Summary and Quick Revision

Chapter Summary

This section is a concise summary of all of Chapter 1 — perfect for a one-glance review the day before the exam.

Key Concepts of the Chapter

This chapter introduced you to the world of chemical reactions. We learned —

1. What a chemical reaction is and how to identify it (5 signs)
2. Writing and balancing chemical equations
3. Five types of reactions — combination, decomposition, displacement, double displacement, oxidation-reduction
4. Two energy types — exothermic and endothermic
5. Effects of oxidation in daily life — corrosion and rancidity

All these concepts are interconnected and essential for the board exam.

Key Definitions — At a Glance

1. Chemical Reaction

A process in which atomic bonds in the reactants break, new bonds form, and new substances are produced.

2. Reactants and Products

• Reactants: On the left of the arrow — what reacts.
• Products: On the right of the arrow — what is formed.

3. The 5 Signs of a Reaction — "S-C-G-T-P"

1. Change in State
2. Change in Colour
3. Evolution of Gas
4. Change in Temperature
5. Formation of Precipitate

4. Law of Conservation of Mass

In a chemical reaction, mass is neither created nor destroyed.

5. Balanced Equation

An equation in which the number of atoms of each element is the same on both sides.

6. Five Types of Reactions

Type	Form	Example
Combination	$A + B \rightarrow AB$	$CaO + H_2O \rightarrow Ca(OH)_2$
Decomposition	$AB \rightarrow A + B$	$CaCO_3 \rightarrow CaO + CO_2$
Displacement	$A + BC \rightarrow AC + B$	$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$
Double displacement	$AB + CD \rightarrow AD + CB$	$BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 + 2NaCl$
Oxidation/Reduction	Gain/loss of O/H	$CuO + H_2 \rightarrow Cu + H_2O$

7. Energy Types

• Exothermic: Heat is released (e.g., $CaO + H_2O$)
• Endothermic: Heat is absorbed (e.g., $Ba(OH)_2 + NH_4Cl$)

Must-Memorise Reactions

Combination Reactions

1. $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat}$ ⭐
2. $2Mg(s) + O_2(g) \rightarrow 2MgO(s) + \text{Heat}$
3. $C(s) + O_2(g) \rightarrow CO_2(g)$
4. $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$
5. $N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$ (Haber process)
6. $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$ (whitewash shine)

Decomposition Reactions

1. $CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g)$ (cement industry) ⭐
2. $2FeSO_4(s) \xrightarrow{\Delta} Fe_2O_3(s) + SO_2(g) + SO_3(g)$ (green → brown)
3. $2Pb(NO_3)_2(s) \xrightarrow{\Delta} 2PbO(s) + 4NO_2(g) + O_2(g)$ (brown smoke)
4. $2H_2O(l) \xrightarrow{\text{electricity}} 2H_2(g) + O_2(g)$
5. $2AgCl(s) \xrightarrow{\text{light}} 2Ag(s) + Cl_2(g)$

Displacement Reactions

1. $Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$ ⭐
2. $Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow$
3. $Cu + 2AgNO_3 \rightarrow Cu(NO_3)_2 + 2Ag$ (silver refining)
4. $Fe_2O_3 + 2Al \rightarrow 2Fe + Al_2O_3 + \text{Heat}$ (thermite)

Double Displacement Reactions

1. $BaCl_2 + Na_2SO_4 \rightarrow BaSO_4\downarrow + 2NaCl$ (white precipitate) ⭐
2. $Pb(NO_3)_2 + 2KI \rightarrow PbI_2\downarrow + 2KNO_3$ (yellow)
3. $AgNO_3 + NaCl \rightarrow AgCl\downarrow + NaNO_3$ (white)
4. $NaOH + HCl \rightarrow NaCl + H_2O$ (neutralisation)

Redox Reactions

1. $CuO + H_2 \rightarrow Cu + H_2O$ (H₂ oxidised, CuO reduced)
2. $ZnO + C \rightarrow Zn + CO$ (C oxidised, ZnO reduced)

⭐ — most frequently asked

Corrosion and Rancidity — Key Facts

Three Famous Examples of Corrosion

Metal	Corrosion product	Colour
Fe	$Fe_2O_3 \cdot xH_2O$ (rust)	Reddish-brown
Ag	$Ag_2S$	Black
Cu	$Cu(OH)_2 \cdot CuCO_3$	Green

Conditions for Rusting (Both Required)

1. Oxygen
2. Water/moisture

Methods to Prevent Corrosion

• Painting
• Galvanisation (Zn coat)
• Chrome plating
• Alloying (stainless steel)
• Cathodic protection

Rancidity

Oxidative spoilage of fat-containing foods by atmospheric $O_2$.

Methods to Prevent Rancidity

• Adding antioxidants (Vitamin E, C, BHA, BHT)
• Air-tight packaging
• Filling with nitrogen gas (chip packets!)
• Refrigeration
• Protection from light

Precipitate Colours — Memorise in a Minute

Precipitate	Colour
$BaSO_4$	White
$AgCl$	White
$AgBr$	Pale yellow
$AgI$	Yellow
$PbI_2$	Bright yellow (Golden Rain)
$PbCl_2$	White (soluble in hot water)
$Cu(OH)_2$	Blue
$Fe(OH)_3$	Brown
$CaCO_3$	White

Activity Series — Memorise This Order

$$K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au$$

Rule: A metal higher in the series can displace those below it.

State Symbols

Symbol	Meaning
$(s)$	Solid
$(l)$	Liquid
$(g)$	Gas
$(aq)$	Dissolved in water
$\uparrow$	Gas evolved
$\downarrow$	Precipitate formed
$\Delta$	Heating

Board's 'Golden' Questions — Read These Before the Exam

Most Important (Asked Every Year)

1. Why is a magnesium ribbon cleaned with sandpaper?

• Answer: To remove the MgO layer formed on its surface, so that pure Mg can react directly with air.

1. Why are iron objects painted?

• Answer: To protect from rust — paint isolates the iron from $O_2$ and moisture.

1. Why are oils/fatty foods flushed with nitrogen?

• Answer: To prevent rancidity — $N_2$ is inert and keeps $O_2$ out.

1. Why is respiration considered an exothermic reaction?

• Answer: Respiration produces new substances and releases energy (as ATP).

1. Why is the volume of $H_2$ twice that of $O_2$ in the electrolysis of water?

• Answer: $2H_2O \rightarrow 2H_2 + O_2$ — ratio 2:1; by Avogadro's law.

1. Why does a whitewashed wall develop a shine after 2-3 days?

• Answer: $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$ — shiny $CaCO_3$ forms.

1. What happens when ferrous sulphate is heated?

• Answer: Green crystals turn into brown $Fe_2O_3$ + pungent smell of $SO_2 + SO_3$.

Important (Frequent)

8. Difference between combination and decomposition
9. Difference between displacement and double displacement
10. Definition and examples of precipitation reactions
11. Definitions of oxidation and reduction
12. Activity 1.10 experiment
13. Difference between corrosion and rancidity
14. Use of the thermite reaction

Numerical (1-2 Sure in Board)

15. Unknown mass from conservation of mass
16. Mass of product from mole concept
17. Volume of gas at STP

Numerical Tricks

1. Conservation of Mass

Total mass of reactants = Total mass of products

Example: If 5 g $CaCO_3$ → 2.8 g $CaO$, then $CO_2 = 5 - 2.8 = 2.2$ g.

2. Mole Ratio

• 1 mole = molar mass in g
• $H_2 = 2$ g, $O_2 = 32$ g, $N_2 = 28$ g, $H_2O = 18$ g, $CO_2 = 44$ g

Example: $C + O_2 \rightarrow CO_2$ 12 g C + 32 g O₂ → 44 g CO₂ (always this ratio).

3. Volume at STP

• 1 mole of any gas = 22.4 litres (at STP)
• 22.4 L of $H_2$ = 2 g
• 22.4 L of $O_2$ = 32 g

Example: $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$ 65 g Zn (1 mol) → 22.4 L $H_2$

4. The General Ratio Rule

Mass-ratio (or volume-ratio) formula: $$\frac{\text{Mass of given reactant}}{\text{Molar mass of reactant}} = \frac{\text{Mass of product}}{\text{Molar mass of product}}$$

(combined with the mole ratio)

Exam Strategy

The Night Before

1. Read this summary section twice.
2. Once-write all the must-memorise reactions to test yourself.
3. Memorise the activity series and precipitate colours.

On Exam Day — Time Management

• 15-20 minutes: MCQ and very short questions
• 30-40 minutes: 2-3 mark questions
• 40-50 minutes: 5-mark long-answer questions
• 10 minutes: Revision and checking

Rules for Writing Answers

1-mark: Direct answer, no reasoning needed.

2-mark: A 1-2 line definition or explanation + one example.

3-mark:

• Definition (1 mark)
• Explanation (1 mark)
• Example/equation (1 mark)

5-mark:

• Definition / introduction (1 mark)
• Detailed explanation (2 marks)
• Two examples or a diagram (1 mark)
• Application or conclusion (1 mark)

Remember

• Always write balanced equations.
• Include state symbols $(s), (l), (g), (aq)$.
• Write neatly.
• Show every step in calculations.
• End with a conclusion.

🧠 Final Mega Memory Capsule

This is the climax summary of the entire chapter — read it 1 hour before the exam.

5 Signs — "S-C-G-T-P"

State | Colour | Gas | Temperature | Precipitate

5 Types of Reactions

1. Combination ($A+B \rightarrow AB$) — combustion, slaking lime
2. Decomposition ($AB \rightarrow A+B$) — cement industry, photography
3. Displacement ($A + BC \rightarrow AC + B$) — Fe+CuSO₄, thermite
4. Double displacement ($AB + CD \rightarrow AD + CB$) — precipitation, neutralisation
5. Redox — oxidation + reduction together

Energy

• Exothermic: Heat released (beaker hot) — usually combination
• Endothermic: Heat absorbed (beaker cold) — usually decomposition

O/H Gain-Loss Rules

• Oxidation: O+ or H-
• Reduction: O- or H+

Activity Series

$K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au$

Precipitate Colours

• White: BaSO₄, AgCl, PbCl₂
• Yellow: PbI₂, AgI
• Blue: Cu(OH)₂
• Brown: Fe(OH)₃

Corrosion

• Fe → red rust ($Fe_2O_3 \cdot xH_2O$)
• Ag → black ($Ag_2S$)
• Cu → green ($Cu(OH)_2 \cdot CuCO_3$)
• Conditions: O₂ + H₂O
• Prevention: paint, galvanisation (Zn)

Rancidity

• Oxidation of fats/oils by $O_2$
• Prevention: antioxidants, $N_2$ packing, refrigeration

The 5 'Golden' Reactions for Board

1. $2Mg + O_2 \rightarrow 2MgO$
2. $CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$
3. $2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$
4. $Fe + CuSO_4 \rightarrow FeSO_4 + Cu$
5. $BaCl_2 + Na_2SO_4 \rightarrow BaSO_4\downarrow + 2NaCl$

The Bottom Line: The reactions are interesting, the structure is simple, and with practice everything is achievable.

Best wishes for your board exam! 🎉

Quick Revision — 5 Solved Examples

Example 1: Most Common Balancing Question

Balance $Mg + O_2 \rightarrow MgO$.

Solution:

• Balance O: $MgO$ × 2: $$Mg + O_2 \rightarrow 2MgO$$
• Balance Mg: $Mg$ × 2: $$2Mg(s) + O_2(g) \rightarrow 2MgO(s)$$ ✓

Example 2: Identify Reaction Types

Give the type of each: (a) $C + O_2 \rightarrow CO_2$ (b) $2H_2O \rightarrow 2H_2 + O_2$ (c) $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$ (d) $NaOH + HCl \rightarrow NaCl + H_2O$

Solution:

• (a) Combination — two → one
• (b) Decomposition — one → two
• (c) Displacement — Zn is a free element
• (d) Neutralisation (double displacement) — acid + base

Example 3: Numerical — Conservation of Mass

How much $CaO$ and $CO_2$ are formed from heating 10 g of $CaCO_3$?

Solution:

1. Reaction: $CaCO_3 \rightarrow CaO + CO_2$
2. Molecular masses: $CaCO_3 = 100$, $CaO = 56$, $CO_2 = 44$
3. Ratio: 100 g → 56 g + 44 g
4. From 10 g CaCO₃:

• CaO = $\frac{56}{100} \times 10 = 5.6$ g
• CO₂ = $\frac{44}{100} \times 10 = 4.4$ g

1. Verification: $5.6 + 4.4 = 10$ ✓

Example 4: Identify Redox

In $ZnO + C \rightarrow Zn + CO$, what is oxidised/reduced?

Solution:

• C gained O (formed CO) → C oxidised
• ZnO lost O (formed Zn) → ZnO reduced
• Oxidising agent: ZnO
• Reducing agent: C
• This is a redox reaction.

Example 5: Combined Question — Corrosion and Rancidity

How do we protect iron from rust? How do we prevent chips from going stale?

Solution:

Methods to protect iron from rust:

1. Painting — isolates iron from $O_2$ and moisture.
2. Galvanisation — Zn coat; Zn corrodes first.
3. Alloying — stainless steel doesn't rust.
4. Oiling/greasing — for machine parts.

Methods to prevent chips from going stale:

1. Filling with nitrogen gas (the main method).
2. Adding antioxidants (BHA, BHT).
3. Air-tight packaging.

Both work on the same principle: Keep the substance away from $O_2$.