Thermal decomposition is a chemical reaction in which a single compound breaks down into two or more simpler substances when heated. Unlike combustion, which requires oxygen, thermal decomposition needs only heat — and because the reaction absorbs rather than releases energy, it is endothermic.
What is thermal decomposition?
Thermal decomposition is a type of chemical reaction in which one reactant absorbs heat energy and breaks apart into two or more products. The general pattern is:
Single compound + heat → Two or more simpler substances
The key features that distinguish thermal decomposition:
- Only one reactant (a compound).
- Requires a continuous input of heat energy to proceed.
- The reaction is endothermic — it takes in energy from the surroundings.
- Produces two or more products, often a metal oxide plus a gas.
It is important not to confuse thermal decomposition with combustion (burning in oxygen) or with decomposition by bacteria or enzymes, which happen at much lower temperatures and use biological catalysts.
What are the most important examples at KS3?
Metal carbonates
Most metal carbonates decompose on heating to form a metal oxide and carbon dioxide gas. This is the most commonly tested example in KS3 and GCSE chemistry.
| Reactant | Products | Observation |
|---|---|---|
| Calcium carbonate (CaCO₃) | Calcium oxide (CaO) + carbon dioxide (CO₂) | White solid remains; gas turns limewater milky |
| Copper carbonate (CuCO₃) | Copper oxide (CuO) + carbon dioxide (CO₂) | Green solid → black solid; gas turns limewater milky |
| Zinc carbonate (ZnCO₃) | Zinc oxide (ZnO) + carbon dioxide (CO₂) | White solid → white/yellow solid (hot); gas turns limewater milky |
| Magnesium carbonate (MgCO₃) | Magnesium oxide (MgO) + carbon dioxide (CO₂) | White solid remains; gas turns limewater milky |
Other examples
- Green verdigris (copper(II) carbonate hydroxide): heated, the compound decomposes — responsible for the green patina on copper statues and roofs.
- Sodium hydrogencarbonate (bicarbonate of soda, NaHCO₃): at lower temperatures than most carbonates. 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ — used in baking to make cakes rise.
- Silver nitrate (AgNO₃): decomposes to silver metal and nitrogen dioxide gas when exposed to light or strong heat — the basis of traditional photography.
How do you write equations for thermal decomposition?
Worked example — copper carbonate decomposition:
Word equation: Copper carbonate → Copper oxide + Carbon dioxide
Symbol equation (balanced): CuCO₃ → CuO + CO₂
This equation is already balanced — one copper, one carbon, and three oxygens on each side.
Worked example — calcium carbonate decomposition:
Word equation: Calcium carbonate → Calcium oxide + Carbon dioxide
Symbol equation: CaCO₃ → CaO + CO₂
Note the pattern: all metal carbonates follow MeCO₃ → MeO + CO₂, where Me is the metal. Learning this pattern means you can write the equation for any metal carbonate without memorising each one separately.
How do you test for carbon dioxide?
Carbon dioxide gas is identified using limewater (calcium hydroxide solution):
- Bubble the gas through limewater using a delivery tube.
- If carbon dioxide is present, the limewater turns milky white (chalky).
The chemistry behind the test: CO₂ reacts with calcium hydroxide (Ca(OH)₂) to produce a white precipitate of calcium carbonate (CaCO₃), which makes the solution appear cloudy.
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
If you continue bubbling CO₂ through, the precipitate eventually dissolves to form soluble calcium hydrogencarbonate — the solution clears again. This is why cave stalactites and stalagmites form: CO₂-rich groundwater dissolves limestone (CaCO₃), and when that water drips into a cave where CO₂ levels are lower, some CO₂ escapes and CaCO₃ deposits again.
Is thermal decomposition endothermic or exothermic?
Thermal decomposition is always endothermic — it absorbs energy from the surroundings (the heat you supply from the Bunsen burner). This is because breaking the bonds in the reactant requires more energy than is released when the new bonds in the products form.
| Energy comparison | What happens | Overall energy change |
|---|---|---|
| Energy to break bonds in reactant | Absorbed (endothermic step) | — |
| Energy released forming bonds in products | Released (exothermic step) | — |
| Net: bond breaking > bond forming | Reaction absorbs energy overall | Endothermic (+ΔH) |
You can confirm it is endothermic by feel: if you hold the tube near the top, the part of the tube being heated would feel hot, but if heat were being released further along, it would warm the tube beyond the heated zone. Because heat is being absorbed, you need to keep heating — the reaction stops if you remove the Bunsen flame.
Frequently asked questions
What is the difference between thermal decomposition and combustion?
Both involve heat, but they are fundamentally different reactions. In thermal decomposition, a single compound breaks down using heat alone — no oxygen is required, and the reaction absorbs energy. In combustion, a substance reacts with oxygen and releases energy (it is exothermic). Burning magnesium ribbon in air is combustion (Mg + O₂ → MgO); heating magnesium carbonate until it breaks down is thermal decomposition (MgCO₃ → MgO + CO₂). Both produce magnesium oxide, but by entirely different routes.
Why does copper carbonate change colour when heated?
Copper carbonate is green; copper oxide is black. When copper carbonate is heated in a test tube, you see the solid change from green to black as the decomposition proceeds. This colour change is a visual indicator that the reaction is happening — alongside the observation of gas bubbles (CO₂) turning limewater milky. The black copper oxide that remains is the product of the decomposition.
Why does sodium hydrogencarbonate decompose at a lower temperature than calcium carbonate?
The stability of a carbonate towards heat broadly follows the reactivity series: the more reactive the metal (i.e. the stronger the metal–oxygen bond in the carbonate), the higher the temperature needed to decompose it. Calcium is more reactive than sodium's carbonate counterpart (NaHCO₃ is a hydrogencarbonate, which decomposes more readily), but the general rule is that carbonates of less reactive metals (like copper, zinc) decompose at lower temperatures than those of very reactive metals (calcium, sodium). This is why potassium carbonate and sodium carbonate are stable enough to be used as food additives, while copper carbonate decomposes readily in a test tube.
How is thermal decomposition used industrially?
The most important industrial application is the production of calcium oxide (quicklime, CaO) by heating limestone (calcium carbonate) in huge rotary kilns at about 1000 °C. Calcium oxide is used to make cement, neutralise acidic soil in agriculture, and treat acidic industrial waste. The CO₂ released is a significant industrial greenhouse gas emission — several per cent of global CO₂ emissions come from lime kilns. Sodium hydrogencarbonate's decomposition is used in baking powder and fire extinguishers (bicarbonate of soda extinguishers).
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