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CBSE Notes Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic and Neutral
In our daily life, we come across many substances such as food items, cleaning agents, and medicines. These substances have different properties, and one important way to classify them is as acids, bases, and neutral substances. This chapter introduces students to these three types and helps them understand their characteristics.
Students learn that acids are substances that have a sour taste, like lemon juice and vinegar. Bases are bitter in taste and feel soapy, like soap solution. Neutral substances do not show the properties of either acids or bases. To identify these substances, we use indicators such as litmus paper. Acids turn blue litmus red, while bases turn red litmus blue.
This CBSE Class 7 chapter notes also explain natural indicators like turmeric and china rose, which can also be used to test substances. This makes learning interesting and practical for students.
1.0Download CBSE Class 7 Science Chapter 2 Notes: Free PDF
Get the CBSE Class 7 Science Chapter 2 notes on "Exploring Substances: Acidic, Basic, and Neutral." PDF for clear and organised explanations of acids, bases, indicators, and how to neutralise them. The notes are easy to read and include real-life examples and important ideas. Great for quick review and getting ready for tests, it helps students understand concepts better.
2.0Exploring Substances: Acidic, Basic and Neutral Class 7 Science Chapter 2 – Revision Notes
Substances are classified as acidic, basic, or neutral based on their chemical nature and behavior with indicators. Natural and synthetic indicators show distinct color or odor changes, helping identify solutions and apply neutralization in daily life.
Key Concepts:
- Indicators are substances that change color or smell to identify if a solution is acidic, basic, or neutral.
- Acids taste sour, turn blue litmus red, and are found in many edible fruits.
- Bases taste bitter, feel slippery, and turn red litmus blue.
- Alkalies are bases that dissolve in water, such as calcium hydroxide.
- Neutral substances show no color change with litmus because they are neither acidic nor basic.
- Litmus is a natural indicator dye extracted from lichens.
- Olfactory indicators change their smell in acidic or basic solutions, such as onion, clove oil, and vanilla.
- Neutralization is a reaction between an acid and a base that produces salt, water, and heat.
Common Acids in Food:
Citric acid (lemons/oranges), Lactic acid (curd), Formic acid (ant stings), Tartaric acid (tamarind), Acetic acid (vinegar), and Oxalic acid (tomatoes).
Common Bases in Daily Use:
- Lime water (calcium hydroxide), Soaps (sodium/potassium hydroxide), Window cleaners (ammonium hydroxide), and Milk of magnesia (magnesium hydroxide).
Indicator Color Changes:
- Litmus: Blue turns red in acid; Red turns blue in base.
- Turmeric: Stays yellow in acid; turns reddish-brown in base.
- Red Rose & Hibiscus: Turns bright red/magenta in acid; turns green in base.
- Synthetic Indicators: Phenolphthalein is colorless in acid and pink in base; Methyl orange is red in acid and yellow in base.
- Olfactory Indicators: Onion and clove oil keep their smell in acid, but lose their smell in base.
Daily Applications of Neutralization (Acid+BaseSalt+Water+Heat):
- Ant Stings: Neutralize acidic formic acid with basic baking soda or calamine lotion.
- Soil Treatment: Add slaked lime (base) to acidic soil, and organic manure (acidic) to basic soil.
- Indigestion Relief: Neutralize excess stomach acid with antacids like baking soda or milk of magnesia.
- Tooth Care: Basic toothpaste neutralizes acids produced by mouth bacteria to prevent enamel decay.
- Factory Waste: Treat acidic factory waste with basic substances before discharging into water bodies.
- Vinegar & Eggshells: Vinegar (acetic acid) reacts with calcium carbonate in eggshells, releasing carbon dioxide bubbles.
3.0Chapter 2 Terminologies to Know – Exploring Substances: Acidic, Basic and Neutral
- Indicator: A chemical substance that changes color or odor in acidic, basic, or neutral solutions.
- Acid: A sour-tasting substance that turns blue litmus red.
- Base: A bitter-tasting, slippery substance that turns red litmus blue.
- Alkali: A water-soluble base.
- Neutral Solution: A solution that is neither acidic nor basic and causes no litmus color change.
- Olfactory Indicator: A substance whose smell changes or disappears in an acidic or basic medium.
- Neutralization: The exothermic reaction between an acid and a base forming salt and water.
4.0Learning Guide for Class 7 Science – Exploring Substances: Acidic, Basic and Neutral
- Litmus Testing: Place drops of test liquids on blue and red litmus paper. Observation: Acid turns blue litmus red, base turns red litmus blue, and neutral samples show no change.
- Red Rose Indicator: Mix crushed red rose petals with hot water and add filtrate to test samples. Observation: Extract turns bright red in acid and green in base.
- Turmeric Paper Activity: Coat filter paper with turmeric paste, dry, cut into strips, and test. Observation: Basic solutions turn yellow turmeric paper reddish-brown, while acidic and neutral samples cause no change.
- Olfactory Indicator Test: Store cloth strips with chopped onions overnight and test with tamarind water and baking soda. Observation: Onion smell remains with acidic tamarind water but disappears with basic baking soda.
- Neutralization Experiment: Add drops of blue litmus to dilute lemon juice until red, then add lime water dropwise. Observation: The solution changes from red back to blue as lime water neutralizes the acid.
Everyday Examples:
A yellow turmeric stain on a shirt turns red when washed with soap. Taking baking soda relieves stomach burning caused by indigestion.
Revision Tips:
- Acid turns blue litmus red; base turns red litmus blue. Turmeric changes color only in basic solutions.
Safety Rules:
- Prepare rose extract and indicator rangoli under adult supervision. Handle turmeric paste carefully to avoid staining clothes.
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8.0Key Features of Chapter 2: Exploring Substances: Acidic, Basic and Neutral
- Classification of Substances: This helps you figure out if a substance is an acid, a base, or neutral by looking at its properties.
- Indicators: Talks about how to use natural and synthetic indicators, such as turmeric, litmus, and phenolphthalein, to test things.
- Properties of Acids and Bases: Talks about how acids taste sour and bases feel slippery.
- Neutralisation Reaction: Shows how acids and bases react to make salt and water, which often gives off heat.
- Everyday Uses: Links ideas to things we do every day, like using antacids to treat acidity and treating soil on farms.
- Safety Measures: Highlights the importance of using and handling chemicals safely to avoid harm.