Thermal decomposition of metal carbonates
This experiment involves a comparison between the thermal stabilities of carbonates of reactive metals, such as sodium and potassium, and the carbonates of less reactive metals, such as lead and copper.
This experiment can be carried out as a class exercise, individually or in pairs. Because of the toxicity of lead compounds, it may be best to leave lead carbonate out with less reliable classes.
The experiment should take about 40 - 45 minutes.
Limewater (calcium hydroxide solution), 10 cm3 per carbonate
About 2 g each of following solids:
Copper carbonate (HARMFUL)
Lead carbonate (TOXIC, DANGEROUS FOR THE ENVIRONMENT)
Potassium carbonate (IRRITANT)
Sodium carbonate, anhydrous (IRRITANT)
Refer to Health & Safety and Technical notes section below for additional information.
Each group will require:
Test-tubes, 2 (per carbonate)
Delivery tube (right-angled)
Clamp and stand
Health and Safety and Technical notes
Wear eye protection. It is important not to inhale dust of lead carbonate or the oxide formed. Wash hands after using lead compounds.
Limewater (calcium hydroxide solution), Ca(OH)2(aq), (treat as IRRITANT) -see CLEAPSS Hazcard and Recipe Book.
Copper carbonate, CuCO3.Cu(OH)2(s), (HARMFUL) - see CLEAPSS Hazcard.
Lead carbonate, PbCO3(s), (TOXIC, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.
Potassium carbonate, K2CO3(s), (IRRITANT) - see CLEAPSS Hazcard.
Sodium carbonate, anhydrous, Na2CO3(s), (IRRITANT) - see CLEAPSS Hazcard.
Zinc carbonate, ZnCO3(s) - see CLEAPSS Hazcard.
a Put a large spatula measure of the carbonate to be tested in a test-tube.
b Fit a delivery tube and then clamp the test-tube so that the delivery tube dips into a second test-tube containing 2-3 cm3 limewater.
c Heat the solid gently at first, then more strongly.
d Lift the delivery tube from the limewater before, or as soon as, the heating is stopped. This is to avoid "suck-back".
e Write down all observations. Notice what happens to the limewater and how long it takes to turn milky (if at all). Notice whether any melting occurs in the heated test-tube and any colour changes taking place. Write your results in tabular form.
f Repeat the experiment with the other metal carbonates supplied, and in each case write down your observations.
g Wash your hands thoroughly at the end of these experiments, since some of the metal carbonates are toxic.
It is important to emphasize how "suck-backs" are avoided before the students begin, otherwise there are bound to be mishaps.
It is also important to ensure that students wash their hands after using lead carbonate, and to ensure that dust is not raised when this solid is being used.
The relative ease with which the carbonates of some of the less reactive metals are decomposed has been used in the extraction of these metals from ores that contain the metal as a carbonate, for example zinc carbonate (calamine).
|Carbonate||Colour before heating||Colour after heating||Gas evolved||Ease of decomposition|
|Potassium carbonate||white||white||none||very difficult|
|Sodium carbonate||white||white||none||very difficult|
|Zinc carbonate||white||yellow when hot, white when cool||carbon dioxide||fairly easy|
|Lead carbonate||white||some melting / yellow||carbon dioxide||fairly easy|
|Copper carbonate||green||black||carbon dioxide||easy|
Students should find that sodium and potassium carbonates give no carbon dioxide or any other sign that decomposition has taken place, even after prolonged heating.
Those metal carbonates which do decompose leave a residue of the metal oxide and evolve carbon dioxide in the process:
eg ZnCO3(s) → ZnO(s) + CO2(g)
At an elementary level, the relative thermal stability of the carbonates of the metals cannot easily be explained in terms of simple ideas of bonding in these compounds. A simple relationship between the reactivity of the metal and the stability of its compounds, such as the carbonate here, will have to suffice.
With abler and older students it may be appropriate to refer to the polarization (distortion) of the electron cloud of the carbonate ion by the metal ion, and that this is bound to be more pronounced when the metal ion is doubly, rather than singly charged, and small. Polarization eventually leads to abstraction of oxygen from the carbonate ion, producing the oxide ion and a carbon dioxide molecule. The greater the polarization, the lower the temperature needed to decompose the carbonate.
Health & Safety checked August 2008
Page last updated on 16 December 2011