📖 Lesson 5: Classification of Substances

👩🏽‍🏫 Introduction

Hi science champion! 👋🏽

By now, you’ve learned how to identify acids, bases, and salts, how they behave, and even how to prepare them. But what happens when you’re given a substance and asked to figure out what it is—an acid, a base, or a salt?

In this lesson, you’ll learn how to classify substances based on simple tests, observations, and reactions. You’ll also discover how to use indicators and pH levels to make accurate conclusions—just like real scientists!

📘 Core Concepts

🔹 Why Classify Substances?

Classification helps us:

• Predict a substance’s behavior in reactions.

• Handle it safely.

• Use it for the right purpose (e.g., medicine, farming, cleaning).

🔹 Tests for Classification

We can use simple tests to classify unknown substances.

1. Litmus Paper Test

2. pH Test (Using Universal Indicator or pH Meter)

Note: Some salts may be slightly acidic or basic depending on their source.

3. Reaction with Metals or Carbonates

• Acids: React with metals (like zinc) to release hydrogen gas.

• Acids: React with carbonates (like baking soda) to release carbon dioxide gas.

• Bases: Usually do not react with metals or carbonates.

• Salts: Generally unreactive unless heated or mixed with specific substances.

🔹 Classifying Common Examples

🧪 Real-Life Example

🔹 Household Cleaning Products:

• Acids like vinegar remove rust or limescale.

• Bases like bleach and soap remove grease and stains.

• Salts like sodium carbonate soften hard water.

Knowing what each product is helps you avoid dangerous reactions—never mix acids and bleach! 🚫

🧩 Sample Problem Walkthrough

🧠 Problem:
A student tests an unknown liquid with blue and red litmus paper. The blue litmus turns red, but the red litmus stays red. What type of substance is this?

✅ Step-by-Step Solution:

• Step 1: Blue litmus turns red — indicates an acid.

• Step 2: Red litmus remains red — supports that it’s an acid.

• Step 3: Conclusion — the substance is acidic.

✔️ Answer:
The unknown liquid is an acid.

✍🏽 Practice Exercises

1. Multiple Choice

Which of these will turn red litmus blue?

• (A) Vinegar

• (B) Lemon juice

• (C) Soap solution

• (D) Distilled water

Answer:
(C) Soap solution

2. Match the substance to its classification:

Answers:

• Orange juice → C. Acid

• Toothpaste → B. Base

• Sodium chloride → A. Salt

3. Short Answer:

What two tests can you perform to determine whether a solution is an acid, base, or salt?

Sample Answer:
Use litmus paper to check for color change and a pH indicator to measure its acidity or basicity. These tests help identify if it’s an acid, base, or neutral salt.

🌀 Recap

In this lesson, you’ve learned:

• How to classify substances as acids, bases, or salts.

• Tools like litmus paper, pH indicators, and reaction tests help in classification.

• Real-life examples like toothpaste, lemon juice, and soap show how these substances behave.

💭 Reflection Prompt

Find 3 items at home—like shampoo, orange juice, or detergent. Predict whether each is an acid, base, or salt. How could you test your prediction safely? Write it down in your science journal!