Acids, Bases & pH: The Chemistry of Sour and Slippery

Acids taste sour (think lemon juice), bases feel slippery (think soap), and the pH scale is the ruler that measures how strongly a solution leans one way or the other. The whole topic rests on a single particle: the hydrogen ion. Acids flood a solution with hydrogen ions; bases remove them. The pH scale just translates that concentration into a convenient number from 0 to 14.

This guide covers definitions, the pH scale, strength, and neutralization, with worked and practice questions matched to the level seen in ACT Science and chemistry at Northside Tutoring.

Why Acids & Bases Matter

Acid-base chemistry appears in ACT Science passages and is fundamental to chemistry and biology (blood pH, digestion, the environment). The pH scale is also a nice real-world use of logarithms. (The SAT has no science section.)

What Acids & Bases Are

An acid is a substance that releases hydrogen ions (H⁺) in solution — hydrochloric acid (HCl) is a classic example. A base releases hydroxide ions (OH⁻) or accepts hydrogen ions — sodium hydroxide (NaOH) is a classic base. The more H⁺ in a solution, the more acidic it is.

The pH Scale

The pH scale measures acidity from 0 to 14:

pH	Type	Examples
0–6	acidic	lemon juice, stomach acid
7	neutral	pure water
8–14	basic (alkaline)	soap, bleach

Strong vs. Weak

Strength describes how completely an acid or base releases its ions. A strong acid (like HCl) dissociates almost completely; a weak acid (like vinegar's acetic acid) only partly. Strength is different from concentration — a strong acid can be dilute, and a weak acid can be concentrated.

Neutralization

When an acid and a base mix, they neutralize each other, producing water and a salt:

For example, HCl + NaOH → H₂O + NaCl. The H⁺ from the acid and the OH⁻ from the base combine to form water, moving the solution toward neutral pH 7.

Indicators

An indicator is a substance that changes color depending on pH, letting you estimate acidity. Litmus paper turns red in acid and blue in base; universal indicator shows a range of colors across the scale. ACT Science passages often present indicator or pH data in tables and graphs.

Where You'll See This — Test by Test

Acid-base chemistry supports ACT Science passages; the SAT has no science section and the SSAT doesn't test it. It's core high-school and AP Chemistry.

Watch the Lesson

Sometimes a diagram needs a voice. In the short video below, one of our Northside tutors walks through the core idea and works through test-style problems in real time.

Worked Example Problems

These problems are calibrated to the difficulty you'll actually see on test day. Try each one before opening the solution.

Common Mistakes to Avoid

Practice Problems — You Try

Three problems below. Work each before checking the solution.

The Northside Method — How We Teach This 1-on-1

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