Overview
This lecture covers the chemical properties, reactions, and uses of acids, bases, and salts, including their identification, the concept of pH, and applications in daily life and industry.
Identification and Properties of Acids and Bases
- Acids taste sour and turn blue litmus red; bases taste bitter and turn red litmus blue.
- Litmus, turmeric, methyl orange, and phenolphthalein are common indicators.
- Olfactory indicators (like onion, vanilla, clove) change odour in acidic or basic solutions.
Chemical Reactions of Acids and Bases
- Acids react with metals to produce a salt and hydrogen gas.
- Bases also react with certain metals to form hydrogen gas and salts (e.g., sodium zincate).
- Acids react with metal carbonates/hydrogencarbonates to form salt, carbon dioxide, and water.
- Acid + base reactions (neutralisation) produce salt and water.
- Metal oxides react with acids to form salt and water (metallic oxides are basic).
- Non-metallic oxides react with bases to form salt and water (non-metallic oxides are acidic).
Ions in Solutions and pH Scale
- Acids release H⁺ (as hydronium, H₃O⁺) ions in water; bases release OH⁻ ions.
- Only aqueous acids/bases conduct electricity (not dry).
- Mixing concentrated acids/bases with water is highly exothermic; always add acid to water.
- pH scale ranges from 0 (strong acid) to 14 (strong base); 7 is neutral.
Importance of pH in Everyday Life
- Human blood pH: 7.0–7.8; deviation can harm organisms.
- Acid rain (pH < 5.6) lowers river water pH, endangering aquatic life.
- Plants require a specific soil pH for healthy growth.
- Stomach acid helps digestion; excess acid causes indigestion, treated by antacids (mild bases).
- Tooth decay occurs when mouth pH < 5.5; brushing with basic toothpaste prevents decay.
- Bee stings (acidic) are treated with mild bases like baking soda.
Salts: Formation, Properties, and Uses
- Salts form by neutralisation of acids and bases; examples include sodium chloride, copper sulphate, etc.
- Salts from strong acid and strong base are neutral (pH 7); strong acid + weak base are acidic; strong base + weak acid are basic.
- Common salt (NaCl) is a raw material for sodium hydroxide, baking soda, washing soda, and bleaching powder.
Chemicals from Common Salt
- Electrolysis of brine produces sodium hydroxide, chlorine, and hydrogen (chlor-alkali process).
- Bleaching powder is made from chlorine and slaked lime.
- Baking soda (NaHCO₃) is used in cooking and as an antacid.
- Washing soda (Na₂CO₃·10H₂O) is used in cleaning, softening water, and industry.
Water of Crystallisation
- Some salts contain fixed water molecules (e.g., CuSO₄·5H₂O, CaSO₄·2H₂O).
- Heating removes water of crystallisation, changing the salt’s appearance.
- Plaster of Paris (CaSO₄·½H₂O) forms by heating gypsum and hardens when mixed with water.
Key Terms & Definitions
- Indicator — substance that changes colour to identify acids or bases.
- Neutralisation — reaction of acid with base forming salt and water.
- pH Scale — numerical scale (0–14) indicating acidity or basicity of a solution.
- Alkali — a base that dissolves in water to give OH⁻ ions.
- Acid Rain — rainwater with pH less than 5.6 due to dissolved acids.
- Water of Crystallisation — fixed water molecules included in a crystalline salt.
- Chlor-alkali Process — electrolysis of brine to yield NaOH, Cl₂, H₂.
Action Items / Next Steps
- Complete exercises and tables on reactions, indicators, and pH values.
- Prepare natural indicators (e.g., beetroot extract) and test household substances.
- Perform group activity: prepare a soda-acid fire extinguisher.
- Review formulas for salts and practice writing balanced chemical equations.