1Breaking down with heat
Thermal decomposition: a compound breaks down when it is heated. Green copper carbonate makes black copper oxide and carbon dioxide.
thermal decompositioncarbonate
Lesson 11.3 notes
Thermal decomposition
Compounds that break down when they are heated: carbonates, limewater, and where the mass goes.
2One reactant, two products
In thermal decomposition one reactant breaks into two or more products: CuCO₃ → CuO + CO₂.
product
3Testing for carbon dioxide
Carbon dioxide turns limewater cloudy. Bubbling the gas through limewater shows that a carbonate has decomposed.
limewater
4Where does the mass go?
The solid gets lighter because carbon dioxide escapes. 12.4 g of copper carbonate makes 8.0 g of copper oxide and 4.4 g of carbon dioxide.
conservation of mass
5Heating to constant mass
Heat, cool and weigh again until the mass stops changing. Then you know all the carbonate has decomposed.
constant mass
6Limestone
Heating limestone (calcium carbonate) makes quicklime (calcium oxide) and carbon dioxide. Lime kilns have done this for hundreds of years.
limestonequicklime
7Baking soda
Baking soda (sodium hydrogencarbonate) decomposes in a hot oven, giving off carbon dioxide. The gas bubbles help cakes rise.
sodium hydrogencarbonate
Explore: Carbonate lab
Heat different carbonates and see which ones turn the limewater cloudy, and how quickly.
Common mistake: Thermal decomposition is not burning
Both need heat, but burning is a reaction with oxygen. In thermal decomposition one compound breaks apart on its own: copper carbonate would decompose even with no air at all.
Remember
- Thermal decomposition: one compound breaks down into two or more products when it is heated.
- Metal carbonate → metal oxide + carbon dioxide. Limewater turns cloudy with carbon dioxide.
- The solid gets lighter because carbon dioxide escapes, but the total mass is conserved.
Going further at GCSE
GCSE: Conservation of mass → GCSE: Mass changes with gases → GCSE: Chemical changes →