Experiment 5 – Periodic Nature
Name __________________
Lab Section __________________
Experiment 5 – Periodic Nature
Introduction
The periodic table, in its most basic form, was developed only recently in history. It was in 1869
that Dmitri Mendeleev, a Russian chemist, devised the ordering of elements by increasing mass
(subatomic particles had yet to be discovered). In doing so, he noticed that a repeating pattern, a
periodic nature, was observed. This table was improved upon in 1913 by Henry Moseley, an
English physicist. Owing to the discoveries of the electron, proton, and neutron, the periodic
table was ordered by increasing number of protons.
In addition to this ordering, certain elements were put into groups, based on their chemical
(reactivity) and physical properties. These groupings resulted in the seven rows on the periodic
table and the eighteen columns. The rows are called periods, and the columns are called
groups. There are four groups in particular that are given specific names, they are: alkali
metals (group 1), alkali earth metals (group 2), halogens (group 17), and noble gases (group
18). Each of these groups, referred to some as “families”, are grouped based on their reactivity.
For example, each element in the alkali metal family will react with oxygen in a 2:1 ratio (e.g.
Na2O or K2O).
In this experiment, we will use each element’s unique color, as well as its reactivity to determine
the composition of an unknown compound. When a salt, a metal combined with a nonmetal, is
placed in water, it can dissociate into the ions that comprise it. The positively charged metal ion
portion is called the cation. This experiment will look at the cations Li+, Na+, K+, Ca2+, Sr2+, and
Ba2+. The negatively charged nonmetal portion is called the anion. The anions Cl-, Br-, and Iwill be investigated.
Each cation will have its own distinct color when excited (remember experiment 4). In this case,
a flame test will be used to excite the electrons. Here, a sample of the dissolved salt will be
burned in order to cause the electrons to become excited, and the resulting color will be seen. In
addition, the similar chemical reactivity of each of the groups will be studied. Each anion will
also be determined by its color, though in a different fashion to the cations.
In this experiment keen observations and careful recording of data will allow us to determine the
composition of an unknown compound by comparing it to the behavior of known compounds.
Experiment 5 – Periodic Nature
Name __________________
Lab Section __________________
Procedure
Identifying Cations
Flame Test Colors
Place six test tubes into a test tube rack and label them for each of the six solutions being put
inside of them. Refer to Figure 5.1.
Put 20 drops of each known solution into its test tube.
Clean a flame-test wire by dipping it in hydrochloric acid (HCl) and burning off the solution
in the flame. Do this three times, or until no color is seen in the flame.
Test each of the solutions by lightly dipping the flame-test wire into the solution and then
placing the wire into the hottest portion of the flame. Note the color that it produces. It may
take a few times to see the color.
Be sure to thoroughly clean the flame-test wire between solutions.
Figure 5.1: The six cations
Experiment 5 – Periodic Nature
Name __________________
Lab Section __________________
Solubility of Solutions
Place six test tubes into a test tube rack and label them for each of the six solutions being put
inside of them.
Put 20 drops of each known solution into its test tube.
Add 2-3 drops of ammonium carbonate, (NH4)2CO3, to each test tube. Note whether or not a
precipitate forms.
Rinse the test tubes into a waste beaker for later disposal.
Repeat this procedure using ammonium sulfate, (NH4)2SO4, and ammonium phosphate,
(NH4)3PO4.
Put all waste into a waste beaker for later disposal as indicated by your instructor.
Identifying Anions
Place three test tubes into a test tube rack and label them for each of the three solutions being
put inside of them (e.g. Cl2, Br2, I2).
Put 20 drops of each solution into its test tube.
Add 20 drops of hexane, C6H14, to the test tube containing the chlorine solution.
Add 2 drops of nitric acid, HNO3, then add 2-3 drops of chlorine water (bleach, NaClO) to
this test tube
Gently shake this test tube as demonstrated by your instructor. Note the colors of each layer
in the test tube. The top layer contains your anion.
Pour the waste into your waste beaker and repeat for each other solution.
Figure 5.2: The three anions
Experiment 5 – Periodic Nature
Name __________________
Lab Section __________________
Identifying the Unknown Solution – Cation and Anion
Obtain an unknown solution from your instructor and record its number.
Cation Test
Perform the flame test on your sample as before
Place 20 drops of your unknown solution into three test tubes and check for its solubility with
each of the ammonium carbonate, (NH4)2CO3, ammonium sulfate, (NH4)2SO4, and
ammonium phosphate, (NH4)3PO4 solutions.
Anion Test
Put 20 drops of each solution into its test tube.
Add 20 drops of hexane, C6H14, to the test tube containing the chlorine solution.
Add 2 drops of nitric acid, HNO3, then add 2-3 drops of chlorine water (bleach, NaClO) to
this test tube.
Gently shake this test tube. Note the colors of each layer in the test tube. The top layer
contains your anion.
Compare these results for the unknown sample with those of the known solutions to determine
the composition of the unknown solution
Experiment 5 – Periodic Nature
Name __________________
Lab Section __________________
Prelaboratory Questions
1) What are the names of the three groups on the periodic table that will be investigated in this
lab. Which elements belong to each of those groups?
Group Name:
Group 1
__________
Group 2
___________
Group 17
__________
Group 2
___________
Group 17
__________
Example Elements:
Group 1
__________
2) Look up the flame test colors for the six cations being investigated. What are the expected
colors for each of these elements?
Li
__________
Na
__________
K
__________
Ca
__________
Ba
__________
Sr
__________
3) Look up the colors of the three anions being investigated. What are the expected colors for
each of these elements?
Cl2
__________
Br2
__________
I2
__________
Experiment 5 – Periodic Nature
Name __________________
Lab Section __________________
Data Table
Cation Tests for Known Solutions
Flame Test - Color of Flame
Known Solution
Color Observed
Lithium (Li)
______________________________
Sodium (Na)
______________________________
Potassium (K)
______________________________
Calcium (Ca)
______________________________
Strontium (Sr)
______________________________
Barium (Ba)
______________________________
Solubility Test – PPT or No Reaction
Ammonium Carbonate
(NH4)2CO3
Ammonium Sulfate
(NH4)2SO4
Ammonium Phosphate
(NH4)2PO4
Lithium (Li)
__________
__________
__________
Sodium (Na)
__________
__________
__________
Potassium (K)
__________
__________
__________
Calcium (Ca)
__________
__________
__________
Strontium (Sr)
__________
__________
__________
Barium (Ba)
__________
__________
__________
Known Solution
Anion Test – Color of Layers
Chloride
Bromide
Iodide
____________________
____________________
____________________
Experiment 5 – Periodic Nature
Name __________________
Lab Section __________________
Unknown Solution – ___________
Cation Test
Flame Test - Color of Flame
Color Observed
Unknown Solution
______________________________
Solubility Test – PPT or No Reaction
Unknown Solution
Ammonium Carbonate
(NH4)2CO3
Ammonium Sulfate
(NH4)2SO4
Ammonium Phosphate
(NH4)2PO4
__________
__________
__________
Anion Test – Color of Layers
Color Observed
Unknown Solution
______________________________
Composition of Unknown Solution
_______________
Experiment 5 – Periodic Nature
Name __________________
Lab Section __________________
Postlaboratory Questions
1) To which group does each of the following elements belong? (family name)
Na ___________
F ____________
Mg ____________
Xe ____________
2) Which element is similar to the one listed below? (circle the appropriate answers)
Element
Similar to:
Ca
Fe
Be
Na
Cd
Cu
Co
Cr
Ag
Zn
O
S
N
Ar
Li
K
Na
Br
Li
Kr
3) An unknown solution is subjected to the same flame-test, solubility test, and anion test that
you just did in lab. It gave a light green color in the flame test and a precipitate with each of the
ammonium salts. In addition, it gave an orange color in the hexane layer. What was this
compound?
4) An unknown solution is subjected to the same flame-test, solubility test, and anion test that
you just did in lab. It gave a red color in the flame test and a precipitate with each of the
ammonium salts except ammonium sulfate. In addition, it gave no color in the hexane layer.
What was this compound?
5) Use arrows on the following blank periodic tables to indicate the directions in which each of
the following properties increases:
Ionic Radius
Electronegativity
Metallic Nature