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Teacher Portal

Properties of Matter: Investigation 2 –

PostLab

 

 

 

 

 

 

 

ZERO-IN

Italicized font represents information to be shared orally or physically completed with the students at this time.

The non-italicized font represents additional information included to support the teacher’s understanding of the content being introduced within the CELL.

ANALYZE IT

  • Review the latest experiments as a class. 
  • Discuss how students may have combined information from each observation to draw conclusions about the substances, as well as how students’ investigations are similar or may have differed from investigations that practicing scientists would lead if faced with similar questions about unknown substances.
  • Discuss the following questions as a class. Use the suggested responses to guide students’ answers. 

Note: Questions marked with a triangle (∆) are included to enrich students’ understanding. These questions do not appear in students’ SDRs but should be used as additional discussion points during the PostLab.

  • ∆ Based on the physical and chemical properties you observed throughout your investigation, were you able to determine which substances were elements and which were compounds? Why or why not? It is likely that students will not have been able to discern the difference between substances that were elements and those that were compounds during the course of their investigation. Based on previous knowledge, students may have looked for properties that would differentiate elements from compounds. Often the difference between elements and compounds is described as elements as substances that cannot be broken down by ordinary means. However, this was not a property that students explored in their investigation.
  • ∆ How do the physical and chemical properties of the compounds compare to the physical and chemical properties of elements? Student discussion should focus on the concept that elements and compounds share many similar properties such as similar colors, textures, and densities as well as reactivity with other compounds.
  • ∆ Were you able to determine the identities of the substances? If so, describe how. Did you think any of the substances were the same? Why? Students may or may not have suggested possible identities of the substances based on similarities of color and texture to substances that they encounter in everyday life. For example, students may have recognized the solid, cylindrical shape of chalk, the bronze color and luster of copper, or the metallic silver color of aluminum foil. However, without a direct comparison to known substances, students may not have been confident in their predictions. In addition, students should suggest that properties such as density may be helpful in the determination of an unknown substance’s identity if compared with substances with known densities. Students may have used similar logic to suggest that some of the substances were the same. For example, students may have predicted that the copper shot sample and the copper foil were similar based on similarities in color, that two substances were the same because their densities were similar, or because each reacted in a similar manner with a silver nitrate solution. Discussion should focus on students’ abilities to recognize that when taken in combination, the physical and chemical properties of an element or compound are distinctive. The greater the number of properties, both physical and chemical, that can be tested or measured, the more likely a scientist is to determine the identity of the substance, especially when those properties can be compared to a list of properties for known substances.
  • ∆ Did the investigation of chemical properties, in addition to physical properties, influence your investigation? Students should suggest that investigation of the chemical properties of the substances was helpful in the investigation. While two substances may have had different physical forms, such as the copper foil and copper shot, their chemical reactivities with certain compounds were the same. The use of chemical tests is helpful because compounds or elements may take different forms, but their chemical properties are generally independent of their physical forms.
  • Discuss the concept of density with students. Explain that the density of an element or compound is independent of its forms. 
  • ∆ During your investigation, did you observe that the density of an element or compound was independent of its forms? If not, what may have accounted for any differences? Students likely saw that for many of the samples in which there were two physical forms, the densities were similar. However, students may have also observed cases in which the densities were seemingly dissimilar. One reason for the discrepancies is the method used to determine a substance’s volume. For many of the substances that were powders, volume was measured simply by adding the powders to a graduated cylinder. In such a case, the space between each individual powder granule was not taken into account and the volume may have been lower than in reality. In addition, when using the volume displacement method, students may not have used a graduated cylinder that effectively measured small differences in volume. The use of a graduated cylinder with smaller graduations would have made the volume determination more accurate.

GET FOCUSED

Instruct students to complete the Focus Questions in their SDRs then discuss them as a class. Use the suggested responses below to guide students’ answers.

  • Can you tell the difference between an element and a compound by calculating density and observing chemical reactions? It is likely that students will not have been able to discern the difference between substances that were elements and those that were compounds during the course of their investigation. Based on previous knowledge, students may have looked for properties that would differentiate elements from compounds. Often the difference between elements and compounds is described as elements as substances that cannot be broken down by ordinary means. However, this was not a property that students explored in their investigation. Student discussion should focus on the concept that elements and compounds share many similar properties such as similar colors, textures, and densities as well as reactivity with other compounds.
  • Can you determine the identity of an unknown substance by calculating its density and observing chemical reactions? Students may or may not have suggested possible identities of the substances based on similarities of color and texture to substances that they encounter in everyday life. For example, students may have recognized the solid, cylindrical shape of chalk, the bronze color and luster of copper, or the metallic silver color of aluminum foil. However, without a direct comparison to known substances, students may not have been confident in their predictions. In addition, students may suggest that properties such as density and chemical reactivity would be helpful in the determination of an unknown substance’s identity if compared with substances with known densities and chemical relativities. Discussion should focus on students’ abilities to recognize that when taken in combination, the physical and chemical properties of an element or compound are distinctive. The greater the number of properties, both physical and chemical, that can be tested or measured, the more likely a scientist is to determine the identity of the substance, especially when those properties can be compared to a list of properties for known substances.