In-Depth Notes on Acids and Alkalis

Acids and Alkalis

Definition and Properties

Acids:
  • Definition: Substances that react with alkalis, characterized by a sour taste.
  • Types:
    • Strong Acids: Corrosive, can damage surfaces and living tissue.
    • Weak Acids: Non-hazardous, safe for consumption.
Alkalis:
  • Definition: Substances that neutralize acids to form salt and water, and have a bitter taste.

Sources of Acids and Alkalis

  1. In the Kitchen:

    • Acids: Lemon, oranges, apples, onions, tomatoes, aspirin, vinegar.
    • Alkalis: Alcohol, soap, antacids (e.g., Eno), baking soda, wood ash.

  2. In the Bathroom:

    • Acids: Dettol, hand wash.
    • Alkalis: Soap, toothpaste, caustic soda, Harpic.

  3. In Our Bodies:

    • Acids: Hydrochloric acid in the stomach aids digestion and kills germs in food.
    • Alkalis: Bile produced in the liver assists with digestion.

Hazard Symbols

  • Definition: Warning signs indicating dangerous substances.
  • Common Symbols:
    • Harmful: Can cause illness if swallowed, inhaled, or absorbed through skin.
    • Irritant: Causes reddening and blistering on skin.
    • Corrosive: Destroys surfaces and living tissue.
    • Flammable: Easily catches fire.

Indicators

  • Definition: Substances that change color in acids and alkalis.
  • Sources: Derived from plant materials (e.g., leaves, petals, roots).
Experiment to Create an Indicator:
  1. Materials: Red cabbage, beetroot, various colored petals, mortar and pestle.
  2. Procedure:
    • Crush the cabbage or beetroot, add water, filter, then test with acid and alkali.
    • Record the color changes for different solutions.
Results and Observations:
  • The indicator demonstrates different colors when tested with acids (e.g., lemon juice) and alkalis (e.g., baking powder).

Common Indicators in the Lab

IndicatorColor in AcidColor in NeutralColor in Alkali
Litmus paperRedNo changeBlue
Litmus solutionRedPurpleBlue
PhenolphthaleinColorlessColorlessPink
Methyl orangeRedOrangeYellow
Universal indicatorPink to RedLight GreenDark Green to Purple

Common Acids and Alkalis in the Lab

  • Common Acids:

    • Hydrochloric Acid (Strong)
    • Sulphuric Acid (Strong)
    • Nitric Acid (Strong)
    • Citric Acid (Weak)

  • Common Alkalis:

    • Ammonium Hydroxide (Weak)
    • Calcium Hydroxide (Weak)
    • Sodium Hydroxide (Strong)
    • Potassium Hydroxide (Strong)

The pH Scale

  • Definition: Ranges from 0 to 14; used to measure acidity or alkalinity.
  • Guidelines:
    • Acids: pH 0-6.9; Strong acids (0-3), Weak acids (4-6.9).
    • Neutral: pH 7.
    • Alkalis: pH 7.1-14; Weak alkalis (8-10), Strong alkalis (11-14).

Antacids

  • Definition: Weak alkalis that neutralize stomach acid (hydrochloric acid) to relieve indigestion.
  • Common Antacids: Eno, Actals, Tumbocids.
Experiment to Determine the Effectiveness of Antacids:
  1. Materials: Dropper, measuring cylinder, hydrochloric acid, antacid.
  2. Procedure:
    • Mix antacid in water, add phenolphthalein, then gradually add hydrochloric acid until color change occurs.
    • Record the drops needed to neutralize the antacid.
Results:
  • Eno: 75 drops to neutralize
  • Actals: 160 drops to neutralize
  • Conclusion: Eno is the better antacid as fewer drops were needed.

Uses of Acids and Alkalis

  1. Food Preservation: Vinegar (acid) used in pickling to kill bacteria and prevent decay.
  2. Hair and Skin Care: Weak acids in skin care products.
  3. Treating Stings:
    • Bee Stings (acidic): Treated with weak alkali (e.g., calamine lotion).
    • Wasp Stings (alkaline): Treated with weak acid (e.g., vinegar).
  4. Soil Treatment: Acidic soil treated with lime (alkali) and alkaline soil neutralized with animal manure (acid).