Acids, Bases and Salts

We encounter acids, bases, and salts every day. The sour taste of lemon juice, the bitter taste of baking soda, and the common salt we add to our food are all examples of these chemical compounds. Understanding their properties, how they react with each other, and how we measure their strength is crucial in chemistry.

1. What are Acids and Bases?

Acids

Acids are substances that are sour in taste and change the color of blue litmus to red. * Chemical Definition: According to the Arrhenius theory, an acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions (H⁺) or hydronium ions (H₃O⁺). * Examples: Hydrochloric acid (HCl) in our stomach, Acetic acid (CH₃COOH) in vinegar, Citric acid in lemons. * Strong vs. Weak Acids: Strong acids (like HCl, H₂SO₄) completely dissociate into ions in water. Weak acids (like acetic acid) only partially dissociate.

Bases

Bases are substances that are bitter in taste, soapy to touch, and change the color of red litmus to blue. * Chemical Definition: A base is a substance that produces hydroxide ions (OH⁻) when dissolved in water. Bases that are soluble in water are specifically called alkalis. * Examples: Sodium hydroxide (NaOH) used in soap, Magnesium hydroxide (Mg(OH)₂) used as an antacid, Calcium hydroxide (Ca(OH)₂).

2. Indicators

How do we test if a substance is acidic or basic without tasting it (which is dangerous in a lab!)? We use indicators. An indicator is a 'dye' that changes color when it is put into an acid or a base.

• Natural Indicators:
  • Litmus: Extracted from lichens. Acids turn blue litmus red; bases turn red litmus blue.
  • Turmeric: Remains yellow in acidic solutions but turns reddish-brown in basic solutions (ever noticed a curry stain turn red when washed with soap?).
• Synthetic Indicators:
  • Phenolphthalein: Colorless in acid, turns pink in a base.
  • Methyl Orange: Shows red color in acidic medium and yellow color in basic medium.
• Olfactory Indicators: Substances whose odor changes in acidic or basic media. Examples include onion, vanilla extract, and clove oil. The smell of onion can be detected in an acid but not in a base.

3. Chemical Properties of Acids and Bases

Reaction with Metals

When an acid reacts with a metal, it displaces hydrogen from the acid, releasing hydrogen gas and forming a compound called a salt. * Acid + Metal → Salt + Hydrogen gas * Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑ * Test for Hydrogen: Bring a burning candle near the gas; it burns with a 'pop' sound.

Some active metals also react with strong bases to produce hydrogen gas. * Example: Zn + 2NaOH → Na₂ZnO₂ (Sodium zincate) + H₂↑

Reaction with Metal Carbonates and Metal Hydrogencarbonates

Acids react with metal carbonates and metal hydrogencarbonates to form salt, carbon dioxide gas, and water. Bases generally do not react with these. * Acid + Metal Carbonate/Hydrogencarbonate → Salt + CO₂ + H₂O * Example: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑ * Test for CO₂: Pass the gas through lime water (calcium hydroxide solution). The lime water turns milky due to the formation of insoluble calcium carbonate.

Reaction Between Acids and Bases (Neutralization)

When an acid and a base react, they cancel out each other's effects, producing salt and water. This is called a neutralization reaction. * Base + Acid → Salt + Water * Example: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) * Concept: The H⁺ ions from the acid combine with the OH⁻ ions from the base to form water (H₂O).

Reaction of Metallic and Non-Metallic Oxides

• Metallic Oxides with Acids: Metallic oxides (like CuO, MgO) react with acids to give salt and water. Therefore, metallic oxides are basic in nature.
  • Example: CuO + 2HCl → CuCl₂ + H₂O
• Non-Metallic Oxides with Bases: Non-metallic oxides (like CO₂, SO₂) react with bases to give salt and water. Therefore, non-metallic oxides are acidic in nature.
  • Example: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

4. The pH Scale: Measuring Strength

The strength of an acid or a base depends on the number of H⁺ ions or OH⁻ ions produced, respectively. To quantify this, we use the pH scale (where 'p' stands for potenz in German, meaning power).

The pH scale ranges from 0 to 14. * pH < 7: Acidic solution. The lower the value, the stronger the acid. * pH = 7: Neutral solution (e.g., pure water). * pH > 7: Basic (alkaline) solution. The higher the value, the stronger the base.

Universal indicator, a mixture of several indicators, shows different colors at different concentrations of hydrogen ions, providing a visual representation of the pH value.

Importance of pH in Everyday Life

• Plants and Animals: Most living organisms can survive only in a narrow range of pH change (usually 7.0 to 7.8). Acid rain (pH less than 5.6) can lower the pH of river water, making the survival of aquatic life difficult.
• Digestive System: Our stomach produces hydrochloric acid (pH ~ 1.2) which helps in digesting food. Excess acid causes indigestion and pain. We use bases called antacids (like magnesium hydroxide) to neutralize the excess acid.
• Tooth Decay: Tooth decay starts when the pH of the mouth is lower than 5.5. Bacteria produce acids by degrading sugar and food particles. Using toothpaste (which is basic) neutralizes the acid and prevents decay.
• Self-defense by Animals/Plants: Bee stings inject methanoic acid causing pain. Applying a mild base like baking soda provides relief. Nettle leaves have stinging hair that injects methanoic acid.

5. Salts

Salts are ionic compounds formed by the neutralization reaction of an acid and a base. * Family of Salts: Salts having the same positive or negative radicals belong to a family (e.g., NaCl and Na₂SO₄ belong to the family of sodium salts). * pH of Salts: * Strong Acid + Strong Base → Neutral Salt (pH = 7) [e.g., NaCl] * Strong Acid + Weak Base → Acidic Salt (pH < 7) [e.g., NH₄Cl] * Weak Acid + Strong Base → Basic Salt (pH > 7) [e.g., Na₂CO₃]

Important Chemicals from Common Salt (NaCl)

Common salt is not just for food; it is an important raw material for various materials of daily use:

1. Sodium Hydroxide (NaOH): Produced by the chlor-alkali process. Electricity is passed through an aqueous solution of sodium chloride (brine). It decomposes to form NaOH, chlorine gas (Cl₂), and hydrogen gas (H₂).
2. Bleaching Powder (CaOCl₂): Produced by the action of chlorine on dry slaked lime (Ca(OH)₂). Used for bleaching cotton and linen, and for disinfecting drinking water.
3. Baking Soda (NaHCO₃): Sodium hydrogencarbonate. Used in cooking to make cakes fluffy (when heated, it releases CO₂). Also used as an antacid.
4. Washing Soda (Na₂CO₃·10H₂O): Sodium carbonate decahydrate. Used in glass, soap, and paper industries, and for removing permanent hardness of water.
5. Plaster of Paris (CaSO₄·½H₂O): Calcium sulfate hemihydrate. Obtained by heating Gypsum (CaSO₄·2H₂O) at 373 K. Used by doctors as plaster for supporting fractured bones. When mixed with water, it sets into a hard solid mass (gypsum again).

Summary

Acids (H⁺ donors) and bases (OH⁻ donors) are fundamental chemical categories identified by their taste, effect on indicators, and pH values. They undergo characteristic reactions with metals, carbonates, and with each other (neutralization) to form salts. The pH scale is a vital tool for understanding not just lab chemicals, but the biological and environmental systems around us. Furthermore, the neutralization product, salt, specifically sodium chloride, serves as the cornerstone for manufacturing numerous essential industrial and household chemicals.