Chemical Reactions in Everyday Life: Practical Chemistry Class 10

In CBSE Class 10 Science board examinations, practical-based questions (PBQs) carry $12$ marks across Section B and Section C. Unlike theoretical questions that ask for textbook definitions, practical questions present laboratory observations: "A student heats a green crystalline powder in a dry test tube... A gas with the smell of burning sulfur is evolved... A white precipitate turns milky..."

To secure full marks on observational chemistry questions, students must memorize the diagnostic confirmatory tests for evolved gases, characteristic flame colors, and color changes of common inorganic salts.

In this master guide, we compile the definitive Qualitative Chemistry Diagnostic Guide for Class 10 Science.

What You Will Learn

• Confirmatory identification tests for all $5$ major laboratory gases: $H_2, CO_2, O_2, SO_2,$ and $Cl_2$
• The Lime Water Test for Carbon Dioxide: Why excess $CO_2$ causes milkiness to disappear
• Color transformations of common salts upon heating (Thermal Decomposition Diagnostics)
• The action of acids on metals, metal carbonates, and metal oxides
• High-yield observational question templates and common exam pitfalls

1. Confirmatory Tests for Common Laboratory Gases

    Evolved Gas              Sensory Clue                       Confirmatory Laboratory Test
    ----------------------------------------------------------------------------------------------------------
    Hydrogen (H₂)            Colourless, odourless              Burns with a characteristic 'POP' SOUND 
                                                                when a burning wooden splinter is brought near!
    
    Carbon Dioxide (CO₂)     Colourless, brisk effervescence    Turns clear LIME WATER MILKY!
                                                                (Milkiness disappears on passing EXCESS CO₂).
    
    Oxygen (O₂)              Colourless, odourless              REKINDLES a glowing wooden splint.
    
    Sulphur Dioxide (SO₂)    Suffocating, CHOKING smell         Turns acidified potassium dichromate (K₂Cr₂O₇) 
                             of burning sulfur                  solution from ORANGE to GREEN!
    
    Chlorine (Cl₂)           Greenish-yellow gas, pungent       BLEACHES moist litmus paper (acts as bleach).
    ----------------------------------------------------------------------------------------------------------

2. The Complete Chemistry of the Lime Water Test ($CO_2$)

The reaction of carbon dioxide with lime water is the most frequently tested experimental question in CBSE Chemistry:

    Step 1: Passing Limited CO₂ (MILKINESS APPEARS!)
    Ca(OH)₂ (aq) [Clear Lime Water]  +  CO₂ (g)  ───>  CaCO₃ (s) ↓ [WHITE PRECIPITATE]  +  H₂O (l)
                                                       (Causes the cloudy milkiness!)

    Step 2: Passing Excess CO₂ (MILKINESS DISAPPEARS!)
    CaCO₃ (s)  +  H₂O (l)  +  CO₂ (g) [Excess]  ───>  Ca(HCO₃)₂ (aq) [SOLUBLE Calcium Bicarbonate]
                                                      (Completely dissolves, turning solution CLEAR again!)

Important: <u>Why does the milkiness disappear when excess carbon dioxide is passed through lime water? Because insoluble white calcium carbonate ($CaCO_3$) reacts with excess $CO_2$ and water to form calcium hydrogen carbonate $[Ca(HCO_3)_2]$, which is completely SOLUBLE in water!</u>

3. Salt Transformations upon Heating (Thermal Decomposition)

When white or colored powders are heated in a dry boiling tube:

4. Reactions of Acids with Metals and Carbonates

1. Metal + Acid $\longrightarrow$ Salt + Hydrogen Gas ($ext{Pop Sound}$)

$\mathbf{Zn\text{ (s)} + H_2SO_4\text{ (aq)} \longrightarrow ZnSO_4\text{ (aq)} + H_2\text{ (g)} \uparrow}$

• Zinc granules react with dilute sulphuric acid; bubbles of hydrogen gas emerge, which burn with a pop sound when tested with a candle.

2. Metal Carbonate + Acid $\longrightarrow$ Salt + Water + Carbon Dioxide ($ext{Effervescence}$)

$\mathbf{Na_2CO_3\text{ (s)} + 2HCl\text{ (aq)} \longrightarrow 2NaCl\text{ (aq)} + H_2O\text{ (l)} + CO_2\text{ (g)} \uparrow}$

• Produces brisk effervescence; passing the gas into lime water turns it milky.

5. Summary and Examination Tips

Exam Tip: In questions where an unknown metal reacts with dilute acid to evolve a gas: if the gas burns with a pop sound, it is Hydrogen ($H_2$)! If it turns lime water milky, the starting substance was a Carbonate or Bicarbonate ($CO_3^{2-} / HCO_3^-$) evolving Carbon Dioxide ($CO_2$)!

Common Mistake: Confusing the gas that turns lime water milky ($CO_2$) with the gas that turns acidified potassium dichromate green ($SO_2$). Both are non-metal oxides, but $SO_2$ has a strong choking smell, whereas $CO_2$ is odorless!