4CH1

Gases in the Atmosphere

Inorganic Chemistry · 1 question type

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable
Rare
Stable5%

Air composition, greenhouse effect and climate implications appear in most series.

What thermal decomposition is

  • Thermal decomposition is a reaction in which a single compound breaks down into two or more simpler products when heated
  • Metal carbonates from the lower half of the reactivity series break down readily on heating to give the metal oxide plus carbon dioxide
  • General equation:

metal carbonate → metal oxide + carbon dioxide

  • The carbon dioxide given off can be confirmed by bubbling it through limewater (calcium hydroxide solution), which turns milky/cloudy

Other examples

  • ZnCO3(s) → ZnO(s) + CO2(g) (the white solid turns yellow when hot and back to white once cool)
  • CaCO3(s) → CaO(s) + CO2(g) (the same reaction used industrially to make lime from limestone)
  • Group 1 carbonates (Na2CO3, K2CO3) do not decompose at Bunsen-burner temperatures because their carbonates are very stable
Thermal decomposition of green copper carbonate heated in a test tube, forming black copper oxide and releasing carbon dioxide gas
Source: Thermal decomposition by Save My Exams
Exam tip

Thermal decomposition of copper(II) carbonate — colour change and equation

What comes up: "Give the colour change that occurs" when copper(II) carbonate is heated, or "Name this type of reaction."

Write (two marks): (1) The solid changes colour from green to black. The mark scheme requires both colours and they must be stated in the correct order. (2) The reaction type is thermal decomposition.

Watch out: starting from the wrong colour (e.g. "blue to black") is the most common error — copper(II) carbonate is green, not blue (blue is copper(II) sulfate crystals). Write the equation as CuCO₃ → CuO + CO₂; state symbols are not required for the mark.