Identification of Cations in Solution

Purpose

To identify the ions in an unknown solution through the application of chemical tests.

Connections to What You Already Know About in Life

Detectives in mystery novels often rush evidence from the crime scene to the lab for analysis. Medical technicians perform many chemical tests on blood and urine samples to determine what is hidden in them. In this experiment, you will become a chemical detective. You will conduct laboratory analyses to determine the ionic composition of an unknown solution. As you complete this analysis, remember that careful observation and logical reasoning are the keys to being a good detective. Who knows what ions lurk in your unknown solutions?

Vocabulary

Precipitate, decant, centrifuge, soluble, insoluble

Background

The process of determining the composition of a sample of matter by conducting chemical test is called qualitative analysis. Solutions of unknown ions can be subjected to chemical tests and the results can be compared to the results given by known ions. By conducting the appropriate tests and applying logic, the identities of the ions present in an unknown solution can be determined.

In this video we'll do an overview of the Inorganic lab.

The analyses you perform are based on the idea that no two ions produce the same set of chemical reactions. Each ion reacts in its own characteristic way. In this experiment, you will observe several types of chemical reactions commonly used as tests in qualitative analysis. These reactions include the color of a flame as the chemical is placed in the flame and the formation of a precipitate (solid).

Procedure Section 1

To start this activity, click this link for Identification of Cations in Solution. The lab will load in a new tab. Click back to this tab to read further instructions and complete the questions below.
Double-click on a test tube from the box on the stockroom counter to place it on the metal tube stand. You can then click on a bottle of metal ion solution on the shelf to add it to the test tube. When you have added one metal ion, drag the test tube to the test tube rack on the bench. Repeat this process with a new metal ion. Continue doing this until you have sent one test tube for each of the following metal ions to the test tube rack: Na⁺, K⁺, and a Na⁺/K⁺ Fill one test tube with just water by clicking on the bottle of distilled water.
You can drag your cursor over each test tube to identify what cation is in each test tube and see a picture of what it looks like in the Live Data display.
You will use two of the lab procedures across the bottom, Flame and Flame w/ Cobalt (blue glass held in front of the flame). A test tube must be moved from the test tube rack to the test tube stand in order to perform the flame test. You can drag a test tube from the blue rack to the metal test tube stand to switch places with a test tube in the metal test tube stand.
Flame test sodium ion only. Flame w/ Cobalt test sodium ion only. Record your observations in the data table.
Flame test potassium ion only. Flame w/ Cobalt test potassium ion only. Record your observations.
Flame and Flame w/ Cobalt test a mixture of sodium and potassium. Record your observations.
Flame test a blank (distilled water) with and without cobalt glass to get a feel for what it looks like with no cations other than water. Record your observations.
Click on the red disposal bin to clear the bench. Click on the Unknowns menu on the right and expand the Practice Unknowns menu and click the blue Create Unknown button. Click on the Na⁺ and K⁺ boxes and set the minimum = 0 and maximum = 2. This means that you could have only water or any combination of the ions up to both. Click on the Create button. An unknown test tube labeled Practice Unknown will be placed in the blue unknown rack on the stockroom counter. Drag the practice unknown from the blue rack to the blue test tube rack on the lab bench.
Flame test the Practice Unknown and determine if it contains sodium or potassium or both or neither. To check your results, in the Unknowns menu click Reveal and it will show what ions were actually in the test tube. To test another Practice Unknown, drag that practice test tube to the red disposal bin or click Clear Lab. Generate another Practice Unknown and do the flame tests to predict what it is. Reveal the answers when you have made your prediction. Continue until you are able to reliably predict what is in every Practice Unknown you create.

Flame Test Flame w/ Cobalt Test

Na⁺

K⁺

Na⁺& K⁺ mixture

Distilled Water

Practice: nothing, Na⁺, K⁺ or both

Procedure Section 2

Clear the lab by clicking on the red disposal bucket. Select a new test tube and place in it the three ions: Ag⁺, Hg₂²⁺, and Pb²⁺. (There is Hg²⁺ and Hg₂²⁺ on the shelf. Make sure you obtain Hg₂²⁺.) As you proceed with the chemical analysis watch the Live Data display to see the chemistry involved in the chemical reactions.
Click the reagent bottle NaCl to add chloride to the test tube. What do you observe? Click the Centrifuge What happened?

Each of the three ions form insoluble precipitates (solids) with chloride. If the solution turns cloudy white it indicates that at least one of the three ions is present. Now, you can learn how to determine which ion is present.

Turn the heat on with the Heater icon (the hot plate image). Observe the results in the Live Data area. Observe what happened and record your observations. If you cannot tell, turn the heat on and off while observing the viewer. With the heat turned on, click Decant. Drag your cursor over the new test tube in the rack. Record your observations from the viewer and the picture window.

This is the test for Pb²⁺. If heated, it is soluble. When cooled it becomes insoluble.

Turn off the Heater. With the test tube containing the two remaining ions in the test tube stand, click the NH₃ bottle in the reagents area on the bench. Record your observations.

Addition of ammonia produces a diammine silver complex ion which is soluble. The mercury produces a black solid. This is the test for mercury.

Centrifuge and then Decant to pour the silver ion into another test tube. Move the tube with the black mercury solid to the red disposal bucket. Move the tube containing silver back to the test tube stand. Click the pH 4 reagent bottle. Record your observations.

The silver ion is soluble as the diammine silver complex ion in pH 10 and is insoluble as AgCl in pH 4. You can click alternately on each of the pH bottles to confirm this test for silver ion.

Now create an unknown containing Ag⁺, Hg₂²⁺, and Pb²⁺. Use minimum = 0 and maximum = 3. Move the Practice Unknown to the test tube stand. Run the tests explained above to determine if you can correctly identify which of these three ions are Present. Click Reveal in the Unknowns menu when you have come to your conclusion of what is in the test tube. Make more practice unknowns and practice again if you didn't get the first one right. When you are confident that you can correctly identify Ag⁺, Hg₂²⁺, and Pb²⁺ proceed to the next section.

Tests to Confirm Presence Observations

Ag⁺

Hg₂²⁺

Pb²⁺

Practice: nothing, Ag⁺, Hg₂²⁺, Pb²⁺ or all three

	Flame Test	Flame w/ Cobalt Test
Na⁺
K⁺
Na⁺& K⁺ mixture
Distilled Water
Practice: nothing, Na⁺, K⁺ or both

	Tests to Confirm Presence	Observations
Ag⁺
Hg₂²⁺
Pb²⁺
Practice: nothing, Ag⁺, Hg₂²⁺, Pb²⁺ or all three