involves:
•
•
interpreting observations by recognising the formation of complex ions
writing balanced equations for these reactions
This student has made links between the observations and the reactions for the
ion identified (1), has justified the identification of the ion by showing how the
identification process eliminates or include ions for most of the steps (2), and has
correctly balanced precipitation and complex ion equations (3).
For a more secure Excellence, the student could show their justification for each
step in the identification of the ion by showing which ions are eliminated or
included for each step of the identification process.
© NZQA 2014
Description of test(s) carried out
Observations
Test for anion:
Test with red litmus
No colour change
Add Ba(NO 3 ) 2
Test for cation:
Add 2 drops NaOH
Add excess NaOH
NEW SAMPLE
Add 2 drops NH 3
Add excess NH 3
NEW SAMPLE
Add dilute H 2 SO 4
Name of
precipitate
Equations
White precipitate formed
Barium
sulfate
Ba2+ (aq) + SO 4 2- (aq) → BaSO 4(s)
White precipitate formed
Precipitate disappears
Aluminium
hydroxide
White precipitate forms
Precipitate remains
Aluminium
hydroxide
Al3+ (aq) + 3OH- (aq) → Al(OH) 3(s)
Al3+ (aq) +4OH- (aq) → [Al(OH) 4 ]- (aq)
(3)
Colourless solution
Al3+ (aq) + 3OH- (aq) → Al(OH) 3(s)
Justification:
The anion was SO 4 2- and the cation was Al3+.
I knew that the anion was SO 4 2- because when I tested it with the red litmus there was no colour change. When I added the Ba(NO 3 ) 2 a white
precipitate (BaSO 4(s) ) formed so therefore it had to be SO 4 2-(1).
I knew that the cation was Al3+ because when I add the NaOH a white precipitate formed which then disappeared ([Al(OH) 4 ]- (aq) ) when I added excess
NaOH. This told me that it was either Al3+, Zn2+ or Pb2+ (2). I then got a new sample and when I added 2 drops of NH 3 a white precipitate (Al(OH) 3(s) )
formed (1) which remained when I added excess NH 3 . This told me that I had either Al3+ or Pb2+. I got another new sample and added dilute H 2 SO 4
and the solution remained colourless which told me I had Al3+ (2).
Exemplar for internal assessment resource Chemistry for Achievement Standard 91162
Grade Boundary: High Merit
2.
For Merit, the student needs to carry out procedures to justify the identification of
ions present in solution.
This involves writing balanced equations for all the reactions where precipitates
are formed.
This student has identified all of the precipitates for the observations (1), correctly
balanced all of the precipitation equations (2), has made links between the
observations to the reactions (3), and has shown how the identification process
used eliminates or includes ions for some of the key steps (4).
To reach Excellence, the student could complete the complex ion equation
correctly, and link each step in the identification process to the ions eliminated or
included.
© NZQA 2014
Description of test(s) carried out
Observations
Test for anion:
Test with red litmus
No colour change
Add Ba(NO 3 ) 2
White precipitate formed
Test for cation:
Add 2 drops NaOH
Add excess NaOH
NEW SAMPLE
Add 2 drops NH 3
Add excess NH 3
NEW SAMPLE
Add dilute H 2 SO 4
Name of
precipitate
Equations
Barium
sulfate
Ba2+ (aq) + SO 4 2- (aq) → BaSO 4(s)
(1)
White precipitate formed
Precipitate disappears
Aluminium
hydroxide
White precipitate forms
Precipitate remains
Aluminium
hydroxide
(2)
Al3+ (aq) + 3OH- (aq) → Al(OH) 3(s)
Al3+ (aq) +2OH- (aq) → [Al(OH) 2 ]- (aq)
Al3+ (aq) + 3OH- (aq) → Al(OH) 3(s)
Colourless solution
Justification:
The anion was SO 4 2- and the cation was Al3+.
I knew that the anion was SO 4 2- because when I tested it with the red litmus there was no colour change. When I added the Ba(NO 3 ) 2 a white
precipitate (BaSO 4(s) ) formed (3) so therefore it had to be SO 4 2-.
I knew that the cation was Al3+ because when I add the NaOH a white precipitate formed which then disappeared when I added excess NaOH. I then
got a new sample and when I added 2 drops of NH 3 a white precipitate (Al(OH) 3(s) ) formed which remained when I added excess NH 3 . This told me
that I had either Al3+ or Pb2+ (4). I got another new sample and added dilute H 2 SO 4 and the solution remained colourless which told me I had Al3+.
Exemplar for internal assessment resource Chemistry for Achievement Standard 91162
Grade Boundary: Low Merit
3.
For Merit, the student needs to carry out procedures to justify the identification of
ions present in solution.
This involves writing balanced equations for all the reactions where precipitates
are formed.
This student has identified all of the precipitates for the observations (1), correctly
balanced most of the precipitation equations (2), and made some links between
the observations and the reactions (3).
For a more secure Merit, the student could complete all of the precipitation
reactions correctly, and fully link the observations to the precipitation reaction
involved.
© NZQA 2014
Description of test(s) carried out
Observations
Test for anion:
Test with red litmus
No colour change
Add Ba(NO 3 ) 2
White precipitate formed
Test for cation:
Add 2 drops NaOH
Add excess NaOH
NEW SAMPLE
Add 2 drops NH 3
Add excess NH 3
NEW SAMPLE
Add dilute H 2 SO 4
Name of
precipitate
Equations
Barium
sulfate
Ba2+ (aq) + SO 4 2- (aq) → BaSO 4(s)
(1)
(2)
White precipitate formed
Precipitate disappears
Aluminium
hydroxide
Al3+ (aq) + 3OH- (aq) → Al(OH) 3(s)
White precipitate forms
Precipitate remains
Aluminium
hydroxide
Al3+ (aq) + OH- (aq) → Al(OH) 3(s)
Colourless solution
Justification:
The anion was SO 4 2- and the cation was Al3+.
I knew that the anion was SO 4 2- because when I tested it with the red litmus there was no colour change and when I added the Ba(NO 3 ) 2 a white
precipitate formed (BaSO 4(s) ) (3) so it had to be SO 4 2-.
I knew that the cation was Al3+ because when I add the NaOH a white precipitate formed which then disappeared when I added excess NaOH. I then
got a new sample and when I added 2 drops of NH 3 a white precipitate formed which remained when I added excess NH 3 . I got another new sample
and added dilute H 2 SO 4 and the solution remained colourless which told me I had Al3+.
Exemplar for internal assessment resource Chemistry for Achievement Standard 91162
Grade Boundary: High Achieved
4.
For Achieved, the student needs to carry out procedures to identify ions present in
solution.
This involves:
•
•
collecting primary data
using these observations to identify ions in solution using a procedure
provided.
This student has stated the observations for each step of the identification process
(1) and correctly identified all of the ions (2) and the precipitates formed (3) (with
either words or formulae).
To reach Merit, the student could make links between the observations and the
reactions for the steps in the identification process, and correctly complete
balanced equations for the precipitation reactions.
© NZQA 2014
Description of test(s) carried out
Observations
Test for anion:
Test with red litmus
No colour change
Add Ba(NO 3 ) 2
Test for cation:
Add 2 drops NaOH
Add excess NaOH
NEW SAMPLE
Add 2 drops NH 3
Add excess NH 3
NEW SAMPLE
Add dilute H 2 SO 4
Name of
precipitate
Equations
White precipitate formed
Barium
sulfate
Ba2+ + SO 4 2- → BaSO 4
White precipitate formed
Precipitate disappears
Aluminium
hydroxide
Al3+ + OH- → AlOH 3
White precipitate forms
Precipitate remains
Aluminium
hydroxide
Al3+ + OH- → AlOH 3
Colourless solution
(1)
(3)
Justification:
Anion is SO 4 2- (2)
When I tested it with red litmus nothing happened but when I added barium nitrate it made a white precipitate.
Cation is Al3+ (2)
2 drops of sodium hydroxide made a white precipitate and when I added more sodium hydroxide it went away. When I used the ammonia it made a
white precipitate that didn’t go away when I added excess. It also did nothing when I added the sulphuric acid
Exemplar for internal assessment resource Chemistry for Achievement Standard 91162
Grade Boundary: Low Achieved
5.
For Achieved, the student needs to carry out procedures to identify ions present in
solution.
This involves:
•
•
collecting primary data
using these observations to identify ions in solution using a procedure
provided.
This student has correctly identified the ions (1), recorded the observations for
each step of the process (2), and identified most of the precipitates formed (3)
(with either words or formulae).
For a more secure Achieved, the student could identify all of the precipitates
formed for each step in the process, for example by including aluminium hydroxide
for the addition of NH 3 .
© NZQA 2014
Description of test(s) carried out
Observations
Test for anion:
Test with red litmus
No colour change
Add Ba(NO 3 ) 2
Test for cation:
Add 2 drops NaOH
Add excess NaOH
NEW SAMPLE
Add 2 drops NH 3
Add excess NH 3
NEW SAMPLE
Add dilute H 2 SO 4
Justification:
Anion was SO 4 2- (1)
The reactions matched sulphate
Cation was Al3+ (1)
The reactions matched aluminium
Name of
precipitate
Equations
White precipitate formed
Barium
sulfate
Barium + sulphate → barium sulphate
White precipitate formed
Precipitate disappears
Aluminium
hydroxide
White precipitate forms
Precipitate remains
Colourless solution
(3)
Aluminium
(2) sulfate
Exemplar for internal assessment resource Chemistry for Achievement Standard 91162
Grade Boundary: High Not Achieved
6.
For Achieved, the student needs to carry out procedures to identify ions present in
solution.
This involves:
•
•
collecting primary data
using these observations to identify ions in solution using a procedure
provided.
This student has identified all of the ions (1) and provided the correct observations
for each step of the process (2).
To reach Achieved, the student could identify the precipitates formed, for example
aluminium hydroxide and barium sulfate.
© NZQA 2014
Description of test(s) carried out
Observations
Test for anion:
Test with red litmus
No colour change
Add Ba(NO 3 ) 2
White precipitate formed
sulfate
White precipitate formed
Precipitate disappears
Aluminium
Test for cation:
Add 2 drops NaOH
Add excess NaOH
NEW SAMPLE
Add 2 drops NH 3
Add excess NH 3
NEW SAMPLE
Add dilute H 2 SO 4
Justification:
Anion is SO 4 2- (1)
Cation is Al3+ (1)
Name of
precipitate
White precipitate forms
Precipitate remains
Colourless solution
(2)
Equations
Al + OH → AlOH