Exploring Substances: Acidic, Basic and Neutral | CBSE Class 7 Science Notes

This note covers acidic, basic and neutral substances; litmus, red rose, turmeric and smell-based indicators; preparation and interpretation of indicator tests; neutralisation; and its uses in ant bites, soil treatment and acidic factory waste.

What are acidic, basic and neutral substances?

Substances around us can be grouped by their acidic, basic or neutral nature. An acidic substance turns blue litmus paper red. A basic substance turns red litmus paper blue. A neutral substance is neither acidic nor basic and does not change either litmus paper.

Litmus is a natural testing substance that shows different colours in acidic and basic solutions. A solution is a liquid mixture in which a substance is dissolved. Lemon juice, soap solution and sugar solution provide examples of the three groups respectively.

How are the three groups recognised?

Lime water means a solution of calcium hydroxide in water. It belongs to the basic group. It must not be confused with juice from lime, the fruit similar to lemon. Similar-sounding names do not establish the nature of a sample.

The grouping depends on observations from tests. First record what happens to each colour of litmus, then connect the change with a group. An observation such as “blue litmus turns red” provides the evidence for the conclusion “the sample is acidic”.

Definition: An acid-base indicator is a substance that shows different colours in acidic and basic solutions. It helps identify the nature of a substance through an observable change.

How can litmus be used to test household samples?

Litmus comes from lichens, associations of two kinds of living organisms. It is available as a solution and as paper strips. Litmus paper is available in blue and red forms, both useful for classifying the samples.

Lichens grow on rocks and trees in regions with abundant rainfall and clean air. Their connection with litmus illustrates how a natural material can supply a laboratory indicator. Other indicators may be natural or synthetic, meaning made in laboratories.

What steps should the test follow?

1. Collect the samples: lemon juice, soap solution, amla juice, tamarind water, vinegar, baking soda solution, lime water, tap water, washing powder solution, sugar solution and salt solution.
2. Cut blue litmus paper into small pieces and spread the pieces on a clean, dry white tile.
3. Use a dropper to put one drop of each sample on separate pieces. A dropper is a device for transferring liquid in drops.
4. Record the colour of each piece after the sample is added. Keep each observation connected with the correct sample name.
5. Repeat the test with pieces of red litmus paper. Record these results alongside the blue-litmus observations.
6. Sort the samples into acidic, basic and neutral groups using the colour changes from both tests.

Two illustrations show pupils using droppers over pieces of litmus paper on white tiles. Sample containers stand on the tables. Part (a) shows the blue-paper activity and part (b) the red-paper activity.

See Fig. 2.2 in your NCERT textbook

Both starting colours matter. A sample that does not change red litmus has not thereby been shown to be neutral. Check its effect on blue litmus too. Neutral samples in the activity leave both colours unchanged; acidic samples turn blue litmus red.

What do taste and touch tell us about acids and bases?

Familiar edible substances such as lemon juice, amla juice, tamarind water and vinegar taste sour. Substances that taste sour tend to contain acids and are acidic in nature. This connects a familiar property of food with the results of litmus testing.

An acid is a substance associated here with acidic behaviour, including changing blue litmus to red. A base shows basic behaviour, including changing red litmus to blue. Indicator tests help classify substances without relying on tasting an unfamiliar sample.

Why is bitterness not a sufficient test?

Basic substances are generally slippery to touch. Baking soda solution feels soapy or slippery when rubbed between the fingers in the activity. Bases also generally taste bitter, but everything that tastes bitter may not contain a base.

Bitter gourd, also called karela, provides the important exception: it has a bitter taste but is not basic in nature. Therefore, “bitter” and “basic” cannot be treated as interchangeable descriptions. The word “generally” is essential when describing this property of bases.

Note: Do not taste an unknown substance. Do not taste anything during an investigation unless specifically asked to do so. Use the indicator observations to decide the nature of the sample.

What is the difference between lime water and lime fruit?

Lime water is the calcium hydroxide solution used as a basic sample. Lime, or chuna, is calcium oxide in the preparation described here. Mixing it with water, leaving it undisturbed for some time, say an hour, and filtering the liquid produces lime water.

The word lime can also refer to a fruit similar to lemon. This shared name does not make lime water a fruit juice. Keep the substance name and the indicator result together: lime water changes red litmus to blue and is basic.

How is a natural indicator prepared from red rose petals?

A red rose extract is the coloured liquid obtained from red rose petals. An extract is a preparation containing substances drawn out of a material. Here, hot water takes colour from crushed petals, producing a liquid that can be tested as an indicator.

Use fallen flowers or petals from the surroundings. Do not pluck flowers for the activity. The preparation involves washing, crushing, adding hot water, waiting and filtering. Each stage leads towards obtaining the coloured liquid used for the later tests.

What is the preparation sequence?

1. Collect a fistful of fallen red rose petals and wash them with water.
2. Crush the washed petals using a mortar and pestle, the bowl and crushing tool used to grind materials.
3. Transfer the crushed petals into a glass tumbler. Add enough hot water to immerse them completely, with adult supervision for this step.
4. Cover the tumbler with a lid and leave it for 5 to 10 minutes, until the water becomes coloured.
5. Filter the mixture. Filtration is the separation step used here to obtain the liquid from the petal mixture.
6. Collect the filtrate, the liquid obtained after filtration. This is the flower extract to use as an acid-base indicator.

A test tube held upright in a stand contains coloured liquid near its lower end. The drawing labels the test tube, test tube stand and red rose extract.

See Fig. 2.7 in your NCERT textbook

The petals are the starting material, while the filtrate is the prepared indicator. Keeping this distinction clear prevents confusion when describing the activity. It is the collected liquid that is divided between the containers for the tests with lemon juice and soap solution.

How does red rose extract reveal the nature of a solution?

The prepared red rose extract can be compared in acidic and basic samples. In this activity, A and B are labels for two test tubes, not names of chemicals. Each tube first receives some of the same prepared extract.

How are the comparison tests carried out?

1. Place 10 to 20 drops of red rose extract in each of two small transparent bottles or test tubes.
2. Label the containers A and B so that the samples and their observations remain identifiable.
3. Add 20 to 30 drops of lemon juice to A. Add 20 to 30 drops of soap solution to B using droppers.
4. Observe and record the changes in the extract. Repeat the investigation with the other household samples used in the litmus activity.

Red rose extract seems to give red colour in an acidic solution and green colour in a basic solution. Compare the samples giving a shade of red with those that turned blue litmus red. Compare those giving a shade of green with the red-litmus-to-blue group.

Two test tubes labelled A and B stand in a holder. Tube A contains red-coloured liquid after lemon juice is added; tube B contains green-coloured liquid after soap solution is added.

See Fig. 2.8 in your NCERT textbook

How does this compare with litmus?

Other natural materials that can act as acid-base indicators include beetroot, purple cabbage, turmeric, Indian blackberry and red hibiscus. Indian blackberry is also called jamun, and red hibiscus is called gudhal. Test and record their observations rather than assuming every natural indicator has the same colours.

How is turmeric paper prepared, and what can it identify?

Turmeric, or haldi, is a natural indicator that can be applied to paper. Its yellow colour turns red in a basic solution. In acidic and neutral solutions, the yellow colour remains unchanged. This makes turmeric useful for recognising basic substances.

How are the yellow strips made?

1. Place a spoonful of turmeric in a petri dish or another container. A petri dish is a shallow dish used to hold materials during an investigation.
2. Add a little water to form a paste. A piece of fresh turmeric may also be ground for the preparation.
3. Dip a piece of filter paper carefully into the paste until it becomes yellow. Filter paper is paper used in separating materials by filtration.
4. Remove the coloured paper and allow it to dry.
5. Cut it into thin strips under adult supervision. These dried yellow strips are called turmeric paper.
6. Put a drop of each sample on separate pieces, one sample at a time, and record the colour after adding it.

Part (a) shows yellow turmeric paste in a shallow dish. Part (b) shows yellow paper strips beside a larger piece of yellow paper.

See Fig. 2.9 in your NCERT textbook

What is the limitation of yellow turmeric paper?

Turmeric cannot differentiate between acidic and neutral substances in this yellow-paper test. A yellow result is therefore not enough to declare a sample acidic. Use the information from the litmus tests to distinguish these two groups.

A curry stain that changes colour after soap is applied connects this indicator behaviour with daily experience. The turmeric in the stain responds to the basic soap. The colour change is evidence of an interaction between the indicator and the substance added to it.

A similar idea can make coloured writing. Coat white paper with turmeric paste and dry it. A basic solution such as soap solution can be used to write on the yellow paper, producing a red message where the solution touches it.

How can changes in smell be used in an investigation?

Some substances change their odour, meaning their smell, in an acidic or basic medium. Here, a medium means the acidic or basic surroundings in which the substance is tested. Such substances are called olfactory indicators, indicators investigated through changes in smell.

The onion activity uses strips of cotton cloth or filter paper exposed to chopped onions. These strips are then treated with different samples. The investigation compares the smell before and after treatment, rather than looking for a colour change.

What is the onion-strip procedure?

1. Put finely chopped onions into a container with some clean cotton cloth strips or filter paper strips.
2. Close the container tightly and leave it overnight.
3. Remove two strips and check their odour before adding the test liquids.
4. Place the strips on a clean surface. Add a few drops of tamarind water to one and a few drops of baking soda solution to the other.
5. Allow the drops to spread across the strips, then check the odour again.
6. Record the observations. Investigate other acidic and basic samples in the same way, noting any change in smell.

Tamarind water supplies the acidic sample, while baking soda solution supplies the basic sample. The comparison uses samples from two known groups. Record what is actually observed before drawing a conclusion about the response of the strips.

The important distinction is between the property being observed and the sample being tested. Litmus, red rose and turmeric tests observe colour. The onion-strip investigation observes odour. Both approaches involve comparing a property before and after contact with a test substance.

What happens when lemon juice is mixed with lime water?

Lemon juice is acidic and lime water is basic. Their interaction can be investigated with blue litmus solution. Unlike the paper tests, the indicator is added directly to the liquid mixture, allowing its colour to be observed as lime water is added.

What steps show the change?

1. Put one drop of lemon juice into a test tube and add around twenty drops of water. Observe the liquid's colour.
2. Add one drop of blue litmus solution. The liquid turns red, showing the acidic nature of the lemon juice solution.
3. Add lime water slowly, drop by drop, using a dropper. Swirl the test tube well as it is added.
4. Continue observing. A stage is reached when the colour changes from red to blue.
5. Add one drop of lemon juice again and observe the mixture. Consider how adding an acidic substance affects the colour response.

Part (a) shows a test tube containing red liquid after blue litmus solution is added to the lemon juice solution. Part (b) shows blue liquid after lime water is added.

See Fig. 2.10 in your NCERT textbook

The initial red colour identifies an acidic solution. The later blue colour shows that the mixture is no longer acidic. The lime water has neutralised the effect of the acid. The observed colour must be interpreted alongside the substance that was added.

Does a blue result mean the mixture is neutral?

Neutral means neither acidic nor basic. A blue result following the addition of lime water should not be described as proof of exact neutrality. It shows the mixture is no longer acidic; enough basic solution has been added to produce the blue indicator response.

The observation and the general idea are related but distinct. The activity follows a colour change as a base is added. Neutralisation explains how an acid and a base can counteract each other's effects when mixed in sufficient quantity.

What is neutralisation, and what does its word equation mean?

Neutralisation is the reaction between an acid and a base. When their solutions are mixed in sufficient quantity, the resulting solution is neither acidic nor basic. The qualification about quantity matters when interpreting a mixture made during an indicator activity.

A reaction is a process in which substances interact to form products. Here the products are salt and water. Salt means the substance formed along with water in this acid-base reaction. Heat is also released during neutralisation.

Definition: In a neutralisation reaction, an acid and a base react to form salt and water, with the evolution of heat. “Evolution of heat” means the release of heat.

How is the reaction represented in words?

Acid + Base → Salt + Water + Heat

In this word equation, the plus sign joins substances or effects listed together. The arrow means “produces”. Acid and base appear on the left because they react. Salt and water appear on the right because they form, and heat is listed because it is released.

The equation connects the mixing activity with its explanation. The indicator reveals changes in the nature of the solution; the word equation describes the reaction and its products. A colour change and the release of heat are different ideas and should be described separately.

Do not replace “in sufficient quantity” with “in any quantity”. Mixing an acid and a base does not justify calling the mixture neutral without considering the amounts added and the indicator observations. In the lemon juice activity, the final blue colour is described as showing a solution that is no longer acidic.

How does neutralisation help in treating acidic and basic soils?

Fertilisers are substances added to soil to help plants grow better. Excessive use of chemical fertilisers can make soil acidic, as in the farmer's problem. When soil is too acidic, plants do not grow well, and its nature needs attention.

How are the two kinds of soil treated?

Acidic soil can be treated with lime, which is a base. The basic material counteracts the soil's acidity. This application follows the same principle as the mixing activity: an acidic substance and a basic substance can neutralise one another.

If the soil is basic, organic matter such as manure and composted leaves is added. Organic matter here includes these materials of living origin. It releases acids that neutralise the basic nature of the soil.

Does poor plant growth prove that soil is acidic?

No. Nutrients are substances plants need for growth. Sometimes soil may be neutral, yet plants in it may still show poor health, which can be due to a deficiency of nutrients. A deficiency means an insufficient supply of something needed.

Therefore, poor growth alone does not identify acidic soil. The acidic or basic nature of a soil sample can be investigated with an indicator. Keep the evidence about soil nature separate from the observation that the plants are unhealthy.

The proposed treatment must match the diagnosed condition. Lime is linked with acidic soil; manure and composted leaves are linked with basic soil. The possibility of nutrient deficiency prevents treating every unhealthy plant as evidence of a neutralisation problem.

Where else do indicators and neutralisation explain daily observations?

An ant bite, factory waste and a hidden message on paper involve different observations. The first two connect with neutralisation, while the message uses the colour response of an indicator. Identify the acidic or basic substance and the change before explaining the situation.

Why can moist baking soda relieve an ant bite?

When an ant bites, it injects an acidic liquid containing formic acid into the skin. Formic acid is the acid involved in this example. The acid's effect can be neutralised by rubbing moist baking soda, a base, on the affected area.

In the red-ant example, the bite leaves redness and stinging pain, and moist baking soda relieves the pain. The explanation depends on the opposite acidic and basic natures of the substances. It is an application of neutralisation to the particular example.

How can acidic factory waste be treated?

A decline in fish numbers in a lake may be due to factory waste entering it. If that waste is acidic, it can be neutralised by adding basic substances before releasing it into the lake. The treatment targets the acidic nature of the waste.

The condition “if the waste is acidic” is important. The example does not establish that every factory waste is acidic or that every decline in fish numbers has the same cause. It presents a possible cause and a treatment for its acidic component.

How can an apparently blank sheet reveal a message?

One possibility is to write with soap solution and spray the paper with turmeric solution. The basic soap changes turmeric's yellow colour to red, making the writing visible. This is a possible explanation for the science-fair message appearing when liquid is sprayed.

The message demonstrates an indicator response. The ant-bite, soil and waste examples show how some daily problems can be attempted to be managed through neutralisation. In each case, connect the stated conditions with the conclusion rather than extending it to every situation.

Glossary

• Acidic substance — A substance that changes blue litmus paper to red during an indicator test.
• Basic substance — A substance that changes red litmus paper to blue during an indicator test.
• Neutral substance — A substance that is neither acidic nor basic and changes neither colour of litmus paper.
• Acid-base indicator — A substance that displays different colours in acidic and basic solutions.
• Litmus — A natural indicator obtained from lichens, available as a solution or coloured paper strips.
• Lichen — An association of two kinds of living organisms, from which litmus is obtained.
• Lime water — A solution of calcium hydroxide in water, used as a basic sample.
• Filtrate — The liquid obtained after filtration, such as the prepared red rose extract.
• Turmeric paper — Paper coloured with turmeric, dried and cut into strips for testing samples.
• Olfactory indicator — A substance whose odour changes in an acidic or basic medium.
• Neutralisation — An acid-base reaction forming salt and water with the release of heat.
• Organic matter — Materials such as manure and composted leaves that release acids and can neutralise basic soil.

Common errors and misconceptions

• Misconception: Any bitter substance must be a base. Correct: Bases generally taste bitter, but bitter gourd is bitter without being basic.
• Misconception: A sample is neutral if red litmus does not change. Correct: Test blue litmus too; an acidic sample also leaves red litmus unchanged.
• Misconception: Yellow turmeric paper proves that a sample is acidic. Correct: Both acidic and neutral samples leave yellow turmeric unchanged.
• Misconception: Lime water is juice from lime fruit. Correct: Lime water is calcium hydroxide solution and is basic.
• Misconception: Mixing any amounts of acid and base guarantees a neutral mixture. Correct: The solutions must be mixed in sufficient quantity; use the indicator observations when interpreting the mixture.
• Misconception: The blue stage of the lemon juice and lime water activity proves exact neutrality. Correct: It shows that the mixture is no longer acidic.
• Misconception: Unhealthy plants prove the soil is acidic. Correct: Sometimes neutral soil supports unhealthy plants because there may be a nutrient deficiency.
• Misconception: Unknown substances should be tasted to identify acids. Correct: Never taste unknown substances; use the appropriate indicator investigation.

Exam-style questions with model answers

Q1. A sample turns blue litmus red and leaves red litmus unchanged. Identify its nature and state the decisive observation. [2 marks]

1. The sample is acidic because it shows the characteristic indicator response of an acidic substance.
2. The decisive observation is the change from blue litmus to red; unchanged red litmus is consistent with this conclusion.

Q2. Three solutions are labelled A, B and C. A turns red litmus blue, B turns yellow turmeric red, and C turns red rose extract green. Classify each solution and justify each classification using its given test. [3 marks]

1. Solution A is basic. The given change from red litmus to blue is the indicator response used to identify a basic solution.
2. Solution B is basic. Yellow turmeric changes to red in a basic solution, which matches the observation supplied for this sample.
3. Solution C is basic. Red rose extract seems to give green colour in a basic solution, so the observed green colour supports this classification.

Q3. A liquid leaves red litmus unchanged, turns blue litmus red and leaves yellow turmeric unchanged. Explain each observation and state why the turmeric result alone is insufficient to classify it. [4 marks]

1. Unchanged red litmus is consistent with an acidic liquid, but this observation alone is insufficient because a neutral liquid also leaves red litmus unchanged.
2. The change from blue litmus to red identifies the liquid as acidic. This is the decisive result among the given tests.
3. The unchanged yellow turmeric is also consistent with an acidic liquid, since turmeric retains its yellow colour in an acidic solution.
4. Turmeric alone cannot distinguish an acidic sample from a neutral one: both leave it yellow. Therefore, the blue-litmus observation supplies the necessary distinction.

Q4. A fistful of fallen red rose petals, water, a mortar and pestle, a glass tumbler, a lid and filtration materials are available. The hot-water soaking time is 5 to 10 minutes. Describe five stages for preparing a red rose indicator, including the hot-water precaution and the liquid collected. [5 marks]

1. Collect a fistful of the fallen red rose petals and wash them with water. Use fallen petals rather than plucking flowers for the activity.
2. Crush the washed petals with the mortar and pestle. Transfer the crushed material to the glass tumbler for the next preparation stage.
3. Add hot water until the crushed petals are completely immersed. Carry out this hot-water step under adult supervision, as required for the activity.
4. Cover the glass tumbler with its lid. Wait for 5 to 10 minutes until the water takes on colour from the petals.
5. Filter the mixture and collect the filtrate, meaning the liquid obtained after filtration. This coloured red rose extract is the prepared acid-base indicator.

Q5. Three samples are known to be vinegar, baking soda solution and sugar solution. Fresh yellow turmeric paper turns red with one sample and stays yellow with the other two. Identify the red-result sample, explain the two yellow results and state a further indicator test that separates those two samples. [3 marks]

1. The red-result sample is baking soda solution. It is basic, and a basic solution changes the yellow colour of turmeric to red.
2. The two yellow-result samples are vinegar and sugar solution. Vinegar is acidic and sugar solution is neutral; neither changes fresh yellow turmeric paper.
3. Test those two samples with blue litmus paper. Vinegar turns it red, while sugar solution leaves it unchanged, allowing the two samples to be identified.

Q6. One drop of lemon juice is mixed with around twenty drops of water. One drop of blue litmus solution makes it red. Lime water is added slowly while swirling, and the mixture eventually turns blue. Explain the initial result, the role of lime water and the meaning of the final colour. [3 marks]

1. The initial red colour shows that the lemon juice solution is acidic. Adding blue litmus to the acidic liquid produces the stated red response.
2. Lime water is basic. As it is added, it neutralises the effect of the acid in the lemon juice solution.
3. The final blue colour shows that the mixture is no longer acidic. It should not be described as proof of exact neutrality, because neutral means neither acidic nor basic.

Q7. Apply neutralisation to these five situations: an ant injects acidic formic acid; soil is too acidic; soil is basic; neutral soil supports unhealthy plants; and factory waste is confirmed acidic before discharge to a lake. For each, give the relevant treatment or explanation and its reason. [5 marks]

1. For the ant-bite example, moist baking soda can neutralise the acid's effect. Baking soda is a base, so it counteracts the acidic liquid injected into the skin.
2. Too-acidic soil can be treated with lime. Lime is basic and therefore counteracts the acidic nature that prevents the plants from growing well.
3. Basic soil can be treated with organic matter such as manure and composted leaves. This material releases acids that neutralise the soil's basic nature.
4. Unhealthy plants in neutral soil may be suffering from nutrient deficiency. Poor health alone does not prove an acidic or basic condition needing neutralisation.
5. The confirmed acidic factory waste can be neutralised with basic substances before release into the lake. The proposed treatment addresses its stated acidic nature.

Q8. A white sheet has been written on with soap solution and then sprayed with yellow turmeric solution. Predict the colour of the writing and explain the indicator response. [2 marks]

1. The writing appears red where the soap solution meets the turmeric indicator.
2. Soap solution is basic, and turmeric changes from yellow to red in a basic solution.

Key takeaways

• Acidic samples turn blue litmus red, basic samples turn red litmus blue, and neutral samples change neither colour.
• Substances tasting sour tend to contain acids; bases generally taste bitter, but bitter gourd is not basic.
• Prepare red rose extract from washed, crushed petals using hot water, a covered waiting period and filtration.
• Red rose extract seems to give red colour in acidic solutions and green colour in basic solutions.
• Yellow turmeric paper turns red in basic solutions but cannot distinguish acidic samples from neutral samples.
• Neutralisation forms salt and water and releases heat; the quantity of acid and base matters.
• Lime treats acidic soil, while organic matter releases acids that help neutralise the basic nature of soil.
• Indicator observations support conclusions about sample nature; a single unchanged colour may leave more than one possibility.

Test yourself

What is the source of litmus, and in which forms is it available?

Litmus comes from lichens and is available as a solution or as blue and red paper strips.

Why does bitter gourd challenge a taste-based classification of bases?

Bases generally taste bitter, but bitter gourd is bitter without being basic. Bitterness alone does not identify a base.

What is the filtrate in the red rose preparation?

It is the coloured liquid collected after filtering the petal mixture, used as the red rose indicator.

What does an unchanged yellow turmeric strip establish?

It does not distinguish an acidic sample from a neutral sample, because both leave yellow turmeric unchanged.

What property is investigated with onion-treated cloth strips?

The investigation compares odour before and after adding acidic or basic substances to the onion-treated strips.

What forms during neutralisation, and what happens to heat?

Salt and water form during neutralisation, and heat is released as the acid and base react.

Why might plants remain unhealthy even in neutral soil?

Sometimes neutral soil may lack nutrients, so the plants' poor health can be due to nutrient deficiency.

When should acidic factory waste be neutralised in the lake example?

It should be neutralised by adding basic substances before it is released into the lake.