Acids are substances that exhibit sour taste, turn blue litmus paper red, and react with metals to produce hydrogen gas. They are important in chemistry, industry, and biological processes.

Arrhenius Definition: An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water.
Example: $$ \text{HCl}_{(aq)} \rightarrow \text{H}^+_{(aq)} + \text{Cl}^-_{(aq)} $$

Brønsted–Lowry Definition: An acid is a proton (H⁺) donor, while a base is a proton acceptor.
Example: $$ \text{NH}_3 + \text{H}_2\text{O} \rightarrow \text{NH}_4^+ + \text{OH}^- $$
Here, water acts as an acid because it donates a proton to ammonia.

Lewis Definition: An acid is a substance that can accept a pair of electrons to form a covalent bond.
Example: $$ \text{BF}_3 + \text{NH}_3 \rightarrow \text{F}_3\text{B–NH}_3 $$

1. Based on Source

• Organic Acids: These are acids erived from living matter(plants and animals), often weak acids.
  Examples: Citric acid, acetic acid, lactic acid.
• Inorganic (Mineral) Acids: These are acids obtained from minerals and usually strong acids.
  Examples: Hydrochloric acid, nitric acid, sulfuric acid.

2. Based on Concentration

• Concentrated Acids: Contain a high proportion of acid in water (e.g., concentrated H₂SO₄).
• Dilute Acids: Contain a small proportion of acid in water (e.g., dilute HCl).

3. Based on Strength

• Strong Acids: These are acids which completely ionize in water to release hydrogen ions.
  Examples: HCl, HNO₃, H₂SO₄.
  Equations:
  $$ \text{HCl}_{(aq)} \rightarrow \text{H}^+_{(aq)} + \text{Cl}^-_{(aq)} $$
• Weak Acids: These are acids that partially ionize in water, producing few hydrogen ions.
  Examples: CH₃COOH, H₂CO₃, HCN.
  Equations:
  $$ \text{CH}_3\text{COOH}_{(aq)} \rightleftharpoons \text{CH}_3\text{COO}^-_{(aq)} + \text{H}^+_{(aq)} $$ $$ \text{HCN}_{(aq)} \rightleftharpoons \text{H}^+_{(aq)} + \text{CN}^-_{(aq)} $$

Note: HCN (hydrogen cyanide) is a weak acid because it partially ionizes in water — only a small fraction of its molecules donate protons (H⁺) to form hydronium ions (H₃O⁺). The ionization can be represented as: $$ \text{HCN}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightleftharpoons \text{H}_3\text{O}^+_{(aq)} + \text{CN}^-_{(aq)} $$

The basicity of an acid is defined as the number of hydrogen ions (H⁺) that one molecule of the acid can donate or replace in a chemical reaction. In other words, it is the number of ionizable hydrogen atoms present in one molecule of an acid. The basicity of an acid depends on its molecular structure and the number of replaceable hydrogen atoms attached to oxygen.

For example: $$ \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- \quad (\text{Monobasic}) $$ $$ \text{H}_2\text{SO}_4 \rightarrow 2\text{H}^+ + \text{SO}_4^{2-} \quad (\text{Dibasic}) $$ $$ \text{H}_3\text{PO}_4 \rightarrow 3\text{H}^+ + \text{PO}_4^{3-} \quad (\text{Tribasic}) $$

• Always wear safety goggles and laboratory gloves to protect the eyes and skin.
• Use lab coats or aprons to prevent acid spills from damaging clothing.
• Always pour acid into water, never water into acid, to prevent violent splashing or explosions.
• Handle concentrated acids in a fume cupboard to avoid inhaling harmful fumes.
• Use tongs or pipettes when transferring acids; avoid direct contact with hands.
• Neutralize acid spills immediately using sodium bicarbonate or slaked lime.
• Store acids in clearly labeled, corrosion-resistant containers.
• Wash hands thoroughly with soap and water after handling acids.
• Never taste or directly smell acids.
• Dispose of acid waste according to laboratory safety procedures.