Willer Academy
Nation Builders Through Education | Founded by Er. Rahul Kumar Dubey Sir
Chapter 5: Acids, Bases and Salts
Subject: Science (Chemistry)
Class: 7
Series: Base Builder for Moderate to Hard
Learning Objectives
By the end of this chapter, you, the future scientist and nation-builder, will be able to:
- Understand the properties of acids, bases, and salts
- Differentiate between acids and bases using indicators
- Explain the process of neutralization and its applications
- Understand the pH scale and its importance
- Solve numerical problems related to acids, bases, and salts
- Apply knowledge to real-life situations involving acids and bases
5.1 Introduction: Acids, Bases and Salts
Acids, bases, and salts are fundamental substances in chemistry that we encounter in our daily lives. From the food we eat to the cleaning products we use, these compounds play crucial roles.
Quick Check
Think about three things you used today that might contain acids or bases. For example, citrus fruits contain citric acid, and soap is basic.
5.2 Acids and Their Properties
Acids are substances that taste sour and turn blue litmus paper red. They release hydrogen ions (H⁺) when dissolved in water.
Common Acids
- Hydrochloric acid (HCl)
- Sulfuric acid (H₂SO₄)
- Nitric acid (HNO₃)
- Acetic acid (CH₃COOH)
- Citric acid (C₆H₈O₇)
Properties of Acids
- Sour taste
- Turn blue litmus red
- Conduct electricity in solution
- React with bases to form salts
- React with metals to produce hydrogen gas
Numerical & Application 1: Acid Concentration
- Scenario: Vinegar contains about 5% acetic acid by volume.
- Problem: How many milliliters of acetic acid are present in a 500mL bottle of vinegar? If you dilute this vinegar with 1500mL of water, what is the new concentration?
- Solution:
- Acetic acid in 500mL vinegar = 5% of 500mL = 0.05 × 500 = 25mL
- Total volume after dilution = 500mL + 1500mL = 2000mL
- New concentration = (25mL / 2000mL) × 100 = 1.25%
This shows how dilution affects acid concentration!
Task 1
Test various household substances with blue litmus paper. Try lemon juice, vinegar, orange juice, and any other safe liquids. Record which ones turn the litmus paper red, indicating they are acidic.
5.3 Bases and Their Properties
Bases are substances that taste bitter and feel slippery. They turn red litmus paper blue and release hydroxide ions (OH⁻) when dissolved in water.
Common Bases
- Sodium hydroxide (NaOH)
- Calcium hydroxide (Ca(OH)₂)
- Ammonium hydroxide (NH₄OH)
- Magnesium hydroxide (Mg(OH)₂)
- Potassium hydroxide (KOH)
Properties of Bases
- Bitter taste
- Turn red litmus blue
- Feel slippery
- Conduct electricity in solution
- React with acids to form salts
Experiment: Testing with Turmeric
Turmeric is a natural indicator that turns red in basic solutions. Make a turmeric solution by mixing turmeric powder with water. Use this solution to test various household substances like baking soda solution, soap solution, and lemon juice. Observe the color changes.
5.4 pH Scale and Indicators
The pH scale measures how acidic or basic a solution is. It ranges from 0 to 14:
0-6
Acidic
7
Neutral
8-14
Basic
Indicators are substances that change color in the presence of acids or bases:
Litmus
Acid: Red
Base: Blue
Phenolphthalein
Acid: Colorless
Base: Pink
Methyl Orange
Acid: Red
Base: Yellow
Numerical & Application 2: pH Calculation
- Scenario: The pH of a solution is calculated using the formula: pH = -log[H⁺], where [H⁺] is the hydrogen ion concentration.
- Problem: If lemon juice has a hydrogen ion concentration of 0.01 mol/L, what is its pH? Is it acidic or basic?
- Solution:
- [H⁺] = 0.01 = 10⁻² mol/L
- pH = -log(10⁻²) = -(-2) = 2
- Since pH < 7, lemon juice is acidic
This shows how pH relates to hydrogen ion concentration!
5.5 Neutralization Reactions
When an acid and a base react, they neutralize each other, forming salt and water. This is called a neutralization reaction.
Acid + Base → Salt + Water
Example: Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water
HCl + NaOH → NaCl + H₂O
Common Salts
- Sodium chloride (NaCl)
- Calcium sulfate (CaSO₄)
- Ammonium nitrate (NH₄NO₃)
- Potassium chloride (KCl)
- Copper sulfate (CuSO₄)
Uses of Neutralization
- Treating acid indigestion
- Treating ant bites
- Soil treatment
- Industrial waste treatment
- Fire extinguishers
Task 2: Neutralization in Action
Add a few drops of lemon juice (acid) to a small amount of baking soda (base) in a bowl. Observe the fizzing, which is the release of carbon dioxide. This is an example of a neutralization reaction.
Section B: MCQ Challenge (20 Questions)
Test your knowledge and find where you need to improve!
1. Which of these turns blue litmus red?
a) Baking soda solution
b) Soap solution
c) Lemon juice
d) Lime water
2. The pH of a neutral solution is:
a) 0
b) 7
c) 14
d) 1
3. Which of these is a natural indicator?
a) Litmus
b) Phenolphthalein
c) Methyl orange
d) All of these
4. Which of the following is a strong acid?
a) Acetic acid
b) Citric acid
c) Formic acid
d) Hydrochloric acid
5. Bases feel slippery because they react with:
a) Acids
b) Fats and oils on skin
c) Water
d) Metals
6. Phenolphthalein is colorless in:
a) Base
b) Neutral solution
c) Acid
d) Alcohol
7. Which of these salts is formed by neutralization?
a) Sodium chloride
b) Baking soda
c) Gypsum
d) Ammonium nitrate
8. What is the pH of a basic solution?
a) 0-6
b) 7
c) 6-7
d) 8-14
9. Which indicator turns red in acid?
a) Phenolphthalein
b) Litmus
c) Methyl violet
d) Bromothymol blue
10. What gas is released when acid reacts with metals?
a) Oxygen
b) Carbon dioxide
c) Hydrogen
d) Nitrogen
11. Calcium hydroxide is a:
a) Base
b) Acid
c) Salt
d) Neutral substance
12. Vinegar contains which acid?
a) Sulfuric acid
b) Citric acid
c) Hydrochloric acid
d) Acetic acid
13. Which is used to test for the presence of acid?
a) Red litmus
b) Blue litmus
c) Methyl orange
d) Phenolphthalein
14. Soap solution is:
a) Acidic
b) Neutral
c) Basic
d) Depends on water
15. Lemon juice has pH around:
a) 2
b) 5
c) 7
d) 9
16. Which acid is used in car batteries?
a) Hydrochloric acid
b) Sulfuric acid
c) Acetic acid
d) Nitric acid
17. Baking soda reacts with lemon juice to produce:
a) Oxygen gas
b) Carbon dioxide gas
c) Hydrogen gas
d) Water
18. Which of these is used in fire extinguishers?
a) Sodium chloride
b) Calcium sulfate
c) Sodium bicarbonate
d) Ammonium nitrate
19. Which natural indicator is yellow in acid?
a) Turmeric
b) Litmus
c) Red cabbage
d) Methyl orange
20. Neutralization reaction produces:
a) Acid + Base
b) Salt only
c) Water only
d) Salt + Water
Section C: Answer in Detail (For Answer Writing Practice)
Q1. Explain the process of neutralization with examples. How is this reaction useful in everyday life?
Hint: Include the general equation and at least three practical applications.
Q2. Describe the pH scale. How does it help in identifying acids and bases?
Hint: Explain the range of the scale and what different values indicate.
Q3. What are indicators? Name three natural indicators and explain how they work.
Hint: Include examples like litmus, turmeric, and red cabbage.
Section D: Tackle These! (Higher Order Thinking Skills - HOTS)
Q1. Farmers often add slaked lime to acidic soil. Explain the chemistry behind this practice. What would happen if they add too much lime?
Think about: Neutralization reactions and pH balance.
Q2. Why is it recommended to use a baking soda paste on ant bites? Explain the chemical reaction involved.
Think about: Acid-base reactions and formic acid.
Q3. Design an experiment to create a natural pH indicator using red cabbage. How would you use it to test various household substances?
Think about: Extraction process, color changes, and pH range.
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