Question

Q4. Level 1 (Unit 18, L.G. 1) Which of the following is the correct dissociation reaction for calcium hydroxide in water? A. CaOH → Cat + OH B. CaOH → Cat + O2- + H+ C. Ca(OH)2 → Ca2+ + 2OH- D. Ca(OH)2 → Ca2+ + 202- + 2H+ E. Ca(OH)2 → Ca2+ + (OH-)2

Answer 1

Many ionic compounds have the tendency to dissociate into their constituent ions when dissolved in water. This happens when the energy released in the reaction by the interaction of the ionic compound with water is enough to break the ionic bonds in the ionic compound and can also separate the water molecules in the solution. So when ionic bonds between the constituent ions of an ionic compound break , the ions exist in the solution in dissociated forms.

For example : Sodium hydroxide ( NaOH ) dissociates in water as

NaOH_{​​​ (aq)} → Na⁺_(aq) + OH^{​​​​-}_​(aq)

Thus NaOH dissociates in water into its constituent ions , Na+ and OH- .

Note : (aq) in the above chemical reaction represents the aqueous phase which means solution of water .

Now in the given question, we have calcium hydroxide as the ionic compound.

The correct formula of calcium hydroxide is Ca(OH)₂ . So clearly options A and B are incorrect as they represent the wrong formula of calcium hydroxide.

Now calcium hydroxide is composed of calcium ion (Ca^{​​​​​2+} ) and hydroxide ions (OH^{​​​​​-} ). One molecule of calcium hydroxide is composed of 1 calcium ion and 2 hydroxide ions as is clear by looking at its formula Ca(OH)₂ . So calcium hydroxide will dissociate into its constituent ions , calcium and hydroxide ions when dissolved in water as : Ca(OH)₂ → Ca^{​​​​​2+} + 2OH^-

(The two OH^{​​​​​-}ions exist as separate ions in the solution after dissociation of calcium hydroxide and not as a combined entity as shown in option E as (OH^{​​​​​-}​)₂ , so option E is incorrect. Also D is also an incorrect option as hydroxide ion does not split further into oxide ion and hydrogen ion )

So option C is the correct answer.