Summary and Quick Revision

Chapter Summary

This section is a concise summary of all of Chapter 2 — perfect for a one-glance review the day before the exam.

Key Concepts of the Chapter

1. Indicators: identifying acids and bases
2. Chemical properties: reactions with metals, carbonates, oxides
3. Neutralisation: acid + base → salt + water
4. Ionic nature: acid = $H^+$; base = $OH^-$
5. pH scale: 0 to 14, importance in daily life
6. Salts: family, pH, water of crystallisation
7. Chemicals from common salt: 5 important chemicals

Key Definitions — At a Glance

1. Acids and Bases

• Acid: a substance that releases $H^+$ (or $H_3O^+$) in water. Sour. Blue litmus → red.
• Base: a substance that releases $OH^-$ in water. Bitter, slippery. Red litmus → blue.

2. Three Types of Indicators

Type	Examples
Natural	Litmus, turmeric, red cabbage
Synthetic	Phenolphthalein, methyl orange
Olfactory	Onion, vanilla, clove oil

3. Indicator Colours

Indicator	Acid	Base
Red litmus	Red	Blue
Blue litmus	Red	Blue
Phenolphthalein	Colourless	Pink
Methyl orange	Red	Yellow

4. Neutralisation

Acid + Base → Salt + Water Ionic: $H^+ + OH^- \rightarrow H_2O$

5. The pH Scale

• 0 to 14
• pH < 7: acidic
• pH = 7: neutral (pure water)
• pH > 7: basic
• $\text{pH} = -\log[H^+]$

Must-Memorise Reactions

Acid-Metal Reactions

1. $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow$ ⭐
2. $Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow$
3. $Mg + 2HCl \rightarrow MgCl_2 + H_2\uparrow$

Base-Metal (Amphoteric)

4. $Zn + 2NaOH \rightarrow Na_2ZnO_2 + H_2\uparrow$

Acid-Carbonate

5. $Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2\uparrow$ ⭐
6. $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow$
7. $NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2\uparrow$

Neutralisation

8. $NaOH + HCl \rightarrow NaCl + H_2O$ ⭐
9. $H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O$
10. $Mg(OH)_2 + 2HCl \rightarrow MgCl_2 + 2H_2O$ (antacid)

Oxide Reactions

11. $CuO + 2HCl \rightarrow CuCl_2 + H_2O$ (black → bluish-green)
12. $Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O$ (lime water turns milky)

Production of Important Chemicals

13. $2NaCl + 2H_2O \xrightarrow{\text{electricity}} 2NaOH + Cl_2 + H_2$ (chlor-alkali)
14. $Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$ (bleaching powder)
15. $2NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2$ (heated baking soda)
16. $Na_2CO_3 + CaCl_2 \rightarrow CaCO_3\downarrow + 2NaCl$ (softening hard water)
17. $CaSO_4 \cdot 2H_2O \xrightarrow{373 K} CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O$ (POP)

⭐ — most frequently asked

pH Values in Daily Life

Substance	pH
Stomach juice	1-2
Lemon	2-3
Vinegar	3
Tomato	4-4.5
Milk	6.4-6.8
Pure water	7
Blood	7.4
Sea water	7.5-8.4
Baking soda	8-9
Ammonia	11-12
Conc. $NaOH$	14

Preventing Tooth Decay

Tooth enamel dissolves below pH 5.5.

Prevention:

• Rinse mouth after meals
• Use basic toothpaste
• Reduce sugar
• Fluoride-containing toothpaste

Treating Stings

Sting	Nature	Treatment
Bee	Acidic	Baking soda
Ant	Acidic (formic)	Baking soda
Wasp	Basic	Vinegar/lemon

Acid Rain

• Normal rain pH ~ 5.6
• Acid rain pH < 5.6
• Causes: $SO_2$, $NO_2$
• Effects: fish, soil, the Taj Mahal

Correcting Soil pH

• Acidic soil: slaked lime ($Ca(OH)_2$)
• Alkaline soil: gypsum ($CaSO_4 \cdot 2H_2O$)

Salts — Four pH Types

Parents	Salt's nature	Examples
Strong + strong	Neutral (pH = 7)	$NaCl, K_2SO_4$
Strong + weak	Acidic (pH < 7)	$NH_4Cl, CuSO_4$
Weak + strong	Basic (pH > 7)	$Na_2CO_3, CH_3COONa$
Weak + weak	Mixed	$CH_3COONH_4$

Families of Salts

• Chloride family: $NaCl, KCl, CaCl_2, NH_4Cl$
• Sulphate family: $Na_2SO_4, K_2SO_4, CuSO_4, FeSO_4$
• Sodium family: $NaCl, Na_2SO_4, NaNO_3, Na_2CO_3, NaHCO_3$

Famous Salts with Water of Crystallisation

Salt	Formula	Water molecules
Blue vitriol	$CuSO_4 \cdot 5H_2O$	5
Washing soda	$Na_2CO_3 \cdot 10H_2O$	10
Gypsum	$CaSO_4 \cdot 2H_2O$	2
POP	$CaSO_4 \cdot \frac{1}{2}H_2O$	1/2
Green vitriol	$FeSO_4 \cdot 7H_2O$	7

Five Chemicals from Common Salt

Chemical	Formula	Common name	Use
Sodium hydroxide	$NaOH$	Caustic soda	Soap
Bleaching powder	$CaOCl_2$	—	Bleaching, water purification
Baking soda	$NaHCO_3$	—	Cakes, antacid
Washing soda	$Na_2CO_3 \cdot 10H_2O$	Soda ash	Laundry
Plaster of Paris	$CaSO_4 \cdot \frac{1}{2}H_2O$	POP	Plaster casts

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

Most Important (Yearly)

1. What colour does phenolphthalein turn in $HCl$ and $NaOH$?

• Answer: colourless in $HCl$; pink in $NaOH$.

1. What precaution when diluting concentrated acid?

• Answer: always add acid to water — never water to acid!

1. Treatment for acidity and chemical reaction?

• Answer: milk of magnesia. $Mg(OH)_2 + 2HCl \rightarrow MgCl_2 + 2H_2O$

1. How does tooth decay happen? Prevention?

• Answer: sugar + bacteria → lactic acid → pH drops → enamel dissolves. Prevention: brush, less sugar.

1. What are the effects of acid rain?

• Answer: river pH falls, fish die, marble dissolves.

1. What to apply on a bee sting? Why?

• Answer: baking soda. Because the sting is acidic (formic) — base neutralises.

1. Three products of the chlor-alkali process?

• Answer: $NaOH$ (in solution), $Cl_2$ (anode), $H_2$ (cathode).

1. Formula, preparation, and use of bleaching powder?

• Answer: $CaOCl_2$. $Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$. Bleaching, water purification.

1. Baking soda vs washing soda?

• Answer: $NaHCO_3$ vs $Na_2CO_3 \cdot 10H_2O$. No water of crystallisation/10. pH 8.5/11. Uses: kitchen/laundry.

1. Why store POP away from moisture?
   • Answer: moisture turns POP back into gypsum — POP becomes useless.

Important (Frequent)

• pH-scale introduction and examples
• Heating $CuSO_4 \cdot 5H_2O$
• Definition of water of crystallisation
• Treating acidic/alkaline soils

Numerical (1-2 sure in board)

• pH from $[H^+]$
• Numerical neutralisation problems
• pH-difference ratios (logarithmic)
• Percentage of water of crystallisation

Numerical Tricks

1. pH Calculation

$$\text{pH} = -\log_{10}[H^+]$$ $$[H^+] = 10^{-\text{pH}}$$

Example: $[H^+] = 10^{-3}$ M → pH = 3 (acidic)

2. Ratio of pH Difference

A 1-unit change in pH = 10× change in $[H^+]$.

Example: pH 2 is 1000 times more acidic than pH 5.

3. Conservation of Mass

Mass of reactants = mass of products.

Example: $NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$

• Molar masses: 84 + 36.5 = 58.5 + 18 + 44 = 120.5 ✓

4. Mole Ratio for Products

Example: $H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O$

• 1 mol $H_2SO_4$ + 2 mol $NaOH$ → 1 mol $Na_2SO_4$ + 2 mol $H_2O$
• 98 g + 80 g → 142 g + 36 g

5. Percentage of Crystal Water

$$\% H_2O = \frac{n \times 18}{\text{total molar mass}} \times 100$$

Example: $CuSO_4 \cdot 5H_2O$ — total = 250; water = 90 → 36% water

Exam Strategy

The Night Before

1. Read this summary section twice.
2. Once-write all the must-memorise reactions to test yourself.
3. Memorise pH values, indicator 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 $(s), (l), (g), (aq)$.
• $\uparrow$ for gas, $\downarrow$ for precipitate.
• 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.

Indicator Colours

• Litmus: acid red; base blue
• Phenolphthalein: acid colourless; base pink
• Methyl orange: acid red; base yellow

Acid/Base Essentials

• Acid = $H^+$
• Base = $OH^-$
• $H^+ + OH^- \rightarrow H_2O$ (neutralisation)

pH Scale

• 0 to 14
• 7 = neutral
• $\text{pH} = -\log[H^+]$

Universal Indicator Colours

• 1-3: red; 7: green; 11-14: violet

Daily-life pH

• Stomach 1-2; blood 7.4; acid rain < 5.6

Stings and Treatment

• Bee acidic → baking soda
• Wasp basic → vinegar

Soil Correction

• Acidic → slaked lime
• Alkaline → gypsum

Four pH Types of Salts

• Strong+strong: neutral
• Strong+weak: acidic
• Weak+strong: basic
• Weak+weak: mixed

Five Examples of Crystal Water

• Blue vitriol $CuSO_4 \cdot 5H_2O$ (5)
• Washing soda $Na_2CO_3 \cdot 10H_2O$ (10)
• Gypsum $CaSO_4 \cdot 2H_2O$ (2)
• POP $CaSO_4 \cdot \frac{1}{2}H_2O$ (1/2)
• Green vitriol $FeSO_4 \cdot 7H_2O$ (7)

Five Chemicals from NaCl

1. $NaOH$ (caustic soda) — soap
2. $CaOCl_2$ (bleaching powder) — bleaching/water purification
3. $NaHCO_3$ (baking soda) — cakes/antacid
4. $Na_2CO_3 \cdot 10H_2O$ (washing soda) — laundry
5. $CaSO_4 \cdot \frac{1}{2}H_2O$ (POP) — bone plaster

Board's 5 'Golden' Reactions

1. $NaOH + HCl \rightarrow NaCl + H_2O$ (neutralisation)
2. $Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2$ (carbonate test)
3. $NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$ (acidity)
4. $Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O$ (lime water)
5. $2NaCl + 2H_2O \rightarrow 2NaOH + Cl_2 + H_2$ (chlor-alkali)

Precautions

• Concentrated acid → into water, never the reverse
• POP → away from moisture
• Caustic soda → goggles/gloves

The Bottom Line: Acid sour-$H^+$, base bitter-$OH^-$, neutralisation = salt + water. pH 7 is neutral, present everywhere in daily life.

Best wishes for your board exam! 🎉

Quick Revision — 5 Solved Examples

Example 1: Indicator Colours

State the colour of phenolphthalein in — (a) dilute $HCl$ (b) dilute $NaOH$ (c) pure water

Solution:

• (a) dilute $HCl$ (acid): colourless
• (b) dilute $NaOH$ (base): pink
• (c) pure water (neutral): colourless

Example 2: Numerical — pH Calculation

If $[H^+] = 10^{-5}$ M, what is the pH? What is the nature?

Solution:

$$\text{pH} = -\log(10^{-5}) = 5$$

Solution: weakly acidic (pH < 7).

Examples: coffee, tomato.

Example 3: Treating Acidity

Why is baking soda taken for acidity? Chemical reaction.

Solution:

Acidity = excess $HCl$ in stomach. Baking soda is basic — neutralises the acid.

$$NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2\uparrow$$

Stomach pH normalises → burning eases.

Example 4: Nature of Salts

State the nature of $NaCl$, $NH_4Cl$, $Na_2CO_3$.

Solution:

Salt	Parent Acid/Base	Nature
$NaCl$	Strong + strong	Neutral (pH=7)
$NH_4Cl$	Strong + weak	Acidic (pH<7)
$Na_2CO_3$	Weak + strong	Basic (pH>7)

Example 5: Five Chemicals from Common Salt

Name 5 important chemicals derived from common salt and one use each.

Solution:

Chemical	Formula	Use
1. Caustic soda	$NaOH$	Soap industry
2. Bleaching powder	$CaOCl_2$	Water purification
3. Baking soda	$NaHCO_3$	Cakes/biscuits
4. Washing soda	$Na_2CO_3 \cdot 10H_2O$	Laundry
5. Plaster of Paris	$CaSO_4 \cdot \frac{1}{2}H_2O$	Plaster casts for broken bones

Lesson: Common salt is the foundation of the chemical industry.