Predict Group 2 thermal stability from decomposition data
Compare nitrate and carbonate heating evidence using the shared Group 2 stability trend.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Group trends are qualitative unless data are supplied. Show physical states and balance equations; distinguish reaction from dissolution and precipitation. This sequence does not replace supervised practical experience. Written practice is self-checked
Before you begin
- Use Group 2 formulas, ionic charges, state symbols, balanced equations and acid reaction patterns.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How does thermal stability change for Group 2 nitrates and carbonates down the group?
What you will learn
- Compare nitrate and carbonate thermal stabilities
- Interpret qualitative heating observations
- Separate AS trend predictions from A Level explanations
Both decomposition series become more stable down Group 2
For both carbonates and nitrates, thermal stability increases Mg → Ca → Sr → Ba. A lower-group compound decomposes more readily; a higher-group compound needs stronger heating under comparable conditions.
Carbonates produce MO + CO₂; nitrates produce MO + NO₂ + O₂. The products differ, but the direction of the stability trend is the same.
Use controlled comparisons and stay within the syllabus level
A meaningful comparison keeps sample size, heating method and observation time similar. If one sample starts decomposing later, that supports greater thermal stability; a single uncontrolled demonstration is weaker evidence.
At AS, state and use the trend. The later A Level Topic 27 asks for a qualitative explanation using cation size and polarisation of the large anion. Do not pass that explanation off as an AS outcome.
Pause and explain
Which products identify a heated Group 2 carbonate?
Worked example
In comparable tests, BaCO₃ remains unchanged longer than MgCO₃. State the inference and give the carbonate equation.
Show the worked solution
- Barium is lower in Group 2 than magnesium.
- Carbonate thermal stability increases down the group.
- Decomposition forms the metal oxide and carbon dioxide.
Answer BaCO₃ is more thermally stable than MgCO₃. BaCO₃(s) → BaO(s) + CO₂(g).
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Rank MgCO₃, CaCO₃ and BaCO₃ in increasing thermal stability.
Give me a hint
Move from the top to the bottom of Group 2.
Compare my reasoning
- The group order is Mg, Ca, then Ba.
- Carbonate stability increases down the group.
- Write the compounds in that same increasing order.
MgCO₃ < CaCO₃ < BaCO₃ in thermal stability.
Look for these in your work
- Order follows group position
- Trend direction correct
- Property named
A nitrate decomposes at a lower temperature than a second Group 2 nitrate under the same apparatus conditions. What does this suggest about their relative positions?
Give me a hint
The less stable nitrate is expected higher in the group.
Compare my reasoning
- Thermal stability increases down Group 2.
- The nitrate that decomposes earlier is less stable.
- It is likely higher in the group than the more stable sample.
The nitrate that decomposes earlier is likely higher in Group 2 than the more thermally stable nitrate.
Look for these in your work
- Observation interpreted
- Trend applied
- Inference qualified
A learner explains the AS trend by saying the syllabus requires a detailed polarisation mechanism. Correct the syllabus-level claim.
Give me a hint
Separate the AS learning outcome from the later A Level Topic 27 outcome.
Compare my reasoning
- AS Topic 10 requires equations and the trend in thermal stability.
- A Level Topic 27 adds a qualitative explanation involving ionic radius and polarisation.
- A response should match the exam level and question wording.
AS Topic 10 requires describing and predicting the stability trend; the detailed ionic-polarisation explanation is developed in A Level Topic 27.
Look for these in your work
- AS scope identified
- A Level extension separated
- Exam-level boundary made clear
Common mistakes
- Reversing the down-group trend
- Treating different sample sizes as a controlled comparison
- Mixing nitrate and carbonate gas products
- Requiring the A Level polarization mechanism for an AS trend question
What can you explain now?
How does thermal stability change for Group 2 nitrates and carbonates down the group?
Compare with the explanation
It increases
Both series become more thermally stable from Mg towards Ba.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What is the shared thermal-stability direction for Group 2 carbonates and nitrates, and what extra mechanism is reserved for A Level?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.