Acid Bases
Acids
Definition
Acids are substances that dissociate in water to produce hydrogen ions (H⁺).
Physical Properties
Taste: Sour
Indicator: Turns blue litmus paper red
Conductivity: Good conductors of electricity; strong acids act as strong electrolytes while weak acids act as weak electrolytes.
Chemical Properties
Reaction with Metals:
Acids react with metals to form hydrogen gas (H₂) and a metallic salt.
Example:
H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)
HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)
CH₃COOH(aq) + Zn(s) → (CH₃COO)₂Zn(aq) + H₂(g)
Reaction with Metal Carbonates/Hydrogen Carbonates:
Produces carbon dioxide gas (CO₂), water (H₂O), and a salt.
Example:
Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
Reaction with Bases:
Acids react with bases to form salt and water.
Example:
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Reaction with Metal Oxides:
Produces salt and water.
Example:
CuO + 2HCl → CuCl₂ + H₂O
Bases
Definition
Bases are substances that dissociate in water to produce hydroxide ions (OH⁻).
Physical Properties
Taste: Bitter
Indicator: Turns red litmus paper blue
Conductivity: Good conductors of electricity.
Chemical Properties
Reaction with Metals:
Bases can react with metals to produce hydrogen gas (H₂) and salts.
Example:
2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g)
Reaction with Non-Metallic Oxides:
Bases react with non-metallic oxides to form salts and water.
Example:
CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)
Indicators
Definition
Indicators are substances that change color based on the acidity or basicity of a solution.
Types of Indicators
Natural Indicators:
Litmus: Neutral color is purple, extracted from lichen (Thallophyta).
Red cabbage, turmeric, colored petals of flowers (e.g., Hydrangea).
Synthetic Indicators:
Methyl orange, phenolphthalein.
Color changes:
Acid: Red
Base: Blue
Olfactory Indicators:
Vanilla, onion, clove oil (smell changes in different pH conditions).
Observations with Indicators
Red Litmus - No change in acids; Blue Litmus - Remains blue in bases.
Neutralisation Reaction
The reaction between an acid and a base producing a salt and water is called a neutralization reaction.
Example:
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Alkali Definition
A water-soluble base.
Dilution Process
Mixing an acid or base with water decreases concentration and increases pH value.
Caution: Water should be added to acid, not the reverse, to avoid accidents.
pH Scale
Used to measure hydrogen ion concentration.
Acidic: pH < 7
Neutral: pH = 7
Basic: pH > 7
Important Acids and Their Sources
Acetic acid: Vinegar
Tartaric acid: Tamarind
Citric acid: Oranges, lemons
Methanoic acid: Ants (sting)
Importance of pH in Daily Life
pH affects plant growth and aquatic life; healthy pH for humans is 7.0-7.8.
Acid rain: pH < 5.6, harmful to rivers and aquatic ecosystems.
Tooth decay occurs with pH < 5.5 (calcium hydroxyapatite is corroded).
Common Salts and Their Uses
Common salt (NaCl): used in food and soap manufacturing.
Sodium hydroxide (NaOH): used in various industries and household cleaning.
Calciums oxychloride (Bleaching powder): used in textile, paper industries and water treatment.
Sodium bicarbonate (Baking soda): used in baking and as an antacid.
Washing soda (Sodium carbonate): used for cleaning and in glass production.
Hydrated Salts and Their Uses
CuSO₄ • 5H₂O (Copper sulphate pentahydrate): used as a fungicide.
Plaster of Paris: used for medical casts, toys, and surface smoothing.
Exam Questions and Answers
What happens when an acid is mixed with a base?
(ii): The temperature increases; (iv): Salt formation occurs.
Strong acid examples: HCl, HNO₃, H₂SO₄. Weak acid examples: Citric acid, acetic acid.
Sodium hydroxide (NaOH) absorbs moisture from air and becomes sticky. It reacts with acidic oxides to produce salt and water.