Chemical tests

Welcome to Chemical Detective Work!

Welcome to the exciting world of Chemical Tests! In this chapter, you will learn how to identify unknown substances—gases, cations, anions, and water—using specific chemical and physical tests.
This knowledge is crucial for practical laboratory investigations and exams. Let's break down each test into clear, memorable steps!

Section 1: Tests for Common Gases

Identifying gases is often the most visual part of chemical testing:

1. Testing for Oxygen (\(O_2\))

• The Test: Place a glowing splint into the gas sample.
• The Result: The splint relights.
• Explanation: Oxygen supports combustion, allowing the smouldering wood to burst back into flame.

2. Testing for Hydrogen (\(H_2\))

• The Test: Place a lit splint into the gas sample.
• The Result: You hear a distinctive squeaky pop.
• Explanation: Hydrogen burns rapidly in oxygen to produce water: \(2H_2 (g) + O_2 (g) \rightarrow 2H_2O (l)\).

3. Testing for Carbon Dioxide (\(CO_2\))

• The Test: Bubble the gas through limewater (aqueous calcium hydroxide, \(Ca(OH)_2\)).
• The Result: The limewater turns cloudy or milky.
• Explanation: Insoluble calcium carbonate precipitate forms:
  \(Ca(OH)_2 (aq) + CO_2 (g) \rightarrow CaCO_3 (s) + H_2O (l)\)

4. Testing for Ammonia (\(NH_3\))

• The Test: Hold a piece of damp red litmus paper in the gas.
• The Result: The litmus paper turns blue.
• Explanation: Ammonia is an alkaline gas that dissolves in the water on the damp paper to form hydroxide ions.

5. Testing for Chlorine (\(Cl_2\))

• The Test: Hold a piece of damp blue litmus paper in the gas.
• The Result: The litmus paper turns red and is then bleached white.
• Explanation: Chlorine is acidic in solution and acts as a powerful bleaching agent.

Section 2: Flame Tests for Cations

Flame tests identify positive metal ions based on the characteristic colour they impart to a non-luminous (roaring blue) Bunsen flame.

Method: Clean a platinum or nichrome wire by dipping it into concentrated hydrochloric acid and placing it in a hot flame. Dip the wire into the solid sample or solution and hold it in the edge of a non-luminous flame.

• Lithium (\(Li^+\)): Red flame
• Sodium (\(Na^+\)): Yellow flame
• Potassium (\(K^+\)): Lilac flame
• Calcium (\(Ca^{2+}\)): Orange-red flame
• Copper(II) (\(Cu^{2+}\)): Blue-green flame

Section 3: Precipitate Tests for Cations (Using Sodium Hydroxide)

Adding aqueous sodium hydroxide (\(NaOH\)) allows us to identify metal cations by the insoluble hydroxide precipitates they form, as well as testing for ammonium ions.

1. Transition Metal Cations

• Copper(II) (\(Cu^{2+}\)): Forms a blue precipitate of copper(II) hydroxide.
  \(Cu^{2+} (aq) + 2OH^- (aq) \rightarrow Cu(OH)_2 (s)\)
• Iron(II) (\(Fe^{2+}\)): Forms a green precipitate of iron(II) hydroxide.
  \(Fe^{2+} (aq) + 2OH^- (aq) \rightarrow Fe(OH)_2 (s)\)
• Iron(III) (\(Fe^{3+}\)): Forms a red-brown precipitate of iron(III) hydroxide.
  \(Fe^{3+} (aq) + 3OH^- (aq) \rightarrow Fe(OH)_3 (s)\)

2. Ammonium Ion (\(NH_4^+\))

• The Test: Add aqueous sodium hydroxide solution to the sample and warm gently.
• The Result: Ammonia gas is produced, which turns damp red litmus paper blue.
  \(NH_4^+ (aq) + OH^- (aq) \rightarrow NH_3 (g) + H_2O (l)\)

Section 4: Tests for Anions (Negative Ions)

1. Testing for Carbonates (\(CO_3^{2-}\))

• The Test: Add dilute hydrochloric acid to the solid or solution.
• The Result: Effervescence (fizzing) occurs. Bubble the produced gas through limewater; the limewater turns milky, confirming carbon dioxide gas.
• \(CO_3^{2-} (s\text{ or }aq) + 2H^+ (aq) \rightarrow CO_2 (g) + H_2O (l)\)

2. Testing for Halide Ions (\(Cl^-\), \(Br^-\), \(I^-\))

• Step 1: Add dilute nitric acid (\(HNO_3\)) to acidify the solution (this removes any carbonate ions that would give a false positive precipitate).
• Step 2: Add aqueous silver nitrate (\(AgNO_3\)).
• Observations:
  • Chloride (\(Cl^-\)): White precipitate of silver chloride (\(AgCl\)).
  • Bromide (\(Br^-\)): Cream precipitate of silver bromide (\(AgBr\)).
  • Iodide (\(I^-\)): Yellow precipitate of silver iodide (\(AgI\)).

3. Testing for Sulfate Ions (\(SO_4^{2-}\))

• Step 1: Add dilute hydrochloric acid (\(HCl\)) to acidify the solution (to eliminate interfering carbonate ions).
• Step 2: Add aqueous barium chloride (\(BaCl_2\)).
• The Result: A white precipitate of barium sulfate (\(BaSO_4\)) forms.
  \(Ba^{2+} (aq) + SO_4^{2-} (aq) \rightarrow BaSO_4 (s)\)

Section 5: Tests for Water

1. Chemical Test for the Presence of Water

• The Test: Add the liquid to white anhydrous copper(II) sulfate (\(CuSO_4\)).
• The Result: The white powder turns blue (forming hydrated copper(II) sulfate, \(CuSO_4 \cdot 5H_2O\)).
• Note: This confirms the presence of water, but does NOT show if the water is pure.

2. Physical Test for Water Purity

• The Test: Measure the boiling point or freezing/melting point of the sample.
• The Result: Pure water boils at exactly 100 °C and freezes at 0 °C at standard atmospheric pressure (1 atm). Any dissolved impurities raise the boiling point and depress the freezing point.