Top 25 Important Chemical Reactions and Equations for Class 10 Science

Important: <u>Mastering these 25 balanced chemical equations, along with their reaction types, color changes, and state symbols, is the single most effective way to secure full marks in Class 10 Chemistry!</u>

In CBSE Class 10 Science, chemistry accounts for approximately $25$ marks. If you examine the past ten years of board question papers, you will notice a consistent pattern: examiners do not invent new reactions. Almost every single chemical question revolves around a specific roster of $25$ core chemical equations, their visual color changes, balanced stoichiometric coefficients, and associated precipitate identities.

Whether you are asked to identify "compound $X$ that turns green crystals into a brown solid with choking sulfur fumes", or write the balanced equation for the "manufacture of baking soda", this comprehensive guide compiles all $25$ mandatory chemical reactions into an organized, high-yield study sheet.

What You Will Learn

• The 4 fundamental reaction types: Combination, Decomposition, Displacement, and Double Displacement
• Characteristic colour changes, gas evolutions, and precipitates
• Important industrial processes: Chlor-Alkali Process, Thermite Process
• Reactions of household salts: Bleaching powder, Baking soda, Washing soda, and Plaster of Paris
• Carbon Chemistry: Esterification, Saponification, and Combustion
• The exact balanced chemical equations required for full marks in board exams

1. Combination and Exothermic Reactions

1. Slaking of Quicklime (Vigorous Exothermic Reaction)

$\mathbf{CaO\text{ (s)} + H_2O\text{ (l)} \longrightarrow Ca(OH)_2\text{ (aq)} + \text{Heat}}$

• Observation: Quicklime ($CaO$) reacts vigorously with water with a hissing sound, producing slaked lime ($Ca(OH)_2$) and releasing enormous heat.

2. White-Washing Reaction (Formation of Shiny Marble Finish)

$\mathbf{Ca(OH)_2\text{ (aq)} + CO_2\text{ (g)} \longrightarrow CaCO_3\text{ (s)} \downarrow + H_2O\text{ (l)}}$

• Observation: Slaked lime applied to walls reacts slowly with atmospheric carbon dioxide, forming a thin, shiny layer of calcium carbonate ($CaCO_3$) after 2 to 3 days.

2. Decomposition Reactions (Thermal, Photolytic, Electrolytic)

3. Thermal Decomposition of Ferrous Sulphate (Green Crystals $o$ Brown Residue)

$\mathbf{2FeSO_4\text{ (s)} \xrightarrow{\Delta} Fe_2O_3\text{ (s)} + SO_2\text{ (g)} \uparrow + SO_3\text{ (g)} \uparrow}$

• Observation: Pale green ferrous sulphate crystals ($FeSO_4 \cdot 7H_2O$) turn white upon losing water of crystallization, then decompose into a reddish-brown solid ($Fe_2O_3$) with the characteristic choking smell of burning sulfur ($SO_2, SO_3$).

4. Thermal Decomposition of Lead Nitrate (Brown Fumes)

$\mathbf{2Pb(NO_3)_2\text{ (s)} \xrightarrow{\Delta} 2PbO\text{ (s)} + 4NO_2\text{ (g)} \uparrow + O_2\text{ (g)} \uparrow}$

• Observation: White lead nitrate powder decomposes to leave a yellow residue of lead monoxide ($PbO$) and evolves dense brown fumes of nitrogen dioxide ($NO_2$).

5. Photolytic Decomposition of Silver Chloride (Black & White Photography)

$\mathbf{2AgCl\text{ (s)} \xrightarrow{\text{Sunlight}} 2Ag\text{ (s)} + Cl_2\text{ (g)} \uparrow}$

• Observation: White silver chloride turns grey in sunlight due to the formation of elemental silver metal.

6. Electrolysis of Water (2

Volume Ratio)

$\mathbf{2H_2O\text{ (l)} \xrightarrow{\text{Electric Current}} 2H_2\text{ (g)} + O_2\text{ (g)}}$

• Observation: The volume of hydrogen gas collected at the cathode is twice the volume of oxygen gas collected at the anode.

3. Displacement and Precipitation Reactions

7. Iron in Copper Sulphate Solution

$\mathbf{Fe\text{ (s)} + CuSO_4\text{ (aq)} \longrightarrow FeSO_4\text{ (aq)} + Cu\text{ (s)}}$

• Observation: Blue copper sulphate solution fades to light green (ferrous sulphate), and a reddish-brown coating of copper deposits on the iron nail.

8. Double Displacement / Precipitation of Barium Sulphate

$\mathbf{Na_2SO_4\text{ (aq)} + BaCl_2\text{ (aq)} \longrightarrow BaSO_4\text{ (s)} \downarrow + 2NaCl\text{ (aq)}}$

• Observation: Instant formation of a white precipitate of barium sulphate ($BaSO_4$).

9. Lead Nitrate and Potassium Iodide (Yellow Precipitate)

$\mathbf{Pb(NO_3)_2\text{ (aq)} + 2KI\text{ (aq)} \longrightarrow PbI_2\text{ (s)} \downarrow + 2KNO_3\text{ (aq)}}$

• Observation: Instant formation of a brilliant yellow precipitate of lead iodide ($PbI_2$).

4. Industrial Processes & Metallurgy

10. The Thermite Reaction (Welding Railway Tracks)

$\mathbf{Fe_2O_3\text{ (s)} + 2Al\text{ (s)} \longrightarrow 2Fe\text{ (l)} + Al_2O_3\text{ (s)} + \text{Heat}}$

• Observation: An intensely exothermic reduction reaction where iron is produced in a molten liquid state, used to join cracked railway tracks.

11. Roasting of Zinc Sulphide (Excess Air)

$\mathbf{2ZnS\text{ (s)} + 3O_2\text{ (g)} \xrightarrow{\Delta} 2ZnO\text{ (s)} + 2SO_2\text{ (g)} \uparrow}$

12. Calcination of Zinc Carbonate (Limited Air)

$\mathbf{ZnCO_3\text{ (s)} \xrightarrow{\Delta} ZnO\text{ (s)} + CO_2\text{ (g)} \uparrow}$

5. Acids, Bases, and Important Salts

13. The Chlor-Alkali Process (Electrolysis of Brine)

$\mathbf{2NaCl\text{ (aq)} + 2H_2O\text{ (l)} \xrightarrow{\text{Electricity}} 2NaOH\text{ (aq)} + Cl_2\text{ (g)} + H_2\text{ (g)}}$

• Products: $Cl_2$ at anode, $H_2$ at cathode, $NaOH$ near cathode.

14. Preparation of Bleaching Powder

$\mathbf{Ca(OH)_2\text{ (s)} + Cl_2\text{ (g)} \longrightarrow CaOCl_2\text{ (s)} + H_2O\text{ (l)}}$

15. Preparation of Baking Soda (Solvay Process Step)

$\mathbf{NaCl + H_2O + CO_2 + NH_3 \longrightarrow NH_4Cl + NaHCO_3\text{ (s)}}$

16. Heating of Baking Soda During Cooking

$\mathbf{2NaHCO_3\text{ (s)} \xrightarrow{\Delta} Na_2CO_3\text{ (s)} + H_2O\text{ (g)} + CO_2\text{ (g)} \uparrow}$

• Evolved $CO_2$ gas causes bread and cakes to rise, making them soft and spongy.

17. Preparation of Plaster of Paris (POP)

$\mathbf{CaSO_4 \cdot 2H_2O \xrightarrow{373\text{ K } (100^\circ\text{C})} CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O}$

6. Organic Chemistry (Carbon Compounds)

18. Combustion of Ethanol

$\mathbf{C_2H_5OH + 3O_2 \longrightarrow 2CO_2 + 3H_2O + \text{Heat and Light}}$

19. Oxidation of Ethanol to Ethanoic Acid

$\mathbf{CH_3CH_2OH \xrightarrow[\text{or Acidified } K_2Cr_2O_7]{\text{Alkaline } KMnO_4 + \Delta} CH_3COOH}$

20. Esterification Reaction (Sweet-Smelling Ester)

$\mathbf{CH_3COOH + C_2H_5OH \xrightarrow{\text{Conc. } H_2SO_4} CH_3COOC_2H_5\text{ (Ethyl Acetate)} + H_2O}$

21. Saponification (Preparation of Soap)

$\mathbf{CH_3COOC_2H_5 + NaOH \longrightarrow CH_3COONa\text{ (Sodium Acetate / Soap)} + C_2H_5OH}$

22. Hydrogenation of Vegetable Oils (Addition Reaction)

$\mathbf{R_2C=CR_2 + H_2 \xrightarrow[473\text{ K}]{\text{Nickel Catalyst}} R_2CH-CHR_2\text{ (Vanaspati Ghee)}}$

7. Summary and Examination Tips

Exam Tip: In questions describing "A compound X used in white washing...": Compound $X$ is Quicklime ($CaO$); when water is added, it forms compound $Y$, which is Slaked lime ($Ca(OH)_2$)!

Common Mistake: Heating gypsum above $373\text{ K}$ ($100^\circ ext{C}$). If heated above $373\text{ K}$, gypsum loses all water of crystallization to become "dead burnt plaster" ($CaSO_4$), losing its setting property!