Acid-Base Reactions

An acid releases hydrogen ions (H⁺) in water, and a base releases hydroxide ions (OH⁻). The sour taste of lemon links to acids and the slippery feel of soap to bases, and pH turns that strength into a number. pH 7 is neutral, below 7 is acidic, above 7 is basic, and indicator colors like litmus, BTB, and phenolphthalein help you tell them apart. On this page, change the solution type and pH to watch the ions and indicator colors shift.

What are Acids and Bases?

🧪 Intuition — Sour vs Slippery

①Lemon juice tastes sour → acidic (H⁺)

②Soap feels slippery → basic (OH⁻)

③Acid: dissolves in water and releases H⁺

④Base: dissolves in water and releases OH⁻

Solution0

📝 Mode Guide

①0 = Acidic (HCl → H⁺ + Cl⁻)

②1 = Basic (NaOH → Na⁺ + OH⁻)

③2 = Neutral (NaCl solution)

Indicators & pH

🌈 Telling Acid from Base

①Litmus: acid → red, base → blue

②BTB: acid → yellow, neutral → green, base → blue

③Phenolphthalein: acid → colorless, base → pink

pH3

pH Scale

📏 What is pH?

①pH: how acidic or basic the solution is

②pH 7 = neutral (pure water)

③pH < 7 = acidic (smaller = stronger)

④pH > 7 = basic (larger = stronger)

Summary

Definition of an Acid

Acid → releases H⁺

e.g., HCl → H⁺ + Cl⁻

Definition of a Base

Base → releases OH⁻

e.g., NaOH → Na⁺ + OH⁻

pH Scale

pH < 7 acidic | pH = 7 neutral | pH > 7 basic

Smaller = stronger acid, larger = stronger base

🎯 Exam Points

①Acid: releases H⁺, sour taste, reacts with metals

②Base: releases OH⁻, slippery, dissolves protein

③pH 7 = neutral, pH↓ = more acidic, pH↑ = more basic

④Litmus: acid → red, base → blue

⑤Phenolphthalein turns pink only in base

Examples and Unit Test

Example 1

When dissolved in water, which ion does an acid release and which does a base release, and how would you show an example as an equation?

An acid dissolves in water and releases hydrogen ions (H⁺): HCl → H⁺ + Cl⁻.

A base dissolves in water and releases hydroxide ions (OH⁻): NaOH → Na⁺ + OH⁻.

▸ Acid → H⁺ (e.g., HCl → H⁺ + Cl⁻) / base → OH⁻ (e.g., NaOH → Na⁺ + OH⁻)

Acidity comes from H⁺ and basicity from OH⁻; these are their shared properties.

Example 2

Is a solution of pH 2 and one of pH 12 acidic or basic, and how does the color change when phenolphthalein is added to each?

Since pH 7 is neutral, pH 2 is acidic and pH 12 is basic.

Phenolphthalein is colorless in acid and pink in base, so pH 2 stays colorless and pH 12 turns pink.

▸ pH 2 = acidic (phenolphthalein colorless) / pH 12 = basic (phenolphthalein pink)

A lower pH means a stronger acid and a higher pH a stronger base, told apart by indicator color.

Unit Test

Which statement about acids and bases is correct?

①An acid dissolves in water and releases OH⁻

②A pH greater than 7 is acidic

③In a basic solution phenolphthalein turns pink

④An acidic solution makes blue litmus paper even bluer

⑤The lower the pH, the weaker the acid

▸ ③ In a basic solution phenolphthalein turns pink

An acid releases H⁺ (① wrong, OH⁻ is from a base), and a pH greater than 7 is basic (② wrong).

An acidic solution turns blue litmus red (④ wrong), and a lower pH means a stronger acid (⑤ wrong); in a basic solution phenolphthalein turns pink (③ correct).

← Previous

Ions & Electrolysis

Neutralization

Was this helpful? Support seegongsik