ACT Science: Understand Acids, Bases, and Neutralization Reactions

Acids: substances that donate hydrogen ions (H+); pH less than 7; taste sour; turn blue litmus paper red. Bases: substances that accept hydrogen ions (or donate hydroxide ions, OH-); pH greater than 7; taste bitter; turn red litmus paper blue. Neutral: pH=7 (pure water). The pH scale runs 0-14: acids are below 7, bases are above 7, and 7 is neutral.

Example: Hydrochloric acid (HCl) has pH 1 (very acidic). Sodium hydroxide (NaOH) has pH 14 (very basic). When you mix these in the right proportions, they neutralize each other: HCl+NaOH → NaCl+H2O (salt and water). The reaction produces a neutral product.

Question type 1: Identify a substance as acid or base based on pH or properties. Question type 2: Predict the product of an acid-base neutralization reaction. Question type 3: Use pH to determine concentration or strength (lower pH = more acidic; higher pH = more basic). Most questions test whether you understand pH scale and neutralization logic, not complex chemistry calculations.

Common misconception: "Mixing an acid and base always produces water." Not quite. Neutralization produces salt+water: acid+base → salt+H2O. The specific salt depends on which acid and base you mix.

Classify each as acidic, basic, or neutral: (1) Vinegar (pH≈3), (2) Lemon juice (pH≈2), (3) Water (pH=7), (4) Baking soda (pH≈8), (5) Ammonia (pH≈11). For each, predict whether adding an acid or base would increase or decrease pH. Answers: (1) acidic, (2) acidic, (3) neutral, (4) basic, (5) basic. Adding an acid to acidic substances makes them more acidic (lower pH). Adding a base makes them less acidic (higher pH, toward neutral).

If you classified all five correctly and predicted pH changes, you understand acid-base chemistry at the ACT level.

Acids and bases are fundamental to chemistry and biology. ACT Science tests basic understanding, not advanced calculations. Knowing pH scale, what acids and bases are, and how they neutralize gives you 1-2 reliable Chemistry points per test.

Spend 20 minutes learning pH basics and drilling the five-substance drill. By test day, acid-base questions will feel straightforward.