Understanding the pH Scale, Acids, and Alkalis

What is pH?

• A measure of how acidic or alkaline a solution is
• Scale from 0 to 14:
  • Low numbers = more acidic
  • High numbers = more alkaline
  • pH 7 = Neutral (e.g., pure water)

Examples of pH Levels

• Stomach acid: ~pH 2
• Acid rain: ~pH 4
• Washing up liquid: ~pH 9
• Bathroom bleach: ~pH 12

Measuring pH

Indicators

• Chemical dyes that change color depending on pH
• Wide Range Indicators: Mix of dyes, show gradual color change
• Universal Indicator: Common example, wide range of colors from deep red (acidic) to blue-purple (alkaline)

pH Probe and Meter

• Probe dipped into solution, gives numerical reading
• More accurate and precise
• Removes human error in color judgment

Acids and Bases

Acids

• Forms aqueous solutions with pH < 7
• Release hydrogen ions (H⁺) in water

Bases

• Substances with pH > 7

Alkalis

• Soluble in water
• Subgroup of bases
• Form hydroxide ions (OH⁻) in water

Neutralization Reactions

• Acid + Base → Salt + Water
• Example: Hydrochloric acid + Sodium hydroxide → Sodium chloride (salt) + Water
• Hydrogen ions (H⁺) + Hydroxide ions (OH⁻) → H₂O
• Products of neutralization are neutral (pH 7)

Common Acids and Bases

Acids

• Hydrochloric acid (HCl)
• Sulfuric acid (H₂SO₄)
• Nitric acid (HNO₃)

Bases

• Hydroxides (e.g., Sodium hydroxide NaOH)
• Carbonates (e.g., Calcium carbonate CaCO₃)

Note: It’s useful to remember these common acids and bases as they frequently appear in the course.