Section 1 of 5Reaction with Acids
Carbonates react with dilute acids in a fast reaction that produces a salt, water and carbon dioxide.
Word equation
- Word equation Metal carbonate plus acid gives a salt, water and carbon dioxide, with no other products.
Effervescence
- Effervescence Bubbles of carbon dioxide gas fizz out of the solution as the carbonate reacts.
Salt produced
- Salt produced Hydrochloric acid gives chlorides, nitric acid gives nitrates and sulfuric acid gives sulfates.
Limewater test
- Limewater test Carbon dioxide turns limewater milky, which confirms the gas being released is carbon dioxide.
Section 2 of 5Thermal Decomposition
Heating a carbonate breaks it down into a metal oxide and carbon dioxide, and the reaction is more difficult for some carbonates than others.
1
Heating a metal carbonate breaks it down into a metal oxide and carbon dioxide gas.
2
Calcium carbonate decomposes on strong heating: CaCO₃(s) → CaO(s) + CO₂(g).
3
The carbon dioxide released can be identified by bubbling it through limewater, which turns milky.
4
Group 1 carbonates such as sodium carbonate are too stable to decompose easily on normal heating.
Section 3 of 5Thermal Stability of Group 2 Carbonates
The thermal stability of Group 2 carbonates increases as you go down the group, so a higher temperature is needed.
Thermal Stability of Group 2 Carbonates
Magnesium carbonate
- Magnesium carbonate Decomposes at the lowest temperature in Group 2, so it is the least thermally stable.
Calcium carbonate
- Calcium carbonate Needs a higher temperature than magnesium carbonate to decompose completely.
Barium carbonate
- Barium carbonate Needs the highest temperature in Group 2, so it is the most thermally stable.
| Carbonate | Behaviour on heating | Relative stability |
|---|---|---|
| MgCO₃ | Decomposes at the lowest temperature of the group | Least stable |
| CaCO₃ | Decomposes at a higher temperature than MgCO₃ | Intermediate |
| BaCO₃ | Needs the highest temperature of the group | Most stable |
Section 4 of 5Worked Example
Question
- Calculate the mass of carbon dioxide produced when 20 g of magnesium carbonate decomposes completely.
- (Relative atomic masses: Mg = 24, C = 12, O = 16.)
- Relative formula mass of MgCO₃ = 24 + 12 + (3 × 16) = 84.
- Moles of MgCO₃ = 20 ÷ 84 = 0.238 mol (to 3 significant figures).
- The equation MgCO₃ → MgO + CO₂ has a 1 : 1 mole ratio, so 0.238 mol of CO₂ forms.
- Relative formula mass of CO₂ = 12 + 32 = 44, so mass of CO₂ = 0.238 × 44 = 10.5 g.
Answer 10.5 g of carbon dioxide.
Section 5 of 5Exam Checklist & Tips
8.3.3 – Key Points for the Exam
- Thermal decomposition A reaction in which a substance breaks down into simpler substances when heated.
- Limewater Calcium hydroxide solution, which turns milky in the presence of carbon dioxide.
- Effervescence Fizzing caused by gas bubbles escaping from a liquid.
- Thermal stability How resistant a compound is to breaking down when heated.
- Group 2 carbonate A compound containing a metal from Group 2 and the carbonate ion, CO₃²⁻.
Common Mistakes
- Saying carbonates decompose into a metal and oxygen, instead of a metal oxide and carbon dioxide.
- Confusing the limewater test with the test for oxygen, which relights a glowing splint.
- Stating that the thermal stability of Group 2 carbonates decreases going down the group, which is the wrong way round.
- Treating the mass lost on heating as something other than the mass of carbon dioxide released.