Unit 18: Qualitative Analysis

INTRODUCTION

Qualitative analysis is that in which one only tells about the nature of a substance and not its quantity. These analysis methods have different reliability. Some of them are very reliable and reach up to confirmatory tests.

18.1 DETECTION OF GASES

18.1.1 Detection of Ammonia

Ammonia is a basic gas whose pH is more than 7. It is used for making urea fertilizer. It can be manufactured by Haber's process using hydrogen and nitrogen gases.

Procedure for Detection

1. Take a red litmus paper.
2. Moist the filter paper with water spray.
3. Place the litmus paper at the mouth of the test tube or flask containing ammonia gas.
4. The red litmus paper will turn blue.

Turning the litmus paper blue confirms the presence of any basic gas like ammonia.

18.1.2 Detection of Carbon Dioxide (CO₂)

Procedure of Production and Detection

1. Take some marble pieces in a conical flask.
2. Cork it with the help of the cork with two holes.
3. From one hole, insert a thistle funnel, and from the other hole, a glass tube of U-shape.
4. Insert the other end of the tube into a test tube containing lime water (Ca(OH)₂) aqueous.
5. Pour hydrochloric acid into the thistle funnel.
6. Carbon dioxide gas formed will travel through the tube into lime water.
7. The lime water will turn milky due to the production of calcium carbonate, which is insoluble.

The reaction is:
CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

This method is a confirmatory test for Carbon dioxide.

18.1.3 Detection of Chlorine Gas

Detection Procedure

1. Prepare Damp Litmus Paper: Take a piece of blue litmus paper and dampen it with distilled water. Blue litmus paper is typically used for acidic gases like chlorine.
2. Expose to the Gas: Place the damp blue litmus paper in the test tube of chlorine gas.
3. Observe Colour Change: If chlorine gas is present, the blue litmus paper will turn red. This colour change occurs because chlorine gas is acidic and reacts with the red litmus paper, causing it to change from blue to red and then bleach it to white.

Interestingly, blue litmus after turning red turns white as it bleaches the paper. Chlorine gas is a bleaching agent.

Damp blue litmus paper turns RED then BLEACHES.

18.1.4 Detection of Hydrogen Gas Using Lighted Splint (Pop Reaction)

Procedure

1. Put some zinc metal pieces in a wolf bottle.
2. The wolf bottle has two openings: from one opening using a cork pass a thistle funnel, from the other opening pass a U-shaped tubing.
3. Make the apparatus air-tight by using wax.
4. Take a test tube to collect hydrogen gas coming from the tubing and place it vertically inverted over the tubing.
5. Pour sulphuric acid from the thistle funnel.
6. The test tube will be filled with the hydrogen gas.
7. Bring a burning splint after the test tube is filled with hydrogen.
8. A popping sound with a flame is observed which will confirm the presence of hydrogen gas.

This happens as a reaction of hydrogen with oxygen in the presence of flame and the formation of water vapours:
2H₂ + O₂ → 2H₂O

18.1.5 Detection of Oxygen by Using a Glowing Splint

Materials Required

• A glowing wooden splint (a small stick or piece of wood)
• A source of oxygen (e.g., oxygen gas tank or hydrogen peroxide solution)
• A test tube
• A match stick

Procedure

1. Ensure you are in a well-ventilated area and follow safety precautions when handling flammable materials.
2. Prepare your source of oxygen: use hydrogen peroxide, pour a small amount into the test tube; it generates oxygen gas automatically.
3. Ignite the wooden splint using a matchstick or lighter until it's glowing at the tip. Carefully insert the glowing splint into the test tube containing the oxygen gas.
4. Observe what happens to the splint inside the test tube; if oxygen is present, the splint will burst into flames, burning brightly.

Oxygen gas helps in combustion, thus it will suddenly enflame the glowing splint.

18.1.6 Detection of Sulphur Dioxide

Procedure

1. Prepare a solution of potassium permanganate (KMnO₄) in water; it will be of bright purple color.
2. Pass the gas sample containing SO₂ through the KMnO₄ solution.
3. The following reaction occurs in which the purple color of KMnO₄ will be discharged, indicating the presence of SO₂ gas.

The reaction is:
2KMnO₄ + 3SO₂ + 2H₂O → 2MnO₂ + 2K₂SO₄ + 2H₂SO₄

This method is often used in analytical chemistry and environmental monitoring to detect and measure SO₂ levels in gases.