Chapter 4: Acids, Bases and Salts
4.1 Acids and Bases
Core Concepts
- Acids: Substances that taste sour and have an acidic chemical nature. The word originates from the Latin word acere (meaning sour).
- Bases: Substances that taste bitter and feel soapy to the touch. Their chemical nature is basic.
- Indicators: Special substances used to test whether a solution is acidic or basic by changing their color.
- Neutral Solutions: Solutions that do not change the color of either red or blue litmus paper; they are neither acidic nor basic.
Common Acids and Their Sources
- Acetic acid: Found in Vinegar
- Formic acid: Found in Ant's sting
- Citric acid: Found in Citrus fruits (oranges, lemons)
- Lactic acid: Found in Curd
- Oxalic acid: Found in Spinach
- Ascorbic acid (Vitamin C): Found in Amla, Citrus fruits
- Tartaric acid: Found in Tamarind, grapes, unripe mangoes
Common Bases and Their Sources
- Calcium hydroxide: Found in Lime water
- Ammonium hydroxide: Found in Window cleaner
- Sodium hydroxide / Potassium hydroxide: Found in Soap
- Magnesium hydroxide: Found in Milk of magnesia
4.2 Natural Indicators Around Us
Litmus: A Natural Dye
- Extracted from lichens. It has a mauve (purple) color in distilled water.
- Turns red in acidic solutions.
- Turns blue in basic solutions.
- Commonly available as red and blue litmus paper strips.
Turmeric
- Remains yellow in acidic and neutral solutions.
- Turns red when exposed to basic solutions (such as soap solution).
China Rose (Gudhal)
- Turns acidic solutions to dark pink (magenta).
- Turns basic solutions to green.
4.3 Neutralisation
The Neutralisation Reaction
- Neutralisation: The reaction between an acid and a base. It produces salt, water, and evolves heat.
- General Equation: Acid + Base → Salt + Water (Heat is evolved)
- Example: Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H₂O)
- Phenolphthalein: A synthetic indicator used in neutralisation. It turns pink in basic solutions and remains colorless in acidic solutions.
Acid Rain
- Rain becomes acidic when carbon dioxide, sulphur dioxide, and nitrogen dioxide pollutants dissolve in raindrops to form carbonic acid, sulphuric acid, and nitric acid.
- Acid rain causes severe damage to buildings, historical monuments, plants, and aquatic life.
4.4 Neutralisation in Everyday Life
Real-World Applications
- Indigestion: Excess hydrochloric acid in the stomach is neutralised by taking antacids like milk of magnesia (containing magnesium hydroxide).
- Ant Bite: Ant stings inject formic acid. This is neutralised by rubbing moist baking soda (sodium hydrogencarbonate) or calamine solution (zinc carbonate).
- Soil Treatment: Acidic soil is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide). Basic soil is treated with organic matter (compost) which releases acids.
- Factory Wastes: Acidic industrial wastes are neutralised with basic substances before being discharged into water bodies to protect aquatic life.
Did You Know?
- Every cell in our body contains deoxyribonucleic acid (DNA), which controls genetic features.
- Proteins in our body are made of amino acids.
- Fats in our body contain fatty acids.