ACIDS, BASES AND SALTS

Chapter 02 | High-Fidelity Board Study Module

1. The Chemistry of Indicators

Indicators are chemical substances that change their properties (color or smell) when exposed to an acid or a base.
Deep-Cut Fact: Litmus is a purple dye extracted from Lichen, a plant belonging to the Thallophyta division.

Activity 2.1: Laboratory Indicators

Procedure: Test various acid and base samples (HCl, NaOH, etc.) with different indicators.

Observation:
� Acids: Blue Litmus ? Red; Phenolphthalein ? Colorless.
� Bases: Red Litmus ? Blue; Phenolphthalein ? Pink.

Indicator Type	Example	Color in Acid	Color in Base
Natural	Litmus Solution	Red	Blue
Natural	Turmeric	Yellow (No Change)	Reddish-Brown
Synthetic	Phenolphthalein	Colorless	Deep Pink
Synthetic	Methyl Orange	Red/Pink	Yellow
Natural (Flowers)	Hydrangea	Blue	Pink
Natural (Flowers)	Petunia / Geranium	Reddish/Violet	Blue

Activity 2.2: Olfactory Indicators

Definition: Substances whose odour (smell) changes in acidic or basic media.

Procedure: Test finely chopped onions, vanilla essence, and clove oil with dilute HCl and NaOH.

Key Observations (Board Favorite):

Onion: Characteristic smell disappears in basic solution (NaOH).
Vanilla Essence: Smell disappears in NaOH; retained in HCl.
Clove Oil: Smell disappears in NaOH.

Fig: Color variations in common indicators

2. Chemical Properties (Reaction Mechanics)

Understanding how acids and bases interact with different substances is crucial for predicting products and identifying unknown compounds.

Activity 2.3 & 2.4: Reaction with Metals

Procedure: Add Zinc granules to dilute $H_2SO_4$ (Acid) and then to $NaOH$ (Base).

Key Observations:

With Acid: Hydrogen gas evolves. It burns with a 'Pop' sound when tested with a candle.
Equation: $Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)\uparrow$
Acid + Metal $\rightarrow$ Salt + Hydrogen gas

With Base: Zinc reacts with Sodium Hydroxide to form Sodium Zincate and Hydrogen.
Equation: $2NaOH(aq) + Zn(s) \rightarrow Na_2ZnO_2(aq) + H_2(g)\uparrow$
Base + Metal $\rightarrow$ Salt + Hydrogen gas

Note: Such reactions are not possible with all metals.

Activity 2.5: Carbonates & Bicarbonates

Procedure: Add HCl to Sodium Carbonate (Test Tube A) and Sodium Bicarbonate (Test Tube B). Pass the gas through Lime Water.

The Chemical Reactions:

Test Tube A: $Na_2CO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)$
Test Tube B: $NaHCO_3(s) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l) + CO_2(g)$

The Lime Water Test (CBSE Classic):

Step 1 (Milky): Lime water turns milky due to the formation of a white precipitate of Calcium Carbonate.
Reaction: $Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) \text{ (White ppt)} + H_2O(l)$
Step 2 (Soluble): On passing excess $CO_2$, the milkiness disappears as soluble Calcium Bicarbonate is formed.
Reaction: $CaCO_3(s) + H_2O(l) + CO_2(g) \rightarrow Ca(HCO_3)_2(aq) \text{ (Soluble in water)}$

Summary Reaction

Metal carbonate/hydrogencarbonate + Acid $\rightarrow$ Salt + CO2 + H2O

Note: Limestone, chalk and marble are different forms of Calcium Carbonate ($CaCO_3$).

Activity 2.6: Neutralization Reaction

Concept: When an acid and a base react, they cancel each other's effects to form Salt and Water.

Reaction: $NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l) + \text{Heat}$

Base + Acid $\rightarrow$ Salt + Water

Observation: The reaction is exothermic. Using phenolphthalein, the pink color disappears when the acid neutralizes the base.

Activity 2.7: Metallic Oxide + Acid

Observation: Black Copper Oxide ($CuO$) dissolves in HCl to form a blue-green solution of Copper Chloride ($CuCl_2$).

Metal Oxide + Acid $\rightarrow$ Salt + Water

Conclusion: Since metallic oxides react with acids to give salt and water, they are Basic in nature.

Board Favorite

Nature of Oxides

Metallic Oxides (e.g., $MgO, CuO$): Basic in nature. (Metal Oxide + Acid $\rightarrow$ Salt + Water)
Non-Metallic Oxides (e.g., $CO_2$): Acidic in nature. (Non-metal Oxide + Base $\rightarrow$ Salt + Water)

3. What Happens to an Acid or a Base in a Water Solution?

This section explores the behavior of acids and bases at the molecular level, focusing on ion formation.

Activity 2.8: Do all compounds containing hydrogen show acidic behavior?

Experiment: Test conductivity of Glucose, Alcohol, HCl, and NaOH solutions.

Result:
� The bulb glows for HCl and NaOH because they dissociate into ions ($H^+$ and $OH^-$).
� The bulb does NOT glow for Glucose ($C_6H_{12}O_6$) and Alcohol ($C_2H_5OH$) as they do not form ions.

Inference: Only substances that ionize in water act as acids or bases.

Activity 2.9: Is water necessary for acidity?

Experiment: Test dry HCl gas with dry and wet blue litmus paper.

Observation:
� Dry Litmus: No color change.
� Wet Litmus: Turns Red.

Conclusion (Board Exam Point): HCl dissociates into $H^+$ ions only in the presence of water. Separation of $H^+$ ions from $HCl$ molecules cannot occur in the absence of water.

Formation of Hydronium Ions:

$HCl + H_2O \rightarrow H_3O^+ + Cl^-$
Hydrogen ions cannot exist alone; they exist after combining with water molecules:
$H^+ + H_2O \rightarrow H_3O^+$

Dissociation of Bases

When a base is dissolved in water, it generates Hydroxide ($OH^-$) ions:

$NaOH(s) \xrightarrow{H_2O} Na^+(aq) + OH^-(aq)$
$KOH(s) \xrightarrow{H_2O} K^+(aq) + OH^-(aq)$
$Mg(OH)_2(s) \xrightarrow{H_2O} Mg^{2+}(aq) + 2OH^-(aq)$

Bases generate hydroxide (OH-) ions in water. Bases which are soluble in water are called alkalis.

Activity 2.10: The Safety Protocol (Dilution)

Concept: Dissolving an acid or a base in water is highly Exothermic.

The Rule: Always add Acid to Water slowly with constant stirring.
Warning: Never add water to acid! The extreme heat generated can cause the mixture to splash out (causing burns) or break the glass container.

Concept

Dilution

Mixing an acid or base with water results in a decrease in the concentration of ions ($H_3O^+$ or $OH^-$) per unit volume. This process is called Dilution.

4. The pH Scale: Strength of Acids & Bases

A scale for measuring hydrogen ion concentration in a solution, called pH scale, has been developed. The 'p' in pH stands for potenz in German, meaning power.

Fig: The Universal pH Scale Gradient

Activity 2.11: Testing pH

Procedure: Test various solutions with Universal Indicator paper.

Substance	Approx. pH	Nature
Gastric Juice	1.2	Highly Acidic
Lemon Juice	2.2	Acidic
Pure Water/Blood	7.4	Slightly Basic
Milk of Magnesia	10.0	Basic
Sodium Hydroxide	14.0	Highly Basic

Activity 2.12: pH of Soil

Concept: Plants require a specific pH range for healthy growth. If soil is too acidic, it is treated with Quick Lime ($CaO$) or Slaked Lime ($Ca(OH)_2$). If too basic, organic matter is added.

Science Trivia

The Atmosphere of Venus

Did you know? The atmosphere of Venus is made of thick white and yellowish clouds of Sulphuric Acid. Survival of life is impossible there!

Importance of pH in Everyday Life

Context	Board Importance & Facts
Acid Rain	When rain water pH is less than 5.6. It makes survival of aquatic life difficult.
Digestive System	Stomach produces HCl. During indigestion, excess acid causes pain. Antacids (like Milk of Magnesia) are mild bases used to neutralize it.
Tooth Decay	Starts when mouth pH falls below 5.5. Tooth enamel is made of Calcium hydroxyapatite (crystalline calcium phosphate)�the hardest substance in the body. It doesn't dissolve in water but corrodes when pH is low.
Self Defense	Honey-bee sting leaves Methanoic Acid. Rubbing a mild base like Baking Soda provides relief.
Nettle Sting	Nettle leaves inject Methanoic acid. Traditional remedy: Rubbing the leaf of a Dock Plant (Basic) which often grows beside Nettle in the wild.

Nature's Chemistry

Naturally Occurring Acids

For board exams, you must memorize the source and the specific acid present in these natural substances:

Natural Source	Acid	Natural Source	Acid
Vinegar	Acetic acid	Curd (Sour Milk)	Lactic acid
Orange	Citric acid	Lemon	Citric acid
Tamarind	Tartaric acid	Ant sting	Methanoic acid
Tomato	Oxalic acid	Nettle sting	Methanoic acid

Do You Know?

Nature Provides Neutralisation Options

Nettle is a herbaceous plant which grows in the wild. Its leaves have stinging hair, which cause painful stings when touched accidentally. This is due to the Methanoic acid secreted by them.

Traditional Remedy: Rubbing the area with the leaf of the Dock Plant, which often grows beside the nettle in the wild. Since it cures the sting, we can guess that the dock plant is Basic in nature.

5. More About Salts

Salts are formed by the neutralization of an acid and a base.
NCERT Note: Rock Salt crystals are often brown due to impurities. These salt beds were formed when seas of "bygone ages" dried up. It is mined like coal.

Activity 2.13 & 2.14: Families & pH

Concept: Salts can be classified into families based on their radicals (e.g., Chloride family, Sulphate family).

Salt Combination	Nature	pH
Strong Acid + Strong Base	Neutral	7
Strong Acid + Weak Base	Acidic	< 7
Weak Acid + Strong Base	Basic	> 7

Chemicals from Common Salt ($NaCl$)

Common salt is a critical raw material for various chemicals used in daily life and industry.

Fig: The NaCl Derivative Ecosystem (The Salt Family)

1. Chlor-Alkali Process

Method: Electrolysis of aqueous solution of $NaCl$ (called Brine).

Equation: $2NaCl(aq) + 2H_2O(l) \rightarrow 2NaOH(aq) + Cl_2(g) + H_2(g)$

At Anode (+): Chlorine gas ($Cl_2$) - Used for water treatment, PVC, disinfectants.
At Cathode (-): Hydrogen gas ($H_2$) - Used as fuel, margarine, ammonia for fertilizers.
Near Cathode: $NaOH$ solution - Used for degreasing metals, soaps, paper making.

Chemical 02

Bleaching Powder ($CaOCl_2$)

Preparation: Action of chlorine on dry slaked lime [$Ca(OH)_2$].

Equation: $Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$

Uses:
� Bleaching cotton/linen in textiles and wood pulp in paper.
� Oxidizing agent in chemical industries.
� Disinfecting drinking water.

Chemical 03

Baking Soda ($NaHCO_3$)

Chemical Name: Sodium Hydrogen Carbonate.

Preparation: $NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3$

Heating Effect: $2NaHCO_3 \xrightarrow{\text{Heat}} Na_2CO_3 + H_2O + CO_2\uparrow$

Uses: Making baking powder (Soda + Tartaric acid), Antacid, Soda-acid fire extinguishers.

Chemical 04

Washing Soda ($Na_2CO_3 \cdot 10H_2O$)

Preparation: Recrystallization of Sodium Carbonate.

Equation: $Na_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O$

Uses:
� Glass, soap, and paper industries.
� Manufacture of Borax.
� Removing permanent hardness of water.

Chemical 05

Plaster of Paris (P.O.P)

Chemical Name: Calcium Sulphate Hemihydrate ($CaSO_4 \cdot \frac{1}{2}H_2O$).

Preparation: Heating Gypsum ($CaSO_4 \cdot 2H_2O$) at 373 K.

The "Half Molecule" Logic:

Q. How can you get half a water molecule?

Ans: It is written in this form because two formula units of $CaSO_4$ share one molecule of water . This is why it's called a hemihydrate.

Setting (Reaction with Water): On mixing with water, it changes back to Gypsum, giving a hard solid mass.

$CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O \text{ (Gypsum)}$

Uses: Supporting fractured bones, making toys, decoration, smooth surfaces.

6. Are the Crystals of Salts really Dry?

Activity 2.15: Water of Crystallization Heating Copper Sulphate Setup

Experiment: Heat crystals of Copper Sulphate in a dry test tube.

Observation:
� Blue color of crystals turns White.
� Water droplets appear on the sides of the tube.
� On adding water again, the Blue color reappears.

Definition: Water of Crystallization is the fixed number of water molecules present in one formula unit of a salt.

Example: Hydrated Copper Sulphate ($CuSO_4 \cdot 5H_2O$).

Senior Teacher's Desk

Sure-Shot Board Topic: The Lime Water Test

Expect a question on why milkiness disappears with excess $CO_2$. Always mention the formation of soluble Calcium Bicarbonate [$Ca(HCO_3)_2$].

Common Confusion: Alkali vs Base

Remember: "All Alkalis are Bases, but all Bases are not Alkalis." Only water-soluble bases like $NaOH, KOH, Ca(OH)_2$ are alkalis.

Conceptual Logic: Ionization

Acids do NOT show acidic behavior in organic solvents like Toluene or Alcohol because they don't produce $H^+$ ions there. Water is a MUST.

7. Vardaan Practice Zone (Board-Exam Focused)

Q & A Q2. What is the common name of $CaOCl_2$? Write its reaction with water.
Ans: Common name is Bleaching Powder. When it reacts with water (or atmospheric moisture), it releases Chlorine gas: $CaOCl_2 + H_2O \rightarrow Ca(OH)_2 + Cl_2\uparrow$.

Q & A Q3. A substance X is used as an antacid. When heated, it releases gas Y which turns lime water milky. Identify X and Y.
Ans: X is Baking Soda ($NaHCO_3$) and Y is Carbon Dioxide ($CO_2$).

HOTS Challenge Q4. Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9 respectively. Arrange them in increasing order of $H^+$ ion concentration.
Ans: Higher the pH, lower the $H^+$ concentration. Order of pH: $C (11) > E (9) > D (7) > A (4) > B (1)$. Increasing order of $H^+$: C < E < D < A < B.

HOTS Challenge Q5. A compound 'X' is obtained from a salt 'Y' by the chlor-alkali process. 'X' reacts with 'Z' (dry slaked lime) to form bleaching powder. Identify X, Y, and Z.
Ans: � Y: Sodium Chloride ($NaCl$) - raw material for Chlor-Alkali.
� X: Chlorine gas ($Cl_2$) - product of Chlor-Alkali.
� Z: Calcium Hydroxide [$Ca(OH)_2$] - reacts with $Cl_2$ to form Bleaching Powder.

Q & A Q6. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid? (CBSE 2022)
Ans: The process of dissolving an acid in water is highly exothermic. If water is added to concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating. Adding acid to water slowly with stirring allows the heat to dissipate safely.

Q & A Q7. Plaster of Paris should be stored in a moisture-proof container. Why? (NCERT)
Ans: POP is a white powder that, on mixing with water (even moisture from air), sets into a hard solid mass called Gypsum. This makes the POP useless for its intended purposes.
$CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$

Q & A Q8. What is the role of Tartaric Acid in baking powder?
Ans: Baking powder is a mixture of baking soda ($NaHCO_3$) and a mild edible acid like tartaric acid. When heated or mixed in water, the tartaric acid reacts with the soda to neutralize the bitter taste of the sodium carbonate ($Na_2CO_3$) formed during heating, and it helps release $CO_2$.

Q & A Q9. Why does distilled water not conduct electricity, whereas rain water does? (Board 2018)
Ans: Distilled water is a pure form of water and does not contain any dissolved salts or ions. Rain water, while falling through the atmosphere, dissolves gases like $CO_2$ and $SO_2$ to form acids (like carbonic acid), which dissociate into ions. These ions allow the conduction of electricity.

Q & A Q10. What are Strong and Weak Acids? Give examples. (CBSE 2019)
Ans:
� Strong Acids: Acids that dissociate completely into ions in water. E.g., $HCl$, $H_2SO_4$, $HNO_3$.
� Weak Acids: Acids that dissociate only partially in water. E.g., $CH_3COOH$ (Acetic acid), $H_2CO_3$ (Carbonic acid).

Q & A Q11. Name the acid present in the following: (Board 2021)
Ans:
� Vinegar: Acetic Acid.
� Curd: Lactic Acid.
� Lemon/Orange: Citric Acid.
� Tamarind: Tartaric Acid.

Q & A Q12. What is the nature of Sodium Carbonate and Sodium Hydrogen Carbonate?
Ans: Both are Basic salts because they are formed from a strong base ($NaOH$) and a weak acid ($H_2CO_3$).

Q & A Q13. Write the chemical equation for the preparation of Baking Soda.
Ans: It is produced using the Solvay process:
$NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3$

Q & A Q14. Why is tartaric acid added to baking soda to make baking powder?
Ans: When baking soda is heated, it forms sodium carbonate which has a bitter taste. Tartaric acid reacts with this sodium carbonate to neutralize it and produce a pleasant-tasting sodium tartrate salt.

Q & A Q15. Why does an aqueous solution of an acid conduct electricity?
Ans: An aqueous solution of an acid dissociates into ions (like $H^+$). These ions act as charge carriers and allow the flow of electric current through the solution.