The Young Scientist Program
http://ysp.wustl.edu
Washington University Medical School
Funding provided by The Leon Lowenstein Foundation
Periodicity
Chemistry Teaching Team
References: Adapted from the Oakland Schools Chemistry Resource Unit: Periodic Table. Andrew D. Hulbert.
Bloomfield Hills Schools. Andover High School. Activity #5 – Alkaline Earth Metal Demosntration.
See https://www.oakland.k12.mi.us/Portals/0/Learning/PeriodicTable.pdf.
Main Concepts:
Periodicity of elements
Module Overview: The periodic table is arranged such that elements with similar chemical properties are grouped
together in columns. Nevertheless, elements in different rows of the same column can be more or less reactive than
the other elements in the column. These differences arise due to the fact that elements further down the column
have more electrons than the preceding elements. This demonstration shows the differences in the rate of reaction
with water of magnesium and calcium.
Materials:
Magnesium Turnings
Calcium Turnings
Beakers (smaller is better)
Water
Phenophthalein
Spatulas/Forceps
Safety: Students MUST to wear gloves and goggles for this demonstration. The calcium reaction creates a basic
solution which could splatter due to the intense bubbling. The beaker for the calcium solution should be several times
the volume of water and students should stand back and carefully add the calcium with a spatula. In addition, the
reaction also releases hydrogen gas which could catch fire. Therefore, this reaction should be performed away from
sources of heat such as hot plates or Bunsen burners.
Methods/Protocol:
1. Add water to the beakers. 20 mL of water is sufficient. Label one beaker magnesium, and the other calcium.
2. Add about 3 drops of phenolphthalein to each beaker. Make sure the students understand that the
phenolphthalein functions as a pH indicator (just like litmus paper).
3. Add a small quantity of magnesium and calcium to the water.
For 20 mL water, less than 0.2 g of either metal works well. In practice, adding a few, small pellets of
magnesium and one, larger pellet of calcium should be sufficient.
The Young Scientist Program
http://ysp.wustl.edu
Washington University Medical School
Funding provided by The Leon Lowenstein Foundation
4. Observe the reactions.
The magnesium reaction may not react immediately. Over time the magnesium will begin to turn the water
around it pink due to the production of hydroxide ions.
The calcium will react immediately and somewhat violently. The entire solution will turn pink and bubble. The
calcium should react completely very quickly. Note: buildup of CaO due to reaction with the atmosphere may
slow the reaction somewhat.
Discussion:
1. Why does the calcium react more quickly than the magnesium?
The calcium reacts more quickly because it has a lower ionization energy than the magnesium does. This means
that it takes less energy input to get the calcium atoms to give up two electrons to water than it takes to do
the same for magnesium atoms. The result is that the calcium reacts far more quickly than magnesium. This
answer can be combined with ideas about other periodic trends such as atomic radius and electronegativity.
2. The chemical reactions are the following:
Ca (s) + 2 H2O (l)  Ca(OH)2 (aq) + H2 (g)
Mg (s) + 2 H2O (l)  Mg(OH)2 (aq) + H2 (g)
Disposal and cleanup:
Reseal the calcium and magnesium containers. This limits the amount of reaction with air and the buildup of oxides
that will slow the reaction with water.
Do not dump any unreacted magnesium or calcium down the drain. Save leftover magnesium pellets in a sandwich bag
and dispose of them by allowing them to react completely or by putting them in a biohazard bin. The solutions may be
poured down the drain.