Identifying Ions In Solution With Precipitation

A molecule is the smallest part of a chemical element or compound that holds the same chemical properties as that chemical or compound. These molecules are made up entirely of atoms that are held together through chemical bonds and when these bonds form, electrons or either shared or exchanged between the atoms. In this lab, there was an unknown solution that had either silver, mercury (I), or lead ions inside of it and it was the students’ job to use the precipitate reactions to find out which one was present.

The materials used in this assignment were a mixture of one or more of the elements Ag+, Hg22+, Pb2+, Bi3+, Cu2+, Sn4+, Fe3+, Mn2+, Ni2+, Al3+, Ba2+, Ca2+, Na+, and NH4+, boiling water, a glass and plastic test tube, a test tube clamp, a centrifuge, and distilled water. At the start of the experiment the students were told to start a boiling water bath in a 150-mL beaker. Next, they were told to prepare a solution that had 10 drops of each cation: Ag+, Hg22+, Pb2+, that were presented as nitrates. They were then told to mix these in a plastic test tube and then add 12 drops of 6 M HCL before placing it in the centrifuge to be mixed. Once it was ready to be taken out of the centrifuge, they had to decant the supernatant into a designated waste beaker to discard at the end of the experiment. They then had to add 15 drops of distilled water into the white precipitate that was left in the plastic beaker that just came out of the centrifuge before putting a test tube clamp around it and holding it into the boiling hot water that was heated at the start of the experiment. Once the test tube stayed in the water for about a minute, the students were told to decant the supernatant into a glass test tube and save the precipitate. The next step was to add 3 drops of K2CrO4 to the supernatant while I t was still hot. The purpose of this was to see if lead was present. If lead was present, a yellowish color precipitate would form and the students had to repeat the procedure from step 6 and on several times to get all the lead out of it. If no lead was present then only a yellow color would be visible meaning that there would be no precipitate in the test tube. If this was the case, students had to add 10 drops of 6 M NH3 to the plastic test tube that was heated in step 6. The purpose of doing this was to see if any Mercury is present. If some was present, the precipitate will automatically turn a dark gray color. Students then had to decant the supernatant from the test tube after centrifuging it for a minute or so. The next step was to add 20 drops of 6 M HNO3 to see if a white cloudiness would form inside of the test tube. If it did show, that meant that there was silver present in the test tube.

When my group had did the experiment, we did it with the group 1 #4 unknown. Throughout the process of the experiment, we found that lead was present, mercury was present, and that there was no silver present in the tube because no cloudiness appeared. We also had to go back several times to step 6 because we could not get all of the lead out of the test tube. That was the hardest part about doing the experiment.