BEFORE HEATING:
    
    🔥 Flame
     ↑
     |
  ┌──┴──┐
  │     │  ← Boiling tube held with tongs
  │     │
  │ ⚪  │  ← White crystalline lead nitrate powder
  │ ⚪⚪ │     (Pb(NO₃)₂)
  └─────┘


DURING HEATING:

    🔥 Flame (continuous heating)
     ↑
     |
  ┌──┴──┐     ☁️ ☁️  ← Brown fumes (NO₂)
  │  ☁️ │              escaping
  │ ☁️  │
  │ 🟡  │  ← Color changing from white to yellow
  │🟡⚪ │     Crackling sounds heard
  └─────┘


AFTER HEATING:

         (Fumes dispersed)
         
  ┌─────┐
  │     │
  │     │
  │ 🟡  │  ← Yellow residue (PbO - Lead monoxide)
  │ 🟡🟡│     remaining in the tube
  └─────┘

Products Formed:
• Yellow solid residue → Lead monoxide (PbO)
• Brown fumes → Nitrogen dioxide (NO₂)
• Colorless gas → Oxygen (O₂) - not visible

NCERT Solutions for Class 10 Science Chapter 1 Activity 1.2Check This Also

Initial color: White crystalline powder
Final color: Yellow residue
Reason: Lead nitrate \(\text{Pb(NO}_3\text{)}_2\) decomposes to form yellow lead monoxide (PbO)

Observation: Reddish-brown colored fumes are released
Gas identified: Nitrogen dioxide \(\text{(NO}_2\text{)}\)
Important note: These fumes are toxic and should not be inhaled. This is why the experiment must be performed in a well-ventilated area.

Heating Ferrous Sulphate Crystals - Activity 1.5 | NCERT Class 10 Science Chapter 1Check This Also

Observation: Crackling or popping sounds are heard during heating
Reason: The crystalline structure of lead nitrate breaks down rapidly, and gases are released suddenly, creating these sounds.

Observation: Colorless, odorless oxygen gas is also released (though not visible)
Test: You can test for oxygen by bringing a glowing splint near the mouth of the tube—it will rekindle
Note: This is often missed by students but is an important product of the reaction.

Model Answer for Board Exams:

Observations when lead nitrate is heated:

1. Color change: The white crystalline lead nitrate powder turns into a yellow residue (lead monoxide).
2. Evolution of brown fumes: Reddish-brown colored nitrogen dioxide gas \(\text{(NO}_2\text{)}\) is released.
3. Crackling sound: Crackling or popping sounds are heard during heating.
4. Oxygen gas: Colorless oxygen gas \(\text{(O}_2\text{)}\) is also evolved (can be tested with glowing splint).

Chemical equation:

Type of reaction: Thermal decomposition reaction

📊 Marking Scheme Breakdown (3 Marks):

• 1 mark: Correct observations (color change + brown fumes)
• 1 mark: Balanced chemical equation with correct formulas
• 1 mark: Identifying type of reaction OR mentioning all products correctly

💡 Exam Writing Tips:

• Always write observations in past tense (“was observed”, “turned yellow”)
• Use proper chemical formulas with correct subscripts and superscripts
• Don’t forget to mention the Δ symbol above the arrow in the equation
• Include state symbols (s), (g) in the equation for extra marks
• If asked for safety precautions, mention “well-ventilated area” and “do not inhale fumes”

Q1. Why does lead nitrate turn yellow when heated?

Lead nitrate decomposes to form lead monoxide (PbO), which is naturally yellow in color. The yellow color is an intrinsic property of PbO. The transformation from white \(\text{Pb(NO}_3\text{)}_2\) to yellow PbO is a clear visual indicator that a chemical reaction has occurred. This color change is permanent and the yellow residue will not turn back to white even after cooling.

Q2. What are the brown fumes produced in this reaction?

The brown fumes are nitrogen dioxide \(\text{(NO}_2\text{)}\) gas. This gas has a characteristic reddish-brown color and a pungent odor. It’s important to note that \(\text{NO}_2\) is toxic and should not be inhaled. This is why the experiment must be performed in a well-ventilated area or under a fume hood. The brown color is so distinctive that it’s often used as a test to identify nitrogen dioxide in chemical analysis.

Q3. How can we test for the presence of oxygen gas in this reaction?

Oxygen gas can be tested using the glowing splint test. Take a wooden splint, light it, then blow it out so it’s glowing (not flaming). If you bring this glowing splint near the mouth of the test tube during the reaction, the splint will rekindle (burst into flames again). This is because oxygen supports combustion. This test is a standard method to confirm the presence of oxygen gas in any chemical reaction.

Q4. Why is this called a decomposition reaction?

This is called a decomposition reaction because a single compound (lead nitrate) breaks down into two or more simpler substances (lead monoxide, nitrogen dioxide, and oxygen). Specifically, it’s a thermal decomposition reaction because heat energy is required to break the chemical bonds in lead nitrate. The general form of decomposition reactions is AB → A + B, where one reactant produces multiple products.

Q5. What precautions should be taken while performing this activity?

Several important precautions must be followed: (1) Never inhale the brown fumes as nitrogen dioxide is toxic; (2) Hold the test tube with tongs at a slight angle to prevent moisture from flowing back and cracking the hot tube; (3) Perform the experiment in a well-ventilated area or under a fume hood; (4) Wear safety goggles to protect your eyes from any spattering; (5) Do not touch the tube immediately after heating as it remains very hot; (6) Use only a small amount (about 2g) of lead nitrate to avoid excessive fume production.