NCERT Solutions for Class 7 Science Chapter 2 Exploring Substances Acidic Basic and Neutral (PDF) – 2027

Yes. Only some of the samples change the blue litmus pieces to red; the rest leave them blue.

The blue pieces turn red with lemon juice, amla juice, tamarind water and vinegar. They stay blue with soap solution, baking soda solution, lime water, washing powder solution, tap water, sugar solution and salt solution.

Now exactly the opposite set of samples reacts. The red litmus pieces turn blue with soap solution, baking soda solution, lime water and washing powder solution, and stay red with everything else.

Putting both halves of the test together gives the completed Table 2.1.

Sorting the twelve entries of Table 2.1 gives Table 2.2.

In the words the chapter uses two pages later: Group A is acidic, Group B is basic, Group C is neutral.

Mix lime (chuna, i.e. calcium oxide) in water, leave the mixture undisturbed for some time — say an hour — then filter the clear liquid into another container. That filtrate is lime water.

Litmus paper is ordinary paper soaked in litmus, a natural dye obtained from lichens. Two versions are sold — the dye in its blue form and the dye in its red form.

They change colour because litmus is an acid-base indicator: an acid turns blue litmus red, and a base turns red litmus blue.

They are neutral — neither acidic nor basic — because they change neither blue litmus nor red litmus.

Look on the shaded side of old tree trunks, on compound walls and on stone surfaces after the monsoon. Lichens look like flat, crusty or leafy patches — dull green, grey-green, yellow or orange — that seem painted onto the bark.

Sample answer: “In our locality I found grey-green crusty patches on the trunk of an old mango tree and on the boundary wall of the school, but none on the trees along the main road.”

Yes — every substance in Group A is edible: lemon juice, amla juice, tamarind water and vinegar are all used in Indian kitchens. And every one of them tastes sour.

It feels soapy or slippery between the fingers. Basic substances are generally slippery to touch.

Yes — plenty of them, and most are already in your kitchen or garden. Any strongly coloured plant part whose colour changes in acid or base will work.

Steps to prepare the red rose extract:

1. Collect fallen petals of red roses (do not pluck flowers).
2. Take a fistful of petals and wash them with water.
3. Crush them with a mortar and pestle.
4. Put them in a glass tumbler and pour hot water till the petals are completely immersed (under adult supervision).
5. Cover the tumbler with a lid and wait 5–10 minutes till the water becomes coloured.
6. Filter it. The filtrate is the red rose extract — the acid-base indicator.

The two test tubes end up looking completely different, exactly as in Fig. 2.8.

• Test tube A (lemon juice): the extract turns a clear red.
• Test tube B (soap solution): the extract turns green.

Repeating with the other samples of Activity 2.1 fills in Table 2.3.

Yes — exactly the same set: lemon juice, amla juice, tamarind water and vinegar, that is, all of Group A.

Yes — soap solution, baking soda solution, lime water and washing powder solution, that is, all of Group B.

Yes — tap water, sugar solution and salt solution, that is, all of Group C.

Applying the rule: lemon juice, amla juice, tamarind water and vinegar are acidic; soap solution, baking soda solution, lime water and washing powder solution are basic; tap water, sugar solution and salt solution are neutral. The completed column is shown in the table of Q1 above.

Yes, they can — and gardeners actually do this.

• To get blue flowers, the gardener makes the soil more acidic — for example by adding organic matter such as composted leaves or pine needles.
• To get pink or red flowers, the gardener makes the soil more basic — for example by mixing in lime (chuna).

Yes. The recipe is always the same: get a strongly coloured plant extract, soak plain paper in it, dry it, and cut it into strips.

Four steps and you have your own indicator paper.

1. Put a spoonful of turmeric in a petri dish and add a little water — just enough for a thick paste. Fresh turmeric, ground, works equally well.
2. Dip a piece of filter paper in the paste until it is evenly yellow.
3. Take it out and let it dry completely.
4. Cut the dry yellow paper into thin strips — this is turmeric paper. Do this under adult supervision.

No. Only some samples change it — and every one of them turns the yellow paper red.

The samples that leave the turmeric paper yellow are: lemon juice, amla juice, tamarind water, vinegar, tap water, sugar solution and salt solution.

Compared with Table 2.2, that is all of Group A plus all of Group C — acidic and neutral substances lumped together. Only Group B (the bases) is separated out.

No. Turmeric paper can be used to test basic substances only. It cannot differentiate between acidic and neutral substances, because both leave it yellow.

Because of its bright golden-yellow colour and the value it has long been given in Indian homes and in Ayurveda.

• Turmeric is a member of the ginger family, grown in India and other countries.
• Beyond the taste and colour it gives to food, it is being researched for other benefits.
• In the Ayurvedic system of medicine it is considered to have numerous health benefits and is used in many traditional home remedies.

Any basic solution from Table 2.4 — soap solution is the easiest, and baking soda solution, lime water or washing powder solution would work just as well.

Yes. Substances whose odour changes in an acidic or basic medium are called olfactory indicators.

• Onion — the sharp onion smell on a cloth strip is strongly reduced by a basic solution such as baking soda solution, while an acidic solution such as tamarind water leaves it much as it was.
• Vanilla essence and clove oil behave the same way — their characteristic smell disappears in a base.

After a night in the closed container the strips smell strongly of onion, even though no onion is touching them any more.

Yes, there is a clear difference between the two strips.

Repeat with more samples from Activity 2.1 and the pattern holds. A sample record:

The diluted lemon juice by itself is colourless. The moment a drop of blue litmus solution goes in, the whole solution turns red (Fig. 2.10a).

Nothing seems to happen for the first several drops — the solution stays red. Then, quite suddenly, a stage comes when the colour changes from red to blue (Fig. 2.10b). This happens because lime water, a base, neutralises the acid of the lemon juice.

The solution goes back to red, because the extra lemon juice makes it acidic again.

The reason for every colour change here is the same: the litmus is simply reporting whether acid or base is in excess at that moment.

The ant bite injects an acid, and the baking soda is a base that neutralises it.

Most traditional remedies turn out to be either a base that neutralises the acid or something that soothes the swelling.

Sample answer: “In my area people rub moist baking soda or a little chuna on an ant bite, and elders sometimes press a crushed tulsi leaf on it.”

Treat the field with lime, which is a base (Fig. 2.12). The lime neutralises the excess acid and brings the soil back towards neutral, after which the plants grow well again.

The most likely cause is factory waste being released into the lake.

• If the waste is acidic or basic, it changes the nature of the lake water. Fish can survive only in water that is close to neutral.
• The waste may also make the water dirty and cut down the oxygen dissolved in it.
• Other everyday causes worth mentioning: sewage from houses, and fertiliser washed off nearby fields into the lake.

Neutralise the waste before it is released. Basic substances are added to the acidic factory waste in a treatment tank, and only after it has become neutral is it allowed into the lake.

Yes. The message was written with a base and made visible by an indicator:

1. The message was written on the white sheet beforehand with a soap solution (basic) and allowed to dry. Dry soap solution on white paper is practically invisible.
2. At the gate the volunteer sprayed a turmeric solution on the sheet. The whole sheet turned pale yellow.
3. Wherever the turmeric met the dried base, it turned red. So the letters appeared in red on a yellow background.

(iii) Vinegar

(iv) Basic, basic, and basic

Read each strip against the two rules for red rose extract — red in acid, green in base, unchanged in neutral.

The liquid is acidic. It turns blue litmus red, which only an acid does; and it changes neither red litmus nor turmeric, so it is not basic.

Manya should use an olfactory indicator — for example onion strips, vanilla essence or clove oil. Being blindfolded, she cannot see a colour change, but she can smell a change in odour: the onion smell almost disappears in a basic solution and stays in an acidic one.

The rule is simple: one of the two must be the indicator and the other must be the acid or the base. Here are combinations that work.

The mixture will change from red to green.

Yes. Orange juice is acidic (citric acid), so the grandmother needs an indicator that shows a clear colour in acid. Red rose extract is the best choice; blue litmus solution or purple cabbage extract works too.

1. Let the orange-juice writing dry fully — it becomes almost invisible on white paper.
2. Spray or gently brush red rose extract over the whole sheet.
3. The letters appear in red, because the acid of the orange juice turns the extract red wherever it was written.

A natural indicator is prepared by taking out the coloured juice of a plant part (flower petals, fruit, turmeric) in water, or by soaking paper in it. Example — red rose extract:

1. Collect fallen red rose petals and wash a fistful of them with water.
2. Crush them with a mortar and pestle.
3. Put them in a glass tumbler and pour hot water till they are completely immersed (under adult supervision).
4. Cover with a lid and wait 5–10 minutes till the water becomes coloured.
5. Filter. The filtrate is the red rose extract — red in acid, green in base.

Only partly — turmeric paper can pick out the baking soda solution, but it cannot separate the vinegar from the sugar solution.

Liquid X is basic. When excess amla juice is added, the green will change to red.

Here is the completed flowchart.

Because eggshell and marble are made of the same substance — calcium carbonate — and calcium carbonate reacts with an acid such as vinegar, giving off carbon dioxide gas, which forms the bubbles.

Soap solution is basic. A base does not react with calcium carbonate in this way, so no gas is produced and no bubbles appear.

Sample plan: brush turmeric paste in a rangoli design on white paper and let it dry. Draw petals, lines and dots on it with soap or baking soda solution — every stroke turns red. For more colours, use a layer of red rose or purple cabbage extract and draw with lemon juice (red) or soap solution (green).

1. Collect samples in clean bottles and label each with the source, date and place — rainwater, tap water, river or pond water, hand-pump water, boiled water, bottled water.
2. Test each with blue litmus and then with red litmus, using a fresh strip every time.
3. Record the results in a table and discuss them with your class.

Sample results (your results may differ):

Method

1. Collect a little soil from two or three spots, a few centimetres below the surface, and mix it.
2. Stir two spoonfuls of it in a glass of clean water and let the soil settle.
3. Dip a blue litmus strip and a red litmus strip in the clear water above the soil.