Solutions-NCERT-Class-7-Science-Curiosity-Chapter-2-Exploring Substances: Acidic, Basic, and Neutral-CBSE

The acid present in the following substances are as follows:

• Lemon- Citric acids
• Curd- Lactic acid
• Tamarind- Tartaric acid
• Vinegar- Acetic acid

Yes, if litmus is not available then there are some natural substances like turmeric, red cabbage juice, red rose extract and hibiscus which can be used as natural acid-base indicators.

Yes, gardeners can change the colour of hydrangea flowers by adjusting the acidity or basicity (pH) of the soil.

• In acidic soil (pH below 6): the flowers usually turn blue.
• In neutral or basic (alkaline) soil (pH above 7): the flowers turn pink.

This happens because the availability of aluminium ions in the soil changes with pH. In acidic soil, aluminium is more available and gives the flowers a blue colour, while in basic soil, aluminium is less available, resulting in pink flowers.

No, turmeric paper cannot be used to test acidic substances because it does not change colour in acids.

Yes, there are some substances whose odour changes in an acidic or basic medium. These are called as olfactory indicators.

When an acidic substance mixes with a basic substance, they react with each other and neutralize the effects of both. This reaction is called a neutralization reaction.

• The acid and base combine to form salt and water.
• The resulting mixture is usually less acidic or less basic than the original substances.

Example: When lemon juice (acid) is mixed with baking soda (base), they react to form salt, water, and carbon dioxide gas (which causes fizzing).

When an ant bites, it injects a small amount of acid into the skin, which causes pain and irritation. People often use simple home remedies to treat ant bites, such as:

• Applying baking soda paste: Mix baking soda with a little water and apply it to the affected area. Baking soda is basic and helps neutralize the acid from the ant bite, reducing pain and itching.
• Using calamine lotion or aloe vera gel: These give a cooling effect and help soothe the skin.
• Rinsing with cold water or applying an ice pack: This helps reduce swelling and burning.
• Using toothpaste or honey: These can help calm irritation and prevent infection.

The factory waste can be neutralised by adding basic substances before releasing into the lake.

One possibility was that they could be using a turmeric solution for the spraying liquid and a soap solution for writing on the paper.

(iii) Vinegar - (Vinegar is acidic in nature and acids turn blue litmus paper back to red.)

(iv) Basic, basic, and basic

Explanation : All three tests show that the solutions A, B, and Care basic in nature as the colour of the red litmus turns blue, turmeric turns red, and rose extract turns green in basic solutions.

Since, red rose extract gives red colour in acidic solutions, green colour in basic solutions and remain same in colour in neutral solution.

Therefore, the solution in

Fig. (a) is basic,

Fig. (b) is neutral

Fig. (c) is acidic.

Since, acid turns blue litmus to red and the colour of turmeric solution remains same in acidic solution, therefore, the given liquid sample is acidic in nature.

The various possible combinations are as follows:

Red rose extract acts as a natural acid-base indicator. It turns red in acidic solutions and green in basic solutions.

• Grape juice is slightly acidic, so when mixed with red rose extract, the solution acquires a reddish tint.
• Baking soda is a mild base. When added to the mixture, it neutralizes the acid and increases the pH, making the solution basic.
• As a result, the red rose extract responds to the basic environment by changing its color from red to green.

Conclusion: Adding baking soda to the mixture will turn the red-colored solution green, indicating a shift from acidic to basic nature.

• Orange juice is acidic. Red rose extract is an indicator that turns red in acidic solutions.
• When the grandmother sprays or applies the red rose extract on the paper then the message written with orange juice will turn red and making it visible.

Natural indicators can be prepared by extracting pigments from vegetables, fruits or flowers, such as beetroot, purple cabbage, turmeric, Indian blackberry (jamun) and red hibiscus (gudhal) flower.

Example : Red Cabbage Indicator :

Step 1: Chop red cabbage leaves.

Step 2: Boil them in water for 10–15 minutes.

Step 3: Cool and strain the purple-colored liquid.

Step 4: Use this extract to test solutions:

• Acidic solution → turns red or pink
• Neutral solution → remains purple
• Basic solution → turns green or yellow

We will put a drop each of vinegar, baking soda solution, and sugar solution on the turmeric indicator paper, one by one.

• The liquid that changes the yellow colour of turmeric paper to red is basic in nature, hence it is the baking soda solution.
• Now, the red turmeric paper (made by the base) can be used to test the remaining two liquids.

When we put one drop each of the remaining liquids (vinegar and sugar solution) on the red turmeric paper:

• The drop of vinegar turns the red turmeric paper back to yellow, because vinegar is acidic and neutralises the basic effect of baking soda.
• The sugar solution causes no colour change, as it is neutral.

• Since, red rose extract gives red colour in acidic solutions and green colour in basic solutions, therefore, the liquid X is basic in nature.
• When excess amla juice (which is acidic) is added to liquid X then the solution will become acidic and the red rose extract will turn red.

(i) The soil can be acidic in nature.

(ii) The soil can be basic in nature.

(iii) The indicator used to test the nature of soil is acid-base indicator (like blue and red litmus paper).

(iv) The acidic soil can be treated with lime.

(v) The basic soil can be treated with organic matter.