Describe Group 2 hydroxides in water and acids
Distinguish hydroxide dissolution from neutralisation and balance reactions with dilute acids.
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 Group 2 hydroxide solubility change from Mg(OH)₂ to Ba(OH)₂?
What you will learn
- Describe the solubility change of Group 2 hydroxides
- Represent dissolution as ions in water
- Balance hydroxide neutralisation equations
Hydroxides dissolve to different extents down the group
Group 2 hydroxide solubility increases from Mg(OH)₂ towards Ba(OH)₂. Magnesium hydroxide is sparingly soluble; barium hydroxide is much more soluble. When a hydroxide dissolves, the ions separate: M(OH)₂(s) ⇌ M²⁺(aq) + 2OH⁻(aq). This is dissolution, not a new neutralisation reaction.
Hydroxide solutions are alkaline because they contain OH⁻ ions. The amount dissolved depends on substance and conditions; do not assign an exact concentration or pH without data.
Hydrochloric and sulfuric acids neutralise hydroxides
The general hydrochloric-acid equation is M(OH)₂ + 2HCl → MCl₂ + 2H₂O. With sulfuric acid it is M(OH)₂ + H₂SO₄ → MSO₄ + 2H₂O.
Balance the two hydroxide groups against two H⁺ equivalents. With sulfuric acid, a low-solubility sulfate may precipitate as water forms. Do not mistake that precipitation for a failure of acid–base neutralisation.
Pause and explain
How many moles of HCl neutralise one mole of M(OH)₂?
Worked example
Write the balanced neutralisation equation for barium hydroxide with hydrochloric acid.
Show the worked solution
- Ba(OH)₂ contains two OH groups.
- Two HCl supply two H atoms and two chloride ions.
- The salt is BaCl₂ and the two H/OH pairs form two waters.
Answer Ba(OH)₂ + 2HCl → BaCl₂ + 2H₂O.
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.
Write the dissolution equilibrium for magnesium hydroxide and explain why a suspension can remain.
Give me a hint
Show separated ions and use the low solubility of Mg(OH)₂.
Compare my reasoning
- Dissolution gives Mg²⁺ and two OH⁻ ions.
- Write Mg(OH)₂(s) ⇌ Mg²⁺(aq) + 2OH⁻(aq).
- Only a limited amount dissolves, so solid can remain.
Mg(OH)₂(s) ⇌ Mg²⁺(aq) + 2OH⁻(aq); its low solubility means undissolved solid may remain.
Look for these in your work
- Ions and charges correct
- Dissolution arrow used
- Low solubility interpreted
Balance the equation for calcium hydroxide with sulfuric acid.
Give me a hint
Use CaSO₄ as the salt and form water from two OH groups.
Compare my reasoning
- Ca(OH)₂ + H₂SO₄ forms CaSO₄ and water.
- Two H₂O molecules balance the four H and two O from the hydroxide groups.
- Check Ca, S, H and O totals.
Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O.
Look for these in your work
- Correct sulfate formula
- Two waters included
- Atoms balanced
Compare a sample of Mg(OH)₂ suspension with Ba(OH)₂ solution. Predict which can supply more OH⁻ to the water if equal amounts are added, and qualify the answer.
Give me a hint
Use the down-group hydroxide solubility trend, not a numerical pH assumption.
Compare my reasoning
- Hydroxide solubility increases down the group.
- Ba(OH)₂ is more soluble than Mg(OH)₂.
- At comparable amounts and conditions, more barium hydroxide can dissolve, but exact pH requires concentration and volume.
Ba(OH)₂ can supply more dissolved OH⁻ under comparable conditions because it is more soluble; exact pH depends on amounts and solution volume.
Look for these in your work
- Solubility trend correct
- OH⁻ linked to dissolution
- Concentration limit stated
Common mistakes
- Treating hydroxide dissolution as acid neutralisation
- Using only one HCl per M(OH)₂
- Saying hydroxide solubility decreases down Group 2
- Assigning a precise pH without concentration
What can you explain now?
How does Group 2 hydroxide solubility change from Mg(OH)₂ to Ba(OH)₂?
Compare with the explanation
It generally increases
The hydroxides become more soluble down the group.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
What is the difference between hydroxide dissolving in water and hydroxide neutralising an acid?
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.