Unit 5: Comprehensive Summary

Acids are defined as substances that produce hydrogen ions ($H^+$) in aqueous solution (Arrhenius) or as proton donors (Brønsted-Lowry).

Bases are defined as substances that produce hydroxide ions ($OH^-$) in aqueous solution (Arrhenius) or as proton acceptors (Brønsted-Lowry). An alkali is a soluble base.

A salt is an ionic compound formed when the $H^+$ of an acid is replaced by a metal or ammonium ion. The reaction between an acid and a base to form a salt and water is called neutralisation.

The strength of an acid or base depends on its degree of dissociation.

• Strong acids/bases dissociate completely in water, producing a high concentration of ions ($H^+$ or $OH^-$). They are strong electrolytes.
• Weak acids/bases only partially dissociate, establishing an equilibrium and producing a low concentration of ions. They are weak electrolytes.

The pH scale (0-14) measures the acidity/alkalinity of a solution. pH < 7 is acidic, pH = 7 is neutral, and pH > 7 is alkaline.

Indicators are substances that change colour at specific pH values. Universal indicator provides an estimate of the pH value across a wide range.

• Amphoteric substances (e.g., $Al_2O_3$, $ZnO$) can act as both acids and bases.
• Salt Hydrolysis: Ions from salts can react with water to produce acidic or basic solutions.
  • Salt of strong acid + weak base → Acidic solution (e.g., $NH_4Cl$)
  • Salt of weak acid + strong base → Basic solution (e.g., $CH_3COONa$)
  • Salt of strong acid + strong base → Neutral solution (e.g., $NaCl$)
• Physical Properties: Some salts are hygroscopic (absorb moisture), deliquescent (absorb moisture and dissolve), or efflorescent (lose water of crystallisation).

Qualitative Analysis uses characteristic reactions to identify ions. Key tests include:

• Acids ($H^+$): Fizz with carbonates to produce $CO_2$.
• Carbonates ($CO_3^{2-}$): Fizz with acids to produce $CO_2$, which turns limewater milky.
• Ammonium ($NH_4^+$): Warm with $NaOH(aq)$ to produce $NH_3$ gas (pungent smell, turns damp red litmus blue).

Salt Preparation:

• Acid + Insoluble Base: Add excess base, filter, then crystallise the filtrate.
• Acid + Soluble Base (Alkali): Use titration to find exact volumes, then mix without indicator and crystallise.

Titration is a quantitative technique to find the unknown concentration of a solution. It involves using a pipette and burette to find the exact volume of a standard solution needed for neutralisation, indicated by an appropriate indicator.