A-level Chemistry: Test for Ions

• Dilute sodium hydroxide

• Dilute sulfuric acid

1. Add 10 drops of barium chloride in test tube

2. Add 10 drops of sodium hydroxide in test tube & mix well

3. Add sodium hydroxide, dropwise, until in excess
   

   • Test tube shouldn’t be more than half-way full

4. Repeat this test with calcium bromide, magnesium chloride and strontium chloride

1. Place about 10 drops of barium chloride in test tube

2. Add 10 drops of sulfuric acid in test tube & mix well

3. Add sulfuric acid, dropwise, until in excess
   

   • Test tube shouldn’t be more than half-way full

4. Repeat this test with calcium bromide, magnesium chloride and strontium chloride

• Ammonia gas is alkaline

• Can test for it using damp piece of red litmus paper

• If ammonia present = paper turns blue

So ammonia gas can dissolve

ammonia gas and water

OH⁻ + NH₄⁺ → NH₃ + H₂O

1. Add dilute sodium hydroxide solution to unknown substance in test tube & gently heat mixture

2. If ammonia given off = ammonium ions must be present

Ba²⁺ + SO₄²⁻ → BaSO₄ (s)

alkaline

1. Dip piece of red litmus paper into solution

2. If hydroxide ions present = paper turns blue

1. 
   Add dilute hydrochloric acid = solution with carbonate ions will fizz

2. Bubble gas (CO2) through test tube of limewater and watch what happens

3. Limewater = cloudy = carbonate ions in solution

∵ carbonate ions react with hydrogen ions in acid to give carbon dioxide

CO₃²⁻ + 2H⁺ → CO₂ + H₂O

1. Add small amount of solid potassium chloride in test tube

2. Add few (2 to 5) drops of concentrated sulfuric acid

3. Test gas produced with moist blue litmus paper
   

   • Make sure litmus paper doesn’t touch side of tube

4. Repeat experiment with solid potassium bromide
   

   • Test gas produced using strip of filter paper dipped in acidified potassium dichromate solution

5. Repeat experiment with solid potassium iodide
   

   • Test gas produced using strip of filter paper dipped in lead nitrate solution

• Gloves must be worn

• Must be done in fume cupboard

When potassium chloride (KCl) reacts with concentrated sulfuric acid (H₂SO₄):

• Reaction: 2KCl+H2SO4→2HCl(g)+K2SO42KCl + H₂SO₄ → 2HCl (g) + K₂SO₄

• Observation: Steamy white fumes of hydrogen chloride (HCl) gas are produced.

• Paper result: When moist blue litmus paper is held near the mouth of the test tube, it turns red, confirming the presence of an acidic gas (HCl).

When potassium bromide (KBr) reacts with concentrated sulfuric acid (H₂SO₄):

• Reaction:

  1. Initial reaction: KBr+H2SO4→KHSO4+HBrKBr + H₂SO₄ → KHSO₄ + HBr

  2. Oxidation of HBr: 2HBr+H2SO4→Br2+SO2+2H2O2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O

• Observations:

  • Steamy white fumes of hydrogen bromide (HBr).

  • Red-brown vapors of bromine (Br₂) due to oxidation of HBr.

• Paper result:

  • Moist blue litmus paper turns red (acidic HBr).

  • Potassium dichromate paper turns green, indicating sulfur dioxide (SO₂) is formed

When potassium iodide (KI) reacts with concentrated sulfuric acid (H₂SO₄):

• Reaction:

8KI+9H2SO4→4I2+8KHSO4+H2S+4H2O8KI + 9H₂SO₄ → 4I₂ + 8KHSO₄ + H₂S + 4H₂O

• Observations:

  • Steamy white fumes of hydrogen iodide (HI)

  • Purple vapors of iodine (I₂)

  • Rotten egg smell from hydrogen sulfide (H₂S)

• Paper result:

  • Moist blue litmus paper turns red (due to acidic HI)

  • Potassium dichromate paper turns green (indicating presence of H₂S)

This reaction is more vigorous than with other halides because iodide ions are strong enough to reduce sulfuric acid to hydrogen sulfide.

1. Add dilute hydrochloric acid and then barium chloride solution, BaCl2(aq)

2. If white precipitate of barium sulfate forms = original compound contained sulfate

To get rid of any lurking sulfites or carbonates which will also produce a white precipitate

1. Add dilute nitric acid (HNO3) and then silver nitrate solution (AgNO3)

2. Precipitate is formed (of silver halide)

No precipitate

White precipitate of silver chloride

Cream precipitate of silver bromide

Yellow precipitate of silver iodide

• You can test your results by adding ammonia solution

• Each silver halide has a different solubility in ammonia

Here’s the general equation for the test for halide ions using silver nitrate:

Halide−+AgNO3→AgHalide(s)+NO3−\text{Halide}^- + AgNO₃ → Ag\text{Halide} (s) + NO₃^-

Where:

• Cl⁻ forms AgCl (white precipitate)

• Br⁻ forms AgBr (cream precipitate)

• I⁻ forms AgI (yellow precipitate)

• Module 1: Completed

• ⏳ Module 2: Incomplete — needs attention

• ✅ Module 3: Completed

To “fill in the gaps,” you’ll want to focus on finishing Module 2. Let me know if you’d like help reviewing or summarizing its content—I can make it quicker and easier to wrap up.

To remove ions which may interfere with test