Overview
This lecture explores the classification of substances as acidic, basic, or neutral, and demonstrates the use of natural indicators and the concept of neutralisation in everyday life.
Substances and Their Nature
- Substances can be classified as acidic, basic, or neutral based on their chemical properties.
- Acidic substances taste sour and turn blue litmus paper red.
- Basic substances feel slippery, often taste bitter, and turn red litmus paper blue.
- Neutral substances do not affect the color of red or blue litmus paper.
Indicators: Litmus and Natural Extracts
- Litmus, derived from lichens, is a common acid-base indicator, available as blue and red strips.
- Blue litmus turns red in acids; red litmus turns blue in bases.
- Natural indicators include extracts from red roses, purple cabbage, turmeric, beetroot, and hibiscus.
- Red rose extract turns red in acid and green in base.
- Turmeric paper turns red in basic solutions but does not change in acids or neutrals.
Olfactory Indicators
- Some substances, like onion, change their odor when exposed to acids or bases; these are called olfactory indicators.
Neutralisation Reaction
- Mixing an acid and base in the right proportions results in a neutralisation reaction, forming salt, water, and heat.
- Neutralisation reaction: Acid + Base → Salt + Water + Heat.
Neutralisation in Daily Life
- Baking soda (a base) relieves pain from ant bites (acidic formic acid).
- Lime (a base) is added to acidic soil to help plant growth; organic matter corrects basic soil.
- Factory waste is neutralised before being released into lakes to protect aquatic life.
Key Terms & Definitions
- Acidic — A substance that tastes sour and turns blue litmus red.
- Basic — A substance that feels slippery, often tastes bitter, and turns red litmus blue.
- Neutral — A substance that does not change the color of litmus paper.
- Indicator — A substance that changes color in the presence of an acid or base.
- Litmus — A natural indicator made from lichens.
- Neutralisation — A chemical reaction where an acid and a base form salt, water, and heat.
- Olfactory Indicator — A substance that changes its odor in acidic or basic environments.
Action Items / Next Steps
- Complete exercises 1–12 at the end of the chapter.
- Try testing household substances with natural indicators like turmeric or rose extract.
- Collect and test water and soil samples to determine if they are acidic, basic, or neutral.
- Observe and record color and odor changes using various indicators for different substances.