Explaining the Concepts of Acids and Bases for Grade 7 Students

Let’s break down the concepts step by step, making them easy to understand:

5.1 Identification of Acids and Bases

Why Do Fruits Have Different Tastes?

• Fruits taste different because they contain different chemical compounds .
• Some fruits like oranges, lemons, pineapples, and tomatoes taste sour . This sourness is due to the presence of acids in them.
• For example:Lemon contains citric acid .Vinegar contains acetic acid .

What Are Acids?

• Acids are substances that taste sour and can make things change color when tested with special tools called indicators .
• Acids are found in many places:In fruits (like lemon and orange).In vinegar.In your stomach (stomach acid helps digest food).

What Are Bases?

• Bases are substances that feel slippery or soapy and can reduce the effects of acids.
• Examples of bases include:Sodium bicarbonate (used to treat bee stings).Milk of magnesia (used for stomach problems).Lime water (used to reduce soil acidity).

Neutral Substances

• Some substances are neither acidic nor basic; they are called neutral substances .
• Examples include:Water.Salt solution.Alcohol.Kerosene.

5.2 Acids and Bases in School Laboratory and Home

Activity 5.1: Identifying Acids, Bases, and Neutral Substances

• To identify whether a substance is an acid, base, or neutral, we use indicators .
• An indicator is a tool that changes color depending on whether it reacts with an acid or a base.
• In this activity, you use shoe flower solution as an indicator:Add shoe flower solution to different liquids (e.g., lime juice, vinegar, soap water).Observe the color change:Acids give one color.Bases give another color.Neutral substances don’t change the color much.

Litmus Paper

• Litmus paper is another common indicator used in labs:Blue litmus paper turns red in acids.Red litmus paper turns blue in bases.Neutral substances don’t change the color of either litmus paper.

Other Natural Indicators

• You can also prepare indicators using natural materials like:Turmeric boiled water.Red cabbage boiled water.Arecanut boiled water.

Activity 5.3: Testing Liquids with Natural Indicators

• Prepare natural indicators and test different liquids (e.g., lime juice, vinegar, soap water, shampoo).
• Record the color changes and classify the liquids as acids, bases, or neutral substances.

pH Papers

• pH papers are special strips that show how acidic or basic a substance is.
• The pH scale ranges from 1 to 14 :1–6 : Acidic (stronger acids have lower numbers).7 : Neutral (e.g., water).8–14 : Basic (stronger bases have higher numbers).
• Dip the pH paper into a liquid and compare the color to the pH chart to determine its nature.

Activity 5.4: Using pH Papers

• Test different solutions (e.g., lime juice, soap water) with pH papers.
• Arrange the solutions based on their pH values (acidic, neutral, or basic).

Summary of Key Points

1. Acids :Sour taste.Turn blue litmus red.Found in fruits, vinegar, and stomach acid.
2. Bases :Slippery or soapy feel.Turn red litmus blue.Found in soap, lime water, and milk of magnesia.
3. Neutral Substances :No effect on litmus paper.Examples: Water, salt solution, kerosene.
4. Indicators :Tools like litmus paper, pH paper, and natural indicators help identify acids and bases.
5. pH Scale :Measures how acidic or basic a substance is (1–14).

Exercise Questions Explained

(01) Which answer contains only acidic substances?

• Correct Answer: 3) Vinegar, lemon, tamarindVinegar, lemon, and tamarind all contain acids.

(02) What turns red litmus into blue?

• Correct Answer: 2) Lime waterLime water is a base, and bases turn red litmus blue.

(03) An indicator used in the laboratory is…

• Correct Answer: 2) Methyl orangeMethyl orange is a chemical indicator used in labs.

(04) What is the color of pH papers?

• Correct Answer: 1) YellowpH papers are usually yellow before testing.

(05) What solution turns phenolphthalein pink?

• Correct Answer: 2) Sodium hydroxidePhenolphthalein turns pink in bases like sodium hydroxide.

Three Containers Problem

Given Table:

Solution

Blue Litmus →

Red Litmus →

A

Blue

Blue

B

Blue

Red

C

Red

Red

1. Which solution shows acidic properties?
2. Which solution shows basic properties?
3. Which solution shows neutral properties?
4. If one vessel contained water, what is the letter given for the solution?

Final Notes

Understanding acids and bases is important because they are everywhere in our daily lives and in science experiments. By using indicators like litmus paper, pH papers, and natural indicators, we can easily identify whether a substance is acidic, basic, or neutral.

General Understanding Questions

1. What are acids? Give two examples.
2. What are bases? Give two examples.
3. What are neutral substances? Provide three examples.
4. Why do fruits like lemons and oranges taste sour?
5. Name two acidic substances found in your kitchen.
6. Name two basic substances used at home.
7. What is the difference between acids and bases?
8. How do acids and bases affect litmus paper?
9. What are indicators? Give examples of natural and laboratory indicators.
10. Why is water considered a neutral substance?

Indicators and Their Uses

1. What is an indicator? How does it help in identifying acids and bases?
2. What happens when blue litmus paper is dipped in an acid?
3. What happens when red litmus paper is dipped in a base?
4. What color does phenolphthalein turn in an acidic solution? In a basic solution?
5. What colors do pH papers show for acidic, basic, and neutral substances?
6. What is the pH scale? Explain its range.
7. How can you prepare a natural indicator using turmeric?
8. Name three natural indicators mentioned in the chapter.
9. What is the difference between red and blue litmus paper?
10. Why is methyl orange used as an indicator in laboratories?

Experiments and Activities

1. Describe Activity 5.1. What is the purpose of using shoe flower solution?
2. How can you identify acids, bases, and neutral substances using litmus paper?
3. What observations would you make if you tested lime juice with red and blue litmus paper?
4. What happens when you dip pH paper into vinegar? Into soap water?
5. What are the steps involved in Activity 5.3 (using natural indicators)?
6. How would you classify shampoo as acidic, basic, or neutral using an indicator?
7. What precautions should you take while handling acids and bases in the lab?
8. Why is it important to rinse your hands with water if you accidentally touch an acid or base?
9. What is the role of the teacher during experiments involving acids and bases?
10. How can you use pH paper to arrange solutions in order of their acidity or basicity?

Applications of Acids and Bases

1. Why is sodium bicarbonate used to treat bee stings?
2. How does milk of magnesia help with stomach problems?
3. Why is lime added to soil in agriculture?
4. Name two acids commonly used in laboratories.
5. Name two bases commonly used in laboratories.
6. What is the importance of identifying acidic and basic substances in daily life?
7. How are acids and bases used in cleaning products?
8. Why is vinegar used in cooking and food preservation?
9. How do antacids work to reduce acidity in the stomach?
10. Why is it dangerous to mix acids and bases carelessly?

pH Scale and Its Significance

1. What does the pH scale measure?
2. What pH value indicates a neutral substance?
3. What range of pH values represents acidic substances?
4. What range of pH values represents basic substances?
5. If a solution has a pH of 3, is it acidic, basic, or neutral?
6. If a solution has a pH of 10, is it acidic, basic, or neutral?
7. What is the pH of pure water?
8. Why is the pH of soil important for farmers?
9. What happens to the pH of a solution when an acid is added to it?
10. What happens to the pH of a solution when a base is added to it?

Problem-Solving Questions

1. You are given three solutions: A, B, and C. Solution A turns blue litmus red, Solution B turns red litmus blue, and Solution C does not change the color of either litmus paper. Identify which solution is acidic, basic, and neutral.
2. A student tested a liquid with pH paper and observed a dark green color. Is the liquid acidic, basic, or neutral?
3. If a solution turns phenolphthalein pink, what can you conclude about its nature?
4. A solution turns methyl orange red. What does this indicate about the solution?
5. You have two unlabeled bottles containing lime water and vinegar. How would you identify them using litmus paper?

Higher-Order Thinking Questions

1. Why do you think some acids are called "weak acids" while others are called "strong acids"?
2. Can a substance be both acidic and basic at the same time? Explain.
3. Why is it important to test the pH of swimming pool water?
4. How does the concept of acids and bases apply to digestion in the human body?
5. What would happen if all the acids in your stomach were neutralized?

Short Answer Questions

1. Define an acid.
2. Define a base.
3. What is a neutralization reaction?
4. What is the role of indicators in chemistry?
5. Why do we use pH papers instead of litmus paper in some cases?

True or False Questions

1. All fruits contain acids. (False)
2. Bases feel slippery to the touch. (True)
3. Water is an example of a neutral substance. (True)
4. Red litmus paper turns blue in acidic solutions. (False)
5. Phenolphthalein turns pink in acidic solutions. (False)

Multiple Choice Questions

1. Which of the following is an acid?

• a) Soap
• b) Lime water
• c) Vinegar
• d) Milk of magnesia

1. Which of the following turns red litmus blue?

• a) Lemon juice
• b) Sodium hydroxide
• c) Vinegar
• d) Tomato juice

1. What is the pH of a neutral substance?

• a) 1
• b) 7
• c) 10
• d) 14

1. Which indicator is colorless in acidic solutions but turns pink in basic solutions?

• a) Litmus
• b) Methyl orange
• c) Phenolphthalein
• d) pH paper

1. Which of the following is a base?

• a) Hydrochloric acid
• b) Sodium hydroxide
• c) Acetic acid
• d) Nitric acid