Thermal decomposition

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Key points

  • Thermal means heat. Decomposing is the process of breaking down.
  • Thermal decomposition is a chemical reaction that happens when a compound breaks down when heated.

Baking powder and self-raising flour contain the chemical sodium hydrogencarbonate. This chemical breaks down when heated. What is released that makes cakes rise?

When sodium hydrogencarbonate is heated in the oven, it breaks down and releases carbon dioxide gas, making the cake rise.

Video - thermal decomposition

Watch this video to find out about thermal decomposition reactions.

Look out for the number of there are in a thermal decomposition reaction.

Thermal decomposition reactions

What are the products when sodium hydrogencarbonate thermally decomposes?

Sodium carbonate, water and carbon dioxide.

What is thermal decomposition?

Thermal means heat. Decomposing is the process of breaking down. Thermal decomposition is a that happens when a compound breaks down when heated.

Thermal decomposition reactions happen at high temperatures. The reactants absorb lots of energy before breaking down into the .

The starting is the . It breaks down to simpler substances, which could be or they could be compounds.

Thermal decomposition are examples of reactions, and are useful when cooking and baking cakes.

Working scientifically

Working safely with thermal reactions

Cooking involves chemical reactions and often takes place at high temperatures. But you don’t usually see cooks and chefs wearing eye protection.

In the science lab, however, you always wear eye protection when working with chemical reactions.

Eye protection is essential when conducting scientific experiments
  • In the science lab, eye protection is worn because of the larger amounts of chemicals, harmful chemicals and unfamiliar equipment surrounded by other people carrying out the same investigations.
  • Wearing eye protection helps reduce the risk of an accident involving laboratory hazards and reminds us of the need to work safely.

Find out more about working safely in the lab.

Equations for thermal decomposition

In a thermal decomposition reaction, there is only one reactant, but two or more products.

For example, when copper carbonate is heated, it breaks down to produce copper oxide and carbon dioxide.

Copper carbonate is green and copper oxide is black. You can see a colour change from green to black during the reaction. The carbon dioxide produced can be detected using limewater, which turns from clear to cloudy.

The word equation for this reaction is:

copper carbonate → copper oxide + carbon dioxide

The symbol equation for this reaction is:

CuCO(s) → CuO(s) + CO(g)

Copper carbonate is easily decomposed by heating

Not all thermal decomposition reactions produce a colour change. When magnesium carbonate is heated, it breaks down to produce magnesium oxide and carbon dioxide. There is no change in colour because magnesium carbonate and magnesium oxide are white solids.

The word equation for this reaction is:

magnesium carbonate → magnesium oxide + carbon dioxide

The symbol equation for this reaction is:

MgCO(s) → MgO(s) + CO(g)

A state symbol is a symbol used in chemical equations to show if a substance is a solid (s), a liquid (l), a gas (g), or an aqueous solution (aq).

What is produced when calcium carbonate is heated?

Calcium oxide and carbon dioxide.

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