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Class 7 Science NCERT Notes – Chapter 2: Exploring Substances – Acidic, Basic, and Neutral (PDF, MindMap, Q&A, Quizzes)
ByAKS
Chapter 2 (Science): Exploring Substances – Acidic, Basic, and Neutral – CBSE Class 7 NCERT Science Detailed Study Notes.
1. Classification of Substances
Substances in our environment can be categorized based on their chemical nature into three main groups:
- Acidic Substances: These are substances that exhibit acidic properties. Examples include lemon juice, amla juice, tamarind water, and vinegar.
- Basic Substances: These substances have properties opposite to acids. Examples include soap solution, baking soda solution, lime water, and washing powder solution.
- Neutral Substances: These are substances that are neither acidic nor basic. Examples include tap water, sugar solution, and salt solution.
Common Acids in Edible Substances
| Substance | Common Acids Present |
| Grapes (unripe) | Tartaric acid |
| Amla (Indian Gooseberry) | Ascorbic acid (Vitamin C) and Citric acid |
| Tomato | Citric acid and Oxalic acid |
| Orange | Citric acid |
| Lemon | Citric acid |
| Curd | Lactic acid (Implicit from context, though not explicitly stated in the source text, it’s a common acid to be known for curd) |
| Tamarind | Tartaric acid |
| Vinegar | Acetic Acid (Implicit from context, a common acid known to be in vinegar) |
2. Acid-Base Indicators
Indicators are special substances that exhibit different colors when added to acidic or basic solutions, thereby “indicating” the nature of the substance. They can be natural or synthetic.
2.1 Litmus: A Natural Indicator
- Source: Litmus is a natural dye extracted from lichens. Lichens are composite organisms formed from the symbiotic association of a fungus and an alga. They typically grow on rocks and trees in areas with clean air and high rainfall.
- How it Works:
- In Acidic Solutions: Acids turn blue litmus paper to red.
- In Basic Solutions: Bases turn red litmus paper to blue.
- In Neutral Solutions: Neutral substances do not change the color of either blue or red litmus paper.
2.2 Red Rose Extract
This is another natural indicator that can be prepared at home.
- Preparation: Crush fallen red rose petals and immerse them in hot water for 5-10 minutes. The colored water, once filtered, serves as the indicator extract.
- How it Works:
- In Acidic Solutions: Red rose extract turns red.
- In Basic Solutions: Red rose extract turns green.
- In Neutral Solutions: The color of the extract remains unchanged.
2.3 Turmeric (Haldi)
Turmeric, a common household spice, also acts as a natural indicator.
- Preparation: A paste of turmeric powder and water can be applied to filter paper and dried to create “turmeric paper” strips.
- How it Works:
- In Basic Solutions: The yellow color of turmeric turns red. This explains why a curry stain (containing turmeric) on a white shirt turns red when washed with soap (which is basic).
- In Acidic and Neutral Solutions: Turmeric remains yellow and shows no color change. This means turmeric paper can identify basic substances but cannot differentiate between acidic and neutral ones.
2.4 Olfactory Indicators
Some substances change their odor in acidic or basic mediums. These are known as olfactory indicators.
- Example: Finely chopped onions stored with cotton cloth strips impart their smell to the cloth. When a basic solution (like baking soda) is added to the strip, the onion smell is diminished or changed, while an acidic solution (like tamarind water) does not significantly affect the smell.
Summary of Indicator Reactions
| Indicator | Color in Acidic Solution | Color in Basic Solution | Color in Neutral Solution |
| Blue Litmus | Red | No Change (Blue) | No Change (Blue) |
| Red Litmus | No Change (Red) | Blue | No Change (Red) |
| Red Rose Extract | Red | Green | No Change |
| Turmeric | No Change (Yellow) | Red | No Change (Yellow) |
3. Properties of Acids and Bases
- Acids: They are responsible for turning blue litmus paper red and turning red rose extract to a shade of red. Many edible acids are found in fruits like lemons, oranges, and tamarind.
- Bases: They are substances that turn red litmus paper blue, turn red rose extract green, and turn turmeric paper red. They are often slippery or soapy to the touch (e.g., baking soda solution) and generally taste bitter. However, not all bitter-tasting substances are basic (e.g., bitter gourd).
4. Neutralization Reactions
When an acidic solution and a basic solution are mixed in sufficient quantities, they neutralize each other’s effects.
- Definition: The reaction between an acid and a base is known as a neutralization reaction.
- Products: The products of this reaction are salt and water.
- Energy Change: Heat is always evolved (released) during a neutralization reaction.
- General Equation:
Acid + Base → Salt + Water + Heat
4.1 Neutralization in Daily Life
Neutralization is a crucial process used to solve many everyday problems.
- Ant Bites: When a red ant bites, it injects formic acid into the skin, causing a stinging pain. This can be neutralized by applying a mild base like moist baking soda, which provides relief.
- Soil Treatment:
- Acidic Soil: Excessive use of chemical fertilizers can make soil too acidic, hindering plant growth. To neutralize the acid, farmers treat the soil with bases like lime (calcium oxide).
- Basic Soil: If the soil is too basic, organic matter (like manure or compost) is added. The organic matter releases acids as it decomposes, neutralizing the soil’s basic nature.
- Factory Waste: Industrial waste is often acidic. Before being discharged into water bodies like lakes or rivers, it must be treated with basic substances to neutralize it. This prevents harm to aquatic life, such as fish populations.
5. Key Scientific Figures
- Acharya Prafulla Chandra Ray (P.C. Ray): Known as the “Father of Modern Indian Chemistry.” He earned a doctorate in the UK, advanced scientific research in India, and established India’s first pharmaceutical company in 1901. He was also a social reformer who advocated for using the mother tongue in education.
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Quiz and Q&A Section
Part A: Short-Answer Questions
(Answer in 2-3 sentences)
- What is an acid-base indicator?
- How is lime water prepared?
- What are lichens, and what important indicator is derived from them?
- Describe what happens when blue litmus paper is tested with lemon juice and soap solution.
- What is a neutral substance? Give two examples from the text.
- How does red rose extract react differently in acidic and basic solutions?
- Why can turmeric paper be used to identify a base but not to distinguish between an acid and a neutral substance?
- What is the general taste and feel of basic substances?
- Explain why a curry stain on a shirt turns red when soap is applied.
- What are olfactory indicators?
- Define a neutralization reaction and state its products.
- What form of energy is released during a neutralization reaction?
- What acid is injected by a red ant’s bite?
- How can the pain from an ant bite be relieved using a common household substance? Explain the principle involved.
- Why might a farmer add lime to the soil?
- If soil is too basic, what can be added to it and why?
- Who is known as the ‘Father of Modern Indian Chemistry’ and what was one of his major contributions?
- What causes the flowers of the Hydrangea plant to be different colors like blue or pink?
- Explain the likely reason for a decline in the fish population in a lake near an industrial area.
- How should factory waste be treated before being released into a lake to protect aquatic life?
- What are the most common acids found in amla juice?
- Name a food item that contains tartaric acid.
- Describe the steps to prepare red rose extract to be used as an indicator.
- What happens when red litmus paper is tested with vinegar and washing powder solution?
- In the welcome sign experiment, what two substances could have been used for the writing and the spray to make the words appear?
Part B: Multiple-Choice Questions
- Which substance turns red litmus paper blue?
- a) Vinegar b) Lemon juice c) Lime water d) Salt solution
- A red ant bite injects which of the following acids into the skin?
- a) Citric acid b) Acetic acid c) Formic acid d) Ascorbic acid
- The reaction between an acid and a base is called:
- a) Oxidation b) Reduction c) Neutralization d) Association
- Which of the following is a neutral substance?
- a) Amla juice b) Sugar solution c) Baking soda solution d) Tamarind water
- Red rose extract turns which color in a basic solution?
- a) Red b) Green c) Blue d) Yellow
- To treat acidic soil, a farmer should add:
- a) Organic matter b) Lime c) More chemical fertilizer d) Salt
- Litmus is extracted from:
- a) Roses b) Turmeric c) Lichens d) Onions
- When turmeric paste is added to a soap solution, its color changes to:
- a) Yellow b) Green c) Red d) It becomes colorless
- What are the products of a neutralization reaction?
- a) Acid and Water b) Base and Water c) Salt and Acid d) Salt and Water
- A substance that feels soapy to the touch is likely:
- a) Acidic b) Basic c) Neutral d) A salt
- Acharya P.C. Ray established India’s first:
- a) University b) Steel company c) Pharmaceutical company d) Textile mill
- Hydrangea flowers are blue in which type of soil?
- a) Basic b) Neutral c) Acidic d) Salty
- Which indicator works based on a change in smell?
- a) Litmus b) Turmeric c) Red Rose Extract d) Onion
- Which of the following is NOT an acidic substance?
- a) Tamarind water b) Vinegar c) Amla juice d) Washing powder solution
- What is the chemical nature of baking soda?
- a) Acidic b) Basic c) Neutral d) Salty
- Adding drops of lime water to lemon juice containing litmus will eventually change the solution’s color from:
- a) Blue to red b) Red to blue c) Red to green d) Green to red
- A solution turns blue litmus paper to red. It will not change the color of:
- a) Red rose extract b) Red litmus paper c) Blue litmus paper d) None of the above
- What is the common name for calcium oxide used by farmers?
- a) Chuna b) Haldi c) Amla d) Karela
- A solution is tested with red litmus, blue litmus, and turmeric. No color change is observed with any of them. The solution is most likely:
- a) Acidic b) Basic c) Neutral d) An indicator itself
- When the soil is basic, what can be added to neutralize it?
- a) Lime water b) Baking soda c) Compost d) Soap solution
Part C: Essay Questions
- Explain the process of neutralization in detail. Describe the general chemical equation and provide two distinct examples from daily life where this process is applied for a beneficial outcome.
- Compare the effectiveness of litmus, red rose extract, and turmeric as acid-base indicators. Discuss their sources, preparation (if applicable), and their specific color changes in acidic, basic, and neutral mediums.
- Describe a step-by-step procedure for preparing a natural indicator from red rose petals. Explain how you would then use this indicator to test and classify lemon juice, soap solution, and tap water.
- A farmer finds that their plants are not growing well. Based on the text, what are the two possible chemical problems with the soil, and what are the corresponding remedies for each?
- What are olfactory indicators? Using the example of onion strips, explain how they can be used to distinguish between an acidic and a basic substance.
- Explain the science behind the “magic” trick at the science fair where the words ‘Welcome to the Wonderful World of Science’ appeared on a seemingly blank sheet of paper. Propose a possible combination of chemicals that could be used.
- Who was Acharya Prafulla Chandra Ray? Discuss his contributions to science and society in India as mentioned in the text.
- Imagine you are given three unlabeled solutions: vinegar, baking soda solution, and sugar solution. Explain how you could use only turmeric paper to identify the baking soda solution, and why it would not be sufficient to distinguish between the other two.
- Explain the relationship between the acidity of soil and the color of Hydrangea flowers. Can a gardener change the color of the flowers? If so, how?
- A factory is releasing its waste into a local river, and the fish are dying. Explain the likely chemical nature of the waste and the scientific process that should be implemented to solve this environmental problem.
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Answer Keys
Answer Key for Short-Answer Questions
- An acid-base indicator is a substance that shows different colors when it is in an acidic or a basic solution. These indicators are used to determine the chemical nature of other substances.
- Lime water is prepared by mixing lime (chuna, or calcium oxide) in water. The mixture is left undisturbed for about an hour, and then the liquid is filtered into a new container.
- Lichens are organisms formed by the association of a fungus and an alga. The natural indicator litmus is derived from them.
- When blue litmus paper is tested with lemon juice (an acid), it turns red. When tested with soap solution (a base), its blue color does not change.
- A neutral substance is one that is neither acidic nor basic. Two examples given are sugar solution and salt solution.
- In acidic solutions, red rose extract gives a red color. In basic solutions, it gives a green color.
- Turmeric paper turns from yellow to red in basic solutions, clearly identifying them. However, it remains yellow in both acidic and neutral solutions, so it cannot be used to tell them apart.
- Basic substances are generally slippery or soapy to the touch. They also generally taste bitter.
- A curry stain contains turmeric. Soap is a basic substance. Turmeric acts as an indicator and changes from yellow to red in the presence of a base, which is why the stain turns red.
- Olfactory indicators are substances whose odors change in an acidic or a basic medium. They can be used to identify the nature of a solution by a change in smell.
- A neutralization reaction is the reaction between an acid and a base. Its products are salt and water, and heat is also evolved.
- Heat is the form of energy that is evolved or released during a neutralization reaction.
- When a red ant bites, it injects an acidic liquid called formic acid into the skin.
- The pain can be relieved by rubbing moist baking soda, which is a base, on the affected area. This is an application of neutralization, where the base (baking soda) neutralizes the effect of the formic acid.
- A farmer might add lime (a base) to the soil if it has become too acidic due to the excessive use of chemical fertilizers. The lime neutralizes the acid, improving conditions for plant growth.
- If soil is too basic, organic matter like manure and composted leaves should be added. This is because organic matter releases acids as it decomposes, which neutralizes the basic nature of the soil.
- Acharya Prafulla Chandra Ray is known as the ‘Father of Modern Indian Chemistry’. One of his major contributions was establishing India’s first pharmaceutical company in 1901.
- The color of Hydrangea flowers depends on the nature of the soil. Acidic soil produces blue-colored flowers, while basic soil results in pink or red flowers.
- The decline in fish population is likely due to factory waste being released into the lake. If this waste is acidic, it changes the water’s nature, making it unsuitable for the fish to survive.
- To protect aquatic life, the factory waste should be neutralized by adding basic substances to it. This should be done before the waste is released into the lake.
- The most common acids found in amla juice are Ascorbic acid (Vitamin C) and Citric acid.
- Unripe grapes and tamarind both contain tartaric acid.
- First, collect fallen red rose petals and wash them. Then, crush the petals, place them in a glass tumbler, and pour hot water over them until they are immersed. After 5-10 minutes, filter the colored liquid, which is the red rose extract.
- When red litmus paper is tested with vinegar (an acid), its red color does not change. When tested with washing powder solution (a base), it turns blue.
- One possibility is that the writing was done with a soap solution (a base). The liquid sprayed on the paper could have been a turmeric solution (an indicator), which turns red upon contact with the basic soap solution, making the words appear.
Answer Key for Multiple-Choice Questions
- c) Lime water
- c) Formic acid
- c) Neutralization
- b) Sugar solution
- b) Green
- b) Lime
- c) Lichens
- c) Red
- d) Salt and Water
- b) Basic
- c) Pharmaceutical company
- c) Acidic
- d) Onion
- d) Washing powder solution
- b) Basic
- b) Red to blue
- b) Red litmus paper
- a) Chuna
- c) Neutral
- c) Compost
Answer Key for Essay Questions
- The process of neutralization is a chemical reaction that occurs when an acid and a base are mixed in sufficient quantities. In this reaction, the acidic and basic properties are nullified, resulting in a solution that is neither acidic nor basic. The general chemical equation is: Acid + Base → Salt + Water + Heat. This shows that the products are always a salt and water, and that heat is released. Two examples from daily life are:
- Treating an Ant Bite: A red ant injects formic acid. Applying moist baking soda (a base) neutralizes the acid and relieves the pain.
- Soil Treatment: If soil becomes too acidic from fertilizers, farmers add lime (a base) to neutralize it, making it more suitable for crops.
- Litmus, red rose extract, and turmeric are all effective natural indicators, but they differ in their source and reactions.
- Litmus: Sourced from lichens, it is highly effective and standard. It comes in two colors: blue litmus turns red in acid, while red litmus turns blue in a base. It clearly distinguishes all three categories.
- Red Rose Extract: Prepared from rose petals, it is also very effective. It turns red in acidic solutions and green in basic solutions, with no change in neutral solutions, allowing it to distinguish all three categories.
- Turmeric: Sourced from the common spice, it is effective but limited. It turns from yellow to red in basic solutions but remains yellow in both acidic and neutral solutions. Therefore, it can identify bases but cannot differentiate between acids and neutral substances.
- Preparation: First, collect a handful of fallen red rose petals and wash them. Next, crush the petals with a mortar and pestle. Place the crushed petals in a glass and pour enough hot water to cover them. Wait for 5-10 minutes for the water to become colored, then filter the liquid. This colored filtrate is the red rose indicator. Testing:
- Add a few drops of the indicator to a sample of lemon juice. The solution will turn red, indicating it is acidic.
- Add a few drops to a sample of soap solution. The solution will turn green, indicating it is basic.
- Add a few drops to a sample of tap water. There will be no color change, indicating it is neutral.
- Based on the text, two possible chemical problems could be causing poor plant growth:
- The soil is too acidic: This often occurs due to the excessive use of chemical fertilizers. The remedy is to treat the soil with bases, such as lime (calcium oxide), which will neutralize the acidity.
- The soil is too basic: This is the opposite problem. The remedy is to add organic matter, such as manure or composted leaves. As this matter decomposes, it releases acids that neutralize the basic nature of the soil.
- Olfactory indicators are substances that change their characteristic smell when mixed with an acid or a base. They are useful for identifying substances by sense of smell. Using onion strips as an example:
- First, strips of cloth are sealed in a container with finely chopped onions overnight to absorb the smell.
- When a few drops of a basic solution (like baking soda solution) are put on one strip, the characteristic smell of onion is altered or destroyed.
- When a few drops of an acidic solution (like tamarind water) are put on another strip, the smell of the onion is not significantly affected.
- By observing whether the smell changes, one can distinguish between a base and an acid.
- The science behind the trick is a chemical reaction between a base and a natural indicator. A likely combination is:
- Writing: The secret message ‘Welcome to the Wonderful World of Science’ was likely written on the white paper using a colorless basic solution, such as soap solution or baking soda solution. Once dried, the writing would be invisible.
- Spray: The liquid in the spray bottle was likely a natural indicator solution that changes color in the presence of a base. A turmeric solution would be a perfect candidate. When the yellow turmeric solution was sprayed, it remained yellow on the blank parts of the paper but turned red where it reacted with the basic writing, making the message magically appear.
- Acharya Prafulla Chandra Ray, known as the ‘Father of Modern Indian Chemistry’, was an eminent Indian scientist and social reformer. According to the text, he earned his doctorate in chemistry from the United Kingdom and returned to India to advance scientific research. His major contributions include establishing India’s first pharmaceutical company in 1901. Beyond science, he was a social reformer who advocated for the use of the mother tongue as the medium of instruction in schools.
- To identify the three unlabeled solutions using only turmeric paper, one would dip a strip of the yellow turmeric paper into each solution.
- When dipped into the baking soda solution (a base), the paper would turn red. This would positively identify that solution.
- When dipped into the vinegar (an acid) and the sugar solution (neutral), the turmeric paper would show no color change; it would remain yellow in both.
- Therefore, turmeric paper is sufficient to find the base, but it cannot be used to distinguish between the acidic vinegar and the neutral sugar solution.
- The text explains that the color of Hydrangea flowers is dependent on the chemical nature of the soil they grow in. In acidic soil, the flowers produced are blue. In basic soil, the flowers are pink or red. Yes, a gardener can alter the color of the flowers by adjusting the soil’s nature. To get blue flowers, they would need to make the soil more acidic. To get pink or red flowers, they would need to make the soil more basic, for instance, by adding a substance like lime.
- The likely chemical nature of the factory waste is acidic. When acidic waste is released into a river, it lowers the water’s pH, making it harmful to aquatic organisms like fish, leading to a population decline. The scientific process to solve this is neutralization. Before discharging the waste, the factory should treat it by adding basic substances. This will neutralize the acid, producing harmless salt and water, thereby making the effluent safe to release into the river.
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Glossary of Key Terms
- Acidic: The nature of substances like lemon juice, amla juice, and vinegar, which turn blue litmus paper to red.
- Acid-Base Indicators: Substances that show different colors in acidic and basic solutions.
- Basic: The nature of substances like soap solution and lime water, which are generally slippery to touch and turn red litmus paper to blue.
- Calcium Hydroxide: The chemical in lime water, prepared by mixing calcium oxide (lime) in water.
- Calcium Oxide: Also known as lime or chuna, a base used to treat acidic soil.
- Formic Acid: An acidic liquid that a red ant injects into the skin when it bites.
- Indicator: A substance used to test whether another substance is acidic or basic, usually by a change in color.
- Lichens: Organisms formed by the association of a fungus and an alga, from which the indicator litmus is extracted.
- Lime Water: A solution of calcium hydroxide in water, which is basic in nature.
- Litmus: A natural indicator extracted from lichens, available as red or blue paper strips.
- Neutral: The nature of substances like sugar solution and salt solution that are neither acidic nor basic and do not change the color of litmus paper.
- Neutralization Reaction: The reaction between an acid and a base, which results in the formation of salt and water with the evolution of heat.
- Olfactory Indicators: Substances whose odors change in an acidic or basic medium.
- Salt: One of the products formed, along with water, when an acid and a base neutralize each other.
- Synthetic Indicators: Indicators that are made in laboratories.
- Turmeric: A common household spice (haldi) that acts as a natural indicator, turning red in basic solutions.