The materials we use such as pickle, lemon, hog plum and pomelo are of a sour taste. Generally the materials that have a sour taste are called acids. The kinds of pickle we eat, and fruits such as apple, orange, lemon and pomelo, are acidic fruits. Because of the acid that is present in foodstuffs, the taste of a great many things is sour.
You now have two completely different ways of finding an acid. Your tongue says sour, and blue litmus paper says red. It is worth stopping to notice that these are not two separate facts about lemon juice; they are two reports of the same underlying thing. Something in the lemon juice is acting on the taste buds of your tongue, and that same something is acting on the colouring matter in the litmus paper, and it is doing the identical job in both places. That is why the two tests never disagree. Every substance that tastes sour turns blue litmus red, and every substance that turns blue litmus red would taste sour if it were safe to try. So we have not swapped one test for a different one. We have swapped a delicate detector that can be destroyed for a cheap one that cannot.
Two kinds of acid, sorted by where they come from
On the basis of their source, acids are of two kinds: organic and inorganic. The acids that are obtained from animals, plants and fruits are organic acids. These acids are mild and are generally considered fit to eat. The acids that are produced from mineral substances are called inorganic acids, and these are also called mineral acids. Hydrochloric acid, sulphuric acid and nitric acid are examples of inorganic acids. These acids must not be eaten.
The word organic is used in two quite different ways, and mixing them up will cost you marks. At the market, organic vegetables means vegetables grown without artificial fertiliser or pesticide. In chemistry it means nothing of the kind. An organic acid is simply one that came from something that was once alive: an animal, a plant or a fruit. That is the whole definition. Vinegar made in a factory is still an organic acid, because it is built on the same kind of substance that living things make. Sulphuric acid dug out of the ground as a mineral is inorganic no matter how naturally it occurred. When the book says organic and inorganic, it is asking one question only, and that question is where did this come from.
Some edible acids and where they are found
| Name of the acid | Source |
|---|---|
| Citric acid | Orange, lemon, tomato |
| Ascorbic acid | Amala, kiwi, sour fruits |
| Tartaric acid | Pomelo, grapes, avocado |
| Malic acid | Apple, cherry, peach |
| Lactic acid | Milk, curd |
| Acetic acid | Vinegar, sour pickle |
| Carbonic acid | Soda water, cold drinks |
| Oxalic acid | Chariamilo, sisnu, spinach |
Ascorbic acid is the one you already know by another name: it is vitamin C, the thing everybody is told to get from citrus fruit and amala. So a vitamin your body cannot do without is, chemically speaking, simply an acid, and the sour taste of an amala is that vitamin announcing itself. Lactic acid is the second. Look back at the last unit, where milk turning into curd was our example of a chemical change. Now you can name the new substance that was made: tiny living things in the starter curd turn the sugar of the milk into lactic acid, the acid makes the milk go sour and set, and there is no way back because a new substance now exists. Two units, one pot of curd, and the second unit has just supplied the missing half of the first.
When ants and bees sting, the burning and the pain at the place they stung are caused by formic acid. The sting of sisnu, the nettle, is also caused by formic acid, whereas when a wasp bites, alkaline substances enter the body.
That small box is doing more work than it looks. A plant and an insect that are not related to each other in any way have both arrived at the same weapon, and it is an acid. Sisnu does not sting because it is rough; each of those fine hairs is a tiny glass needle that snaps off in the skin and injects the acid inside it, which is why brushing past a nettle hurts more than gripping it firmly. What follows from all this is a practical rule, and you will meet it properly in the next topic: an acid sting is calmed by a base, and a base sting is calmed by an acid. Soap on a nettle burn is not an old wives tale, and neither is vinegar on a wasp sting.
Physical properties of acid
| Generally the taste of an acid is sour. |
| It changes blue litmus paper into red. |
| A strong acid burns the skin. |
| An acid changes methyl orange into red. |
| An acid makes phenolphthalein colourless. |
The book says organic acids are mild and inorganic acids must not be eaten, and as a rule of thumb that is a good rule. It is worth knowing what actually lies behind it, because the honest reason is not the origin. Two separate things decide how dangerous an acid is: how strong it is, meaning how fiercely it acts, and how concentrated it is, meaning how much acid there is in how much water. Lemon juice is safe on both counts, because citric acid is a weak acid and the juice is mostly water anyway. Vinegar in the kitchen is safe for the same reason, although the pure acetic acid it is made from would blister skin. And the strongest laboratory acids are dangerous on both counts at once. So the working rule for you at this age is simple and absolute: eat only what has always been eaten as food, and treat everything with a laboratory label as though it will burn, because it will.
Read the list of inorganic acids again. Hydrochloric acid, sulphuric acid, nitric acid, and the book says these must not be eaten. All true, and yet your own stomach is making hydrochloric acid as you read this sentence, and it has been making it every day of your life. It is there to break food down and to kill most of what comes in with it. So how does the stomach survive an acid that would burn skin? Because its inner wall is covered with a thick layer of slime that is replaced constantly, faster than the acid can eat through it. When that layer fails, the acid reaches the wall itself, and the burning pain of gastritis is exactly what that feels like, which is why the medicine for it is a base. Two things follow. First, the difference between food and poison is often where a substance is and how much of it there is, not what it is called. And second, the answer to why do people take antacid tablets is now completely obvious to you.
Points to think about
The book asks two questions here. Why does turmeric turn red when soap water is put into it, and why does a wound sting when vinegar falls on it? You answered the first one in the last topic: turmeric is an indicator and soap is a base. The second one has an answer that everybody has felt and almost nobody has been told. Broken skin exposes the living tissue and the nerve endings underneath, which the outer layer normally protects. Acetic acid in vinegar acts on that exposed tissue directly, and the nerves report it as burning. On unbroken skin the same vinegar does nothing at all, which is the proof: the vinegar has not changed, only what it can reach has changed.
Activity 10.4 asks you to hold moist blue and red litmus paper near the gas that rushes out when a soda water bottle is opened. The blue paper turns red, and at first that seems impossible, because a gas is not a liquid and there is nothing wet coming out of the bottle. Here is what happens. The gas is carbon dioxide, which was forced into the drink under pressure and comes rushing out the moment the pressure is released. Carbon dioxide dissolves in water and makes carbonic acid, which is why the acid table lists soda water and cold drinks. Your litmus paper is damp, so the gas dissolves in the film of water on the paper itself and forms the acid right there, on the surface, where the indicator can find it. Notice the word moist in the instruction; a dry strip would give you almost nothing, and that is not a detail the book explains. Two useful things follow. Fizzy drinks really are acidic, which is why dentists worry about them. And the same reaction happens on a far larger scale in the sky, where carbon dioxide dissolves in rainwater and makes all rain very slightly acidic, everywhere, all the time.
One more everyday question belongs here, although the book raises it later. Why is something sour added when karkalo or pindalu is cooked? Because taro contains sharp needle shaped crystals of a substance related to oxalic acid, and those needles are what makes the throat itch and scratch after eating it badly prepared. Adding lemon, tamarind or amilo while it cooks helps break those crystals down, and the itching goes with them. It is one of the oldest pieces of kitchen chemistry in Nepal, worked out by cooks long before anybody could have explained it, and it is a good reminder that a great deal of what a grandmother knows is chemistry that has not yet been written down.
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