Acids, Bases, and the pH Scale

Some liquids are acids, some are bases, and the pH scale is how we measure where a substance falls between them. It runs from 0 to 14, and it turns up in biology, chemistry, and everyday life — from stomach acid to soap. This lesson covers acids, bases, and how to read the pH scale.

Acids and Bases

An acid is a substance that tastes sour and can be corrosive — lemon juice and vinegar are common acids. A base (also called an alkali when dissolved in water) feels slippery and includes things like soap and baking soda. When an acid and a base combine, they can cancel each other out in a reaction called neutralization, often producing water and a salt.

Reading the pH Scale

The pH scale measures how acidic or basic a substance is on a scale from \(0\) to \(14\). A pH below \(7\) is acidic, a pH of exactly \(7\) is neutral (pure water), and a pH above \(7\) is basic. The further from 7, the stronger the acid or base.

One detail worth knowing: each step on the scale is a tenfold change in strength. A pH of \(4\) is ten times more acidic than a pH of \(5\), and a hundred times more acidic than a pH of \(6\). Formally, pH is defined as \[ \text{pH} = -\log[\text{H}^+], \] but for the test you mainly need to read the scale and know which end is which.

Measuring pH

You can estimate pH with an indicator — a substance that changes color depending on acidity. Litmus paper turns red in acid and blue in base; universal indicator shows a range of colors matched to pH values. When a question shows an indicator color or a pH number, place it on the scale: under 7 acidic, 7 neutral, over 7 basic.

Watch: A Short Video Lesson

FuseSchool walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:

A Routine for pH Questions

1. pH below \(7\) = acidic; pH \(7\) = neutral; pH above \(7\) = basic.
2. The further from 7, the stronger the acid or base.
3. Each step is a tenfold change in strength.
4. Indicators (litmus, universal) reveal pH by color.

Practice

1. What pH value is neutral?
2. Is a substance with pH \(3\) an acid or a base?
3. Is a substance with pH \(11\) an acid or a base?
4. How much more acidic is pH \(4\) than pH \(6\)?
5. What does litmus paper do in an acid?
6. What is it called when an acid and base cancel out?

Answers

1. \(7\).
2. An acid.
3. A base.
4. \(100\) times more acidic (two steps of ten).
5. Turns red.
6. Neutralization.

Where This Fits in Your Science Prep

Acids and bases build on chemical reactions (neutralization is a reaction) and connect to solutions and solubility. See all topics on the Science Topics Hub.