Chapter 2: Exploring Substances: Acidic, Basic, and Neutral
Notes: Exploring Substances (Acidic, Basic, and Neutral)
1. Classification of Substances
Substances in our surroundings are broadly classified into three categories based on their chemical nature:
Acidic Substances: Substances that contain acids. They are generally sour to taste.
Basic Substances: Substances that contain bases. They generally taste bitter and feel soapy or slippery to touch. Note: Not all bitter substances are bases (e.g., bitter gourd tastes bitter but is not basic).
Neutral Substances: Substances that are neither acidic nor basic. They do not alter the colour of indicators.
2. Edible Acids and Their Natural Sources
Many common household foods contain natural organic acids:
Citric acid: Present in oranges and lemons.
Ascorbic acid (Vitamin C) & Citric acid: Present in amla.
Tartaric acid: Present in raw mangoes and tamarind.
Oxalic acid & Citric acid: Present in tomatoes.
Acetic acid: Present in vinegar.
Lactic acid: Present in curd.
3. Acid-Base Indicators
Indicators are special chemical substances (natural or synthetic) that change their colour or odour when mixed with acidic or basic solutions.
A. Natural Indicators (Colour Changing)
Litmus: Extracted from lichens (a symbiotic association of a fungus and an alga).
Acids turn blue litmus to red.
Bases turn red litmus to blue.
Red Rose Extract: Prepared by soaking crushed petals in hot water.
Turns red in acidic solutions.
Turns green in basic solutions.
Turmeric Paper: Prepared by treating paper with a paste of turmeric and water.
Turns red/reddish-brown in basic mediums (like soap).
Remains yellow (unchanged) in acidic and neutral mediums. Hence, it cannot distinguish between an acid and a neutral solution.
Other Examples: Beetroot, purple cabbage, red hibiscus, and jamun.
Fascinating Fact: Hydrangea flowers change colour naturally according to the soil chemistry—growing blue in acidic soil and pink/red in basic soil.
B. Olfactory Indicators
Substances whose odours (smells) change depending on whether they are placed in an acidic or basic medium.
Example: Onion strips retain their pungent smell in an acidic medium but lose their smell when treated with a basic medium.
4. Neutralisation Reactions
When an acidic solution is mixed with an appropriate amount of a basic solution, they destroy each other’s chemical properties, making the final solution neither acidic nor basic. This chemical framework is given by:
Salt: The compound formed during neutralisation.
Evolution of Heat: Neutralisation is an exothermic process; it always releases heat, raising the temperature of the container.
5. Neutralisation in Everyday Life
Ant Bites: Red ants inject formic acid into the skin, causing pain and swelling. Rubbing moist baking soda (sodium bicarbonate, a mild base) neutralises the acid and provides instant relief.
Soil Treatment: Excessive chemical fertilisers make the soil overly acidic, which hurts plant growth. Farmers add lime (calcium oxide/hydroxide, a base) to neutralise it. If the soil becomes too basic, organic matter (manure) is added because it releases natural acids to neutralise the alkalinity.
Industrial Waste: Wastes from factories are frequently highly acidic and can kill marine life if dumped directly into lakes or rivers. They are safely neutralised using basic substances before environmental release.
Chapter Question and Answer Solutions
1. Multiple Choice Questions
Q1. A solution turns the red litmus paper to blue. Excess addition of which of the following solution would reverse the change?
(i) Lime water
(ii) Baking soda
(iii) Vinegar
(iv) Common salt solution
Answer: (iii) Vinegar.
Reasoning: A solution turning red litmus to blue is basic. To reverse this change (turn it back to red), we need to add an excess of an acidic substance. Vinegar contains acetic acid, making it the correct choice.
Q2. Sequence for the nature of unknown solutions A, B, and C based on provided testing values.
(i) Acidic, acidic, and acidic
(ii) Neutral, basic, and basic
(iii) Basic, basic, and acidic
(iv) Basic, basic, and basic
Answer: (iv) Basic, basic, and basic.
Reasoning: Solution A turns red litmus blue $\rightarrow$ Basic. Solution B turns turmeric red $\rightarrow$ Basic. Solution C turns red rose extract green $\rightarrow$ Basic.
3. Image Analysis: Red Rose Paper Strips
Q3. Label the nature of solutions present in each container.
Container Fig 2.13 (Turns Green): Basic Solution.
Container Fig 2.14 (Stays Pink/Unchanged): Neutral Solution.
Container Fig 2.15 (Turns Red): Acidic Solution.
4. Laboratory Sample Testing Table
Q4. A liquid sample shows no change with red litmus, turns blue litmus red, and shows no change on turmeric paper. Identify its nature.
Answer: The liquid sample is Acidic in nature.
Justification: Acids turn blue litmus paper red. They do not change the colour of red litmus paper, nor do they alter the yellow colour of turmeric paper (which only changes colour when exposed to bases).
5. Accessibility in Science
Q5. Manya is blindfolded. She is given two unknown solutions to test. Which indicator should Manya use and why?
Answer: Manya should use an olfactory indicator, such as onion strips.
Reasoning: Since Manya cannot see colour changes, traditional visual indicators like litmus or turmeric will not work. Olfactory indicators change their smell in acidic or basic solutions, allowing her to accurately identify the solutions using her sense of smell.
6. Secret Message & Chemistry Trick
Q6. Suggest materials used for writing the secret message at the beginning of the chapter.
Answer: The paper was written on using a basic solution, and the spray bottle contained a natural indicator. Here is a table of possible functional combinations:
| Message Writing Material (Base) | Spray Bottle Material (Indicator) | Resulting Text Colour |
Soap Solution | Turmeric Solution | Red |
Baking Soda Solution | Red Rose Extract | Green |
Lime Water | Hibiscus Extract | Green/Dark Purple |
7. Chemical Addition Calculations
Q7. Grape juice mixed with red rose extract turns red. What happens if baking soda is added?
Answer: The mixture will turn from red to green.
Justification: The red tint shows that grape juice is acidic. Baking soda is a basic substance. When added to the mixture, it neutralises the acid and makes the solution basic, causing the red rose indicator to turn green.
8. Invisible Ink Processing
Q8. Keerthi wrote a secret message using orange juice. How can her grandmother read it?
Answer: The grandmother can use red rose extract or blue litmus solution as an indicator.
Reasoning: Orange juice contains citric acid. Spraying red rose extract over the message will turn the text area bright red, making the secret message clearly visible against the paper background.
9. Practical Experimentation
Q9. How can natural indicators be prepared? Explain with an example.
Answer: Natural indicators are prepared by extracting the water-soluble pigments from certain flowers, leaves, or spices.
Example (Red Rose Indicator): Collect fallen red rose petals, wash them, and crush them using a mortar and pestle. Place the crushed paste into a tumbler, add hot water, and let it sit for 5–10 minutes until the water absorbs the deep colour. Filter the mixture; the liquid filtrate serves as your natural acid-base indicator.
10. Logical Elimination Strategies
Q10. You have Vinegar, Baking Soda solution, and Sugar solution. Can you identify them using only turmeric paper? Explain.
Answer: Yes, they can be systematically identified through the following steps:
Place a drop of each solution on separate strips of yellow turmeric paper. The solution that turns the paper red is the Baking Soda solution (since turmeric only reacts to bases).
Take the turmeric paper that turned red from the baking soda. Put drops of the remaining two solutions onto this red patch.
The solution that turns the red paper back to yellow is the Vinegar (the acid neutralises the base).
The solution that causes no change to the red patch is the Sugar solution (neutral).
11. Multi-Stage Reactions
Q11. Red rose extract turns Liquid X green. Identify X, and state what happens if excess amla juice is added.
Answer:
Nature of Liquid X: Liquid X is a Base (because red rose extract turns basic solutions green).
Addition of Excess Amla Juice: Amla juice contains high concentrations of ascorbic and citric acids. Adding it in excess will neutralise the base and turn the solution acidic, causing the colour to change from green back to red.
12. Flowchart Completion
Q12. Complete the missing information in the flowchart:
The soil can be Acidic in nature OR The soil can be Basic in nature.
Indicator to test the nature of the soil: Litmus solution or Red Rose extract.
Treatment for Acidic soil: Treated with Lime (a base).
Treatment for Basic soil: Treated with Organic matter/Manure (which releases acids).
Deep Diver Question
Q. Aman spilt vinegar on an eggshell/marble and noticed bubbling, but soap solution caused no bubbles. Why?
Answer: Eggshells and marble are chemically composed of calcium carbonate. Vinegar is an acid. When an acid reacts with calcium carbonate, a chemical reaction occurs that releases carbon dioxide gas, which forms visible bubbles. Soap solution, however, is a base and does not react with carbonates, which is why no bubbles appear.
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