IGCSE Chemistry Acids & Bases | Recap with Best Notes! | CAIE Made Easier

Characteristics of Acids and Bases
📌 Acids are identified by a sour taste and a pH < 7, while bases (alkalis) are bitter and have a pH > 7.
🧪 Litmus paper serves as a basic indicator: blue litmus turns red in acids, and red litmus turns blue in bases.
📊 Universal indicators are superior because they provide an estimate of the specific pH level (0–14) rather than just identifying the substance type.

Chemical Indicators and Color Changes
🎨 Phenolphthalein remains colorless in acidic solutions but turns pink in basic solutions.
🍊 Methyl orange shifts from red in acids to yellow in basic environments.
📉 Concentration vs. Strength: A strong acid (e.g., HCl) remains strong regardless of concentration, whereas a weak acid (e.g., ethanoic acid) remains weak even at identical molar concentrations.

Brønsted–Lowry Theory
⚛️ The Brønsted–Lowry model defines acids as proton (H⁺) donors and bases as proton acceptors.
💦 Water (H₂O) is classified as amphoteric because it can act as either an acid or a base depending on the reaction environment.
🧬 H⁺ ions are essentially bare protons, as a hydrogen atom consists of only one proton and one electron; losing the electron leaves only the nucleus.

Strength: Strong vs. Weak Electrolytes
⚡ Strong acids/bases undergo complete ionization (full dissociation into ions), resulting in high conductivity and lower/higher pH extremes.
⚖️ Weak acids/bases (e.g., ethanoic acid, magnesium hydroxide) undergo partial ionization or have low solubility, leading to lower concentrations of H⁺ or OH⁻ ions.
🧱 Corrosivity: Strong acids and bases are highly corrosive, whereas weak acids and bases are generally not.

Chemical Reactions
💥 Neutralization: The reaction between an acid and a base consistently produces salt and water ($H^+ + OH^- \rightarrow H₂O$).
💨 Acid Reactions: Acids react with metals to produce salt and hydrogen gas, with metal carbonates to produce salt, water, and $CO₂$, and with metal oxides to produce salt and water.
🌬️ Ammonia Displacement: Strong bases react with ammonium salts to release ammonia gas ($NH₃$), which is identified by its pungent smell and ability to turn red litmus paper blue.

Key Points & Insights
➡️ Memorize the Indicator Scale: Use the universal indicator to gauge strength: Red (pH 0–2) represents strong acidity, while Violet/Purple (pH 12–14) represents strong alkalinity.
➡️ Identify Amphoteric Oxides: Remember that Aluminum oxide ($Al₂O₃$), Zinc oxide ($ZnO$), and Lead hydroxide ($Pb(OH)₂$) can behave as both acids and bases.
➡️ Safety Protocol: Never taste unknown substances in a laboratory setting; always rely on chemical indicators like phenolphthalein or litmus paper to identify solutions.

📸 Video summarized with SummaryTube.com on Apr 13, 2026, 20:11 UTC