Testing Mixtures of Two Unknown Solids

To prepare for this lesson, you will create three different mixtures, each containing two different chemicals. 

One-third of the class receives mixture A one-third, B and one-third, C. Working in pairs, students decide which tests they want to perform and in what order. 

Cornstarch Turns purple-black when iodine is added 1  

Management Tip During this lesson, students will be performing a number of different tests simultaneously. Because the heat test requires close supervision to ensure safety in the classroom, it is not formally included in this lesson. If, however, you have another adult in the classroom who can oversee a heat station, you may wish to add this test as an option. 

1 Record Sheet 14-A Unknown Mixtures Teat Results Table 

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The chemical and physical properties of a substance make up a sort of fingerprint that characterizes the substance. In this ChemLab, you will test four unknown solids using three different liquids. The unknowns are common materials that you d probably find in your kitchen. The results of your tests will give you the information you need to unravel the compositions of mixtures of two solids and three solids. 

Your students have carried out two physical tests (water drop and water mixtures) and four chemical tests (vinegar, iodine, red cabbage juice, and heat) on the unknown solids. In this lesson, they review and analyze all the data they have collected as a result of performing these tests to determine the distinctive properties of each unknown. This process provides students with the information and skills they need to solve the mystery in Lesson 12. 

SM-4. Two pure solid substances, A and B, each melt at 87.5 °C. An unknown pure solid substance X also melts at 87.5 C. Melting points are obtained for mixtures of X with A, X with B, and A with B. The melting point of each mixture tested is found to be below 87.5 °C. Which conclusion is justified by these data ... 

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