Thermal stability and flame tests

Thermal decomposition

• Group 2 carbonates: MCO3 -> MO + CO2 (for example CaCO3 -> CaO + CO2)
• Group 2 nitrates: 2M(NO3)2 -> 2MO + 4NO2 + O2 (brown gas)
• Group 1 carbonates (except Li) do not decompose at Bunsen temperatures; Li2CO3 -> Li2O + CO2
• Group 1 nitrates (except Li) give the nitrite and oxygen: 2NaNO3 -> 2NaNO2 + O2
• LiNO3 behaves like a Group 2 nitrate: 4LiNO3 -> 2Li2O + 4NO2 + O2

Explaining the trend

Thermal stability increases down each group. A small, highly charged cation has a high charge density and polarises the large carbonate or nitrate anion, weakening its bonds so it breaks up more easily. Down the group the cations are larger, so polarise less. Group 2 ions (2+) polarise more than Group 1 ions (1+), so Group 2 compounds decompose more easily; Li+ is so small it behaves like Group 2.

Flame tests

Dip a clean nichrome wire, moistened with concentrated HCl, into the solid and hold it in a roaring Bunsen flame.

Why flames are coloured

Heat excites electrons to higher energy levels. When they fall back to lower levels they release energy as light of specific frequencies, which we see as a characteristic colour. Magnesium ions give no visible flame colour.

Worked example

Explain why magnesium carbonate decomposes at a lower temperature than barium carbonate.

1. Mg2+ is much smaller than Ba2+ but has the same charge.
2. So Mg2+ has a higher charge density and polarises the carbonate ion more.
3. This weakens the C-O bonds in the carbonate ion.

Answer: MgCO3 decomposes more easily.

Check yourself

1. Which equation shows the decomposition of calcium nitrate?
   • Ca(NO3)2 -> Ca(NO2)2 + O2
   • Ca(NO3)2 -> CaO + N2 + O2
   • 2Ca(NO3)2 -> 2CaO + 4NO2 + O2
   • Ca(NO3)2 -> Ca + 2NO2 + O2
   Show answer

   2Ca(NO3)2 -> 2CaO + 4NO2 + O2. Group 2 nitrates give the oxide, nitrogen dioxide and oxygen.

2. What forms when sodium nitrate is heated?
   • Sodium nitrite and oxygen
   • Sodium oxide, NO2 and oxygen
   • Sodium and nitrogen
   • No reaction
   Show answer

   Sodium nitrite and oxygen. 2NaNO3 -> 2NaNO2 + O2.

3. Which carbonate is the most thermally stable?
   • MgCO3
   • CaCO3
   • BaCO3
   • SrCO3
   Show answer

   BaCO3. The largest cation polarises the carbonate ion least.

4. A compound gives a lilac flame. Which ion is present?
   • Na+
   • K+
   • Ca2+
   • Ba2+
   Show answer

   K+. Potassium gives lilac.

5. What flame colour does barium give?
   • Brick red
   • Lilac
   • Crimson
   • Pale green
   Show answer

   Pale green. Ba2+ gives a pale (apple) green flame.

6. What causes a flame colour?
   • The metal burning in oxygen
   • Excited electrons falling back to lower energy levels and emitting light
   • Electrons being removed from the atom completely
   • The nucleus emitting radiation
   Show answer

   Excited electrons falling back to lower energy levels and emitting light. Specific energy gaps give light of specific frequencies.

7. Why does lithium carbonate decompose on heating when the other Group 1 carbonates do not?
   • Lithium has a 2+ charge
   • Lithium carbonate is a covalent liquid
   • Lithium is the most reactive Group 1 metal
   • Li+ is very small, so it strongly polarises the carbonate ion
   Show answer

   Li+ is very small, so it strongly polarises the carbonate ion. Li+ has a high charge density for a 1+ ion.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.