Thermal Decomposition of Carbonates

Aim

To investigate the thermal decomposition of metal carbonates, observe the products formed, and compare the ease of decomposition of different metal carbonates.

Apparatus

• Test tubes (hard glass, e.g., borosilicate)
• Delivery tube and bung (to fit the test tube)
• Test tube holder or clamp stand
• Bunsen burner
• Heatproof mat
• Limewater (calcium hydroxide solution) in a separate test tube
• Samples of metal carbonates: copper(II) carbonate (CuCO3), zinc carbonate (ZnCO3), calcium carbonate (CaCO3), sodium carbonate (Na2CO3)
• Stopwatch (if timing decomposition)
• Safety glasses

Method

1. Place a spatula measure of copper(II) carbonate into a hard glass test tube.
2. Fit the delivery tube and bung to the test tube. Place the other end of the delivery tube into a test tube containing limewater.
3. Hold the test tube at an angle (mouth slightly lower than the bottom to prevent condensation running back and cracking the hot glass) and heat strongly with a Bunsen burner.
4. Observe the colour change of the solid and whether the limewater turns milky.
5. Continue heating until no further change occurs.
6. Remove the delivery tube from the limewater before stopping heating (to prevent suck-back of water into the hot tube).
7. Allow the residue to cool and examine it.
8. Repeat with each of the other carbonates, noting how long each takes to show signs of decomposition and whether decomposition occurs at all.

Expected results

Carbonate	Colour before	Colour after	Limewater	Ease of decomposition
Copper(II) carbonate	Green	Black	Turns milky	Easy (decomposes quickly)
Zinc carbonate	White	Yellow when hot, white when cold	Turns milky	Moderate
Calcium carbonate	White	White (calcium oxide)	Turns milky (slowly)	Difficult (needs strong heating)
Sodium carbonate	White	White (no change)	No change	Does not decompose at Bunsen temperatures

Equations:

• CuCO3(s) → CuO(s) + CO2(g)
• ZnCO3(s) → ZnO(s) + CO2(g)
• CaCO3(s) → CaO(s) + CO2(g)

Zinc oxide (ZnO) is white when cool but turns yellow when hot — this is a useful observation to note.

Analysis

The ease of thermal decomposition is related to the position of the metal in the reactivity series. Carbonates of less reactive metals decompose more easily (at lower temperatures) because their ions have a greater polarising power, which weakens the C-O bonds in the carbonate ion.

The trend: Group 1 carbonates (sodium, potassium) do not decompose with a Bunsen burner. Group 2 carbonates (calcium, magnesium) decompose but require strong heating. Transition metal carbonates (copper, zinc) decompose readily.

This pattern can be linked to the stability of the metal oxide formed. If the metal oxide is very stable (e.g., CuO), the decomposition is thermodynamically favourable and occurs at lower temperatures.

Sources of error

• Insufficient heating: Some carbonates require prolonged, strong heating. If the Bunsen burner is not hot enough or heating time is too short, decomposition may appear not to occur.
• Impure samples: If the carbonate sample contains impurities, the colour changes may not be as expected. Use fresh, pure samples.
• Limewater sensitivity: Limewater can become milky from absorbing CO2 from the air. Use freshly prepared limewater and keep the stock bottle sealed.
• Suck-back: If the delivery tube is not removed from the limewater before heating stops, the cooling test tube creates a partial vacuum that sucks limewater back into the hot tube, which can crack it. Always remove the delivery tube first.

Exam tips

Examiners frequently ask for the word and symbol equation for a specific thermal decomposition. The pattern is always: metal carbonate → metal oxide + carbon dioxide. Write the general equation and then apply it to the specific carbonate.

A common question format provides a table of results and asks you to rank the carbonates in order of ease of decomposition. Link this to the reactivity series: the more reactive the metal, the more stable its carbonate and the harder it is to decompose.

When asked how to test for carbon dioxide in this experiment, state: pass the gas through limewater using a delivery tube — the limewater turns milky/cloudy due to the formation of insoluble calcium carbonate: Ca(OH)2 + CO2 → CaCO3 + H2O.

Safety points worth mentioning: wear safety glasses (hot glass can crack), point the test tube away from people, and remove the delivery tube from limewater before stopping heating.