Acids, Bases and Salts

Introduction to Acids and Bases

An acid is a substance that produces hydrogen ions ($\text{H}^+$) in aqueous solution, while a base is a substance that produces hydroxide ions ($\text{OH}^-$) or accepts protons. For board exams, you must thoroughly understand the three primary chemical concepts of acids and bases: the Arrhenius Concept, the Bronsted-Lowry Concept, and the Lewis Concept. According to Bronsted-Lowry, acids are proton donors and bases are proton acceptors, which leads to the formation of conjugate acid-base pairs. Lewis theory focuses on electron pairs, defining acids as electron-pair acceptors and bases as electron-pair donors. Memorizing examples for each theory is frequently tested in both objective and short-question sections.

Properties, $\text{pH}$ Scale, and Indicators

Acids and bases have distinct physical and chemical properties. Acids have a sour taste, turn blue litmus red, and react with metals to liberate hydrogen gas. Bases have a bitter taste, feel slippery, and turn red litmus blue. The strength of acids and bases is measured using the $\text{pH}$ scale, which is the negative logarithm of hydrogen ion concentration ($\text{pH} = -\log[\text{H}^+]$). A $\text{pH}$ of 7 is neutral, below 7 is acidic, and above 7 is basic. You should also know the self-ionization of water, where the ion-product constant of water ($\text{Kw}$) is $1.0 \times 10^{-14}$ at $25^\circ\text{C}$, meaning $\text{pH} + \text{pOH} = 14$. Indicators like litmus, phenolphthalein, and methyl orange are essential for detecting endpoints in titrations.

Salts and Their Preparation

A salt is an ionic compound formed by the neutralization reaction between an acid and a base, replacing the ionizable hydrogen of the acid with a metal or ammonium ion. Salts can be classified as normal, acidic, basic, double, mixed, or complex salts. Board numericals and long questions frequently focus on the methods of preparation of salts, which are divided into soluble and insoluble types. Soluble salts are prepared via direct combination, neutralization, or reaction of an acid with a metal/carbonate. Insoluble salts are typically prepared by precipitation (double decomposition). Additionally, learn the commercial uses of important salts like sodium chloride, sodium carbonate (Solvay's process), and sodium bicarbonate as they are high-yield topics for board examinations.