Acids, Bases and Salts

Acids are substances that release hydrogen ions (H+) in water, turning blue litmus red. Bases release hydroxide ions (OH-) in water, turning red litmus blue. Examples include hydrochloric acid (HCl) and sodium hydroxide (NaOH).

Acids react with metals to produce salt and hydrogen gas, e.g., Zn + 2HCl → ZnCl2 + H2. Bases like NaOH react with metals such as zinc to form salt and hydrogen gas, e.g., 2NaOH + Zn → Na2ZnO2 + H2.

The pH scale measures the acidity or basicity of a solution from 0 (very acidic) to 14 (very alkaline). A pH of 7 is neutral. It's determined by the concentration of H+ ions, with lower pH indicating higher acidity.

Dry HCl gas does not dissociate into H+ and Cl- ions, which are necessary to exhibit acidic properties. Water is required for ionization, enabling the gas to show acidic behavior and change litmus color.

A neutralization reaction occurs when an acid and a base react to form salt and water, e.g., HCl + NaOH → NaCl + H2O. This reaction is exothermic and is used in antacids to relieve acidity.

Metal carbonates react with acids to produce salt, carbon dioxide, and water, e.g., CaCO3 + 2HCl → CaCl2 + CO2 + H2O. This reaction is used in fire extinguishers and baking.

pH is crucial for digestion, agriculture, and water treatment. Stomach acid has a low pH for digestion, while soil pH affects plant growth. Acid rain with pH <5.6 harms aquatic life and buildings.

Olfactory indicators change smell in acidic or basic media. Onion and vanilla lose their odor in bases but retain it in acids. They are used in labs to test solutions without litmus.

Bleaching powder (CaOCl2) is made by reacting chlorine with dry slaked lime, Ca(OH)2 + Cl2 → CaOCl2 + H2O. It's used for bleaching, disinfecting water, and as an oxidizer.

Water of crystallization is fixed water molecules in a salt's crystal structure, e.g., CuSO4.5H2O. Heating removes this water, turning blue CuSO4 to white anhydrous CuSO4, which regains color on adding water.

Plaster of Paris (CaSO4.½H2O) absorbs moisture to form gypsum (CaSO4.2H2O), becoming hard and unusable. Moisture-proof containers prevent this reaction, preserving its quality for medical and decorative uses.

Strong acids like HCl completely dissociate in water, releasing many H+ ions, while weak acids like CH3COOH partially dissociate, releasing fewer H+ ions. This affects their conductivity and reactivity.

Baking soda (NaHCO3) is made by reacting NaCl with CO2 and NH3 in water, NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3. It's used in baking, antacids, and fire extinguishers.

Heating NaHCO3 decomposes it into Na2CO3, CO2, and H2O, 2NaHCO3 → Na2CO3 + CO2 + H2O. This reaction is used in baking to make bread rise and in fire extinguishers.

Acids need water to dissociate into H+ ions, which are responsible for acidic properties. Without water, acids like HCl remain as molecules and cannot exhibit acidity or conduct electricity.

Washing soda (Na2CO3.10H2O) is used in glass, soap, and paper industries, as a water softener, and for cleaning. It removes permanent hardness by precipitating Ca2+ and Mg2+ ions.

Sodium hydroxide is prepared by electrolysis of brine, 2NaCl + 2H2O → 2NaOH + Cl2 + H2. It's used in soap, paper, and textile industries, and as a drain cleaner.

Non-metallic oxides like CO2 react with bases to form salt and water, CO2 + Ca(OH)2 → CaCO3 + H2O. This shows non-metallic oxides are acidic, neutralizing bases similar to acids.

The neutralization reaction releases heat due to the formation of stable water molecules from H+ and OH- ions. This energy release makes the reaction exothermic, often felt as warmth.

Tooth decay starts when mouth pH drops below 5.5, dissolving enamel (Ca10(PO4)6(OH)2). Bacteria produce acids from food sugars, but brushing with basic toothpaste neutralizes acids, preventing decay.

Red litmus turns blue in base, stays red in acid, and remains unchanged in water. Test each solution: the one turning blue is base, staying red is acid, and unchanged is water.

Gypsum is CaSO4.2H2O. Heating at 373K removes 1.5 water molecules, forming Plaster of Paris (CaSO4.½H2O). Adding water reverses the reaction, setting into a hard mass.

The process produces chlorine (chlor) at the anode and alkali (NaOH) at the cathode, hence 'chlor-alkali'. It also yields hydrogen gas, with all three products being industrially valuable.

Baking soda (NaHCO3) is mild, used in cooking and antacids, while washing soda (Na2CO3.10H2O) is strong, used in cleaning and industries. Heating NaHCO3 produces Na2CO3, linking them chemically.

Higher H+ concentration makes a solution more acidic (lower pH), while lower H+ makes it more basic (higher pH). Neutral solutions have equal H+ and OH- concentrations, with pH 7.