Identifying the Unknown Solids

Overview and Objectives By analyzing and discussing their test data in the last lesson, the students identified the unique physical and chemical properties of each unknown solid. In this lesson, students are challenged to identify each unknown by comparing their test data with verified test data from chemists. Students use the process of comparing their test data with a reliable source of information to identify the unknowns. Students read about how the five chemicals they have been testing are used in everyday life. 

Background How do scientists identify unknowns What about an unidentified rock sample from the moon, for example Scientists compare the moon rock s properties with those of known rocks on earth. By seeking correlations—and ideally, a match—scientists can determine the moon rock s composition. In this lesson, students follow a similar process to discover the Identities of their five chemical unknowns. They compare the data they have collected through observing and testing the unknowns (contained in their test summary tables from Lesson 11) with data from a reliable source of information (contained in Record Sheet 12-A Chemical Information Sheet). 

Materials For each student 1 science notebook 1 pencil 1 Record Sheet 12-A Chemical Information Sheet 1 Reading Selection MChemicals Are All Around Us For the class Class Properties Table (from Lesson 3) 

Copy Record Sheet 12-A Chemical Information Sheet for each student. 

Copy Chemicals Are All around Us (pgs. 125-26 of this guide) for each student. 

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Students review and discuss the processes they used to identify the unknown solids. 

Lesson 14 challenges students to go a step beyond identifying one unknown solid as they perform a chemical analysis on a mixture of two unknown solids. Next, students use the red cabbage... 

Finally, Lesson 16 is an embedded assessment of the skills students have been developing throughout the unit. Here, students face a challenging reversal instead of using known test liquids (water, vinegar, iodine, and red cabbage juice) to identify an unknown solid, they use the... 

The hydrazone derivatives, oximes, and semicarbazones discussed in this section are rather specialized compounds. Why mention them Organic chemists rely on spectroscopy (see Chapter 14) to identify the structure of organic compounds, but prior to the development of spectroscopy, chemical reactions were used as an identification tool. One such method converts unknown organic compounds to solid derivatives. The color and melting point of several derivatives are cross-referenced to a library of known derivatives to help identify the unknown. The conversion of aldehydes and ketones to phenylhydrazones, 2,4-dinitrophenylhydrazones, oximes, and semicarbazones is one method used for derivatization. 

Searching the spectrum of an unknown chemical against a spectral library is a routine method used to identify chemicals. Most of the commercial infrared instruments include library search software that has several search algorithms to choose from. The search algorithm can sometimes have a strong effect on the library search result. This is due to the different ways the actual comparison between the spectra is done. Especially when the library and the unknown spectra have been measured differently (e.g. using solid KBr disk and cryodeposition GC/FTIR), the... 

Libraries prepared by application of real combinatorial synthetic methods are usually submitted to screening experiments, either as soluble mixtures or as unknown discrete compounds cleaved from, or tethered to individual beads of the solid support. The task in deconvolution is to identify the substance that has a desired property. The deconvolution methods can be classified into two groups deconvolution of mixtures, cleaved from support and deconvolution of tethered libraries. 

A scientist is given the task of identifying an unknown compound on the basis of its physical properties. The substance is a white solid at room temperature. Attempts to determine its boiling point were unsuccessful. Using Table 3-1, name the unknown compound. 

The laboratory procedures associated with identification comprise a process known as qualitative analysis. In Part A of this experiment, you will examine 11 household products to determine some specific physical and chemical properties and to establish a qualitative analysis scheme for their identification. The physical properties include characteristics associated with their physical appearance, including color, particle size, and texture, as well as their solubility in water, rubbing alcohol, and hot water. The chemical properties include the manner in which the white solids react chemically with various other chemicals. In Part B, you will have three to five unknown household products (taken from those tested in Part A) and your work will involve using your qualitative analysis scheme to identify them. The flow chart that follows the procedure (Figure 3.6) should be filled out while performing Part A and should help when you perform Part B. 

In Lesson 11, students review, analyze, and discuss the information they have collected to help identify the five unknown solids. In the next lesson, students compare this information to a Chemical Information Sheet, a reliable source of outside data, to discover the unknowns identities at last. A reading selection illustrates the important role these and other chemicals play in students everyday lives. Then, in Lesson 13, students articulate the process they have used to solve their mystery and apply their testing, observing, recording, and analyzing skills to a new situation. 

There are two general approaches for computer-aided identification of infrared spectra of unknown compounds [173,196-199,248-250]. The most common approach uses software designed to identify an unknown spectrum by its similarity to a limited number of reference spectra selected from a general or customized library of reference spectra measured under similar conditions (e.g. vapor phase, solid phase, etc.) Commercial... 

To identify the anion present in an unknown sample, silver, barium or lead(II) solutions are generally added to determine whether a precipitate forms and whether the precipitate reacts with acids (carbonate or sulfate(IV)). The unknown sample can also be given in a solid form and students can be asked to heat the substance. When adding acid or when heating, students must be prepared to test for the appropriate gases as discussed in section 9.3. 

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