Chapter 2: Exploring Substances: Acidic, Basic, and Neutral (Chemistry)

The solution is basic (it turned red litmus blue). To reverse the change, we must add an acid in excess: (iii) Vinegar. (Lime water and baking soda are bases; common salt solution is neutral.)

(iii) Vinegar

• A turns red litmus blue → basic.
• B turns turmeric red → basic.
• C turns red rose extract green → basic.

Answer: (iv) Basic, basic and basic.

(iv)

• Fig. 2.13: the strip turns green → the solution is basic.
• Fig. 2.14: the strip stays pink (no change) → the solution is neutral.
• Fig. 2.15: the strip turns dark pink / red → the solution is acidic.

Fig. 2.13: basic (green); Fig. 2.14: neutral (no change); Fig. 2.15: acidic (red).

The liquid is acidic. Acids turn blue litmus red, and they do not change red litmus or turmeric. (A base would have turned red litmus blue and turmeric red.)

Acidic: it turned blue litmus red and did not change red litmus or turmeric.

She cannot see colour changes, so she should use an olfactory indicator, one whose smell changes in an acidic or basic medium, such as onion-treated cloth strips (or clove oil or vanilla essence). For example, the smell of onion on a cloth strip disappears when a basic solution is added, but stays with an acidic solution. So by smelling, Manya can tell which solution is basic and which is acidic.

An olfactory indicator such as onion strips: its smell disappears in a base but not in an acid, so she can identify them by smell.

The message is written with a colourless solution, and an indicator is sprayed on it (or the other way round):

(The message 'Welcome to the Wonderful World of Science' could have been written with baking soda solution and sprayed with turmeric solution, appearing in red.)

E.g. baking soda writing sprayed with turmeric (red) or red rose extract (green); lemon juice writing sprayed with red rose extract (red) or blue litmus (red); soap or lime water writing sprayed with red litmus (blue).

Grape juice is acidic, so the red rose extract turned red. Baking soda is a base. As baking soda is added, it neutralises the acid; the red colour fades, and if enough baking soda is added to make the mixture basic, it turns green.

The red colour fades and finally turns green, because baking soda (a base) neutralises the acidic grape juice and then makes it basic.

Orange juice is acidic. Spray or brush the paper with an indicator that changes colour in acids: red rose extract (the writing turns dark pink or red) or blue litmus solution (the writing turns red). Turmeric will not work, because it does not change colour in acids.

Use red rose extract or blue litmus solution; the acidic orange-juice writing turns red. (Turmeric will not work for an acid.)

Example: red rose extract

1. Take a few petals of red roses and wash them.
2. Crush them in a mortar with a little warm water (or soak them in warm water for some time).
3. Filter the coloured liquid; this is the red rose extract.
4. Add a few drops to the solution to be tested: it turns dark pink or red in acids and green in bases.

Turmeric indicator: mix a little turmeric powder with water to make a paste, apply it on blotting or filter paper and let it dry; cut it into strips. The strips turn red in bases.

E.g. crush washed red rose petals in warm water and filter to get red rose extract; or paint turmeric paste on paper and dry it to make turmeric strips.

Only partly. Turmeric paper turns red only in a basic solution, so the baking soda solution can be identified (it turns the strip red). Vinegar (acidic) and sugar solution (neutral) both leave turmeric unchanged, so turmeric alone cannot tell them apart. (We could then use the baking soda solution itself: mix a drop of each remaining liquid with the identified baking soda solution on turmeric paper; the vinegar will neutralise it and the red colour will fade, while sugar solution will not.)

Turmeric identifies only the baking soda solution (turns red); vinegar and sugar solution both cause no change, so they cannot be told apart with turmeric alone.

Liquid X is basic (red rose extract turns green in bases). Amla juice is acidic. On adding amla juice, the acid neutralises the base; with excess amla juice the mixture becomes acidic, and the colour changes from green to dark pink / red.

X is basic; excess amla juice (an acid) neutralises it and then makes it acidic, so the colour changes from green to red.

• The soil can be acidic in nature. / The soil can be basic in nature.
• Indicator: litmus paper (or turmeric or red rose extract) dipped in the water in which the soil sample has been mixed and allowed to settle.
• The acidic soil can be treated with lime (a base), e.g. quicklime or slaked lime.
• The basic soil can be treated with organic matter like manure and compost, which releases acids.

acidic / basic; litmus (or turmeric, red rose) solution of the soil; acidic soil: lime; basic soil: manure or compost (organic matter).

Eggshell and marble are made of calcium carbonate. Vinegar is an acid; acids react with carbonates to produce carbon dioxide gas, which comes out as bubbles. Soap solution is basic, and bases do not react with carbonates in this way, so no gas and no bubbles are formed.

Eggshell and marble contain calcium carbonate, which reacts with acids (vinegar) to give carbon dioxide bubbles; soap solution is a base and does not react with it.