Exploring Substances: Acidic, Basic, and Neutral - Class 7 Science Free

• Litmus is a natural material obtained from lichens, which are a combination of fungus and algae.
• Lichens grow on rocks and trees, especially in rainy and clean areas.
• Litmus is used in the form of blue and red paper strips to test if a substance is acidic or basic.

• Acids turn blue litmus paper red.
• This color change helps identify the acidic nature of a substance.
• For example, lemon juice, which is acidic, turns blue litmus paper red.

• Bases often feel slippery or soapy to the touch, similar to soap or baking soda solutions.
• This slippery feel helps identify bases compared to acids and neutral substances.
• Bases usually taste bitter, but not all bitter substances are bases.

• Red rose extract changes color depending on the solution’s nature.
• It turns red in acidic substances like lemon juice and green in basic substances like soap solution.
• Neutral substances do not change its color.

• Turmeric paper turns red in the presence of bases but remains yellow with acidic or neutral substances.
• Therefore, turmeric paper cannot distinguish between acids and neutrals.
• This limits its use compared to other indicators like litmus or red rose extract.

• Olfactory indicators like onion juice change smell when mixed with acids or bases.
• For example, the cloth soaked in onion juice loses its smell when exposed to tamarind water (acidic) or baking soda solution (basic).
• This property helps identify acids and bases by smell.

• Neutralization is the reaction between an acid and a base producing salt, water, and heat.
• This process balances the properties of acids and bases, creating a neutral solution.
• It is important in many real-life applications, such as treating ant bites and soil.

• Ant bites inject formic acid causing redness and stinging pain.
• Applying baking soda, a base, neutralizes the acid, reducing pain and swelling.
• This practical use of neutralization helps in everyday first aid.

• Excessive use of fertilizers can make soil acidic, which is harmful to plants.
• Adding lime, a base, neutralizes the acidic soil, making it suitable for healthy plant growth.
• This demonstrates the importance of chemical balance in agriculture.

c) Blue flowers in acidic soil and pink or red flowers in basic soil ✅

Explanation:

• Soil Acidity (pH < 7, acidic) → Hydrangea flowers turn blue.
  This happens because acidic soil allows the plant to absorb more aluminum ions (Al³⁺), which causes the blue pigmentation.

• Soil Alkalinity (pH > 7, basic) → Hydrangea flowers turn pink or red.
  In alkaline soil, aluminum becomes unavailable to the plant, and the flowers display pink/red colors.

• Neutral soil (around pH 7) → Flowers are often purple or a mix of pink and blue, depending on the balance of available aluminum.