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.

1.0What 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:

2.0Characteristics 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:

3.0Characteristics of Bases

• Bitter taste
• Slippery to touch
• Conduct electricity in solution
• React with acids to form salt and water

4.0Natural 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.

5.0Neutralization 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)

6.0Neutralization in Everyday Life

1. 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.
2. 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.

3. 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.
4. 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.