Chapter 2: Acids, Bases, and Salts

Acids

• Taste: Acids have a sour taste.
• Litmus Test: Acids turn blue litmus paper red.
• Chemical Properties: Acids produce H⁺ ions (hydrogen ions) in aqueous solutions.
• Origin of the Term: The word Acid is derived from the Latin word meaning "sour."
  
  

1. Strong Acids:

• HCl (Hydrochloric Acid)
• H₂SO₄ (Sulfuric Acid)
• HNO₃ (Nitric Acid)
  
  

2. Weak Acids:

• CH₃COOH (Acetic Acid, found in vinegar)
• Lactic Acid (found in milk)
• Oxalic Acid (found in plants)
  
  

3. Concentrated Acid:

• A concentrated acid contains a higher amount of acid and less water.

 

4. Dilute Acid:

• A dilute acid contains a lower amount of acid and more water.

 

Bases

• Taste: Bases have a bitter taste.
• Litmus Test: Bases turn red litmus paper blue.
• Chemical Properties: Bases produce OH⁻ ions (hydroxide ions) in aqueous solutions.
  
  

1. Strong Bases:

• NaOH (Sodium Hydroxide)
• KOH (Potassium Hydroxide)
• Ca(OH)₂ (Calcium Hydroxide)

2. Weak Bases:

• NH₃ (Ammonia)

Alkalis:

• When a base dissolves in water, it is called an alkali.
• Examples: NaOH, KOH, Mg(OH)₂

 

Salts

• Salts are formed by the reaction between an acid and a base.
• Examples: NaCl (Common Salt), KCl (Potassium Chloride)

 

Indicators

Indicators are substances that change color or smell in acidic and basic solutions. They are classified into three types:

1. Natural Indicators:

These indicators are obtained from plants.

• Litmus: Turns red in acidic solutions and blue in basic solutions.
• Turmeric: No change in acidic solutions, turns red in basic solutions.
• Hydrangea Flower Extract: Turns blue in acidic solutions and pink in basic solutions.
  
  

2. Synthetic Indicators:

These are chemical substances that help identify acids and bases by specific color changes.

• Phenolphthalein: Colorless in acidic solutions, pink in basic solutions.
• Methyl Orange: Red in acidic solutions, yellow in basic solutions.
  
  

3. Olfactory (Odor) Indicators:

These substances change their smell in acidic or basic media.

• Onion Juice: Strong odor in acidic solutions, no odor in basic solutions.
• Vanilla: Retains the same smell in both acidic and basic solutions.
• Clove Oil: Retains the same smell in both acidic and basic solutions.
  
  

Chemical Reactions of Acids and Bases

Reaction of Metals

 

1. Reaction of Metals with Acids

Formula:

Acid + Metal → Salt + H₂ (Hydrogen Gas)

When a metal reacts with an acid, salt and hydrogen gas are produced.

Example: 2HCl + Zn → ZnCl₂ + H₂↑

(This reaction produces hydrogen gas.)

 

2. Reaction of Metals with Bases

When a base reacts with a metal, salt and hydrogen gas are produced.

Example: 2NaOH + Zn → Na₂ZnO₂ + H₂↑

(This reaction produces sodium zincate and hydrogen gas.)

Pop Test for Hydrogen Gas

To test for the presence of hydrogen gas, the Pop Test is performed.

Procedure:

• When a burning candle is brought near a test tube containing hydrogen gas, a "pop" sound is heard.
• This confirms the presence of hydrogen gas.
  
  
  

Reaction of Metal Carbonate and Metal Bicarbonate

 

1. Reaction with Acid:

Metal carbonate and bicarbonate react with acid to form salt, carbon dioxide (CO₂), and water.

(i) Reaction of Metal Carbonate:

Example: 2HCl + Na₂CO₃(s) → 2NaCl(aq) + CO₂(g) + H₂O(l)

(In this process, sodium chloride, carbon dioxide gas, and water are produced.)

 

(ii) Reaction of Metal Bicarbonate:

Example: HCl + NaHCO₃(s) → NaCl(aq) + CO₂(g) + H₂O(l)

(This reaction also forms salt, carbon dioxide, and water.)

 

2. Reaction with Base:

Metal carbonate and bicarbonate do not react with bases.

 

3. Lime Water Test:

The carbon dioxide gas produced by the reaction of acid with metal carbonate or bicarbonate is identified using lime water.

Test:

When carbon dioxide is passed through lime water, the water turns milky (white).

(i) Na₂CO₃ + HCl → NaCl + H₂O + CO₂↑

(ii) NaHCO₃ + HCl → NaCl + H₂O + CO₂↑

Example:

1. Passing the gas through lime water:

The produced CO₂ gas is passed through lime water [Ca(OH)₂].

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

(In this reaction, a white precipitate (CaCO₃) is formed.)

 

2. Result: CO₂ reacts with lime water and forms calcium carbonate (CaCO₃). It does not dissolve in water and appears as a white precipitate (milky white precipitate).

 

3. Disappearance of Milky Color (After a Long Time):

If CO₂ is passed through lime water for a long time, calcium carbonate converts into calcium bicarbonate [Ca(HCO₃)₂], which is soluble in water.

CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq) (this is soluble in water.)

 

4. Neutralization Reaction of Acid and Base:

Neutralization Reaction:

• When acid and base react, salt and water are formed.
• The effect of acid is neutralized by the base, and the effect of the base is neutralized by the acid, forming salt and water.

Example: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Results of Acid and Base Combination:

• Strong Acid + Weak Base → Acidic Salt + Water
• Weak Acid + Strong Base → Basic Salt + Water
• Strong Acid + Strong Base → Neutral Salt + Water
• Weak Acid + Weak Base → Neutral Salt + Water
  
  

5. Reaction of Acid and Metallic Oxide:

Metallic oxides react with acids to form salt and water.

Example: CuO + 2HCl → CuCl₂ + H₂O

(In this process, copper chloride is formed, making the solution blue.)

• Metallic oxides are basic in nature because they react with acid to form salt and water.
• Examples: CuO, MgO
  
  

6. Reaction of Non-Metallic Oxide and Base:

Non-metallic oxides react with bases to form salt and water.

Example: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

Non-metallic oxides are acidic in nature and conduct electricity.

 

Behavior of Acids and Bases in Aqueous Solution:

1. Behavior of Acid in Aqueous Solution:

• Acids produce H⁺ ions when dissolved in water.
• These H⁺ ions combine with water to form hydronium ions (H₃O⁺).

 

2. Behavior of Base in Aqueous Solution:

• Bases produce OH⁻ ions when dissolved in water.
• Not all bases are soluble in water.
• A base that dissolves in water is called an alkali.

Important Fact:

• All alkalis are bases, but not all bases are alkalis.
  
  

3. Precautions When Mixing Acid or Base with Water:

• When adding concentrated acid or base to water, precautions should be taken.
• Always add acid or base slowly to water and keep stirring the mixture constantly.
• This process is highly exothermic, causing the mixture to heat up.

Warning:

• Never add water to acid.
• Doing so may cause the mixture to splash out due to the heat generated, increasing the risk of burns or injuries.
• The localized high temperature can cause a glass container to break.
  
  

Similarities Between Acids and Bases:

• Acids produce H⁺ ions when dissolved in water.
• Bases produce OH⁻ ions when dissolved in water.
• Both acids and bases become dilute when mixed with water.
  
  

Strength of Acids and Bases:

• The strength of an acid or base depends on how many H⁺ or OH⁻ ions it produces in water.
• Strong acids and bases ionize completely in water.
• Weak acids and bases ionize partially.

Examples:

• Strong Acids: HCl, H₂SO₄
• Weak Acids: CH₃COOH (Acetic Acid)
• Strong Bases: NaOH, KOH
• Weak Bases: NH₄OH

Identification of Acids and Bases:

• The strength of an acid or base is measured using a universal indicator.
• A universal indicator is a mixture of various indicators and shows different colors in acidic or basic solutions.

Note:

• The pH of an acidic solution is less than 7.
• The pH of a basic solution is greater than 7.
• A solution with a pH of 7 is neutral.
  
  

Importance of pH in Daily Life

1. Plants and Animals Are Sensitive to pH:

• Most biological processes function smoothly at a specific pH level.
• The human body generally functions within the pH range of 7.0 to 7.8.

 

2. Acid Rain and pH:

• The normal pH of rainwater is 5.6.
• When the pH of rainwater falls below 5.6, it is called acid rain.
• Acid rain damages soil, water bodies, and building structures.

 

3. Soil pH and Its Effect on Agriculture:

• For good crop yield, the pH of the soil must be appropriate.
• If the soil pH is too acidic or too alkaline, farmers adjust it by adding acidic or alkaline substances.
• Balanced pH improves the absorption of nutrients by plants in the soil.

 

4. Importance of pH in Our Digestive System:

• Our stomach produces hydrochloric acid (HCl) for food digestion.
• During indigestion, the stomach produces excessive acid, causing burning sensation and stomach pain.
• To relieve this condition, antacids such as bases are used.
• Milk of Magnesia (Magnesium Hydroxide) is a common antacid that neutralizes acid.

 

5. pH and Tooth Decay:

• Tooth decay can begin when the pH of the mouth falls below 5.5.
• Tooth enamel is made of calcium phosphate, the hardest substance in the body.
• However, when the pH of the mouth drops below 5.5, calcium phosphate starts dissolving, leading to tooth decay.
• Alkaline toothpaste helps neutralize acid and protects the teeth.

 

6. Use of pH for Self-Defense in Animals and Plants:

• A bee sting releases an acidic substance, causing pain and irritation.
• To relieve this irritation, baking soda (NaHCO₃), a base, is used to neutralize the acid.
• The nettle plant releases methanoic acid (formic acid) through its stinging hairs, causing skin irritation.
• Applying dock plant leaves to the affected area provides relief as it has an alkaline effect.

 

pH of Salts and Their Classification

1. Strong Acid + Strong Base → Neutral Salt (pH = 7)

Example: NaCl (Sodium Chloride)

2. Strong Acid + Weak Base → Acidic Salt (pH < 7)

Example: NH₄Cl (Ammonium Chloride)

3. Strong Base + Weak Acid → Basic (Alkaline) Salt (pH > 7)

Example: Na₂CO₃ (Sodium Carbonate)

Chemicals Derived from Common Salt

1. Sodium Hydroxide (NaOH):

Formed by passing electricity through an aqueous solution of sodium chloride (brine).

This process is called the Chlor-Alkali process.

Reaction:

2NaCl (aq) + 2H₂O (l) → 2NaOH (aq) + Cl₂ (g) + H₂ (g)

• At Anode: Cl₂ gas is released.
• At Cathode: H₂ gas is released.
• Near Cathode: NaOH solution is formed.

Uses:

• H₂: Used as fuel, in margarine production.
• Cl₂: Used for water purification, PVC, and CFC production.
• HCl: Used for cleaning steel, in medicine production.
• NaOH: Used for degreasing metals, in soap and paper making.
• Cl₂ + NaOH: Used in bleaching powder production.
  
  

2. Bleaching Powder (CaOCl₂):

Produced by reacting dry slaked lime [Ca(OH)₂] with chlorine gas.

Reaction:

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Uses:

• Used in the textile industry for bleaching cotton and linen.
• Used in the paper industry for bleaching wood pulp.
• Used as an oxidizing agent in chemical industries.
• Used to disinfect drinking water.
  
  

3. Baking Soda (NaHCO₃):

Formed by the reaction of sodium chloride, water, carbon dioxide, and ammonia.

Reaction:

NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃

When Heated:

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Uses:

• Used in making baking powder.
• Used to make bread and cakes rise.
• Used as an antacid.
• Used in soda-acid fire extinguishers.
  
  

4. Washing Soda (Na₂CO₃⋅10H₂O):

Obtained by re-crystallizing sodium carbonate.

It is an alkaline salt.

Reaction:

Na₂CO₃ + 10H₂O → Na₂CO₃⋅10H₂O

Uses:

• Used in glass, soap, and paper industries.
• Used in the production of borax.
• Used in household cleaning.
• Used to remove permanent hardness of water.
  
  

5. Plaster of Paris (CaSO₄⋅1/2H₂O):

Formed by heating gypsum at 373K, which releases water molecules and forms calcium sulfate hemihydrate (Plaster of Paris).

Reaction:

CaSO₄⋅2H₂O → CaSO₄⋅1/2H₂O + 1₁/2H₂O

When Water is Added:

CaSO₄⋅1/2H₂O + 1₁/2H₂O → CaSO₄⋅2H₂O

Uses:

• Used for setting fractured bones.
• Used in making toys and decorative items.
• Used for smooth finishing of walls and ceilings.
  
  

Water of Crystallization

The fixed number of water molecules present in one formula unit of a salt is called water of crystallization.

Example: CuSO₄.5H₂O (Blue Vitriol or Blue Stone).