Exploring Substances: Acidic, Basic, and Neutral

1. Introduction to Acidic, Basic, and Neutral Substances

Substances around us can be classified into three main types based on their chemical nature:

• Acids:

  • Taste sour. The word "acid" originates from the Latin word acere, which means sour.

  • The sour taste of naturally occurring foods like citrus fruits, curd, and tamarind is due to the presence of acids.

  • Caution: Never taste unknown substances or laboratory chemicals, as many acids can be corrosive to skin or toxic.

• Bases:

  • Taste bitter and feel slippery or soapy when rubbed between fingers.

  • Soluble bases in water are specifically called alkalies.

  • Note: Not all bitter-tasting substances are basic (e.g., bitter gourd is bitter but not basic).

• Neutral Substances:

  • Substances that are neither acidic nor basic in nature.

  • Examples include pure tap water, sugar solution, and common salt solution.

2. Indicators

An indicator is a chemical substance that identifies whether a given solution is acidic, basic, or neutral by undergoing a distinct change in colour or odour.

2.1 Natural Indicators

1. Litmus:

   • Extracted from lichens (a symbiotic association of a fungus and an alga).

   • Naturally purple (mauve) in neutral solution.

   • Available as solutions or paper strips in two varieties: blue litmus and red litmus.

   • In Acidic Medium: Turns blue litmus paper red.

   • In Basic Medium: Turns red litmus paper blue.

   • In Neutral Medium: No colour change on either blue or red litmus paper.

2. Turmeric (Haldi):

   • Contains the active chemical component curcumin.

   • In Basic Medium: Turns bright reddish-brown.

   • In Acidic or Neutral Medium: Shows no colour change (remains yellow).

3. Red Rose Extract:

   • Prepared by steeping crushed red rose petals in hot water.

   • In Acidic Medium: Turns red.

   • In Basic Medium: Turns green.

   • In Neutral Medium: Remains unchanged.

4. Other Botanical Indicators:

   • Red Hibiscus (Gudhal): Turns dark pink (magenta) in acids and green in bases.

   • Beetroot Extract: Turns pink/red in acids and yellowish in bases.

   • Purple Cabbage Extract: Remains red in acids and turns green in bases.

   • Indian Blackberry (Jamun): Turns red/pink in acids and greenish-blue in bases.

   • Hydrangea Plant: Produces blue flowers when grown in acidic soil, and pink/red flowers in basic soil.

2.2 Olfactory Indicators

Olfactory indicators undergo a distinct change in smell (odour) depending on whether they are placed in an acidic or basic medium:

• Onion: Retains its characteristic odour in acidic solutions, but its smell weakens or completely disappears in basic solutions.

• Clove Oil and Vanilla: Also show a loss or change of odour in basic media.

2.3 Synthetic Indicators

Indicators manufactured in laboratories are called synthetic indicators:

• Phenolphthalein: Remains colourless in acidic medium and turns pink in basic medium.

• Methyl Orange: Turns red in acidic medium and yellow in basic medium.

3. Naturally Occurring Acids and Common Bases

3.1 Naturally Occurring Acids

3.2 Common Bases

• Preparation of Lime Water: Mix quicklime (calcium oxide, $\text{CaO}$) in water. Stir and leave it undisturbed. Quicklime reacts with water to form slaked lime (calcium hydroxide, $\text{Ca(OH)}_2$). Filter the mixture; the clear filtrate is lime water.

4. Neutralisation Reactions

When an acidic solution and a basic solution are mixed in appropriate proportions, they destroy each other's chemical properties. The resulting mixture is neither acidic nor basic.

4.1 Chemical Reaction

$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water} + \text{Heat}$

• Products formed are salt and water.

• The reaction is exothermic, meaning heat is released, causing the temperature of the mixture to rise.

4.2 Applications of Neutralisation in Daily Life

1. Ant and Bee Stings:

   • Ant and bee stings inject formic acid into the skin, causing severe burning pain.

   • Neutralised by rubbing mild bases such as moist baking soda or calamine solution (containing zinc carbonate, $\text{ZnCO}_3$).

2. Soil Treatment:

   • Excessive chemical fertilisers make soil too acidic, retarding plant growth. Acidic soil is treated with basic slaked lime [$\text{Ca(OH)}_2$] or quicklime [$\text{CaO}$].

   • Overly basic soil is treated with organic matter (manure/compost), which releases organic acids to neutralise the excess base.

3. Treatment of Industrial Wastes:

   • Wastes from factories are often acidic and can kill fish and aquatic organisms if discharged directly into lakes or rivers.

   • Factory effluents are neutralised with basic substances before release.

4. Indigestion and Acidity:

   • The stomach produces hydrochloric acid [$\text{HCl}$] to assist digestion. Overeating spicy food or stress causes excess acid production (acidity/indigestion).

   • Neutralised by consuming antacids like milk of magnesia or baking soda solution.

5. Preservation of Milk:

   • Milk vendors in warm regions add a small amount of baking soda to fresh milk to neutralise lactic acid formed by bacterial action, preventing curdling.

6. Prevention of Tooth Decay:

   • Bacteria in the mouth break down food particles, producing acids that corrode tooth enamel (calcium phosphate).

   • Toothpastes are generally basic and neutralise excess mouth acid to stop decay.

5. Chemical Reaction Highlights

• Reaction of Vinegar with Marble or Eggshell:

  • Eggshells and marble consist of calcium carbonate [$\text{CaCO}_3$].

  • Vinegar contains acetic acid [$\text{CH}_3\text{COOH}$].

  • Reaction Equation:

$\text{Acetic acid} + \text{Calcium carbonate} \rightarrow \text{Calcium acetate} + \text{Water} + \text{Carbon dioxide}$

• Carbon dioxide gas [$\text{CO}_2$] bubbles out as effervescence.

• Soap solution does not react with calcium carbonate, so no bubbles appear when soap is poured on marble/eggshells.

6. Historical & Scientific Contributions

• Acharya Prafulla Chandra Ray (1861–1944):

  • Known as the "Father of Modern Indian Chemistry".

  • Earned a doctorate in chemistry from the UK and established India's first pharmaceutical company (Bengal Chemicals & Pharmaceuticals) in 1901.

  • Researched and documented ancient Indian scientific achievements and advocated for teaching science in regional mother tongues.