Comprehensive notes, solved MCQs, Short Questions for Class 9 Chemistry Chapter 7 Acid-Base Chemistry. Covers Arrhenius & Bronsted-Lowry Concepts, Properties, Neutralization, Strong/Weak Acids, and Acid Rain.
Acids: Substances with sour taste that turn blue litmus red. Examples: Lemon (citric acid), Vinegar (acetic acid), Curd (lactic acid).
Bases/Alkalis: Substances with bitter taste and slippery touch that turn red litmus blue. Examples: $NaOH$, $KOH$, $Ca(OH)_2$.
Litmus Test: Acids: Blue litmus $\to$ Red. Bases: Red litmus $\to$ Blue.
1. Natural/Organic Acids: Obtained from natural sources (living organisms). Examples: Acetic acid ($CH_3COOH$), Formic acid ($HCOOH$), Citric acid, Oxalic acid ($C_2O_4H_2$), Tartaric acid, Ascorbic acid (Vitamin C).
2. Mineral/Inorganic Acids: Prepared from minerals. Examples: Hydrochloric acid ($HCl$), Sulfuric acid ($H_2SO_4$), Nitric acid ($HNO_3$).
Uses: Food preservatives, flavoring agents, cleaning agents, lead-acid batteries.
Acids (Arrhenius): Substances that produce hydrogen ions ($H^+$) or hydronium ions ($H_3O^+$) in water.
Examples:
Bases (Arrhenius): Substances that produce hydroxide ions ($OH^-$) in water.
Examples:
Limitation: Only applies to aqueous solutions. Cannot explain bases like $NH_3$, $Na_2CO_3$ (no $OH^-$ in formula but behave as bases).
Acids (Bronsted-Lowry): Proton ($H^+$) donors. Examples: $HCl$, $H_2SO_4$, $HNO_3$.
Bases (Bronsted-Lowry): Proton ($H^+$) acceptors. Examples: $NH_3$, $OH^-$, $Cl^-$.
Conjugate Acid-Base Pairs:
Amphoteric Substance: Can behave as both acid and base. Example: $H_2O$.
Strong Acids: Completely ionize in water (100%). Examples: $HCl$, $HNO_3$, $H_2SO_4$.
Weak Acids: Partially ionize in water. Examples: $CH_3COOH$ (0.132% ionization), $HCOOH$ (1.3% ionization).
Strong Bases: Completely ionize in water. Examples: $NaOH$, $KOH$.
Weak Bases: Partially ionize in water. Example: $NH_4OH$.
Acid Properties: React with (1) Metals: $Zn + H_2SO_4 \to ZnSO_4 + H_2$, (2) Metal Oxides: $CaO + 2HNO_3 \to Ca(NO_3)_2 + H_2O$, (3) Metal Hydroxides: $2KOH + H_2SO_4 \to K_2SO_4 + 2H_2O$, (4) Carbonates: $CaCO_3 + 2HCl \to CaCl_2 + H_2O + CO_2$.
Base Properties: React with (1) Acids: $NaOH + HCl \to NaCl + H_2O$, (2) Ammonium salts: $Ca(OH)_2 + NH_4Cl \to CaCl_2 + H_2O + NH_3$.
Neutralization: Acid + Base $\to$ Salt + Water.
Definition: Rain water with pH < 5.6 (normal rain pH = 5.6).
Formation: Burning fossil fuels releases $SO_2$ and $NO_2$. These react with moisture: $SO_2 + O_2 \to SO_3$, $SO_3 + H_2O \to H_2SO_4$, $2NO_2 + H_2O \to HNO_3 + HNO_2$.
Effects: (1) Makes soil acidic, damages plants, (2) Makes water bodies too acidic for aquatic life, (3) Damages buildings, statues, automobiles made of stone and metal.