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Hydrated Salt
Rubric

Grades 9-12 Performance Task
Contributed by: New York State Education Department (NYSED)
Developed through University of Buffalo and NORC (1991)

Task-Specific Scoring Criteria for Chemistry Task #6: Hydrated Salt

PART A: EXPERIMENT DESIGN

1. Statement of hypothesis Not required for this task
- Effect linked to variables  
- Directionality of effect  
- Expected effect/change  
- Independent variable  
- Dependent variable  
2. Procedures for investigation  
- Detailed procedure/experimentally feasible Procedure includes determination of moles of hydrate, moles of water, and suggests an endpoint for dehydration of the unknown sample.
- Sequence to plan Procedure suggests finding the mass of the crucible and hydrated sample before and after heating.
- General strategy Procedure suggests heating the hydrated sample to drive off the water.
- Safety procedures Use of safety goggles and crucible tongs is suggested
- Use of equipment/diagram Appropriate use of equipment and materials is suggested, especially use of crucible and Bunsen burner.
3. Plan for recording and organizing observations/data  
- Space for manipulation of data Space is allowed to record mass of sample, before and after heating, mass of water lost, moles of sample, moles of water lost. Qualitatively, the substance appears more opaque, dry and powdery after heating.
- Matched to plan Plan is comprehensive so that all observations and data generated by plan may be recorded, e.g., space for repeat trials is included.
- Organized sequentially Organized so that recording follows data as generated.
- Labelled fully (units included) All columns and rows are identified, using appropriate units of measure (g).
- Variables identified Mass of hydrated salt, before and after heating, listed here.

 

PART B: EXPERIMENT REPORT

4. Quality of observations/data  
- Consistent data Mass of water lost should be 40-60% of the mass of the hydrated salt. (Ideally, water is 51% of the hydrated sample.)
- Accurate measurements/observations Mass of salt after heating is less than before heating.
- Completed data table Data for all values determined experimentally are recorded in the first four cells of data table.
- Correct units All values for mass are in grams.
- Qualitative description *
5. Graph Not required for this task
- Curve is appropriate to data trend  
- Points plotted accurately  
- Appropriate scale  
- Axes labelled with variables  
- Variables placed on correct axes  
6. Calculations  
- Calculated accurately All calculations are determined accurately and consistent with significant figure rules.
- Substituted correctly into relationship Converted numerical value for ratio to a whole number ratio of moles X * moles H20.
- Relationship stated or implied Determined numerical value by division for mole ratio of anhydrous salt and water.
- Units used correctly Used gram molecular weight of anhydrous salt and GMW of water to correctly find moles of each.
- Use of all data available Mass of anhydrous salt and mass of water are calculated.
7. Conclusion  
- Consistent with scientific principles Empirical formula represented in conclusion is X * 7 H20.
- Sources of error *
- Consistent with data Moles of water in empirical formula were based on experimental results. (1)
- Relationship among variables stated Formula is represented as a whole number ratio of moles of substance X to moles of H20 (X * H20).
- Variables stated in conclusion Moles of X and moles of water are stated.

 

* These elements were not scored in this task; a "holistic" scoring approach was utilized so that the other four elements had a total value of 5 points.

(1) A number other than 7 is acceptable if it reflects student's data.

 


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