CBSE Notes Class 7 Science Chapter 4 Acids, Bases and Salts
Acids, bases, and salts are essential chemical substances with unique properties that we encounter daily. Indicators such as litmus, turmeric, and phenolphthalein help identify these substances by changing color, making them useful for various everyday applications like soil testing, treating indigestion, and neutralizing ant bites.
1.0Download CBSE Class 7 Science Chapter 4 Acids, Bases and Salts Notes : Free PDF
Download CBSE Class 7 Science Chapter 4 – Acids, Bases and Salts Notes in a free and easy-to-access PDF format. These notes provide a clear summary of key concepts such as the properties of acids and bases, indicators, and neutralization reactions
Class 7 Science Chapter 4 Revision Notes:
2.0What are Acids and Bases?
Acids are substances with a sour taste that can turn blue litmus paper red. They release hydrogen ions (H⁺) when dissolved in water. Here are some common examples:
Acid
Chemical Formula
Uses
Hydrochloric Acid
HCl
Found in the stomach, Break down food, Activate enzymes, Kill bacteria
Sulphuric Acid
H₂SO₄
Used in car batteries and industry
Citric Acid
C₆H₈O₇
Present in citrus fruits like lemons and oranges
3.0Characteristics of Acids
Sour taste
Conduct electricity in solution
React with metals to produce hydrogen gas
React with bases to form salt and water (neutralization reaction)
Bases are substances with a bitter taste and slippery feel. They turn red litmus paper blue and release hydroxide ions (OH⁻) when dissolved in water. Some common examples are:
Base
Chemical Formula
Uses
Sodium Hydroxide
NaOH
Used in soap and detergent making
Calcium Hydroxide
Ca(OH)₂
Used in whitewashing
Ammonium Hydroxide
NH₄OH
Found in household cleaning solutions
4.0Characteristics of Bases
Bitter taste
Slippery to touch
Conduct electricity in solution
React with acids to form salt and water
5.0Natural Indicators Around Us
1. Litmus: A Natural Dye Litmus is the most widely used natural indicator, derived from lichens. In distilled water, it appears mauve (purple). When added to an acidic solution, litmus turns red, and in a basic solution, it turns blue. It is available as a solution or as litmus paper strips, which come in red and blue varieties.
2. Turmeric: Turmeric is another natural indicator. It turns yellow in acidic or neutral solutions and changes to red in basic solutions. It’s often used in household experiments to test for the presence of bases.
3. China Rose as an Indicator: The China Rose (Hibiscus) flower can also be used as a natural indicator. It turns acidic solutions dark pink and basic solutions green, making it a useful and colorful tool for identifying the nature of substances.
6.0Neutralization Reaction
When an acid is mixed with a base, a neutralization reaction occurs, resulting in the formation of salt and water. During this reaction, the acidic and basic properties are neutralized, resulting in a neutral substance. To observe this reaction, we can use an indicator like phenolphthalein, which is colorless in acidic solutions and turns pink in basic solutions.
The reaction between an acid and a base is known as neutralisation. The evolution of heat produces salt and water in this process.
Acid + Base → Salt + Water (Heat is evolved)
7.0Neutralization in Everyday Life
Indigestion: Our stomach naturally produces hydrochloric acid to aid in digestion. However, excessive acid can lead to indigestion, causing discomfort and pain. To alleviate this, we take antacids like milk of magnesia, which contains magnesium hydroxide. This base neutralizes the excess acid, providing relief.
Ant Bites: When an ant bites, it injects formic acid into the skin, causing irritation and pain. To neutralize the effect, substances like moist baking soda (sodium bicarbonate) or calamine lotion, which contains zinc carbonate, can be applied. These bases counteract the acidic venom and reduce discomfort.
Soil Treatment: Overuse of chemical fertilizers can make soil too acidic, hindering plant growth. To correct this, bases such as quick lime (calcium oxide) or slaked lime (calcium hydroxide) are added to the soil to neutralize the acidity. If the soil is too basic, organic matter like compost is used. As it decomposes, it releases acids that balance the soil's pH, promoting healthy plant growth.
Factory Waste: Many factories produce acidic waste that, if released into water bodies, can harm aquatic life by lowering the water’s pH. To prevent this, basic substances are added to the waste before it is released, neutralizing the acids and making it safer for the environment.
8.0Key Features of Class 7 Science Chapter 4 Acids, Bases and Salts
Introduction to Acids and Bases: Clear explanation of what acids and bases are, with examples from everyday life.
Properties of Acids and Bases: Detailed discussion on the physical and chemical properties of acids and bases.
Indicators and Their Uses: Study of natural and synthetic indicators like litmus, turmeric, and phenolphthalein used to identify acids and bases.
Neutralization Reaction: Explanation of the process when an acid reacts with a base to form salt and water, including real-life examples like antacid use.
Everyday Applications: Practical applications of acids, bases, and salts in daily life, such as in food, medicine, and cleaning products.
Important Examples: Common acids (like vinegar, lemon juice) and bases (like soap, baking soda) are discussed to build a strong foundation.
Neutralization is a chemical reaction where an acid and a base react to form water and a salt, neutralizing their properties.
How do antacids help with indigestion?
Antacids contain bases like magnesium hydroxide, which neutralize excess stomach acid, providing relief from indigestion.
What is a natural indicator?
A natural indicator is a substance obtained from natural sources that changes color in the presence of acids or bases, helping to identify the nature of a solution.
How does litmus paper work as an indicator?
Litmus paper, derived from lichens, turns red in acidic solutions and blue in basic solutions, making it easy to identify the nature of a substance.