Testing for carbonates: acid releases carbon dioxide
Definition
Carbonate
A compound that contains the carbonate ion, CO32−\text{CO}_3^{2-}CO32−.
Definition
Dilute acid
An acid solution containing a relatively small amount of acid dissolved in water.
Definition
Limewater
Limewater is a solution of calcium hydroxide in water that is used to test for carbon dioxide.
- A carbonate contains the carbonate ion, CO32−\text{CO}_3^{2-}CO32−.
- Adding dilute acid to a carbonate produces fizzing as carbon dioxide gas is released.
- The general reaction is acid + carbonate →\rightarrow→ salt + water + carbon dioxide.
- For example, Na2CO3+2HCl→2NaCl+H2O+CO2\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2Na2CO3+2HCl→2NaCl+H2O+CO2.
- Passing the gas through limewater turns it cloudy, confirming the gas is carbon dioxide.
Practical
Required practical: identifying a carbonate
- Put about 1 cm1\,\text{cm}1cm depth of the solution, or a small amount of the solid, into a test tube.
- Add dilute hydrochloric acid and watch for bubbles.
- If bubbles form, use a teat pipette to transfer the gas into a test tube of limewater.
- Several pipettes of gas may be needed before the limewater turns cloudy.
- Cloudy limewater confirms carbon dioxide, so the sample is a carbonate.
Reading the result and common slips
- Fizzing alone only suggests a carbonate; confirm it by testing the gas with limewater.
- The limewater turning cloudy is the step that proves the gas is carbon dioxide.
- Collect several pipettes of gas, because a small amount may not cloud the limewater at first.
Self review
- Which ion does a carbonate contain?
- What do you see when dilute acid is added to a carbonate?
- How do you confirm that the gas produced is carbon dioxide?
- Write the equation for sodium carbonate reacting with hydrochloric acid.
