Acids, Bases, and Salts Definitions, examples, properties; pH scale; Neutralization

CHEMISTRY: Acids, Bases, and Salts

Welcome, SS1 students! Today, we will explore the fascinating world of acids, bases, and salts – fundamental substances that are all around us and play crucial roles in chemistry and everyday life. Understanding them helps us explain many chemical reactions.

Core Concepts

1. Acids

Acids are substances that taste sour, are corrosive, and turn blue litmus paper red. In water, acids produce hydrogen ions (H⁺).

• Examples: Hydrochloric acid (HCl) found in stomach acid, sulphuric acid (H₂SO₄) used in car batteries, ethanoic acid (CH₃COOH) found in vinegar, and citric acid in citrus fruits.
• Properties:
  • Sour taste (e.g., lemon juice).
  • Corrosive (can burn skin).
  • Turn blue litmus paper red.
  • React with most metals to produce hydrogen gas.
  • React with carbonates to produce carbon dioxide gas.
  • React with bases to form salt and water (neutralization).

2. Bases

Bases are substances that usually taste bitter, feel soapy or slippery, and turn red litmus paper blue. In water, soluble bases (called alkalis) produce hydroxide ions (OH⁻).

• Examples: Sodium hydroxide (NaOH) used in soap making, potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂) used in limewater, and ammonia (NH₃) used in cleaning agents.
• Properties:
  • Bitter taste (e.g., bitter leaf).
  • Soapy/slippery feel.
  • Corrosive (strong bases can burn skin).
  • Turn red litmus paper blue.
  • React with acids to form salt and water (neutralization).

Image: Litmus paper changing colour to indicate acid (red) or base (blue).

3. Salts

Salts are ionic compounds formed when the hydrogen ion (H⁺) of an acid is replaced by a metal ion or an ammonium ion (NH₄⁺). They are typically formed from the reaction of an acid and a base.

• Examples: Sodium chloride (NaCl) – common table salt, copper(II) sulphate (CuSO₄), potassium nitrate (KNO₃).
• Properties:
  • Most salts are soluble in water.
  • When dissolved in water or molten, many salts can conduct electricity.
  • They have high melting and boiling points.

4. pH Scale

The pH scale measures how acidic or basic a substance is. It ranges from 0 to 14.

• Acidic solutions have a pH less than 7 (0-6.9). The lower the pH, the stronger the acid.
• Neutral solutions have a pH of exactly 7 (e.g., pure water).
• Basic (alkaline) solutions have a pH greater than 7 (7.1-14). The higher the pH, the stronger the base.

Image: The pH scale showing common substances and their typical pH values.

5. Neutralization

Neutralization is a chemical reaction in which an acid and a base react to form a salt and water. This reaction reduces the acidity or alkalinity of both substances, bringing the resulting solution closer to a neutral pH of 7.

• General Equation: Acid + Base → Salt + Water
• Example: Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water
  • HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
• Ionic Equation: H⁺(aq) + OH⁻(aq) → H₂O(l) This shows the fundamental reaction where hydrogen ions from the acid combine with hydroxide ions from the base to form water.

Real-World Example & Practice

When you experience heartburn, it's often due to excess stomach acid (hydrochloric acid). Taking an antacid tablet, which contains a base like magnesium hydroxide, helps neutralize the acid and relieve the discomfort. This is a common application of neutralization in medicine.

Practice/Reflection: Think about how farmers might use lime (calcium hydroxide, a base) to treat overly acidic soil for better crop growth.