The pH Scale

Definition

• The pH scale measures how acidic or alkaline a solution is.
• Ranges from 0 to 14:
  • Low numbers (0-6): Most acidic
  • Neutral (7): Pure water
  • High numbers (8-14): Most alkaline

Examples of pH Values

• Stomach acid: pH 2
• Acid rain: pH 4
• Washing up liquid: pH 9
• Bleach: pH 12

Importance of Context

• Specific examples are for context; exact values not necessary to memorize.

Measuring pH

Methods

1. Indicators

   • Chemical dyes that change color with pH.
   • Different indicators change at different pH levels.
   • Universal indicator: A common wide-range indicator that shows a gradient of colors from deep red (acidic) to bluey purple (alkaline).

2. pH Meter

   • Uses a pH probe to electronically measure pH.
   • Provides numerical readings, more accurate than color indicators.
   • Reduces human error in judgment regarding colors.

Acids and Bases

Definition of Acids

• Any substance forming an aqueous solution with a pH of less than 7.
• Acids release hydrogen ions (H⁺) in water, making the solution acidic.

Definition of Bases

• Any substance with a pH greater than 7.

Alkalis

• Subgroup of bases that are soluble in water.
• Form hydroxide ions (OH⁻) in solutions with a pH greater than 7.

Neutralization Reaction

• Reacting an acid and a base produces a salt and water.
• Example: Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H₂O).
• Neutralization can also be expressed in terms of ions:
  • H⁺ (from acid) + OH⁻ (from base) → H₂O.
• The resulting solution has a pH of 7 (neutral).

Common Acids and Bases

• Common Acids:
  • Hydrochloric acid (HCl)
  • Sulfuric acid (H₂SO₄)
  • Nitric acid (HNO₃)
• Common Bases:
  • Hydroxides (e.g., Sodium hydroxide, NaOH)
  • Carbonates (e.g., Calcium carbonate, CaCO₃)

Conclusion

• Familiarize with common acids and bases, as they frequently appear in discussions.