Acids, Bases, and Indicators

• Many common substances like lemon, tamarind, salt, sugar, and vinegar have different tastes

Acids and Bases

• Curd, lemon juice, orange juice, and vinegar taste sour due to the presence of acids
• The chemical nature of these sour substances is acidic
• The word 'acid' comes from the Latin word 'acere', meaning sour
• Acids in these substances are natural acids
• Substances that taste bitter and feel soapy are generally known as bases, and their nature is termed basic
• Indicators are special substances used to test whether a substance is acidic or basic
• Indicators change color when added to acidic or basic solutions
• Turmeric, litmus, and China rose petals (Gudhal) are naturally occurring indicators

Natural Indicators

• Litmus is a commonly used natural indicator extracted from lichens
• Litmus has a mauve (purple) color in distilled water
• Litmus turns red when added to an acidic solution and blue when added to a basic solution
• Litmus is available as a solution or in the form of red and blue litmus paper
• Turmeric can be made into paper to indicate a base
• Lime water can be prepared by adding some lime (chuna) to water in a tumbler

Phenolphthalein

• When the solution is basic, phenolphthalein gives a pink colour
• On the other hand, when the solution is acidic, it remains colorless

Neutralisation

• When an acidic solution is mixed with a basic solution, they neutralize each other's effects

• In a neutralization reaction, the acidic and basic natures are destroyed, resulting in a solution that is neither acidic nor basic

• Neutralization reactions produce heat, raising the temperature of the reaction mixture

• A new substance, called salt, is formed

• Salts can be acidic, basic, or neutral

• Neutralisation is the reaction between an acid and a base, producing salt and water with the evolution of heat

• Acid + Base → Salt + Water + Heat*

• Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H₂O)

Neutralisation in Everyday Life

Indigestion

• The stomach contains hydrochloric acid for digestion
• Too much acid can cause indigestion
• Antacids, like milk of magnesia (containing magnesium hydroxide), neutralize excessive acid

Ant Bites

• Ant bites inject formic acid into the skin, so neutralise with moist baking soda (sodium hydrogencarbonate) or calamine solution (containing zinc carbonate)

Soil Treatment

• Excessive use of chemical fertilizers can make soil acidic, which impacts plant growth
• Soils that are too acidic are treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide)
• If the soil is basic, organic matter (compost) is added, releasing acids to neutralize the soil

Factory Waste

• Factory wastes often contain acids that can harm aquatic life so; they are neutralized by adding basic substances before disposal

Additional Facts

• Acid rain contains excess acids and is caused by pollutants like carbon dioxide, sulphur dioxide, and nitrogen dioxide dissolving in rain
• These pollutants form carbonic acid, sulphuric acid, and nitric acid
• Acid rain damages buildings, historical monuments, plants, and animals
• Each cell in our body contains deoxyribonucleic acid (DNA)
• DNA controls the body's features, and fats in our body contain fatty acids