In daily life we use many different kinds of material, and their properties are different from one another. If we taste the various things around us that can be tasted or eaten, some are sour, some sweet, some astringent, some bitter and some salty. Such things come into use for many different jobs: sugar for making tea, salt for cooking vegetables, vinegar for making pickle, soap for washing clothes, baking powder for making roti rise. On the basis of their properties those materials have been classified into different groups.
Generally, things with a sour taste are acids. Things with a bitter or astringent taste are bases, while salts are tasteless, although common salt is salty. That is the whole classification in one sentence, and it works for the kitchen. The trouble starts the moment you step out of the kitchen, and dealing with that trouble is what this first topic is about.
Indicators
It is awkward to separate every material by taste alone. Trying to taste some substances is harmful for the body. To study the properties of such things, indicator substances have to be used. The chemical substance that is used to separate whether some material is an acid, a base or a salt is exactly what an indicator is. Litmus paper, methyl orange and phenolphthalein are examples of common indicators. When an indicator is put into or mixed with other substances, a change comes in its colour, and from that change the substance can be recognised as an acid, a base or a salt.
Being an acid is not something you can see. Lemon juice and water look exactly alike in a glass, and so do soap solution and salt solution. The difference between them is a chemical property, and chemical properties are invisible. An indicator is a substance that turns that invisible property into a visible one. It takes a question your eyes cannot answer and rewrites it as a colour, which your eyes can answer instantly. That is why chemists went to the trouble of finding them, and it is why the same idea keeps returning throughout science: when you cannot measure a thing directly, find something that changes when it changes, and measure that instead.
The standard table of indicator colours
| Indicator | In an acid | In a base | In a salt |
|---|---|---|---|
| Red litmus | Red | Blue | Neutral |
| Blue litmus | Red | Blue | Neutral |
| Methyl orange | Red | Yellow | Neutral |
| Phenolphthalein | Colourless | Pink | Neutral |
Every row of the last column says neutral, and students often read it as though neutral were a fourth colour that indicators turn in salt solutions. It is not. Neutral means that nothing happened. Dip blue litmus in salt water and it stays blue; dip red litmus and it stays red; add phenolphthalein and it stays colourless. Nothing changed, and it is precisely that absence of a change which tells you the substance is a salt. This is the one entry in the table that is easy to get wrong in an examination, because it is the only one where the correct answer is no answer.
Look carefully at the first two rows of the table. Red litmus and blue litmus give exactly the same three answers: red in acid, blue in base, no change in salt. That looks like a mistake, and it is not. Litmus ends up red in any acid and blue in any base no matter which colour of paper you started with, because the acid or the base decides the colour, not the paper. So why sell two kinds at all? Because the useful information is not the final colour, it is whether a change happened. Suppose you have only blue litmus and it comes out blue. Was the substance a base, which turned it blue, or a salt, which left it alone? You cannot tell. Now use red litmus as well. If the red one turns blue, it is a base. If the red one stays red and the blue one stays blue, nothing changed either way and it is a salt. Two papers are needed because one paper can only ever answer half the question.
Read the phenolphthalein row again. In an acid it is colourless. In a salt nothing happens, so it is also colourless. Those two are the same thing to look at, which means phenolphthalein cannot tell an acid from a salt at all. It has exactly one talent: it turns bright pink in a base, and it does nothing anywhere else. That makes it useless as a general sorting tool and superb as a base detector, which is exactly the job it is given in laboratories. Methyl orange has the opposite habit and gives you three genuinely different colours, red, yellow and unchanged orange, so it can sort all three groups on its own. Knowing which indicator answers which question is more useful than memorising all twelve boxes of the table.
Making indicators from local materials
Indicators can be prepared easily by taking the juice out of the coloured parts of various plants that are found locally, such as a red rose flower, the leaf of red cabbage, pieces of onion, turmeric powder and beetroot. Lichen is used to prepare litmus itself.
The book asks why a piece of clothing stained with turmeric goes red when it is washed with soap, and why a mixture of ash and turmeric rubbed on a skinned hide turns deep red. Both questions have the same answer, and once you have it you will notice it happening in kitchens all your life. Turmeric is a natural indicator. It is yellow in acids and in neutral things, and it turns bright red in a base. Soap is a base, and so is wood ash. So the moment soap touches the turmeric stain, the stain reports what it has met by turning red, and the red patch is not a new stain at all but the old one holding up a sign. Rinse the soap away thoroughly and the yellow comes back, which is a small proof that no new substance was made. Try it deliberately: put a drop of turmeric water on white cloth, dry it, and then touch one corner with soap and another with lemon juice.
Litmus itself is not made in a factory. It comes from lichen, the flat crusty growth you have seen clinging to old walls and to the bark of trees in damp places, which is a fungus and an alga living together as one. The colour is squeezed out of the lichen and soaked into paper. So the neat strip in the laboratory box began life on a rock, and there is no difference in principle between it and the onion juice you are about to boil in Activity 10.2.
Sorting things you already own
Here is what the litmus test says about the things listed in Activity 10.3, together with the group each one belongs to. Work down the table and notice how badly guessing by taste would have done on the last two rows.
| Substance | Red litmus | Blue litmus | Which group |
|---|---|---|---|
| Lemon juice | No change | Turned red | Acid |
| Curd | No change | Turned red | Acid |
| Tomato juice | No change | Turned red | Acid |
| Bitter gourd juice | Turned blue | No change | Base |
| Soap water | Turned blue | No change | Base |
| Water | No change | No change | Neutral |
| Common salt solution | No change | No change | Salt |
On the basis of that observation the conclusion is written like this. Substances that change blue litmus paper to red are acids. Substances that turn red litmus paper blue are bases. And substances that cannot bring any change at all to the colour of litmus paper are salts.
It is worth saying plainly what this whole topic is quietly built on. Taste is an indicator too. It changes when a chemical property changes, which is exactly the definition, and it is the one the human race used for thousands of years before anything better existed. The reason it has been retired is not that it fails, but that it fails dangerously. Sulphuric acid is sour in the same way that lemon juice is sour, and a drop of it on the tongue would do damage that no amount of being right about the answer could repair. Sodium hydroxide, which the book lists as a base, will burn skin. So the rule in a laboratory is absolute: never taste anything, never smell anything directly, and use the paper. Litmus costs almost nothing and cannot be hurt by the strongest acid in the room, and that is the whole point of it.
Our daily life materials are studied by dividing them into three groups in this way: acid, base and salt, where salt is the neutral one. The next three topics take those groups one at a time, and every one of them leans on what you have just learned, because an indicator is the only tool any of them can be tested with.
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