Excess Alkali Mystery

Why it matters

A white precipitate of zinc hydroxide does not dissolve when only a little sodium hydroxide is added, but vanishes completely when you add excess sodium hydroxide.

The idea in plain words

Map the atoms: show how Zn(OH)₂ and NaOH rearrange to form Na₂ZnO₂ and water.

Predict first

Why does the white Zn(OH)₂ precipitate dissolve in excess NaOH?

Zinc hydroxide is amphoteric: it acts as an acid towards strong bases like NaOH, forming soluble sodium zincate.

• It is an amphoteric hydroxide reacting to form a soluble salt — correct
• The heat from mixing melts the solid
• NaOH is a solvent that dissolves all solids
• The precipitate evaporates into the solution

What you do

Add excess NaOH to both Zn(OH)₂ and Pb(OH)₂. Observe which precipitates dissolve and note the clear solutions formed.

Check yourself

What is the chemical formula of the soluble salt formed when Zn(OH)₂ dissolves in excess NaOH?

The reaction Zn(OH)₂ + 2NaOH → Na₂ZnO₂ + 2H₂O produces sodium zincate (Na₂ZnO₂).

Which condition is strictly required for Pb(OH)₂ to dissolve in NaOH according to the reaction data?

The reaction data for Pb(OH)₂ + 2NaOH → Na₂PbO₂ + 2H₂O specifies 'conc': 'excess' as a condition.

Why does Zn(OH)₂ dissolve when excess NaOH is added?

Zn(OH)₂ is amphoteric: with excess strong alkali it behaves as an acid, reacting with NaOH to form the soluble salt sodium zincate, Na₂ZnO₂.

• Na₂ZnO₂ — correct
• ZnNaO₂
• NaZn(OH)₄
• Zn(OH)₄
• Excess concentration of NaOH — correct
• Heating the mixture
• Presence of light
• Electrolysis
• It is an amphoteric hydroxide reacting with excess alkali to form a soluble salt (sodium zincate) — correct
• The strong alkali simply melts the solid precipitate
• NaOH acts as a universal solvent that dissolves every precipitate
• The precipitate reacts with water to evaporate away