Introduction
Let me tell you about an interesting story that happened on National Science Day, 28 February. Two siblings, Ashwin and Keerthi, went to a science fair and got white paper sheets at the entry gate. A person at gate sprayed some liquid on their sheets, something magical happened – the message “Welcome to the Wonderful World of Science” appeared on the paper.
They were really curious about how this message appeared out of nowhere. They went to a stall called “Colourful World of Substances” where they saw how different substances change colors when mixed together. This got them interested in learning about acidic, basic, and neutral substances.
Just like Ashwin and Keerthi, you’re going to learn all about these substances and understand the science behind that mysterious appearing message.
Nature – Our Science Laboratory
Nature gives us many tools to study chemistry, and one of the most imp tools is indicators. These are special substances that help us identify whether something is acidic, basic, or neutral by changing colors.
Litmus as an Indicator
Litmus is a natural substance that comes from lichens (those small plant-like organisms you see growing on rocks and trees). It’s available as blue and red litmus paper or as a liquid solution, and it acts as an acid-base indicator.
How litmus paper works: Using litmus paper, we can classify all substances into three main groups based on how they react:
Acidic Substances:
• Turn blue litmus paper red
• Common examples include lemon juice, amla juice, tamarind water, and vinegar
• Most acidic substances taste sour and are often edible
• The sourness you taste in citrus fruits comes from their acidic nature
Basic Substances: • Turn red litmus paper blue
• Examples include soap solution, baking soda solution, lime water, and washing powder solution
• Basic substances feel slippery when you touch them
• They often taste bitter.
Neutral Substances:
• Don’t change the color of either blue or red litmus paper.
• Common examples are tap water, sugar solution, and salt solution.
• These substances are neither acidic nor basic .
• They’re safe and don’t have strong chemical properties.
| Substance Type | Effect on Blue Litmus | Effect on Red Litmus | Common Examples | Taste/Feel |
|---|---|---|---|---|
| Acidic | Turns Red | No Change | Lemon juice, Vinegar | Sour taste |
| Basic | No Change | Turns Blue | Soap, Baking soda | Slippery feel, Bitter taste |
| Neutral | No Change | No Change | Water, Sugar solution | No specific taste/feel |
Red Rose as an Indicator
Did you know that red rose petals can work as a natural acid-base indicator? This is a beautiful example of how nature provides us with scientific tools.
How to make red rose indicator: • Collect fallen red rose petals and wash them properly • Crush the petals to release their natural pigments • Soak the crushed petals in hot water for 5-10 minutes • Filter the colored liquid – this is your indicator!
How red rose extract behaves: • In acidic solutions: The extract turns red (like with lemon juice or vinegar) • In basic solutions: The extract turns green (like with soap or baking soda solution) • In neutral solutions: The extract remains unchanged (like with sugar solution)
The red rose extract is quite reliable and matches the results you get from litmus tests. Other natural indicators that work similarly include beetroot, purple cabbage, red hibiscus, and Indian blackberry (jamun).
Turmeric as an Indicator
Turmeric, that golden spice found in every Indian kitchen, can also be used as an indicator! This shows how everyday items around us have scientific applications.
Preparing turmeric indicator paper: • Make a paste with turmeric powder and water • Apply this paste on white paper strips • Let it dry – now you have turmeric paper!
How turmeric indicator works: • With basic substances: Turmeric paper turns red (like when exposed to lime water) • With acidic substances: No color change occurs (stays yellow like with vinegar) • With neutral substances: No color change occurs (stays yellow like with salt solution)
One limitation of turmeric is that it can only identify basic substances clearly. It cannot distinguish between acidic and neutral substances since both keep the turmeric yellow.
Turmeric is called the “golden spice” in Ayurveda because of its health benefits and use in traditional remedies. It’s been used for thousands of years not just for cooking but also for medicinal purposes.
Olfactory Indicators
Here’s something really interesting – some substances work as indicators through smell rather than color change! These are called olfactory indicators.
How olfactory indicators work:
• Take cloth strips soaked in onion juice
• When you expose these strips to acidic substances like tamarind water, the onion smell changes
• When exposed to basic substances like baking soda solution, the smell changes differently
• This gives you another way to identify whether a substance is acidic or basic
This method is particularly useful when you can’t see color changes clearly or when working in low light conditions. The human nose is very sensitive and can detect these subtle changes in odor.
What Happens When Acidic Substances Mix with Basic Substances?
When you mix an acid with a base in the right amounts, something really interesting happens – they neutralize each other! This process is called neutralization.
The neutralization process step by step:
• Start with an acidic solution (like lemon juice) that turns blue litmus red
• Slowly add a basic solution (like lime water) to it • As you add the base, the solution gradually becomes less acidic
• At the perfect point, the solution becomes neutral
• If you continue adding base, the solution becomes basic and turns red litmus blue
The neutralization reaction: The chemical reaction can be written as: Acid + Base → Salt + Water + Heat
During this process:
• The acidic and basic properties cancel out each other
• A salt is formed (not necessarily table salt, but a chemical salt)
• Water is produced • Heat is released, making the solution warm
Neutralisation in Daily Life
Neutralization isn’t just a lab experiment – it happens around us every day and solves many practical problems.
Ant Bites
When red ants bite you, they inject formic acid into your skin. This acid causes pain, redness, and swelling.
The solution: • Apply moist baking soda (a base) to the bite area • The baking soda neutralizes the formic acid • This reduces pain and inflammation • The neutralization reaction helps heal the bite faster
Soil Problems
Soil can become too acidic or too basic, affecting plant growth badly.
Acidic Soil Problems: • Excessive use of chemical fertilizers makes soil acidic • Plants can’t absorb nutrients properly from acidic soil • Root growth gets affected
Solution for acidic soil: • Add lime (calcium oxide or calcium hydroxide) to the soil • Lime is basic and neutralizes the excess acid • This brings soil pH back to neutral, helping plants grow better
Basic Soil Problems: • Some soils naturally have too much alkaline content • This also prevents proper nutrient absorption
Solution for basic soil: • Add organic matter like cow manure or compost • Organic matter releases mild acids as it decomposes • These acids neutralize the basic soil gradually
Industrial Waste Management
Factories often produce acidic waste that can harm the environment.
The problem: • Acidic factory waste gets discharged into rivers and lakes • This kills fish and other aquatic life • The ecosystem gets damaged
The solution: • Add basic substances to neutralize the acidic waste before discharge • This makes the waste harmless to aquatic life • Environmental protection through chemistry!
Real-life applications table:
| Problem | Cause | Solution | Result |
|---|---|---|---|
| Ant bite pain | Formic acid injection | Apply baking soda | Pain relief through neutralization |
| Poor plant growth | Acidic soil | Add lime | Neutral soil, better growth |
| Alkaline soil | Natural/over-liming | Add organic matter | Balanced soil pH |
| Water pollution | Industrial acid waste | Add basic chemicals | Safe water for aquatic life |
Imp Points
Classification of substances: • All substances can be classified as acidic, basic, or neutral based on their properties • This classification helps us understand how they behave and interact
Different types of indicators: • Litmus: Blue turns red in acids, red turns blue in bases • Red rose extract: Turns red in acids, green in bases • Turmeric: Turns red in bases only, stays yellow in acids and neutral solutions • Olfactory indicators: Change smell in different pH conditions
Neutralization reactions: • Acids and bases react to form salt, water, and heat • This process cancels out both acidic and basic properties • The resulting solution becomes neutral
Practical applications: • Medical: Treating ant bites and stomach acidity • Agriculture: Balancing soil pH for better crop growth • Environmental: Treating industrial waste before disposal • Household: Understanding cleaning products and food preservation
Questions and Answers
Question 1: Reversing Litmus Changes
Problem: A solution turns red litmus paper blue. Which solution would reverse this change? Options: (i) Lime water (ii) Baking soda (iii) Vinegar (iv) Common salt solution
Solution process: • If red litmus turns blue, the original solution is basic • To reverse this change, we need to add an acid • Let’s check each option:
- Lime water: Basic (won’t reverse)
- Baking soda: Basic (won’t reverse)
- Vinegar: Acidic (will neutralize and reverse)
- Salt solution: Neutral (no effect)
Answer: (iii) Vinegar – because it’s acidic and will neutralize the basic solution.
Question 2: Multiple Indicator Tests
Problem: Three unknown solutions A, B, and C are tested: • Solution A: Turns red litmus blue • Solution B: Turns turmeric red
• Solution C: Turns red rose extract green
What’s the nature of each solution?
Analysis: • Solution A: Red litmus → blue means it’s basic • Solution B: Turmeric → red means it’s basic • Solution C: Red rose extract → green means it’s basic
Answer: All three solutions (A, B, and C) are basic in nature.
Question 3: Laboratory Liquid Analysis
Problem: A liquid sample shows these results: • Red litmus: No change • Blue litmus: Turns red • Turmeric: No color change
What’s the nature of this liquid?
Step-by-step analysis: • Red litmus shows no change → liquid is not basic • Blue litmus turns red → liquid is acidic • Turmeric shows no change → confirms it’s not basic (turmeric only turns red for bases)
Answer: The liquid is acidic because it turns blue litmus red while not affecting red litmus or turmeric.
Question 4: Blindfolded Testing
Problem: Manya is blindfolded and needs to test if solutions are acidic or basic. Which indicator should she use?
Reasoning: • Manya cannot see color changes, so visual indicators won’t work • She needs an indicator that works through a different sense • Olfactory indicators change smell, not color • Onion-soaked cloth changes odor differently with acids and bases
Answer: Manya should use olfactory indicators like onion-soaked cloth because she can detect the nature of solutions through smell changes instead of color changes.
Question 5: Secret Message Materials
Problem: What materials could create the appearing message effect from the chapter introduction?
Possible combinations:
| Writing Material | Spray Solution | Resulting Color | Explanation |
|---|---|---|---|
| Turmeric paste | Soap solution | Red writing | Turmeric turns red in basic soap solution |
| Red rose extract | Lemon juice | Red writing | Rose extract turns red in acidic lemon juice |
| Red rose extract | Baking soda solution | Green writing | Rose extract turns green in basic solution |
| Phenolphthalein | Lime water | Pink writing | Phenolphthalein turns pink in basic lime water |
The trick works because the writing material (indicator) is invisible when dry but becomes colored when the spray (acid or base) reacts with it.
Question 6: Grape Juice Experiment
Problem: Grape juice mixed with red rose extract turns red. What happens if baking soda is added?
Analysis: • Grape juice is acidic, making red rose extract turn red • Baking soda is basic • Adding baking soda neutralizes the acid and makes the solution basic • Red rose extract turns green in basic solutions
Answer: The mixture will turn green because baking soda makes the solution basic, and red rose extract turns green in basic conditions.
Question 7: Secret Birthday Message
Problem: Keerthi wrote a secret message using orange juice. How can her grandmother reveal it?
Solution approach: • Orange juice is acidic but invisible when dry on paper • We need an indicator that will show contrast • Turmeric solution can be sprayed on the paper • Turmeric stays yellow on acidic orange juice but might turn different color on the paper • This creates a visible contrast revealing the message
Answer: Use turmeric solution as a spray. It will create a color contrast between the acidic orange juice areas and the rest of the paper, making the secret message visible.
Question 8: Natural Indicator Preparation
Problem: How can natural indicators be prepared? Give an example.
General process: • Natural indicators come from plant materials with pH-sensitive pigments • The preparation involves extracting these pigments • The extract changes color based on acidity or basicity
Red rose extract example: • Collect fresh fallen red rose petals • Wash petals thoroughly with clean water • Crush petals using mortar and pestle to release pigments • Place crushed petals in a glass container • Add hot water to completely immerse the petals • Cover and wait 5-10 minutes for color extraction • Filter the colored liquid through cloth or filter paper • The filtered liquid is your natural indicator
Answer: Natural indicators are prepared by extracting color pigments from plants. For example, red rose extract is made by crushing washed rose petals, soaking them in hot water, and filtering the colored solution, which then acts as an indicator turning red in acids and green in bases.
Question 9: Turmeric Paper Limitation
Problem: Can turmeric paper alone identify vinegar, baking soda solution, and sugar solution?
Testing each solution: • Vinegar (acidic): Turmeric paper stays yellow • Baking soda solution (basic): Turmeric paper turns red • Sugar solution (neutral): Turmeric paper stays yellow
Analysis: • Baking soda solution is easily identified by the red color change • Both vinegar and sugar solution keep turmeric yellow • Turmeric cannot distinguish between acidic and neutral substances
Answer: Turmeric paper can identify the baking soda solution (turns red) but cannot distinguish between vinegar and sugar solution since both keep the turmeric paper yellow.
Question 10: Red Rose Extract Analysis
Problem: Red rose extract turns liquid X green. What’s the nature of liquid X? What happens when excess amla juice is added?
First part analysis: • Red rose extract turns green in basic solutions • Therefore, liquid X is basic
Second part analysis: • Amla juice is acidic • Adding excess amla juice to basic liquid X will neutralize it • With excess amla juice, the solution becomes acidic • Red rose extract turns red in acidic solutions
Answer: Liquid X is basic in nature. Adding excess amla juice will make the solution acidic, causing the red rose extract to turn red.
Question 11: Soil Treatment Flowchart
Problem: Complete the missing information about soil treatment.
Complete flowchart information:
Problem identification: • Poor plant growth can be due to wrong soil pH • Soil can be acidic OR basic in nature
Testing method: • Which indicator to use: Litmus paper • Blue litmus turns red for acidic soil • Red litmus turns blue for basic soil
Treatment methods: • For acidic soil: Add lime (calcium oxide/hydroxide) • Lime is basic and neutralizes soil acidity • For basic soil: Add organic matter (manure, compost) • Organic matter releases acids that neutralize basic soil
Answer:
- Indicator for testing: Litmus paper
- Treatment for acidic soil: Lime
- Treatment for basic soil: Organic matter
Download Free Mind Map from the link below
This mind map contains all important topics of this chapter
Visit our Class 7 Curiosity page for free mind maps of all Chapters