Overview: In today's experiments, you will be working with two alkaline earth metals, magnesium and calcium. You find these two elements in the same column on the Periodic Table. That means they are in the same chemical family. To say these two elements are in the same chemical family is to make an analogy to a human family in which some of the characteristics between the members are the same and some are different. By the end of today's lab, you should learn about some characteristic reactions of the alkaline earth metals, see in what ways these two metals are the same, and observe some of the differences.
***Reminder: Your observations, data and answers to your complete lab should be done in your Student Laboratory Notebook. You will turn in the carbon copy pages when the lab is due.
Procedure
A: Alkaline Earth metals and water
1. Obtain
a 7-8 cm piece of magnesium and a piece of calcium metal. Examine their appearances.
You can use a file to “clean off” the surface. Record your observations.
a.
What clues does the surface appearance of these metals give you about the
reactivity of these two metals with air?
2. Fill
two test tubes half-full with distilled water.
Distilled water is pure H2O. Put the tubes in your test-tube
rack. Test the pH (acidic or basic
property of the solution) by dipping a stirring rod into the water and touching
it to a piece of Universal Indicator paper.
Record observations.
3.
Drop
a small piece of calcium metal into one of the test tubes and a small piece of
magnesium into the other. Observe
and record. (Caution the reaction
can be very hot.)
b. From
what you observe in the test tubes, what can you state about the reactivity of
these two metals with water?
c.
You have seen a video clip on the reactivity of the alkali metals with water.
What do you predict about the reactivity of strontium metal with water
and air as compared to magnesium and calcium?
4.
Test the pH of the solutions in the
test tubes again. Record your
observations.
5. The white substance formed in the reaction is an ionic compound. Make a hypothesis about what this product is. Do a pH experiment with the available salt(s) of your choice and see whether the pH result is consistent with the pH measured in Step #4. Observe the solubility of the salt as well. Write a statement about the results of the pH and solubility test regarding the accuracy of your hypothesis. (Four salt samples of Ca(OH)2, CaCl2, MgCl2 and CaO. Are available at the end of the lab bench for you to use. Take very small amounts of these.)
d.
In one of the reactions you should have seen bubbles of gas being formed.
Of what do you suppose these bubbles of gas are made?
Procedure
B: Alkaline
Earth metals and HCl
1.
Obtain another very small piece of
calcium and another small piece of magnesium.
Measure 20 mL of 2M HCl into each of two 50-mL beakers.
Drop a piece of metal into each of the beakers and observe the results. Record your observations below.
e.
What differences do you notice between the reaction of calcium with HCl and the
reaction of calcium and water, which you ran before?
f.
What differences do you notice between the reaction of magnesium with HCl and
the reaction of magnesium and water, which you ran before?
2.
Test the pH of the solutions in these
two reactions after the reactions are completed, and compare them with the pH of
the solutions after the reactions with water.
Record.
g.
What evidence do you now have that one product of the reaction of calcium with
HCl is different that a product of the reaction with water?
h.
What explanation can you give for why the product of calcium with HCl ends up
looking different than the reaction of calcium with H2O?
Use the salts provided to test your hypothesis.
(Hint: The 2 M HCl you are using is mostly water.
Hint 2: Think solubility here.)
i.
What substance(s) do you think have been formed in the reaction of calcium with
HCl?
j.
Based on the fact that calcium and magnesium are in the same family, what
substance do you think has been formed in the reaction of magnesium with HCl?
k.
Predict the strontium containing product of a reaction between strontium metal
and HCl.
3.
CLEAN UP includes dumping all test
tube contents into the sink. Cleaning
and rinsing out the test tubes with distilled water.
Returning the test tubes to your test tube rack and running water into
the sink for a minute to flush down the contents.
Straighten up your work area.
Summary Questions:
l.
What general properties can you state regarding alkaline earth metals (both
chemical and physical) after this exploration?
m.
Your mother has probably told you that calcium is an important part of your
diet. Do you think it would be
reasonable for you to eat calcium metal atoms as a dietary calcium supplement?
Explain.
n.
Do you think your bones and teeth contain calcium metal atoms or calcium ions?
o.
Strontium-90 is a radioactive isotope of strontium that is a dangerous product
of nuclear fallout whether from nuclear weapons’ blasts or from nuclear
reactor accidents. Based upon your
understanding of similarities of properties of elements in a chemical family,
explain why this isotope of strontium existing as ions is so dangerous to humans if it is spread
around in the environment. (Hint:
where do you think the 90Sr2+ would go in your body?)
