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The final characteristic reaction of an acid is that it will produce carbon dioxide gas when reacted with a metal carbonate. The carbon dioxide gas which is released during the reaction will cause limewater to change from a colourless solution to milky.

Apparatus and chemicals

• Measuring cylinder
• Boiling tube, test-tube
• Disposable pipette/dropper
• Test-tube rack
• HCl 0.5 mol/dm³
• Calcium carbonate (3 g)
• Limewater (5 cm³)

Method

1. Using a measuring cylinder, measuring 15 cm³ of hydrochloric acid and place into the boiling tube.
2. Using a clean measuring cylinder, measure 5 cm³ of limewater and place into a test-tube. Place the test-tube and boiling tube side by side in a test-tube rack.
3. Carefully add the calcium carbonate to the acid – record your observations.
4. Using the disposable pipette/dropper, collect the gas produced by opening and closing the dropper above the reaction in the boiling tube.
5. Once the gas has been collected in the disposable pipette/dropper, bubble the gas through the limewater and record your observations.

Observations

During this reaction, it is very obvious that gas is produced as the reaction fizzes vigorously. The carbon dioxide gas is more dense than air which allows us to use a dropper to collect it – it will not rise out of the boiling tube quickly and so can be collected and bubble through the limewater. Once the carbon dioxide gas goes through the limewater it reacts with it and makes a solid (precipitate), this causes the limewater to go ‘milky’.