Predict Group 2 compound reactions from a property table
Combine reaction, decomposition and solubility trends to make evidence-based predictions for Group 2 compounds.
Section 10 · Group 2
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
Which Group 2 compound trend is opposite to hydroxide solubility?
What you will learn
- Select the relevant trend from observations
- Predict products and qualitative observations
- State the limits of a prediction from incomplete data
Translate observations into a Group 2 prediction
Start with the known sequence Mg → Ca → Sr → Ba. Ask what is being compared: metal reactivity, reaction products, thermal stability, hydroxide solubility or sulfate solubility. Each property has its own direction.
Reactivity and thermal stability increase down the group. Hydroxide solubility increases; sulfate solubility decreases. Use the correct trend for the observation rather than treating all properties as one general increase.
Support deductions with a formula and an evidence limit
A useful answer names the property, applies the group order and gives a chemical reason or balanced equation. For example, a vigorous water reaction suggests a lower Group 2 metal than a less vigorous one, if conditions are comparable.
A single colorless solution, white solid or reaction rate rarely identifies a unique metal. Consider concentration, surface area, temperature, sample purity and possible precipitate coatings. State the strongest conclusion supported by all observations.
Pause and explain
An unknown carbonate is more thermally stable than CaCO₃. What is the best trend-based prediction?
Worked example
An unknown Group 2 carbonate decomposes less readily than CaCO₃, and its sulfate is less soluble than CaSO₄. What position is suggested?
Show the worked solution
- Greater carbonate thermal stability suggests a lower position than calcium.
- Lower sulfate solubility also increases down Group 2.
- Both clues support a position below calcium, but do not identify the exact element alone.
Answer The evidence suggests a position below calcium in Group 2; the clues support a trend-based position, not a unique identity without more data.
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.
A Group 2 metal reacts with water more vigorously than calcium. Predict its likely position and write the general cold-water equation.
Give me a hint
Reactivity increases down the group; use the hydroxide and hydrogen products.
Compare my reasoning
- The metal is likely below calcium.
- Cold-water reaction forms M(OH)₂ and H₂.
- Balance as M + 2H₂O → M(OH)₂ + H₂.
It is likely Sr or Ba, below Ca. M(s) + 2H₂O(l) → M(OH)₂ + H₂(g).
Look for these in your work
- Position trend applied
- Products correct
- General equation balanced
A Group 2 hydroxide sample dissolves more than Ca(OH)₂, while its sulfate dissolves less than CaSO₄. Which direction is supported by each observation?
Give me a hint
Use the two opposite solubility trends independently.
Compare my reasoning
- Greater hydroxide solubility suggests a position below calcium.
- Lower sulfate solubility also suggests a position below calcium.
- The two observations agree, but do not uniquely identify Sr or Ba.
Both observations suggest a position below calcium: hydroxide solubility increases while sulfate solubility decreases down the group.
Look for these in your work
- Hydroxide direction correct
- Sulfate direction correct
- Conclusion appropriately qualified
A property table compares Mg and Ba but gives different temperatures and sample sizes for their water reactions. What can and cannot be concluded about relative reactivity?
Give me a hint
Check whether the comparison controls the conditions before using reaction vigor as evidence.
Compare my reasoning
- The group trend predicts Ba is generally more reactive than Mg.
- Different temperature and sample size make the observed rates an uncontrolled comparison.
- Use the trend as a prediction, not as a rate measurement from this table.
Ba is expected to be more reactive from its lower Group 2 position, but the table's different conditions do not provide a fair quantitative rate comparison.
Look for these in your work
- Trend-based prediction stated
- Confounders identified
- Evidence strength qualified
Common mistakes
- Using hydroxide solubility to predict sulfate behavior
- Ignoring whether a comparison used equal conditions
- Claiming a unique element from one qualitative observation
- Writing an unbalanced equation after predicting the correct product
What can you explain now?
Which Group 2 compound trend is opposite to hydroxide solubility?
Compare with the explanation
Sulfate solubility decreases down the group
Hydroxide solubility rises while sulfate solubility falls.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do you choose the correct Group 2 trend, balance the predicted reaction, and avoid claiming more than the evidence supports?
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.