👩🏽‍🏫 Introduction

Welcome back, enthusiastic scientist! 👋🏽

Imagine you’re trying to find out if the water in your school’s garden is suitable for plants, or checking if milk has gone sour. How would you know? In science, we have special tools called acid-base indicators that help us identify whether substances are acidic or basic.

Today, you’ll discover how scientists use indicators and the pH scale to measure acidity and alkalinity precisely. By the end of this lesson, you’ll confidently test and interpret the acidity or basicity of common substances around you.

📘 Core Concepts

🔹 What are Acid-Base Indicators?

Indicators are substances that change color depending on whether they’re in an acidic or basic environment. They provide an easy way to identify acids and bases quickly.

Common examples include:

• Litmus paper:

  • Acids turn blue litmus red 🔴

  • Bases turn red litmus blue 🔵

• Phenolphthalein:

  • Colourless in acids ⚪️, pink in bases 🌸

• Methyl Orange:

  • Red in acids 🔴, yellow in bases 🟡

🔹 The pH Scale

The pH scale measures how acidic or basic a substance is on a scale from 0 to 14:

Important:

• Lower pH means more acidic (stronger acid).

• Higher pH means more basic (stronger base).

🧪 Real-Life Example

🔹 Testing Soil pH: Farmers regularly test soil pH because different crops grow best at specific pH values. For example:

• Tomatoes 🍅 prefer slightly acidic soil (pH 6).

• Beans 🌱 prefer neutral or slightly alkaline soil (pH 7-8).

Using indicators or digital pH meters, farmers adjust soil conditions by adding lime (a base) or sulfur (an acid), helping crops thrive.

🧩 Sample Problem Walkthrough

🧠 Problem:
You test a liquid with phenolphthalein, and it turns pink. What does this indicate about the liquid’s pH?

✅ Step-by-Step Solution:

• Step 1: Recall the indicator: phenolphthalein is colourless in acidic and neutral solutions, pink in basic solutions.

• Step 2: If the solution turns pink, it must be basic.

• Step 3: Since it is basic, the liquid’s pH is greater than 7.

✔️ Answer:
The pink color indicates that the liquid is basic, with a pH greater than 7.

✍🏽 Practice Exercises

1. Multiple Choice:

If a solution turns methyl orange indicator yellow, it means the solution is:

• (A) Acidic

• (B) Basic

• (C) Neutral

Answer: (B) Basic

2. Fill in the blanks:

(a) A solution with pH of 3 is ______.
(b) Pure water has a pH of ______.
(c) pH above 7 indicates a ______ solution.

Answers:
(a) acidic
(b) 7
(c) basic (alkaline)

3. Short Answer:

Why is it important to know the pH of water used in fish farming?

Sample Answer:
Because fish and aquatic organisms can survive only within specific pH ranges. If the water is too acidic or too basic, it can harm or kill fish. Knowing pH helps maintain healthy conditions for aquatic life.

🌀 Recap

Today, you learned:

• Acid-base indicators help us identify if substances are acidic, basic, or neutral by changing colours.

• Common indicators include litmus paper, phenolphthalein, and methyl orange.

• The pH scale ranges from 0 to 14; lower numbers mean acidic, higher numbers mean basic, and 7 is neutral.

• Understanding pH is crucial in daily life, farming, and environmental management.

💭 Reflection Prompt

Look around your house or classroom. Think of at least three substances (like vinegar, soap, soda, or toothpaste). Predict their pH levels as acidic, basic, or neutral, and explain why you made your predictions. Write your thoughts in your science journal, and discuss them with a friend or family member!