Describe tests using aqueous sodium hydroxide and aqueous ammonia to identify the aqueous cations: (a) aluminium, $mathrm{Al}^{3+}$ (b) ammonium, $mathrm{NH}_4{ }^{+}$ (c) calcium, $mathrm{Ca}^{2+}$ (d) chromium(III), $mathrm{Cr}^{3+}$ (e) copper(II)

Experimental Techniques and Chemical Analysis – Identification of Ions and Gases

🔬 In this lesson we will learn how to identify common cations using aqueous sodium hydroxide (NaOH) and aqueous ammonia (NH₃). These simple tests are like detective clues that help us figure out which ion is hiding in a solution. Let’s dive in! 💡

(a) Aluminium, \$\mathrm{Al^{3+}}\$

• NaOH: Add a few drops of 1 M NaOH → white precipitate of \$\mathrm{Al(OH)_3}\$ (insoluble).
• NH₃: Add a few drops of 1 M NH₃ → white precipitate of \$\mathrm{Al(OH)_3}\$ (does not dissolve in excess NH₃).
• Colour: White. 🔍

(b) Ammonium, \$\mathrm{NH_4^+}\$

• NaOH: No precipitate. Add a little acid (e.g. HCl) → smell of ammonia gas (NH₃).
• NH₃: No precipitate. Add acid → smell of NH₃.
• Colour: No visible colour change. 🚫

(c) Calcium, \$\mathrm{Ca^{2+}}\$

• NaOH: White precipitate of \$\mathrm{Ca(OH)_2}\$ (slightly soluble, dissolves in excess acid).
• NH₃: No precipitate.
• Colour: White. ⚪️

(d) Chromium(III), \$\mathrm{Cr^{3+}}\$

• NaOH: Green precipitate of \$\mathrm{Cr(OH)_3}\$.
• NH₃: Green precipitate of \$\mathrm{Cr(OH)_3}\$.
• Colour: Green. 🌿

(e) Copper(II), \$\mathrm{Cu^{2+}}\$

• NaOH: Blue precipitate of \$\mathrm{Cu(OH)_2}\$.
• NH₃: Deep blue solution of tetraamminecopper(II) complex \$\mathrm{[Cu(NH3)4]^{2+}}\$.
• Colour: Blue (NaOH) → Deep blue (NH₃). 🔵

(f) Iron(II), \$\mathrm{Fe^{2+}}\$

• NaOH: Pale green precipitate of \$\mathrm{Fe(OH)_2}\$.
• NH₃: Pale green precipitate of \$\mathrm{Fe(OH)_2}\$.
• Colour: Pale green. 🟢

(g) Iron(III), \$\mathrm{Fe^{3+}}\$

• NaOH: Orange‑red precipitate of \$\mathrm{Fe(OH)_3}\$.
• NH₃: Yellow precipitate of \$\mathrm{Fe(OH)_3}\$.
• Colour: Orange‑red (NaOH) → Yellow (NH₃). 🧡🟡

(h) Zinc, \$\mathrm{Zn^{2+}}\$

• NaOH: White precipitate of \$\mathrm{Zn(OH)_2}\$.
• NH₃: White precipitate of \$\mathrm{Zn(OH)_2}\$.
• Colour: White. ⚪️

Exam Tips &️ Quick Reference

• Remember the colour of the precipitate: Cu²⁺ → blue, Fe³⁺ → orange‑red (NaOH) or yellow (NH₃), Fe²⁺ → pale green, Cr³⁺ → green, Al³⁺, Ca²⁺, Zn²⁺ → white, NH₄⁺ → no precipitate, smell NH₃ when acidified.
• Use a small drop of reagent (NaOH or NH₃) and observe colour change within seconds.
• Always add acid after the test if you want to confirm NH₄⁺ by smelling NH₃.
• Keep a neat table of observations to avoid confusion during the exam.