Study Guide

Field 004: Chemistry 
Sample Constructed-Response Assignment

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Test Directions for the Constructed-Response Assignment

This section of the test consists of one constructed-response assignment. You will be asked to prepare a written response of approximately 300–600 words on the assigned topic.

Read the assignment carefully before you begin your response. Think about how you will organize your response. You may use the erasable sheet(s) to make notes, write an outline, or otherwise prepare your response.  start bold and italics However, your final response to the assignment must be either:  end bold and italics 

  1. typed into the on-screen response box,
  2. written on a response sheet and scanned using the scanner provided at your workstation, or
  3. provided using both the on-screen response box (for typed text) and a response sheet (for calculations or drawings) that you will scan using the scanner provided at your workstation.

 start bold Instructions for scanning your response sheet(s) are available by clicking the "Scanning Help" button at the top of the screen. end bold 

As a whole, your response must demonstrate an understanding of the knowledge and skills of the field. In your response to the assignment, you are expected to demonstrate the depth of your understanding of the content area through your ability to apply your knowledge and skills rather than by merely reciting factual information.

Your responses to the assignments will be evaluated based on the following criteria.

 start bold PURPOSE: end bold  the extent to which the response achieves the purpose of the assignment
 start bold SUBJECT KNOWLEDGE: end bold  appropriateness and accuracy in the application of subject knowledge
 start bold SUPPORT: end bold  quality and relevance of supporting evidence
 start bold RATIONALE: end bold  soundness of argument and degree of understanding of the subject area

The constructed-response assignment is intended to assess subject matter knowledge and skills, not writing ability. However, your response must be communicated clearly enough to permit valid judgment of the scoring criteria. Your response should be written for an audience of educators in this field. The final version of your response should conform to the conventions of edited American English. Your response must be your original work, written in your own words, and not copied or paraphrased from some other work.

Be sure to write about the assigned topic. Remember to review your work and make any changes you think will improve your response.

Any time spent responding to the assignment, including scanning the response sheet(s), is part of your testing time. Monitor your time carefully. When your testing time expires, a pop-up message will appear on-screen indicating the conclusion of your test session. Only response sheets that are scanned before you end your test or before time has expired will be scored. Any response sheet that is not scanned before testing ends will  start uppercase NOT end uppercase  be scored.

Sample Constructed-Response Assignment

subarea roman numeral 1 
Nature of Science

 start bold Read the information below; then complete the exercise that follows. end bold 

A student is performing a chemistry laboratory investigation to determine the molar mass of an unknown solute from freezing point depression.

The student uses the procedure below to carry out this investigation.

  1. Prepare an ice bath by filling a 500 mL beaker about two-thirds full of ice.
  2. Pipette 25 mL of cyclohexane into a test tube and lower the test tube into the ice bath.
  3. Place a thermometer in the test tube to record the temperature of the cyclohexane.
  4. Observe the test tube, noting the temperature at which solid crystals first begin to form.
  5. Remove the test tube from the ice bath, and reheat the test tube until all of the cyclohexane has melted.
  6. Replenish the ice in the ice bath if necessary.
  7. Measure 2.0 g of the unknown solute and transfer the solute to the test tube containing the cyclohexane.
  8. Lower the test tube into the ice bath and reinsert the thermometer in the test tube.
  9. Observe the test tube, noting the temperature at which solid crystals first begin to form.
  10. Compare the freezing points determined in steps 4 and 9.
  11. Determine the molar mass of the solute from the freezing point depression.

Using your knowledge of laboratory techniques, prepare a response in which you:

Sample Strong Response to the Constructed-Response Assignment

 start bold Please note: The sample response provided below is for review purposes only and should not be used in a response on an operational exam. Use of the exact words and phrases presented in this sample response will result in a score of "U" (Unscorable) due to lack of original work. end bold 

There are several weaknesses in the described procedure. There are a couple of related factors that might call into question the accuracy of the results. First, neither the cyclohexane nor the solute is measured with a high enough degree of accuracy for this experiment. In terms of significant figures,  25 milliliters  of cyclohexane indicates that the measurement could vary by as much as  1 milliliter  above or below  25 milliliters.  The same reasoning applies to the solute—measuring to a tenth of a gram may not be precise enough to allow for accurate calculations of its molar mass. The second related factor that may affect the accuracy of the results is the volatility of cyclohexane. Cyclohexane tends to evaporate, so by not measuring out a fresh  25.00 milliliter  sample of cyclohexane in step 7, there may actually be less than  25.00 milliliters  left in the test tube after completing steps 2–6. So again, there is some uncertainty in the amount of cyclohexane in the test tube when the unknown is added, and this would undermine the validity of the results.

To address these problems associated with accuracy, a couple of modifications should be made. First, the solvent and solute should be measured out with a higher degree of precision. This may mean using different measuring instruments that allow greater precision. Second, a fresh sample of the solvent should be measured out before adding the unknown in step 7. Both of these modifications will produce more accurate and reliable data, enhancing the validity of the investigation's results.

Another weakness of the described procedure is the lack of information on whether the unknown is an electrolyte or nonelectrolyte, and if it is an electrolyte, how many ions are formed when it ionizes. This information is needed in order to calculate the molality of the solution, which is then used along with the amount of solvent to calculate the molar mass of the unknown. The more particles a solute forms in solution, the greater the freezing point depression. So if the student does not have this information about the unknown, then calculations of molar mass based on how much the freezing point is lowered could be wrong depending on what assumptions the student makes.

For the student to be able to make the appropriate calculations that will lead to an accurate determination of the unknown's molar mass, the number of particles the solute forms in solution must be provided. Providing this information will not give away the identity of the unknown, but it will allow a more certain determination of the unknown's molar mass.

There are several safety precautions that should be considered and taken in performing this investigation. When working with any chemical in the laboratory, safety goggles should be worn to protect against splatters and splashes. Also, more specific to cyclohexane, because it is a volatile chemical, it can cause irritation to the respiratory passages and eyes. It is also highly flammable. For both of these reasons, if cyclohexane is to be used as the solvent, then the investigation should be carried out under a fume hood. Given the purpose of the investigation, determining the molar mass of an unknown solute, the best way to minimize some of these safety risks would be to select a less dangerous solvent in place of cyclohexane.

Performance Characteristics

The following characteristics guide the scoring of responses to the constructed-response assignment.

Characteristics that guide the scoring of responses
Purpose The extent to which the response achieves the purpose of the assignment
Subject Matter Knowledge Accuracy and appropriateness in the application of subject matter knowledge
Support Quality and relevance of supporting details
Rationale Soundness of argument and degree of understanding of the subject matter

Scoring Scale

Scores will be assigned to each response to the constructed-response assignment according to the following scoring scale.

Score Scale with description for each score point.
Score Point Score Point Description
4  start bold The "4" response reflects a thorough knowledge and understanding of the subject matter. end bold 
  • The purpose of the assignment is fully achieved.
  • There is a substantial, accurate, and appropriate application of subject matter knowledge.
  • The supporting evidence is sound; there are high-quality, relevant examples.
  • The response reflects an ably reasoned, comprehensive understanding of the topic.
3  start bold The "3" response reflects a general knowledge and understanding of the subject matter. end bold 
  • The purpose of the assignment is largely achieved.
  • There is a generally accurate and appropriate application of subject matter knowledge.
  • The supporting evidence generally supports the discussion; there are some relevant examples.
  • The response reflects a general understanding of the topic.
2  start bold The "2" response reflects a partial knowledge and understanding of the subject matter. end bold 
  • The purpose of the assignment is partially achieved.
  • There is a limited, possibly inaccurate or inappropriate application of subject matter knowledge.
  • The supporting evidence is limited; there are few relevant examples.
  • The response reflects a limited, poorly reasoned understanding of the topic.
1  start bold The "1" response reflects little or no knowledge and understanding of the subject matter. end bold 
  • The purpose of the assignment is not achieved.
  • There is little or no appropriate or accurate application of subject matter knowledge.
  • The supporting evidence, if present, is weak; there are few or no relevant examples.
  • The response reflects little or no reasoning about or understanding of the topic.
U The response is unscorable because it is illegible, not written to the assigned topic, written in a language other than English, or lacking a sufficient amount of original work to score.
B There is no response to the assignment.