Is it correct to say: Balancing equations by changing subscripts inside formulas?

Only coefficients in front of formulas may be changed. Changing H_2O → H_2O_2 gives a different compound.

Is it correct to say: Missing physical states (s), (l), (g), (aq)?

Board answers expect them — especially (aq) for solutions and ↑↓ for gas/precipitate.

Is it correct to say: Writing 'thermite reaction' but forgetting that heat is a PRODUCT (exothermic)?

Add '+ heat' on product side or explicitly label as exothermic.

Is it correct to say: Confusing oxidation with 'adding oxygen' only?

Oxidation = loss of electrons, OR gain of O, OR loss of H. The electron definition is most general.

Is it correct to say: Skipping the arrow condition (Δ, hv, electricity)?

Write the condition above the arrow — Δ for heat, hv for light, 'electrolysis' for current.

Is it correct to say: Treating photosynthesis as exothermic?

Photosynthesis absorbs sunlight energy — endothermic. Respiration releases energy — exothermic.

Chemical Reactions and Equations

Types of chemical reactions, balancing equations, redox, exothermic and endothermic — Maharashtra SSC Science Part 1 Ch 3

📐 12 formulas✏️ 3 examples🎯 6 practice⚖️ 5-6 marks🏫 Maharashtra State Board📚 Class 10✓ 2025–26 syllabus

💡

Board Exam Tips

→SSC always asks 5 balanced equations — practise combination, decomposition, displacement, double displacement, redox.
→Include physical state (s), (l), (g), (aq) — 1 mark is often reserved for these in board answers.
→Marathi terms: संयोग अभिक्रिया (combination), अपघटन (decomposition), विस्थापन (displacement), दुहेरी विस्थापन (double displacement), उपचयन-अपचयन (oxidation-reduction).
→Thermite reaction is a favourite HOTS — memorise it with the exothermic tag and its use for welding rails.
→Corrosion (rust) and rancidity are compulsory 2-mark questions almost every year.

📐 Formulas(12)

Law of Conservation of Mass

Mass of reactants = Mass of products

Combination Reaction★ Board fav

A + B \to A B

Decomposition Reaction★ Board fav

A B \to A + B

Displacement Reaction

A + B C \to A C + B

Double Displacement

A B + C D \to A D + C B

Precipitation Example

B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} ↓ + 2 N a C l

Thermal Decomposition of Calcium Carbonate

C a C O_{3} Δ C a O + C O_{2} ↑

Electrolysis of Water

2 H_{2} O electricity 2 H_{2} ↑ + O_{2} ↑

Photodecomposition of Silver Chloride

2 A g C l light 2 A g + C l_{2}

Thermite Reaction (Redox)★ Board fav

F e_{2} O_{3} + 2 A l \to 2 F e + A l_{2} O_{3} + heat

Oxidation and Reduction

Oxidation: loss of electrons; Reduction: gain of electrons

Photosynthesis (Endothermic)

6 C O_{2} + 6 H_{2} O sunlight C_{6} H_{12} O_{6} + 6 O_{2}

✏️ Solved Examples

1Solved Exampleeasy3 steps

Balance the equation: Fe + H_2O → Fe_3O_4 + H_2.

Count Fe atoms — need 3 on left

3 F e + H_{2} O \to F e_{3} O_{4} + H_{2}

2Solved Exampleboard3 steps

When silver nitrate solution is mixed with sodium chloride solution, a white precipitate is formed. Write the balanced equation and state the type of reaction.

Swap partners (double displacement)

A g N O_{3} + N a C l \to A g C l + N a N O_{3}

3Solved ExampleHOTS4 steps

When a strip of copper is placed in silver nitrate solution, blue colour develops and shiny silver deposits. Write the balanced equation, identify the species oxidised and reduced, and explain why the solution turns blue.

Cu displaces Ag (Cu is above Ag in activity series)

C u + 2 A g N O_{3} \to C u (N O_{3})_{2} + 2 A g

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Balancing equations by changing subscripts inside formulas
✓Only coefficients in front of formulas may be changed. Changing H_2O → H_2O_2 gives a different compound.
2
Missing physical states (s), (l), (g), (aq)
✓Board answers expect them — especially (aq) for solutions and ↑↓ for gas/precipitate.
3
Writing 'thermite reaction' but forgetting that heat is a PRODUCT (exothermic)
✓Add '+ heat' on product side or explicitly label as exothermic.
4
Confusing oxidation with 'adding oxygen' only
✓Oxidation = loss of electrons, OR gain of O, OR loss of H. The electron definition is most general.
5
Skipping the arrow condition (Δ, hv, electricity)
✓Write the condition above the arrow — Δ for heat, hv for light, 'electrolysis' for current.
6
Treating photosynthesis as exothermic
✓Photosynthesis absorbs sunlight energy — endothermic. Respiration releases energy — exothermic.

🎯 Practice Yourself

🎯Practice Yourself6 questions

Q1
Balance: Pb(NO_3)_2 + KI → PbI_2 + KNO_3. State the type.
Q2
Give one example each of exothermic and endothermic reactions.
Q3
Why should copper vessels not be used to store curd?
Q4
Define corrosion and give one preventive method.
Q5
Write the balanced equation for decomposition of lead nitrate on heating.
Q6
Balance: KClO_3 → KCl + O_2.

📝 Notes

Chemical Reactions and Equations

Every change in the material world traces to atoms rearranging into new combinations. Learning to recognise and balance these rearrangements is the goal of this chapter.

SSC angle

Maharashtra SSC Ch 3 (Science Part 1) demands three things:

Classify the reaction — combination / decomposition / displacement / double displacement / redox.
Balance using the coefficient method (never touch subscripts).
Explain exothermic vs endothermic — mention whether heat is a reactant or product.

Balancing checklist

Balance metals first, non-metals next, H and O last.
If odd number of O appears, multiply by 2.
Recount atoms of every element after each step.
Balance polyatomic ions (SO_4, NO_3, CO_3) as one unit if unchanged.

Reaction types — cheat sheet

| Type | Pattern | Example | |---|---|---| | Combination | A + B → AB | 2Mg + O_2 → 2MgO | | Decomposition | AB → A + B | 2H_2O_2 → 2H_2O + O_2 | | Displacement | A + BC → AC + B | Zn + CuSO_4 → ZnSO_4 + Cu | | Double displacement | AB + CD → AD + CB | AgNO_3 + NaCl → AgCl + NaNO_3 | | Redox | Simultaneous ox + red | CuO + H_2 → Cu + H_2O |

Redox at Class 10 level

Oxidation — loss of electrons (LEO).
Reduction — gain of electrons (GER).
Oxidising agent — species that oxidises another; itself gets reduced.
Reducing agent — species that reduces another; itself gets oxidised.

Corrosion and rancidity

Corrosion (गंज): slow oxidation of metals in moist air. Iron rust = Fe_2O_3·xH_2O.
Rancidity (साखलपणा): oxidation of fats/oils in food, producing bad smell. Prevented by antioxidants, refrigeration, N_2 flushing.