Learning focus
Develop precise chemical language, connect equations and practical observations, interpret periodic trends and apply salt-preparation rules to unfamiliar examples.
General equation
Acid + carbonate -> salt + water + carbon dioxide. The reaction produces visible effervescence. For calcium carbonate and hydrochloric acid: CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g).
Ionic explanation
The carbonate ion reacts with hydrogen ions: CO3 2-(aq or s) + 2H+(aq) -> H2O(l) + CO2(g). If an insoluble carbonate is used, the reaction occurs at its surface.
Carbon dioxide test
Bubble the gas through limewater. A white precipitate of calcium carbonate makes the solution milky. Effervescence alone suggests gas production but is not a positive identification.
Preparation use
An insoluble carbonate can be added until fizzing stops and some solid remains. The excess ensures all acid is neutralised. It is then removed by filtration before crystallisation.
Practical or data skill
Compare equal masses of carbonate powder and chips with acid to observe both gas production and the effect of surface area.
Examination tip
Include all three products: salt, water and carbon dioxide.
Review questions and suggested answers
Question 1
What gas forms when an acid reacts with a carbonate?
Suggested answer
Carbon dioxide.
Question 2
How is carbon dioxide tested?
Suggested answer
It turns limewater milky.
Question 3
Why add carbonate until excess remains in salt preparation?
Suggested answer
To ensure all acid has reacted.
