Carbonates are ionic compounds with negatively charged carbonate ions and positively charged metal ions held together by mutual attraction. Carbonates of metals from Group 2 in the periodic table decompose when you heat them, as do many other carbonates. You can make use of this fact to design a fun lab experiment centred on thermal decomposition. Choose from a number of metal carbonates, including perhaps best options of zinc carbonate and copper carbonate, which decompose quite easily.
- Skill level:
- Moderately Challenging
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Things you need
- 2 test tubes
- Bunsen burner
- Rubber hose
- Gloves, goggles, lab coat
- Metal carbonate
- Calcium hydroxide solution
- Right-angled delivery tube
- Test tube stopper
Put on your lab coat, goggles and gloves for safety.
Add a spatula-full of sample to the first test tube. Attach it to the ring stand with a clamp so that the test tube lies horizontally.
Put the stopper in the test tube. The stopper you use should be the kind that has a hole through the middle. Insert the right-angled delivery tube through this stopper, and place the other end of the right-angled delivery tube in a second test tube without a stopper. Place the second test tube in a clamp or a rack so it remains upright throughout the experiment.
Add less than 5ml of calcium hydroxide solution to the second test tube. Remove the right-angled delivery tube if needed to gain access. The second test tube should not be more than half-full. Reinsert the right-angled delivery tube afterward. Be careful with the calcium hydroxide; it's highly alkaline and therefore quite caustic.
Place a Bunsen burner beneath the first test tube containing the metal carbonate. Connect it to the gas supply using the hose.
Turn on the gas and carefully light the Bunsen burner.
Adjust the flame so the test tube with the carbonate is heated gently. As it decomposes, it will release carbon dioxide, which when bubbled through the lime water will form carbonic acid and react with the calcium hydroxide. Record your observations.
Remove the delivery tube from the calcium hydroxide solution shortly before you turn off the Bunsen burner. If you do not, the pressure drop in the test tube with the carbonate can suck some of the calcium hydroxide solution back up the delivery tube.
Tips and warnings
- Generally Group 2 carbonates and those of copper and zinc are better choices than Group 1 carbonates, which only decompose thermally with great difficulty.
- Some metal carbonates, like lead carbonate, are potentially toxic. Calcium hydroxide is a strong base, and it can cause serious injury if you put it in your eyes or on your skin. Be sure you handle these chemicals responsibly. The most serious hazard in this experiment, however, is the open flame. Be very careful while lighting your Bunsen burner and once you have done so, do not leave it unattended. Remain mindful at all times that you are working with an open flame, and do not drape your face or clothing over the burner at any time.
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