Tests of Ions and Gases

Question 1

What is the only alkaline gas?

Answer 1

Ammonia (NH₃), which dissolves in water to form ammonium hydroxide (NH₄OH)

Question 2

What gases are acidic?

Answer 2

• Chlorine: Cl₂ + H₂O → HCl + HOCl
• Sulfur dioxide: SO₂ + H₂O → H₂SO₃
• Nitrogen dioxide: NO₂ + H₂O → HNO₃

Question 3

What are neutral gases?

Answer 3

H₂, O₂, He, and other non-metal gases that do not react with water

Question 4

What happens when a non-metal oxide gas dissolves in water?

Answer 4

It forms an acid

Question 5

Test for Hydrogen (H₂)

Answer 5

• Test: Approach with a lit splint
• Result: Burns with a “pop” sound

Question 6

Test for Oxygen (O₂)

Answer 6

• Test: Insert a glowing splint
• Result: Splint relights

Question 7

Test for Chlorine (Cl₂):

Answer 7

• Test: Pass gas through damp blue litmus paper
• Result: Litmus paper turns red, then bleaches
• Explanation: Cl₂ + H₂O forms HCl (turns red) and HOCl (bleaches)

Question 8

Test for Carbon dioxide (CO₂):

Answer 8

• Test: Bubble gas through limewater
• Result: Limewater turns cloudy
• Explanation: Ca(OH)₂ + H₂O + CO₂ → CaCO₃ (white solid) + H₂O

Observation when excess CO₂ is added:
* Turns cloudy (CaCO₃ forms)
* Turns clear again (CaCO₃ dissolves to form Ca(HCO₃)₂)

Question 9

Test for Water/Steam (H₂O)

Answer 9

• Test: Add anhydrous copper sulfate (white)
• Result: Turns blue (becomes hydrated)

Question 10

Test for Ammonia (NH₃)

Answer 10

• Glass Tube Test: HCl + NH₃ → NH₄Cl (forms white gas)
• Litmus Paper Test: Turns damp red litmus paper blue

Question 11

How is a flame test carried out?

Answer 11

1. Wash platinum wire in HCl (to avoid contamination)
2. Dip platinum wire into sample
3. Insert into a non-luminous (clear) flame
4. Observe flame color

Question 12

Why use platinum wire for flame test?

Answer 12

• It is unreactive
• It has a high melting point

Question 13

What are the colours of the flames for metals?

Answer 13

• Lithium (Li⁺): Red
• Potassium (K⁺): Lilac
• Sodium (Na⁺): Yellow
• Copper (Cu²⁺): Blue-green
• Calcium (Ca²⁺): Brick-red

Question 14

What are the results for sodium hydroixide test?

Answer 14

• Iron (II) Fe²⁺: Green ppt
• Iron (III) Fe³⁺: Brown ppt
• Copper (II) Cu²⁺: Blue ppt
• Ammonium (NH₄⁺): No precipitate, but produces NH₃ gas when warmed (turns damp red litmus blue)

Question 15

Carbonate (CO₃²⁻) Test:

Answer 15

• Test: Add HCl
• Result: Produces CO₂ bubbles that turn limewater cloudy

Question 16

Sulfate (SO₄²⁻) Test:

Answer 16

• Test: Add HCl and barium chloride (acidified BaCl₂)
• Why add HCl? To remove carbonate impurities
• Result: White precipitate forms

Question 17

Halide (Cl⁻, Br⁻, I⁻) Test:

Answer 17

• Test: Add nitric acid (HNO₃) and silver nitrate (AgNO₃)
• Why not use HCl? Cl⁻ would interfere with the test

Halide Ion Precipitate Color (results):
* Cl⁻: White ppt
* Br⁻: Cream ppt
* I⁻: Yellow ppt