Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 8.18 Thermal decomposition and flame tests-Study Notes - New Syllabus

8.18 (i) Experimental Procedures: Thermal Decomposition of Group 1 & Group 2 Compounds

These experiments are used to observe patterns in thermal stability by heating nitrates and carbonates and identifying the gases produced.

(A) Thermal Decomposition of Carbonates

Procedure

• Place solid carbonate (e.g. \( \mathrm{CaCO_3} \)) in a test tube.
• Heat strongly using a Bunsen burner.
• Pass gas produced into limewater.

Observation

• Limewater turns milky/cloudy.

Conclusion

• Gas produced is \( \mathrm{CO_2} \).

\( \mathrm{MCO_3 \rightarrow MO + CO_2} \)

(B) Thermal Decomposition of Nitrates

Procedure

• Place metal nitrate (e.g. \( \mathrm{Mg(NO_3)_2} \)) in a test tube.
• Heat gently then strongly.
• Test gases evolved.

Observations & Tests

Oxygen (\( \mathrm{O_2} \)):

• • Glowing splint relights.

Nitrogen dioxide (\( \mathrm{NO_2} \)):

• • Brown gas.
  • Turns damp indicator paper acidic (red).

Equations

Group 2 Nitrates

\( \mathrm{2M(NO_3)_2 \rightarrow 2MO + 4NO_2 + O_2} \)

Group 1 (except Li)

\( \mathrm{2MNO_3 \rightarrow 2MNO_2 + O_2} \)

Patterns Observed

• Down the group → more heat required.
• Shows increasing thermal stability.

Key Tests to Remember 

• \( \mathrm{CO_2} \) → limewater turns milky
• \( \mathrm{O_2} \) → glowing splint relights
• \( \mathrm{NO_2} \) → brown gas + acidic

Summary

• Heating carbonates produces \( \mathrm{CO_2} \).
• Heating nitrates produces \( \mathrm{O_2} \) and \( \mathrm{NO_2} \) (Group 2).
• Observations confirm decomposition patterns and trends.

Therefore, these experiments provide evidence for thermal stability trends in Groups 1 and 2.

8.18 (ii) Experimental Procedure: Flame Colours of Group 1 and Group 2 Compounds

Flame tests are used to identify metal ions based on the characteristic colour they produce when heated.

Apparatus

• Bunsen burner
• Platinum / nichrome wire loop
• Dilute \( \mathrm{HCl} \)
• Metal salt (solid or solution)

Procedure

• Clean the wire loop by dipping it in dilute \( \mathrm{HCl} \).
• Place it in the Bunsen flame until no colour is seen.
• Dip the clean loop into the sample.
• Place the loop in the blue (non-luminous) flame.
• Observe and record the flame colour.

Observations (Colours)

• \( \mathrm{Li^+} \) → crimson red
• \( \mathrm{Na^+} \) → yellow
• \( \mathrm{K^+} \) → lilac
• \( \mathrm{Ca^{2+}} \) → brick red
• \( \mathrm{Sr^{2+}} \) → red
• \( \mathrm{Ba^{2+}} \) → apple green

Explanation

• Heat excites electrons to higher energy levels.
• Electrons return to lower levels.
• Energy released as light of specific wavelength.

Precautions

• Always clean wire thoroughly to avoid contamination.
• Use blue flame (non-luminous) for clear observation.
• Sodium contamination may mask other colours.

Summary

• Flame tests identify metal ions by colour.
• Caused by electron transitions.
• Simple qualitative analysis method.

Therefore, flame tests provide quick identification of Group 1 and Group 2 ions.