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Chapter 2. Exploring Substances: Acidic, Basic, and Neutral

NCERT CLASS-7 GUIDE
Notes in a Nutshell

1. Substances around us may be classified as acidic, basic, and neutral in nature.
2. Litmus Paper and its Use.
   - Litmus is a natural substance obtained from lichens. It is available both as a solution and in the form of paper strips, known as litmus paper.
   - The litmus paper is available in two colours—blue and red.
   - Substances that turn blue litmus paper to red are acidic in nature, while those that turn red litmus paper to blue are basic in nature.
   
   Acidic substances: turn blue litmus red.
   Basic substances: turn red litmus blue.
   
   - Since litmus shows different colours in acidic and basic solutions, it is called an acid-base indicator.
3. Some substances other than litmus, both natural and synthetic, can also be used as indicators. Synthetic indicators are made in laboratories.
4. The substances such as lemon juice, amla juice, tamarind water, and vinegar turned the blue litmus paper to red, implying that these substances are acidic in nature.
5. The substances such as soap solution, baking soda solution, lime water, and washing powder solution turned the red litmus paper to blue. Hence, these substances are basic in nature.
6. The substances such as tap water, sugar solution, and salt solution, did not change the colour of either litmus paper. These substances are said to be neutral because they are neither acidic nor basic.
7. Some vegetables, fruits, or flowers, such as beetroot, purple cabbage, turmeric, Indian blackberry (jamun), and red hibiscus (gudhal) flower, also act as acid-base indicators.
8. Turmeric paper can be used to test basic substances. However, it cannot differentiate between acidic and neutral substances. So, the yellow colour of turmeric turns red in basic solutions but remains unchanged in acidic and neutral solutions.
9. Red rose extract can also be used to test the nature of the substances; hence, it is another example of an acid-base indicator. The red rose extract seems to give red colour in an acidic solution and green colour in a basic solution.
10. All acidic substances that taste sour tends to contain acids and are acidic in nature.
11. Basic substances are generally slippery to touch. Also, bases generally taste bitter, but everything that tastes bitter may not contain a base. For example, bitter gourd (karela) possesses a bitter taste but is not basic in nature.
12. Can a gardener alter the colour of Hydrangea flowers?
    Hydrangea is a plant that grows in cooler climates in the Himalayan region and the North-eastern states. It gives flowers of different colours, depending on the nature of the soil. Acidic soil produces blue coloured flowers, whereas in basic soil, the flowers are pink or red. From this fact, we can say that a gardener can alter the colour of Hydrangea flowers by adjusting the acidic or basic nature of the soil.
13. There are some substances whose odours change in an acidic or basic medium. These are called olfactory indicators.
14. Initially, when a drop of blue litmus solution is added to the lemon juice solution, the colour of the solution turns red. When lime water is added to this test tube, the colour of the solution eventually changes from red to blue. This shows that the solution in the test tube is no longer acidic. Lime water has neutralised the effect of the acid.
15. When the solution of an acid is mixed with the solution of a base in sufficient quantity, we find that the resulting solution is neither acidic nor basic. Such reactions are called neutralisation reactions. In a neutralisation reaction, salt and water are formed with the evolution (i.e., release) of heat.
    Acid + Base → Salt + Water + Heat
    
16. When an ant bites, it injects an acidic liquid (formic acid) into the skin. The effect of the acid can be neutralised by rubbing moist baking soda, which is a base.
17. When the soil is too acidic, the plants do not grow well. It can be treated with lime, which is a base. If the soil is basic, organic matter like manure and composted leaves are added to it. Organic matter releases acids that neutralise the basic nature of the soil. Sometimes, the soil may be neutral, but the plants growing in it may still show poor health, which can be due to the deficiency of nutrients in the soil.
18. Due to factory waste being released into the lake, if fish population of a lake was declining day by day. It means, the factory waste is acidic in nature, the factory waste can be neutralised by adding basic substances before releasing into the lake.
19. Many day-to-day problems like a red ant bite, acidic or basic nature of soil, and industrial waste can be attempted to be managed by the process of neutralisation.

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References

• NCERT BOOKS.

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Further Reading

• Notes of Chapter 1. The Ever-Evolving World of Science
• Notes of Chapter 3. Electricity: Circuits and their Components
• Notes of Chapter 4. The World of Metals and Non-metals
• Notes of Chapter 5. Changes Around Us: Physical and Chemical
• Notes of Chapter 6. Adolescence: A Stage of Growth and Change

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