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Properties of Matter: Investigation 4 –

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

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

  •  Which property of compounds did you determine? The property of pH.
  • Which compounds were acidic? Vinegar, rubbing alcohol, and vitamin C were all acidic. Each has a pH below 7.
  • Which compounds were basic? Baking soda and vegetable oil were basic compounds. Each had a pH above 7.
  • Which compounds were neutral? Salt, sugar, and water were all neutral compounds. The pH of each was approximately 7.
  •  Were there differences in the degrees of acidity or basicity of the compounds? Yes. Vinegar, vitamin C, and rubbing alcohol all had an acidic pH. However, vinegar and vitamin C would be described as stronger acids than rubbing alcohol because their pH was closer to 0 than 7, whereas the pH of rubbing alcohol was closer to 7 than to 0. Baking soda would be described as a more basic compound or stronger base than vegetable oil because its pH was closer to 14 than the pH of vegetable oil.

 

Ask students to consider some of the methods they used in their investigation of pH.

  • ∆ Why did you test the pH of water? What was the pH of water? It was important to test the pH of water because water was the solvent used when creating the solution of the solid compounds. The pH of the water was neutral. Therefore, when the pH of the solution was tested, we could assume that the pH reflected the pH of the solid solute within the solution rather than the water itself.
  •  Calcium carbonate was a compound you investigated previously. Why do you think it was not included in this investigation of pH? In order to test the pH of a solid, the solid must be dissolved in water, creating a solution. We learned earlier that calcium carbonate is insoluble in water. Therefore, it would have been difficult to determine its pH using the method performed in the laboratory today.

 

Facilitate a discussion concerning the broader conclusions that students may draw from their investigation of pH as well as the other investigations of the Core Experience Learning Lab.

  • ∆ Why is pH a valuable property to test when working with compounds? Students should suggest that pH is a property that can be used to distinguish one compound from another when used in combination with other properties. For example, both rubbing alcohol and vinegar were liquid compounds that were miscible in water. However, the pH of vinegar was 2 whereas the pH of rubbing alcohol was 6.
  •  ∆ Do compounds composed of similar elements have a similar or dissimilar pH? Compounds composed of the same elements may have a pH that is either similar or dissimilar to each other. For example, vinegar and vitamin C are composed of the same elements and have a similar pH in that they are both acidic. However, when vinegar and vitamin C are compared to sugar or vegetable oil, which are also composed of the elements carbon, hydrogen, and oxygen, there is a difference in pH. The sugar solution was neutral and vegetable oil was slightly basic. Students may also suggest that this question could be more accurately answered given the opportunity to investigate a wider ranger of compounds.
  •  Do all liquid compounds have the same pH? Why or why not? The pH of each liquid compound in our investigation was dissimilar from that of the other liquid compounds. Although the number of samples we investigated were limited, we would hypothesize that the pH of liquid compounds is dependent more upon the chemical composition of the compound rather than its state of matter.
  •  Do all solid compounds have the same pH? Why or why not? Yes and no. Salt and sugar each had a pH of approximately 7. However, the pH of vitamin C was acidic and the pH of baking soda was basic as compared to the neutral pH of salt and sugar. Although the number of samples we investigated were limited, we would hypothesize that the pH of solid compounds is dependent more upon the chemical composition of the compound rather than its state of matter.
  •  Based on all of the investigations and experiments in this Core Experience Learning Lab, describe how you would determine the identity of an unknown substance. Student answers will vary. However, students should be able to formulate logical steps to conducting an investigation by drawing upon their work from the Core Experience Learning Lab. Student answers should reflect the need to compare the properties of unknown substances to known substances and to measure or test a variety of properties. Students may even suggest that an investigation is likely to include other properties not explored within the Core Experience Learning Lab.

Note: Students should use their data and the information from the Backgrounds to answer the questions in the Analysis section.

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.

  • Why is pH a valuable property to test when working with compounds? Students should suggest that pH is a property that can be used to distinguish one compound from another when used in combination with other properties. For example, both rubbing alcohol and vinegar were liquid compounds that were miscible in water. However, the pH of vinegar was 2 whereas the pH of rubbing alcohol was 6.
  • Do compounds composed of similar elements have a similar or dissimilar pH? Compounds composed of the same elements may have a pH that is either similar or dissimilar to each other. For example, vinegar and vitamin C are composed of the same elements and have a similar pH in that they are both acidic. However, when vinegar and vitamin C are compared to sugar or vegetable oil, which are also composed of the elements carbon, hydrogen, and oxygen, there is a difference in pH. The sugar solution was neutral and vegetable oil was slightly basic. Students may also suggest that this question could be more accurately answered given the opportunity to investigate a wider ranger of compounds.