CHEMICAL REACTIONS & EQUATIONS

1. The Foundation: Chemical vs. Physical

Every change in our surroundings is either temporary or permanent. For the board exam, you must distinguish between them based on the formation of new substances.

Concept

Standard Observations (The 5 Indicators)

How do we know a chemical reaction has taken place? Look for these signs:

Change in State: E.g., Burning of wax (Solid to Gas).
Change in Colour: E.g., Iron nails in Copper Sulphate (Blue to Green).
Evolution of Gas: E.g., Zinc + Sulphuric Acid (Hydrogen gas bubbles).
Change in Temperature: E.g., Quick lime + Water (Exothermic - beaker gets hot).
Formation of Precipitate: E.g., Barium Chloride + Sodium Sulphate (White solid forms).

Teacher's Tip: The "Candle" Confusion Burning of a candle is both physical (melting of wax) and chemical (burning of wax). If asked in boards, specify this distinction!

2. Representing Reactions: Chemical Equations

A chemical equation is a shorthand representation using symbols and formulas.

Key Knowledge

The 3-Step Representation

Word Equation: Magnesium + Oxygen $\rightarrow$ Magnesium Oxide
Skeletal Equation: $Mg + O_2 \rightarrow MgO$ (Unbalanced)
Balanced Equation: $2Mg + O_2 \rightarrow 2MgO$ (Follows Law of Conservation of Mass)

Law of Conservation of Mass: Mass can neither be created nor destroyed. Atoms on LHS must = Atoms on RHS.

The "Hit and Trial" Balancing Masterclass

Follow the "Heavy Atom First" rule. Balance the atom with the highest count first.

Note: Here, $H_2O$ is in the form of steam, hence the $(g)$ state symbol.

Pro-Tip

The State Symbol Legend

Boards give extra credit for correct state symbols. Memorize these shorthand symbols:

(s): Solid (e.g., metals)
(l): Liquid (e.g., pure water $H_2O$)
(g): Gas (e.g., $CO_2$, $H_2$)
(aq): Aqueous (Substance dissolved in water)
$\downarrow$: Precipitate (Solid settling down)
$\uparrow$: Gas evolving (Bubbles/Fumes)
$\Delta$: Heat supplied (Thermal)

Q & A Q1. Why is it necessary to balance a skeletal chemical equation? (Board 2018)
Ans: To satisfy the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of elements present in the products must be equal to the total mass of elements present in the reactants.

Q & A Q2. Why should a magnesium ribbon be cleaned with sandpaper before burning in air? (CBSE 2020)
Ans: Magnesium is a reactive metal. When stored in air, it reacts with atmospheric oxygen and moisture to form a thin dull layer of basic magnesium carbonate. Cleaning with sandpaper removes this layer, exposing the shiny metal surface so that it can burn completely and properly in air.

3. NCERT Activity Master-Chart (High Yield)

Board exams love asking about observations from activities. Here is the ultimate memory table followed by deep-dives into each experiment:

Activity	Key Observation & Type	Chemical Equation
1.1 Mg Ribbon	Dazzling white flame; white ash of $MgO$ forms Combination	$2Mg + O_2 \rightarrow 2MgO$
1.2 Lead Nitrate + KI	Yellow Precipitate of $PbI_2$ forms Double Disp.	$Pb(NO_3)_2 + 2KI \rightarrow PbI_2\downarrow + 2KNO_3$
1.3 Zinc + Acid	Evolution of Hydrogen gas (Pop sound test) Displacement	$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow$
1.4 CaO + Water	Vigorous reaction; beaker becomes hot (Exothermic) Combination	$CaO + H_2O \rightarrow Ca(OH)_2 + \Delta$
1.5 Ferrous Sulphate	Green crystals → Brown; burning sulphur smell Decomposition	$2FeSO_4 \xrightarrow{\Delta} Fe_2O_3 + SO_2 + SO_3$
1.6 Lead Nitrate Heating	Brown fumes of $NO_2$; yellow $PbO$ residue Decomposition	$2Pb(NO_3)_2 \xrightarrow{\Delta} 2PbO + 4NO_2 + O_2$
1.7 Electrolysis of Water	$H_2:O_2$ volume ratio = 2:1 at electrodes Electrolytic Decomp.	$2H_2O \xrightarrow{\text{Electricity}} 2H_2 + O_2$
1.8 Silver Chloride	White $AgCl$ turns grey in sunlight Photolytic Decomp.	$2AgCl \xrightarrow{\text{Sunlight}} 2Ag + Cl_2$
1.9 Iron Nail + CuSO₄	Blue → Light Green; brown copper on nail Displacement	$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$
1.10 Precipitation	White precipitate of $BaSO_4$ forms Double Disp.	$Na_2SO_4 + BaCl_2 \rightarrow BaSO_4\downarrow + 2NaCl$
1.11 Copper Oxidation	Brown copper turns black Copper Oxide Oxidation	$2Cu + O_2 \xrightarrow{\Delta} 2CuO$
Respiration (Everyday Example)	Energy released for life processes Exothermic	$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + E$

ACTIVITY 1.1: BURNING MAGNESIUM RIBBON

Procedure: Clean Mg ribbon with sandpaper and burn it in air using tongs.

Observation: Dazzling white flame; white ash of magnesium oxide forms.

Equation: $$ 2Mg(s) + O_2(g) \longrightarrow 2MgO(s) $$

Fig: Dazzling white flame of burning Magnesium ribbon

ACTIVITY 1.2: LEAD NITRATE + POTASSIUM IODIDE

Procedure: Mix aqueous lead nitrate and potassium iodide solutions.

Observation: Yellow precipitate of lead iodide forms ($PbI_2$).

Equation: $$ Pb(NO_3)_2(aq) + 2KI(aq) \longrightarrow PbI_2(s)\downarrow + 2KNO_3(aq) $$

ACTIVITY 1.3: ZINC GRANULES + DILUTE ACID

Procedure: Add zinc granules to dilute $H_2SO_4$ or $HCl$ in a conical flask.

Observation: Gas bubbles form; flask becomes hot (exothermic).

Equation: $$ Zn(s) + H_2SO_4(aq) \longrightarrow ZnSO_4(aq) + H_2(g)\uparrow $$

ACTIVITY 1.4: QUICK LIME + WATER

Procedure: Add water slowly to a small amount of calcium oxide (Quick Lime).

Observation: Vigorous reaction with a hissing sound; beaker becomes very hot.

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

ACTIVITY 1.5: HEATING FERROUS SULPHATE

Procedure: Heat green crystals of ferrous sulphate in a dry test tube.

Observation: Colour changes from green to brown ($Fe_2O_3$); smell of burning sulphur.

Equation: $$ 2FeSO_4(s) \xrightarrow{\Delta} Fe_2O_3(s) + SO_2(g) + SO_3(g) $$

ACTIVITY 1.6: HEATING LEAD NITRATE

Procedure: Heat lead nitrate crystals in a test tube.

Observation: Brown fumes of nitrogen dioxide ($NO_2$) evolve.

Equation: $$ 2Pb(NO_3)_2(s) \xrightarrow{\Delta} 2PbO(s) + 4NO_2(g) + O_2(g) $$

ACTIVITY 1.7: ELECTROLYSIS OF WATER

Procedure: Pass electricity through acidified water.

Observation: Volume of hydrogen collected at cathode is double that of oxygen at anode.

Equation: $$ 2H_2O(l) \xrightarrow{\text{Electricity}} 2H_2(g) + O_2(g) $$

Critical Observations

1. The 2:1 Ratio (Activity 1.7)

During electrolysis of water, Hydrogen gas is collected at the Cathode and Oxygen at the Anode. The volume of $H_2$ is double that of $O_2$ because one molecule of water ($H_2O$) contains two atoms of Hydrogen for every one atom of Oxygen.

2. The 'Pop' Sound (H₂ Test)

To test for Hydrogen gas ($H_2$), bring a burning candle near the mouth of the test tube. The gas burns with a pop sound.

ACTIVITY 1.8: PHOTOLYTIC DECOMPOSITION

Procedure: Keep silver chloride ($AgCl$) in sunlight.

Observation: White $AgCl$ turns grey due to formation of silver metal.

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

ACTIVITY 1.9: IRON NAILS IN COPPER SULPHATE

Procedure: Dip iron nails in blue $CuSO_4$ solution.

Observation: Blue solution fades to light green; nails turn brownish.

Equation: $$ Fe(s) + CuSO_4(aq) \longrightarrow FeSO_4(aq) + Cu(s) $$

ACTIVITY 1.10: PRECIPITATION REACTION

Procedure: Mix sodium sulphate and barium chloride solutions.

Observation: White precipitate of $BaSO_4$ forms instantly.

Equation: $ Na_2SO_4(aq) + BaCl_2(aq) \longrightarrow BaSO_4(s)\downarrow + 2NaCl(aq) $

ACTIVITY 1.11: OXIDATION OF COPPER

Procedure: Heat copper powder in a china dish.

Observation: Brown copper turns black Copper Oxide ($CuO$).

Equation: $$ 2Cu(s) + O_2(g) \xrightarrow{\Delta} 2CuO(s) $$

4. Types of Reactions (Simplified Logic)

Classification

A. Combination Reaction ($A + B \rightarrow C$)

Reactants merge into one. Commonly Exothermic.

Crucial Board Question: Whitewashing.
Quick Lime + Water $\rightarrow$ Slaked Lime ($CaO + H_2O \rightarrow Ca(OH)_2$)
Slaked Lime + $CO_2$ $\rightarrow$ Calcium Carbonate ($Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$)
The shiny finish comes from $CaCO_3$ after 2-3 days.

Q & A Q3. Distinguish between Exothermic and Endothermic reactions with examples. (CBSE 2017)
Ans:
• Exothermic Reactions: Reactions in which heat is released along with products. E.g., Burning of natural gas: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Heat}$.
• Endothermic Reactions: Reactions in which energy is absorbed. E.g., Decomposition of Calcium Carbonate: $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$.

HOTS Challenge Q4. A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is also present in bones. On treatment with water, it forms a solution which turns red litmus blue. Identify X and the reaction involved.
Ans:
• X is Calcium Oxide ($CaO$), also known as Quicklime.
• The element is Calcium (Group 2, present in bones).
• Reaction: $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat}$.
• $Ca(OH)_2$ (Slaked Lime) is basic, hence it turns red litmus blue.

Q & A Q5. Explain with a chemical equation why respiration is considered an exothermic reaction. (CBSE 2019, 2022)
Ans: During respiration, glucose obtained from digestion of food reacts with oxygen inside body cells to produce carbon dioxide, water, and releases energy. Since energy is released in the process, respiration is an exothermic reaction.
$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$

Q & A Q6. Why is photosynthesis considered an endothermic reaction? Write its equation. (Board 2022)
Ans: In photosynthesis, green plants absorb energy from sunlight to convert carbon dioxide and water into glucose and oxygen. Since energy is absorbed, it is an endothermic reaction.
$6CO_2 + 12H_2O \xrightarrow{\text{Sunlight/Chlorophyll}} C_6H_{12}O_6 + 6O_2 + 6H_2O$

Classification

B. Decomposition Reaction ($C \rightarrow A + B$)

The "Opposite of Combination". Commonly Endothermic.

Thermal: Heat ($CaCO_3 \rightarrow CaO + CO_2$)
Electrolytic: Electricity ($2H_2O \rightarrow 2H_2 + O_2$)
Photolytic: Sunlight ($2AgBr \rightarrow 2Ag + Br_2$)

Q & A Q7. A student heats Lead Nitrate powder in a test tube. (CBSE 2023)
(a) What colour are the fumes evolved?
(b) Write the balanced chemical equation.
(c) Name the type of chemical reaction.

Ans:
(a) Brown fumes of Nitrogen Dioxide ($NO_2$) are evolved. A yellow residue of Lead Oxide ($PbO$) remains.
(b) $2Pb(NO_3)_2 \xrightarrow{\Delta} 2PbO + 4NO_2 + O_2$
(c) Thermal Decomposition Reaction.

Q & A Q8. Why are decomposition reactions called the opposite of combination reactions? (NCERT)
Ans: In a combination reaction, two or more substances combine to form a single product ($A + B \rightarrow C$). In a decomposition reaction, a single reactant breaks down to give two or more simpler products ($C \rightarrow A + B$). Thus, they are functionally opposite.

Q & A Q9. Why is Silver Chloride stored in dark-coloured bottles? (CBSE 2019)
Ans: Silver chloride ($AgCl$) is light-sensitive. In the presence of sunlight, it undergoes photolytic decomposition to form silver metal and chlorine gas ($2AgCl \rightarrow 2Ag + Cl_2$). Dark bottles block sunlight to prevent this decomposition.

HOTS Challenge Q10. A white salt 'A' on heating gives brown fumes of gas 'B' and a yellow residue 'C'. When an aqueous solution of 'A' is mixed with Potassium Iodide, a yellow precipitate 'D' is formed. Identify A, B, C, and D.
Ans:
• A: Lead Nitrate [$Pb(NO_3)_2$]
• B: Nitrogen Dioxide [$NO_2$] (Brown fumes)
• C: Lead Oxide [$PbO$] (Yellow residue)
• D: Lead Iodide [$PbI_2$] (Yellow precipitate)

Classification

C. Displacement Reaction ($A + BC \rightarrow AC + B$)

Based on Reactivity Series. A stronger metal kicks out a weaker one.

Example: $Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$
Zinc is more reactive than Copper.

Q & A Q11. Why does the colour of copper sulphate solution change when an iron nail is kept immersed in it? (CBSE 2018)
Ans: Iron is higher than copper in the Reactivity Series. It therefore displaces copper from its solution. The blue colour of copper sulphate ($CuSO_4$) fades and turns light green as Ferrous Sulphate ($FeSO_4$) forms. The nail gets a reddish-brown copper coating.
$Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$

HOTS Challenge Q12. On adding dilute HCl to copper oxide powder, the solution becomes blue-green. Identify the new compound formed and write the balanced equation.
Ans: The blue-green color is due to the formation of Copper(II) Chloride ($CuCl_2$).
$CuO(s) + 2HCl(aq) \rightarrow CuCl_2(aq) + H_2O(l)$

Classification

D. Double Displacement ($AB + CD \rightarrow AD + CB$)

Exchange of ions. Usually results in a Precipitate.

Standard Example: $Na_2SO_4 + BaCl_2 \rightarrow BaSO_4\downarrow + 2NaCl$ (White ppt of Barium Sulphate).

5. Redox Reactions (Oxidation & Reduction)

This is where most students lose marks. Use the "Oxygen-Hydrogen Rule".

Concept

Identifying the Agents

Process	Oxygen	Hydrogen
Oxidation	Gain (+)	Loss (-)
Reduction	Loss (-)	Gain (+)

Example Equation: $MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2$

$MnO_2 \rightarrow MnCl_2$: Reduction (Oxygen removed)
$HCl \rightarrow Cl_2$: Oxidation (Hydrogen removed)
Oxidizing Agent: $MnO_2$ (The one that gets reduced)
Reducing Agent: $HCl$ (The one that gets oxidized)

Q & A Q13. In $MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2$, identify the: (a) Substance oxidised (b) Substance reduced (c) Oxidising Agent (d) Reducing Agent. (CBSE Board)
Ans:
(a) Substance Oxidised: $HCl$ — Hydrogen is removed from $HCl$, it loses hydrogen (oxidised).
(b) Substance Reduced: $MnO_2$ — Oxygen is removed from $MnO_2$ (reduced).
(c) Oxidising Agent: $MnO_2$ (it gets reduced itself).
(d) Reducing Agent: $HCl$ (it gets oxidised itself).

Mnemonic: OIL RIG Oxidation Is Loss (of Electrons) | Reduction Is Gain (of Electrons).

6. Everyday Effects of Oxidation

Board Favorite

Corrosion (Destruction of Metals)

Iron: Rust ($Fe_2O_3 \cdot xH_2O$) - Reddish Brown.
Silver: Black Coating ($Ag_2S$) due to sulfur in air.
Copper: Green Coating (Basic Copper Carbonate) - $CuCO_3 \cdot Cu(OH)_2$.

ACTIVITY 1.11: OXIDATION OF COPPER

Observation: Heating copper powder in a china dish turns it black.
Reason: Oxygen is added to copper to form Copper (II) Oxide.
Equation: $2Cu + O_2 \xrightarrow{\Delta} 2CuO$ (Black)
To reverse: Pass Hydrogen gas over hot CuO: $CuO + H_2 \rightarrow Cu + H_2O$ (Returns to brown).

Q & A Q14. A shiny brown element X on heating in air becomes a black compound Y. Identify X and Y, and write the balanced equation. (NCERT Exemplar / HOTS)
Ans: X is Copper ($Cu$) and Y is Copper(II) Oxide ($CuO$).
$2Cu(s) + O_2(g) \xrightarrow{\Delta} 2CuO(s)$ — This is an oxidation reaction.
When Hydrogen gas is passed over the hot black $CuO$, it returns to shiny brown copper: $CuO + H_2 \rightarrow Cu + H_2O$ (Reduction).

Board Favorite

Rancidity (Spoiling of Food)

Oxidation of fats/oils leads to bad smell/taste.

Prevention: Flush packets with Nitrogen gas (Inert gas), use airtight containers, add Antioxidants.

Q & A Q15. Why are packets of potato chips flushed with Nitrogen gas? (CBSE 2021)
Ans: Potato chips contain fats and oils. When they come in contact with oxygen in the air, they get oxidised and become rancid, leading to a bad smell and taste. Nitrogen is an inert gas; it replaces oxygen and prevents oxidation, keeping the chips fresh.

7. Board Exam Scenario Masterclass

Case Study Logic: Why does the color of copper sulphate change when iron is dipped?

Answer: Because iron is more reactive than copper (refer reactivity series), it displaces copper from its salt solution to form Ferrous Sulphate ($FeSO_4$), which is light green in color.

Key Knowledge

Top 3 Reaction Balancings for Boards:

$2Pb(NO_3)_2 \xrightarrow{\Delta} 2PbO + 4NO_2 + O_2$ (Thermal Decomposition of Lead Nitrate)
$6CO_2 + 12H_2O \xrightarrow{Sunlight/Chlorophyll} C_6H_{12}O_6 + 6O_2 + 6H_2O$ (Photosynthesis)
$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Energy}$ (Burning of Methane)