Thermal Decomposition

Introduction

Heat a lump of limestone strongly and it looks almost the same afterwards, yet it has become a completely different substance. Heat green copper carbonate and it turns black. In both cases a single compound has been split into simpler substances by heat alone. This type of reaction is called thermal decomposition , and it is one of the oldest chemical processes used by people: lime kilns have been burning limestone for thousands of years to make building materials.

Core explanation

The idea. "Thermal" means to do with heat; "decomposition" means breaking down. In a thermal decomposition, one reactant is heated and breaks down into two or more products. There is nothing else to react with — the heat supplies the energy needed to break bonds inside the compound so that its atoms can rearrange into new, simpler substances.

The general pattern is:

compound → (heat) → simpler substance A + simpler substance B

Metal carbonates. The best-known examples are metal carbonates. When heated strongly, many carbonates break down into a metal oxide and carbon dioxide gas :

calcium carbonate → calcium oxide + carbon dioxide copper(II) carbonate → copper(II) oxide + carbon dioxide

Copper(II) carbonate is a green powder; copper(II) oxide is black. The colour change is clear evidence of a new substance. With calcium carbonate the solid stays white, so the evidence comes from testing the gas given off: carbon dioxide turns limewater milky.

Why heat is needed all the time. Thermal decomposition reactions are endothermic — they take in energy. If you stop heating, the reaction stops. This is different from burning, which, once started, releases enough energy to keep itself going.

Not all carbonates behave the same. Carbonates of less reactive metals, such as copper, decompose easily with gentle heating. Calcium carbonate needs much stronger heating (around 800–900 °C in industry). Carbonates of very reactive metals such as sodium and potassium hardly decompose at all in a school laboratory. In general, the more reactive the metal, the more stable its carbonate.

Other examples. Some metal hydroxides decompose to an oxide and water, and some metal nitrates decompose on heating to give oxygen and other gases. Hydrated salts losing their water, which you met when studying hydrated and anhydrous salts, is closely related: heat drives water out of the crystal.

Why it is a chemical change. New substances with different properties are formed, and the change is not reversed simply by cooling. Cooling calcium oxide does not turn it back into limestone.

Step-by-step reasoning

To decide whether a reaction is a thermal decomposition:

1. Count the reactants. There should be only one compound being heated. 2. Check that heat is the cause of the change. 3. Count the products. There should be two or more, each simpler than the starting compound. 4. Look for evidence of new substances, such as a colour change or a gas that gives a positive test.

Visual explanation

Imagine a green heap of copper carbonate in a tube. As the flame heats it, the green darkens from the bottom upwards and becomes black powder, while invisible carbon dioxide rises out of the tube and bubbles through limewater, which slowly clouds over like milk being stirred into water.

Real-world analogy

Thermal decomposition is like taking apart a model built from interlocking bricks. The model is one object, but with enough effort you can pull it into two smaller pieces. You have to keep applying effort to separate the pieces — stop pulling, and nothing more comes apart.

Real-world example

Cement and lime production depend on thermal decomposition. Limestone (calcium carbonate) is heated in large rotary kilns to make quicklime (calcium oxide). This process releases large amounts of carbon dioxide, which is why the cement industry is a major source of greenhouse gas emissions worldwide.

Why?

Why does thermal decomposition need continuous heating? Breaking the bonds in the compound takes in more energy than forming the bonds in the products gives out. The difference must be supplied from outside, so once the heat source is removed there is not enough energy to break any more bonds.

Common misconception

"When calcium carbonate is heated and looks the same, nothing has happened." A reaction does not always change the colour. The loss of mass and the carbon dioxide that turns limewater milky show that a new substance, calcium oxide, has formed.

Worked example

Question: 10.0 g of calcium carbonate is heated strongly until no further change occurs. 5.6 g of solid remains. Explain the mass change and name the products.

Reasoning: Calcium carbonate decomposes into calcium oxide and carbon dioxide. The solid remaining is calcium oxide. Carbon dioxide is a gas and escapes into the air, so the solid loses mass. Mass is still conserved overall: 10.0 g = 5.6 g + 4.4 g of carbon dioxide.

Answer: The products are calcium oxide (5.6 g of solid) and carbon dioxide (4.4 g, lost as gas).

Quick check

1. What are the two products when copper(II) carbonate is heated? Answer: Copper(II) oxide and carbon dioxide.

Exam focus

Learn the word equation for heating a metal carbonate: metal carbonate → metal oxide + carbon dioxide. Be ready to describe the limewater test for carbon dioxide and to explain why the solid loses mass. Always say "one reactant breaks down into two or more products".

Advanced insight

Carbonate stability depends on the metal ion. Small ions with a large charge, such as Mg²⁺, distort the electron cloud of the carbonate ion and weaken it, so magnesium carbonate decomposes more easily than barium carbonate. This "polarising power" trend explains why carbonate stability increases down Group 2 of the periodic table.

Summary

Thermal decomposition is a chemical reaction in which heat breaks one compound into two or more simpler substances. Metal carbonates decompose into a metal oxide and carbon dioxide, which turns limewater milky. The reactions are endothermic and stop when heating stops. Carbonates of more reactive metals are harder to decompose. Lime and cement manufacture rely on this reaction.

Practice questions

1. Define thermal decomposition. Answer: A chemical reaction in which a single compound is broken down into two or more simpler substances by heating. 2. Write the word equation for the thermal decomposition of calcium carbonate. Answer: Calcium carbonate → calcium oxide + carbon dioxide. 3. Give two pieces of evidence that heating copper(II) carbonate causes a chemical change. Answer: The green solid turns black, and a gas is given off that turns limewater milky (carbon dioxide). 4. Why does the reaction stop as soon as the Bunsen burner is removed? Answer: Thermal decomposition is endothermic, so it needs a continuous supply of energy to break bonds; without heat it cannot continue. 5. Suggest why sodium carbonate does not decompose in a school Bunsen flame. Answer: Sodium is a very reactive metal, so its carbonate is very stable and needs far more energy to decompose than a Bunsen flame can supply.