Elements that are soft, that do not shine, that burn when they are heated and that do not conduct heat and electricity are non-metals. Sulphur, iodine and carbon are examples. Read that list again and notice its shape: every item on it is a failure. A non-metal is not defined by something it does, but by all the things it does not do, and the whole of this last topic is built on that oddity.
It is easy to read the list of non-metal properties as a list of shortcomings, as though these were the leftover elements that did not make it into the metal group. They are not. Look at the handle of a pressure cooker: it is covered with hard plastic, and it is there precisely because plastic is a bad conductor of heat. If it were a good conductor it would be useless, and you would need a cloth every time. Look at the covering on an electric wire: it is there precisely because it will not carry electricity. In both cases the useful property is a refusal. So the exercise question about the pressure cooker handle is not really about plastic at all; it is about the fact that a substance which will not do something is exactly as valuable as one which will, and any sensible engineer needs both kinds on the shelf.
Sulphur
Symbol: S. Atomic number: 16. Sulphur is a non-metal that is found in nature in a pure form. It does not dissolve in water. It is generally found naturally in the areas where volcanoes go. In the form of a compound it is found as the sulphides of various metals. Sulphur is also found in onion, in garlic and in mustard oil.
| Physical properties | Uses |
|---|---|
| It is a straw yellow coloured shining solid substance | Used to make sulphuric acid |
| It is a non-metal insoluble in water | Used to make the gunpowder kept in guns |
| It is a bad conductor of heat and electricity | Used to make the paste on the head of a matchstick |
| It does not react with acid | Used to make firecrackers |
| When it burns in air it forms sulphur dioxide | Used to make the medicine applied on wounds |
The book slips in a line that is easy to read past: sulphur is also found in onion, in garlic and in mustard oil. That single line explains three things you have known all your life without ever being told why. Cutting an onion breaks open its cells, and the sulphur containing substances inside are released and drift up into the air. When they reach the wet surface of your eyes they irritate them, and the eyes flood in self defence. Cutting under running water, or chilling the onion first, helps because less of it reaches your face. The smell of garlic on your fingers is also sulphur, and it survives ordinary washing because it is not sitting on the skin but has soaked into it. And the sharp, catching smell of raw mustard oil is the same family of substances again. Chemistry is not only in the laboratory bottle: three quarters of the smells in your kitchen are one element making itself known.
One of the exercise questions asks why a flame of fire is sometimes seen on a hill on a dark night, the thing people have long called raanke bhoot. The answer the book gives, and the one to write in an examination, is that sulphur coming out of a sulphur mine on that hill catches fire on contact with the air. It is worth knowing that flames of this kind have been reported in many countries and that the usual scientific explanation elsewhere is gas seeping out of marshy or decaying ground and igniting, rather than sulphur itself, and different places may genuinely have different causes. But the honest thing to notice is not which explanation is right. It is that people saw a light moving on a hillside at night, and in the absence of any other account they gave it a name and a story. Then somebody went and looked. That is the whole difference between folklore and science, and it is not that one is stupid: both are attempts to explain the same real observation, and only one of them can be checked.
Iodine
Symbol: I. Atomic number: 53. Iodine is a useful non-metal, and it is found to have been in use by people since very early times. It is found in seaweed, in rocks and in various kinds of food. When there is an imbalance of it in the body, thyroid disease occurs.
| Physical properties | Uses |
|---|---|
| Iodine is a lustrous non-metal, violet in colour | The body needs iodine to be safe from goitre |
| It is insoluble in water | It is needed in infancy to be safe from intellectual disability |
| It is a bad conductor of heat and electricity | Used to make tincture of iodine, which heals wounds |
| It sublimes: given heat it changes directly from solid into gas | Mixed into common salt, which is why iodised salt is sold |
Almost everything you know goes solid, then liquid, then gas: ice becomes water and water becomes steam. Iodine does not bother with the middle step. Heat a few crystals gently and they go straight from purple black solid to violet vapour, with no liquid at any point, and the vapour will settle back into solid crystals on a cool surface without ever being a liquid either. That is called sublimation, and dry ice does the same thing. Now ask the question from Unit 9: is this a physical change or a chemical one? Physical, and by both tests. No new substance is formed, because the vapour is still iodine, and the change can be reversed simply by cooling it. Only the spacing of the molecules changed, exactly as it did with ice and steam. So sublimation is not a strange third kind of change; it is an ordinary change of state that happens to miss out a stage.
Look at the packet of salt in your own kitchen and you will find the word iodised on it, and that is not a marketing slogan. The thyroid, a small gland in the neck, needs iodine to make the substance that controls how fast the body works. If there is not enough iodine in the diet, the gland swells in an attempt to catch more, and that swelling is goitre. Much worse than the swelling is what happens to a baby: a child that does not get enough iodine before birth and in infancy can be left with lasting damage to its growth and its learning, and that damage cannot be undone afterwards. The reason this matters so much in Nepal is geography. Iodine is washed out of soil by rain over thousands of years, and it ends up in the sea, so hill and mountain soils hold very little of it and the crops grown there carry very little. That is why iodine is deliberately mixed into common salt, which everybody buys and everybody eats a little of every day. It is one of the cheapest and most successful public health measures ever taken anywhere, and the whole of it rests on one non-metal in a packet.
Metal and non-metal, side by side
| Property | Metal | Non-metal |
|---|---|---|
| Hardness | Hard | Soft, and often brittle |
| Lustre | Shines | Generally does not shine, though iodine does |
| Heat and electricity | Good conductor | Bad conductor, which is an insulator |
| When heated | Gets hot, then melts | Often burns |
| When hammered | Flattens, and can be drawn into wire | Shatters into pieces |
| Sound | Rings when struck | A dull thud |
| Examples | Iron, copper, gold, silver, aluminium, zinc | Sulphur, iodine, carbon |
The table is a good working guide, and like all good working guides it is not perfect, so it is worth knowing where it bends. First, iodine has a lustre. The book says so plainly in its own property list, and a lustre is supposed to be a metal property, so here is a non-metal that shines. It is still firmly a non-metal, because it fails every other test: it does not conduct, it is not hard, and it shatters. Second, carbon appears on the non-metal side, and one form of carbon is graphite, the black stuff in your pencil, which conducts electricity quite well. It is still a non-metal too. The lesson is not that the table is wrong but that nature does not arrange itself into tidy boxes for the convenience of textbooks. A classification is a useful tool, and when a real substance does not fit it neatly, the interesting question is why, not which rule to break.
That is the end of the unit, so it is worth seeing what has actually been done in it. It began with a lemon and a bar of soap and the observation that daily materials have different properties. It then built one tool, the indicator, which turns an invisible property into a visible colour, and used that tool to sort every substance in a kitchen into acid, base and salt. Then it changed the question completely, stopped asking about colour changes and started asking about hardness, shine, conduction and sound, and used those five to sort the elements into metals and non-metals. Two completely different classifications, two completely different sets of tests, and one habit running through all of it: never sort things by what they look like, sort them by what they do.
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