Thermal stability and flame coloursEdexcel International A Level Chemistry: Revision notes
Section 1
Flame colours and electron transitions
In a flame test a nichrome wire is cleaned in concentrated hydrochloric acid, dipped in the sample and held in the edge of a blue Bunsen flame.
The heat promotes electrons to higher energy levels (excitation). When they fall back they emit energy as light, with ΔE = hν. Each metal has a different set of energy gaps, so it emits a characteristic frequency and colour.
- Lithium: red
- Sodium: yellow
- Potassium: lilac
- Calcium: brick-red
- Strontium: red (crimson)
- Barium: apple green
- Magnesium: no colour
Do not say electrons are 'lost' or 'burn'. They are promoted to higher levels, then fall back and emit light.
Section 2
Thermal decomposition of Group 1 and 2 carbonates
Group 2 carbonates decompose on heating to the oxide and carbon dioxide: MCO₃ → MO + CO₂. Group 1 carbonates (except lithium carbonate) do not decompose at Bunsen temperatures.
- Li₂CO₃ → Li₂O + CO₂
- MgCO₃ → MgO + CO₂
The thermal stability of the carbonates increases down both groups: magnesium carbonate decomposes at a lower temperature than barium carbonate.
Section 3
Thermal decomposition of Group 1 and 2 nitrates
Group 2 nitrates and lithium nitrate give the oxide, nitrogen dioxide and oxygen:
- 2Mg(NO₃)₂ → 2MgO + 4NO₂ + O₂
- 4LiNO₃ → 2Li₂O + 4NO₂ + O₂
The other Group 1 nitrates give the nitrite and oxygen:
- 2NaNO₃ → 2NaNO₂ + O₂
Stability again increases down each group, so Ba(NO₃)₂ needs a higher temperature than Mg(NO₃)₂.
Brown fumes of NO₂ mean a Group 2 nitrate (or lithium nitrate). Only oxygen means a Group 1 nitrate other than lithium.
Section 4
Explaining the trends: polarisation
A small cation with a high charge has a high charge density. It polarises (distorts) the electron cloud of a nearby anion and weakens the bonds within it, so the compound decomposes more easily.
- Down a group the cation radius increases, so the charge density and polarising power fall, and thermal stability increases.
- Group 2 ions (2+) are smaller than the Group 1 ions in the same period (1+), so they polarise more and their carbonates and nitrates are less stable to heat.
- Li⁺ is the smallest Group 1 ion, so lithium compounds behave like those of Group 2.
Section 5
Tests for carbon dioxide, oxygen and nitrogen dioxide
- Carbon dioxide: bubble through limewater, which turns milky (cloudy): Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.
- Oxygen: relights a glowing splint.
- Nitrogen dioxide: a brown gas that is acidic and turns damp blue litmus paper red.
Method for decomposition: heat a small amount of the solid in a dry hard-glass test tube, pass the gas into limewater or hold a splint or litmus paper in the mouth of the tube. Remove the delivery tube from the limewater before stopping heating, to avoid suck-back.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Thermal stability and flame colours
- A technician identifies the metal in unlabelled solid samples by flame tests. She dips a clean nichrome wire into each sample, holds it in the edge of a blue Bunsen flame and records the colour. The samples contain compounds of lithium, potassium, calcium and barium.Explain, in terms of electrons, how the heated metal ions produce coloured light.2 marks
- Samples of magnesium carbonate and barium carbonate are heated strongly in turn in a hard-glass test tube under identical conditions, and any gas given off is passed through limewater. Magnesium carbonate decomposes at a much lower temperature than barium carbonate.Explain why barium carbonate decomposes at a higher temperature than magnesium carbonate.2 marks
- A student heats small samples of sodium nitrate and calcium nitrate in separate dry test tubes. Both tubes give off a colourless gas that relights a glowing splint, but only one of them also gives off a brown gas.Write equations for the thermal decomposition of sodium nitrate and of calcium nitrate. State which tube gives the brown gas, naming it and giving its effect on damp blue litmus paper.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).