Compare Group 2 hydroxide solubilities
State and apply the increase in hydroxide solubility down Group 2.
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 is more soluble in water in the Group 2 hydroxide trend: Mg(OH)₂ or Ba(OH)₂?
What you will learn
- Order hydroxides by qualitative solubility
- Relate dissolution to alkaline solution formation
- Predict observations without inventing numerical solubilities
Hydroxides become more soluble down the group
The qualitative order is Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂ in solubility. Magnesium hydroxide is sparingly soluble, while barium hydroxide is substantially more soluble.
As M(OH)₂ dissolves it supplies M²⁺ and OH⁻ ions. More dissolved hydroxide can make the solution more alkaline at comparable amounts and volumes. Exact pH also depends on concentration and temperature; no numerical solubility data are implied.
Separate solubility from reaction rate
A more soluble hydroxide does not mean its parent oxide necessarily reacts at the same rate with water. Solubility describes how much compound dissolves at equilibrium; reaction rate describes how quickly change occurs.
When comparing test-tube observations, keep the amount of solid, water volume, temperature and time comparable. Use the supplied evidence before stating a detailed order among close observations.
Pause and explain
Which ion makes a dissolved Group 2 hydroxide solution alkaline?
Worked example
Arrange Mg(OH)₂, Ca(OH)₂ and Ba(OH)₂ in increasing solubility and state what happens to the available OH⁻ at comparable amounts.
Show the worked solution
- Hydroxide solubility increases down Group 2.
- The order is magnesium, calcium, then barium hydroxide.
- At comparable amounts and volume, the more soluble hydroxide can supply more OH⁻ ions.
Answer Mg(OH)₂ < Ca(OH)₂ < Ba(OH)₂ in solubility; a more soluble sample can supply more OH⁻ at comparable amounts and volume.
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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.
Order the four Group 2 hydroxides from least to most soluble.
Give me a hint
Follow Mg, Ca, Sr, Ba down the group.
Compare my reasoning
- Hydroxide solubility increases down Group 2.
- Place Mg(OH)₂ first and Ba(OH)₂ last.
- Keep Ca and Sr in their group order.
Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂.
Look for these in your work
- All four included
- Correct group order
- Increasing-solubility direction stated
Two equal-volume samples contain equal masses of Mg(OH)₂ and Ba(OH)₂. Which is expected to dissolve to a greater extent?
Give me a hint
Use the solubility trend; do not claim exact concentrations from mass alone.
Compare my reasoning
- Ba(OH)₂ is lower in Group 2.
- Hydroxide solubility increases down the group.
- More barium hydroxide is expected to dissolve, subject to the amounts and conditions.
Ba(OH)₂ is expected to dissolve to a greater extent; exact concentrations depend on conditions and the amount added.
Look for these in your work
- Correct compound selected
- Trend applied
- No unsupported numeric claim
A student says barium hydroxide reacts faster with water because it is more soluble. Explain why the evidence does not establish that rate claim.
Give me a hint
Compare equilibrium solubility with time-dependent reaction rate.
Compare my reasoning
- Solubility describes the amount that dissolves at equilibrium.
- Rate describes how quickly a process occurs.
- Greater solubility alone does not prove a faster reaction of the oxide with water.
Greater Ba(OH)₂ solubility does not by itself show that BaO reacts faster with water; solubility and reaction rate are different properties.
Look for these in your work
- Definitions distinguished
- Inference limit stated
- Example linked to Group 2
Common mistakes
- Using the sulfate trend for hydroxides
- Equating solubility with reaction speed
- Claiming equal pH for equal masses without a volume or concentration
- Assigning exact solubility values not provided
What can you explain now?
Which is more soluble in water in the Group 2 hydroxide trend: Mg(OH)₂ or Ba(OH)₂?
Compare with the explanation
Ba(OH)₂
Hydroxide solubility increases down the group.
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Make it stick
Which direction do Group 2 hydroxide solubilities change, and how is that different from reaction rate?
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.