Class 7 Science Chapter 2: Exploring Substances: Acidic, Basic, and Neutral

The Words That Were Not There

On National Science Day a school holds a fair. At the gate Ashwin and Keerthi are each handed a blank white sheet. A few steps on, a volunteer sprays something onto the paper, and a welcome message appears out of nowhere. Nobody wrote it while they watched. By the end of this chapter you will be able to say how it was done, and the trick is a colour change rather than invisible ink.

Three Kinds of Substance

The things around your house sort themselves into three groups. Some are acidic, like lemon juice and tamarind water. Some are basic, like soap solution and lime water. Some are neither, like sugar solution and plain tap water, and those are called neutral. The whole of this chapter is about telling which group something belongs to without tasting it.

Why Tasting Is Not the Method

You already know lemon is sour and you may know soap is bitter. That is not how a scientist decides. Many acidic and basic substances are strong enough to burn your mouth or your skin, and a laboratory sample may be anything at all. So we need a test that works from the outside, using something that changes colour instead of a tongue.

Litmus Comes From a Living Thing

The oldest of these tests uses litmus. It is a natural substance taken from lichens, which are the flat crusty growths you see on old walls and tree bark. Litmus is sold in two forms. One is a solution you add drop by drop. The other is thin paper soaked in it and dried, which is called litmus paper.

Two Papers, Not One

Litmus paper comes in blue and in red, and you need both. One paper alone can only answer half the question. The blue paper is the one that reacts to acids, and the red paper is the one that reacts to bases. Keep them dry and keep them apart until you use them.

What Blue Litmus Tells You

Put a drop of your sample on a piece of blue litmus paper and watch. If the blue turns red, the sample is acidic. Vinegar does this, and so do tamarind water, amla juice and lemon juice. If the blue stays blue, then whatever else the sample is, it is not acidic.

What Red Litmus Tells You

Now take the red paper and do the same thing. If the red turns blue, the sample is basic. Soap solution does this, and so do lime water, baking soda solution and washing powder solution. If the red stays red, the sample is not basic.

When Neither Paper Moves

Some samples leave both papers exactly as they were. The blue stays blue and the red stays red. A sample like that is neutral, which means neither acidic nor basic. Sugar solution, salt solution and tap water normally behave this way. Nothing is wrong with the test when nothing happens.

Three Groups From Two Papers

So two papers sort every sample into three groups. Group A turns blue litmus red and is acidic. Group B turns red litmus blue and is basic. Group C changes neither paper and is neutral. Writing your results into a table in those three columns is what turns a pile of observations into something you can use.

What an Indicator Is

Litmus shows one colour in an acidic solution and a different colour in a basic one. Any substance that behaves like that is called an acid-base indicator. The word indicator is doing real work here. It does not change the sample and it does not fix anything. It only shows you which of the three groups the sample is in.

Lime Water Is Not a Lime

One name trips people up every year. Lime water has nothing to do with the green fruit that looks like a lemon. It is a solution of calcium hydroxide, and you make it by stirring lime, which is chuna or calcium oxide, into water. Leave it to stand for an hour, then filter the clear liquid off and use that.

One Drop Is Enough

A test like this needs very little. Cut the paper into small pieces, spread them on a clean dry white tile, and put one drop of each sample on its own piece with a dropper. A white tile is used so the colour is easy to read, and the pieces are kept separate so no sample runs into the next.

Work Clean or Work Twice

Two habits save the whole experiment. Rinse the dropper between samples, because a trace of lemon juice carried into your soap solution will give you a false reading. And label every piece before you start. Twelve unlabelled drops on a tile are twelve results you cannot use.

Why Record the Boring Ones

Write down every sample, including the ones where nothing happened at all. A no-change result is what identifies the neutral group, so leaving it out destroys a third of your findings. The temptation to record only the exciting colour changes is strong, and it is how a good experiment turns into a useless one.

What Comes Next

Litmus is not the only indicator, and it is not always at hand. The rest of this chapter builds indicators from things in a kitchen and a garden: rose petals, turmeric, and others you can try. Each one tells you something slightly different, and one of them has a limit worth knowing about.

Scenario:

At the science fair a stall hands every visitor a blank white sheet, then sprays a liquid over it, and a welcome message appears. Ashwin and Keerthi are certain there is no hidden ink, because they watched the sheet being handed over. At the next table they find bottles of lemon juice, soap solution, sugar solution and lime water, with blue and red litmus paper beside them. They test all four. Two of the bottles redden blue litmus, one blues red litmus, and one moves neither paper. Keerthi starts to suspect that the message on the sheet was written in something that only shows up when a second liquid reaches it.

Analysis:

The children have sorted four samples into three groups using nothing but two papers, which is the method of this whole section. Their reasoning about the sheet is also sound. A message that appears only when sprayed must have been written in something colourless that reacts, and the spray must be an indicator. The mechanism is a colour change, not invisible ink, and that is the same mechanism they have just seen on the litmus paper. Note that their test cannot yet tell them which substance was used on the sheet. It only tells them that an indicator and a sample of a different nature are involved.

Activity:

Write on a sheet of white paper with a cotton bud dipped in soap solution and let it dry completely so no mark is visible. Then brush or spray turmeric water lightly over the whole sheet and watch the writing appear. Write two sentences saying which liquid was the indicator and which was the sample.

Title:

Sorting the kitchen into three groups

Safety:

Do not taste any sample. Wash your hands afterwards, and keep soap solution away from your eyes.

What You Notice:

Five samples fall into three groups, and the neutral ones are found by nothing happening at all. That is why the unchanged results have to be written down like any other.

Lichens, the source of litmus, are the flat crusty growths found on old walls and tree bark.

Lime water is a solution of calcium hydroxide and has no connection with the fruit called lime.

A white tile is used for litmus testing because a pale background makes a faint colour change easy to read.

Blue to red means acid instead: blue litmus going red marks an acidic sample.

Red to blue, a base for you: red litmus going blue marks a basic sample.

What If There Is No Litmus?

Litmus works, but a packet of litmus paper is not something most homes keep. So here is a fair question. If litmus comes from a living thing, might other living things do the same job? The answer is yes, and some of them are growing within a few steps of where you are sitting. A garden and a kitchen between them hold several usable indicators.

Petals From the Ground

Start with a red rose. You need a fistful of petals, and you should pick up ones that have already fallen rather than pulling flowers off a plant. That is not only politeness. A fallen petal works exactly as well, so there is nothing to gain by damaging the plant, and a garden stripped for an experiment is a poor advertisement for science.

Washing and Crushing

Wash the petals with water first, because dust and insects on them will end up in your extract. Then crush them, using a mortar and pestle if you have one. Crushing matters more than it looks. The colour you want is locked inside the cells of the petal, and breaking them open is what lets the water reach it.

Hot Water Does the Work

Put the crushed petals in a glass tumbler and pour in enough hot water to cover them completely. Hot water pulls the colour out far faster than cold water does. This is the step that needs an adult beside you, because hot water in a glass tumbler is the part of this experiment that can actually hurt you.

Wait, Then Filter

Cover the tumbler with a lid and leave it for five to ten minutes, until the water has taken on a strong colour. Then filter it. The liquid that comes through the filter is called the filtrate, and that filtrate is your indicator. The spent petals left behind in the filter have done their job and can be thrown away.

Reading the Red Rose

Now test it. Put ten to twenty drops of the extract into each of two small bottles or test tubes and mark them A and B. Add lemon juice to A and soap solution to B, about twenty to thirty drops of each. The extract in A turns a shade of red. The extract in B turns a shade of green.

Red for Acid, Green for Base

So the rule for this indicator is short. Red rose extract gives red in an acidic solution and green in a basic one. That is a different pair of colours from litmus, and there is no reason the two should agree. What matters is that each indicator is consistent with itself, so once you know its rule you can read any sample.

Does It Agree With Litmus?

Here is the check worth doing. Run the extract across all the samples you tested with litmus. The ones that turn the extract red should be the same ones that turned blue litmus red. The ones that turn it green should be the ones that blued red litmus. When two independent indicators agree, your grouping is a good deal more trustworthy.

Why Agreement Is the Point

A single indicator could in principle be fooled by something unusual in a sample. Two indicators built from completely different plants are unlikely to be fooled in the same way. So when litmus and rose extract place a sample in the same group, that agreement is evidence about the sample rather than about the paper.

It Is Not Only Roses

The same method works on other coloured things. Beetroot, purple cabbage, jamun and the red hibiscus flower that many people call gudhal all give usable extracts. Crush, soak in hot water, filter, and test. Each one has its own pair of colours, so work out its rule on a known acid and a known base before you trust it on anything unknown.

Calibrate Before You Trust

That last point is the discipline of this topic. A new extract tells you nothing until you have shown it a sample you already know. Lemon juice and soap solution are your two reference samples. Note what colour each produces, write the rule down, and only then use the extract on a sample whose nature you are trying to find.

Hydrangea Reads the Soil

One plant does this without being crushed at all. Hydrangea grows where the hills stay cool, in the north and the north-east, and its flowers come out in different colours depending on the soil it is growing in. In acidic soil the flowers are blue. In basic soil they come out pink or red.

A Gardener With a Lever

That leads somewhere interesting. If the colour depends on whether the soil is acidic or basic, then a gardener who changes the soil should be able to change the colour of the flowers. It is a real question and the answer is yes. The plant is reporting on its soil in the same way an indicator reports on a sample.

Safety Is Part of the Method

Two cautions belong to this topic and not to the last one. Hot water is involved, so an adult should be present for the soaking step. And no extract you make is a drink, however much it looks like one. A tumbler of bright pink rose water on a kitchen counter is exactly the sort of thing somebody picks up by mistake, so label it.

What Is Still Missing

The rose extract separates acidic from basic cleanly, and so does litmus. Neither of them has any trouble telling you a sample is neutral either, since a neutral sample simply leaves the extract alone. The next topic introduces an indicator that is genuinely useful and yet cannot do that last part, and knowing its limit is the whole point of it.

Scenario:

Tashi lives in a hill town where hydrangea grows in many gardens. In his own garden the flowers come out a clear blue every year. Two streets away, his aunt has hydrangea from the same nursery and her flowers are pink. His aunt is certain she is doing something wrong with the watering. Tashi collects a little soil from each garden, stirs each sample in a cup of water, lets the mud settle, and tests the clear water above it with red rose extract. The water from his garden turns the extract red. The water from his aunt garden turns it green.

Analysis:

Tashi has explained the difference without any argument about watering. Hydrangea flowers are blue in acidic soil and pink or red in basic soil, and his two tests match that exactly. Red rose extract reds in an acid, so his own soil water is acidic, which fits his blue flowers. It greens in a base, so his aunt soil water is basic, which fits her pink ones. Notice what makes this work. He used an indicator whose rule he already knew, and he let the mud settle so he was testing clear water rather than reading a colour through dirt. His aunt watering was never the cause.

Activity:

Take a spoonful of soil from two different places, such as a garden bed and a patch beside a wall. Stir each into half a cup of water, leave it to settle for ten minutes, and test the clear water above with litmus paper. Write down which soil is acidic, basic or neutral, and name one plant growing in each place.

Title:

Build your own indicator and prove it works

Safety:

An adult must pour the hot water. Do not taste the extract, and label the tumbler so nobody mistakes it for a drink.

What You Notice:

Bottle A goes red and bottle B goes green, which gives you the rule for your own extract. Now it can be used on a sample you do not know, which is why the two known samples came first.

Hydrangea grows where the hills are cool, in the north and the north-east.

A gardener can change the colour of hydrangea flowers by changing whether the soil is acidic or basic.

Purple cabbage, beetroot, jamun and red hibiscus all give extracts that work as acid-base indicators.

Rose rule: Red for acid, Green for base, and both words start as the colours do.

CSF for making any extract: Crush the petals, Soak in hot water, Filter the liquid.

The Stain Everybody Has Made

A drop of curry lands on a white school shirt. Somebody rubs soap on the yellow mark and it turns red, which looks like the stain getting worse. It is not. The yellow came from turmeric, the soap is basic, and what you have just watched is a chemical test happening by accident on your uniform. This topic is about doing it on purpose.

Making Turmeric Paper

Put a spoonful of turmeric in a small dish and work in just enough water for a thick paste. Fresh turmeric works too if you grind a piece. Dip a sheet of filter paper into the paste until it takes up the yellow colour, lift it out, and let it dry completely. Then cut the dry sheet into thin strips. Those strips are turmeric paper.

Why It Must Dry First

Do not hurry the drying. A strip still wet with paste carries loose turmeric that will smear across whatever you touch, and a wet strip is already holding water, which dilutes the drop of sample you add. A dry strip changes colour only where the sample lands, and that sharp edge is what makes the result easy to read.

Using the Strips

The test itself is the same as with litmus. Take a strip, put a single drop of your sample onto it with a dropper, and look at what happens to the yellow. Work through your samples one at a time, use a fresh strip for each, and write the result down before moving on.

Yellow Turns Red for a Base

Here is the rule. Turmeric yellow becomes red when it meets a base. Soap solution does it. So do lime water, baking soda solution and washing powder solution. If your yellow strip goes red, you have found a base, and that part of the test is as clear and reliable as anything litmus tells you.

And Nothing Happens for Everything Else

Now the other half. Put lemon juice on a turmeric strip and the yellow stays yellow. Put sugar solution on one and the yellow stays yellow. Put plain tap water on one and the yellow stays yellow. An acidic sample and a neutral sample produce exactly the same result, which is no result at all.

What That Means

So turmeric paper can test for basic substances, and it cannot tell an acidic substance apart from a neutral one. That is not a fault in your method or a stale strip. It is what this indicator is. Knowing the limit of a test is as much a part of using it as knowing what it detects.

A Test That Answers Half a Question

Compare that with litmus, where two papers between them reached all three groups. Turmeric gives you one answer and one silence. Yellow going red means basic. Yellow staying yellow means not basic, and stops there. If you need to know which of the other two a sample is, you must bring in a second indicator.

Using the Limit Rather Than Fighting It

A test with a narrow answer is still useful when that is the question you have. Suppose you only want to know whether a cleaning liquid is basic. Turmeric paper settles that in one drop, and the fact that it cannot separate acids from neutrals does not matter, because you were not asking. Choose the test that fits the question.

The Disappearing Message

This is how the trick at the science fair worked. Write on white paper with a cotton bud dipped in soap solution and let it dry. Nothing is visible. Then spray turmeric solution over the whole sheet. Everywhere the soap was, the turmeric turns red, and the writing appears. There was never any invisible ink.

Ashwin Makes a Greeting Card

Ashwin used the same idea for Guru Purnima. He spread turmeric paste over white paper and dried it, giving himself a yellow sheet. Then he wrote his wishes on it with soap solution, and the letters came up red against the yellow. He wrote them in his teacher own mother tongue, which made the card matter more than the chemistry did.

Which One Was the Indicator?

It is worth being precise about the two liquids, because it is easy to get them the wrong way round. The turmeric is the indicator, because it is the substance that changes colour. The soap solution is the sample, because it is the basic substance being detected. Whichever one goes on first, those roles do not swap.

The Golden Spice

Turmeric sits in the same plant family as ginger, and is grown in India and elsewhere. It is in food in most Indian homes, and it gives both colour and taste. Ayurvedic medicine holds that it carries many benefits, and it turns up in a great many home remedies.

Still Being Studied

Turmeric is also being researched now for effects beyond the colour and flavour it adds to cooking. That is worth noticing in a chapter about science being unfinished. A spice that has been in kitchens for centuries is still producing questions that nobody has finished answering.

What You Can Now Do

You have three indicators. Litmus, in two papers, reaches all three groups. Rose extract reds in acid and greens in base. Turmeric finds bases and is silent about the rest. The next topic stops identifying substances and starts changing them, by mixing an acid and a base together.

Scenario:

Meera finds three unlabelled bottles under the kitchen sink and wants to know what she is dealing with before touching anything further. She makes turmeric paper, cuts three strips, and puts one drop from each bottle on its own strip. The first strip turns red. The second and third stay yellow. She writes down that bottle one is basic and that the other two are acidic. Her elder brother reads her notebook and says half of it is a guess. Meera then tests the second and third bottles again, this time with blue and red litmus paper. Bottle two reddens the blue paper. Bottle three moves neither paper.

Analysis:

Meera first test was done properly and her first conclusion was sound. A red strip does mark a base. Her brother is right about the second half, though. Turmeric paper leaves an acidic sample and a neutral sample looking identical, so calling both of the yellow ones acidic went beyond what her evidence could support. The litmus retest is the correct fix, because two litmus papers can separate exactly the distinction turmeric cannot make. It shows bottle two is acidic and bottle three is neutral. Her method was never wrong; her conclusion simply outran her test.

Activity:

Make turmeric paper and test four liquids from your kitchen. Write each result as either basic or not basic, and resist writing anything more than that. Then list which samples you would still need a second test for, and say which indicator you would use.

Title:

Write a message that appears

Safety:

Keep the soap solution away from your eyes, and wash your hands when you have finished.

What You Notice:

The writing appears in red because the turmeric turns red wherever the basic soap is. Nothing was invisible ink. The turmeric is the indicator and the dried soap was the sample all along.

Turmeric sits in the same plant family as ginger, and is grown in India and in several other countries.

Ayurvedic medicine treats turmeric as carrying many benefits, and it turns up in a great many home remedies.

Turmeric is still being researched today for effects beyond the colour and taste it gives to food.

Turmeric speaks once: Red means Base, and yellow means only not a base.

Golden silence: turmeric is silent about acids and neutrals alike, so it cannot tell them apart.

From Naming to Changing

So far this chapter has only identified things. Litmus, rose extract and turmeric all report what a sample is and leave it exactly as it was. Now the question changes. What happens if you stop testing a sample and instead mix an acid and a base together? Something new is made, and the result is not a stronger version of either.

Watch the Colour Go Back

Here is the experiment. Drop a single drop of lemon juice into a test tube and add about twenty drops of water. Add a drop of blue litmus solution and the liquid turns red, as you would expect from an acid. Now add lime water drop by drop, swirling as you go, and keep watching the colour.

The Moment It Turns

For a while nothing much happens. Then a stage comes when the red changes to blue. That matters. The solution in the tube is no longer acidic. The lime water has cancelled out the effect of the acid, and the indicator is telling you so. Add one more drop of lemon juice and the colour will come back the other way.

What Neutralisation Means

Mix enough of an acid solution into a base solution and what you are left with is neither acidic nor basic. A reaction like that is called a neutralisation reaction. The phrase in sufficient quantity is doing real work there. Too little base and the mixture is still acidic. Too much and it has gone over to basic.

What Is Actually Made

Two new substances come out of a neutralisation. One is a salt and the other is water. Heat is also released. So the whole thing is written as a word equation: Acid plus Base gives Salt plus Water plus Heat. Learn that line as four items on the right, because the heat is as real a product as the water.

Why the Tube Feels Warm

If you hold the test tube while enough acid and base are reacting, it warms slightly. That is the heat in the equation, and it is why the book says heat is evolved, meaning given out rather than taken in. A neutralisation does not need warming up to get going. It produces warmth of its own.

The Ant on the Tree Trunk

Keerthi rests her hand on a tree trunk while watching a butterfly, and a red ant bites her. The skin goes red and stings. Her brother puts moist baking soda on the spot and the pain eases. This is not a folk remedy with no reason behind it. It is a neutralisation, and you can now say exactly why it works.

Acid Under the Skin

An ant bite pushes an acidic liquid under the skin, and that liquid is called formic acid. The acid is what produces the sting. Baking soda is a base, so rubbing the moist powder on the bite neutralises the acid that is sitting there. The remedy and the chemistry are the same thing described two ways.

A Farmer With Poor Growth

A farmer writes to an agriculture portal to say his plants are not growing well. After some discussion the cause turns out to be his own fertilisers. Fertilisers are materials mixed into soil so that crops grow more strongly, and using far too much of them had made the soil acidic. Plants do not grow well in soil that is too acidic.

Treating Acidic Soil

The remedy follows from the diagnosis. Acidic soil is treated with lime, which is a base, and the lime neutralises the excess acid. Notice the shape of this. Nobody tested the plants. Somebody worked out the nature of the soil, and the treatment was then whatever would neutralise it.

Treating Basic Soil

Soil can go the other way too. If the soil is basic, organic matter is added to it, such as manure or composted leaves. Organic matter releases acids as it breaks down, and those acids neutralise the basic soil. So both problems have the same logic and opposite cures.

When the Soil Is Not the Problem

Here is the part worth remembering longest. Sometimes the soil is neutral and the plants still do badly. The cause then may be a deficiency of nutrients in the soil, which neutralisation cannot fix at all. A sick plant is not proof of an acidic or basic soil, and adding lime to neutral soil helps nothing.

The Lake Losing Its Fish

Gurbir lives near an industrial area and tells Ashwin that the fish in the local lake are fewer every day. One likely cause is factory waste being released into the water. If that waste is acidic, it changes the lake the fish live in, and the fish are what shows it first.

Treat It Before It Leaves

The fix is to neutralise the waste by adding basic substances to it before it is released into the lake. The word before is the whole point. Once the acid is in the lake it is mixed through a great volume of water and spread among living things. Treating it at the factory is both easier and kinder.

One Idea, Four Problems

Look at what you have just done with a single idea. A sting on the skin, a field that will not grow, and a lake losing its fish are three completely different problems, and the same reaction answers all three. That is what chapter one meant about an idea travelling further than the chapter that raised it.

Scenario:

Gurbir lives beside a lake near an industrial area, and the fish have been getting fewer each year. Ashwin comes to stay and they test the lake water with litmus. The blue paper turns red near the drain where the factory water enters. It does not change at the far end of the lake. They also test water from two fields beside it. The first field, which has had heavy chemical fertiliser for years, reddens blue litmus. The second field moves neither paper, yet its crop is the thinner of the two. The farmer of the second field is ready to buy lime because his neighbour is using it.

Analysis:

Three different situations and one idea runs through all of them. The lake water is acidic at the drain and neutral at the far end. That points at the factory discharge rather than at the lake as a whole. The remedy is to neutralise the waste with basic substances before release. The first field is acidic from the excessive fertiliser, so lime is the correct treatment there. The second field is the interesting one. Its soil is neutral, so there is no excess acid for lime to neutralise. Buying lime would waste his money and could push the soil towards basic. A thin crop on neutral soil points instead to a deficiency of nutrients.

Activity:

Collect water from two different sources, such as a tap and a puddle, and soil water from two different patches of ground. Test all four with litmus paper and write down the nature of each. For any sample that is not neutral, write one sentence saying what you would add to neutralise it and why.

Title:

Find the turning point

Safety:

Keep all of it away from your eyes and mouth, and wash your hands afterwards. Ask an adult before using lime water.

What You Notice:

The colour does not drift slowly. It holds, then turns over within a drop or two. That is what in sufficient quantity means, and it is why the last drop before the change is the interesting one.

The heat given out in a neutralisation is a real product, which is why the test tube warms slightly.

A farmer query about poor growth on an agriculture portal was traced to his own excessive use of fertilisers.

Declining fish numbers in a lake near an industrial area can be the first visible sign of acidic waste.

SWH for the products: Salt, Water and Heat, in that order after Acid plus Base.

Opposites cure: lime for acidic soil, manure for basic soil, baking soda for an acid sting.