Elements, Compounds and Mixtures

In everyday language we use the word pure very loosely. A packet of atta says pure wheat, a tin says pure mustard oil, and the milkman promises pure milk. In all of these, pure only means that nothing extra was mixed in on purpose. Science uses the same word in a much tighter way, and once you take on board the scientific meaning, a great deal of chemistry suddenly falls into place.

A pure substance is made of only one kind of particle all the way through. Any sample you take, from the top of the container or the bottom, has exactly the same composition and exactly the same properties. Distilled water, a lump of pure copper, a diamond, a crystal of common salt and the oxygen inside a hospital cylinder are all pure substances in the scientific sense. A pure substance also melts or boils at one sharp, fixed temperature, and it cannot be split into anything simpler by physical methods such as filtering, sieving, settling, picking by hand or holding a magnet near it.

By that stricter test, milk is not pure at all. Milk carries water, fat, protein, sugar and minerals side by side, and you can pull the fat out simply by churning it into butter. Milk is a mixture. So is air, so is sea water, so is the soil in a field, and so is the brass of a temple bell. In a mixture, two or more substances sit together without joining chemically. Each one keeps its own properties, the substances can be present in any proportion you like, and no new substance appears at the moment of mixing.

Three questions will nearly always tell you which one you are holding.

• Is the composition fixed? Sugar is always sugar, wherever it comes from. Sugar solution can be weak or strong depending on how many spoons you stirred in, so it is a mixture.
• Do the parts keep their own properties? In a mixture of sand and salt, the salt still tastes salty and the sand is still gritty. Nothing has changed inside either of them.
• Can a simple physical method separate it? If a filter paper, a magnet, a sieve or a spell of boiling can pull the sample apart, it was a mixture from the start.

There is a fourth clue that older students use a lot. Pure water boils at 100 degrees Celsius at normal atmospheric pressure and it stays at that temperature until all of it has turned to steam. Stir salt into the water and the boiling behaviour is no longer that neat and single valued. A sharp, unchanging melting or boiling point is the fingerprint of a pure substance.

One caution before we go further. Pure does not automatically mean good, and mixture does not mean spoilt or cheap. Air is a mixture and you cannot live without it. Stainless steel is a mixture and for most jobs it is far more useful than iron on its own, because it is harder and it resists rust. On the other hand pure carbon monoxide is a deadly gas. Chemists use these words to describe how matter is put together, never to praise it or condemn it.

Finally, remember that pure substances themselves come in only two varieties: elements and compounds. Every pure substance in the universe is one or the other, and the rest of this chapter unpacks both, then returns to mixtures and how to take them apart.

Matter is built from tiny particles called atoms. An element is a pure substance in which every single atom is of the same kind. A piece of pure gold contains gold atoms and nothing else. A cylinder of pure helium contains helium atoms and nothing else. Because there is only one kind of atom present, an element cannot be broken down into any simpler substance by ordinary chemical methods. That is exactly what makes elements the starting blocks of all other matter.

At present about 118 elements are known to science. A good number of them occur naturally in rocks, air, water and living bodies, while the rest have been made in laboratories in extremely small amounts and are usually very short lived. You do not need to memorise the whole list at this stage. What helps far more is recognising the common ones and understanding the pattern behind their names and symbols.

Writing out full element names in every experiment would be slow and would differ from language to language, so scientists worldwide agreed on short symbols. The rules are simple and worth getting right, because a wrongly written symbol can mean an entirely different substance.

• A symbol is one or two letters taken from the name of the element: H for hydrogen, C for carbon, O for oxygen, N for nitrogen.
• When two letters are used, the first is a capital and the second is always small: Ca for calcium, Mg for magnesium, Zn for zinc, Al for aluminium.
• Some symbols come from older Latin names rather than the English name. Iron is Fe from ferrum, sodium is Na from natrium, potassium is K from kalium, copper is Cu from cuprum, silver is Ag from argentum, gold is Au from aurum and lead is Pb from plumbum.

Notice how much the capital letter matters. Co stands for the metal cobalt, while CO written with two capitals is carbon monoxide, a poisonous gas made of two different elements. A careless capital letter changes a metal into a gas.

Atoms of an element do not always wander about alone. In many elements the atoms hold hands in small groups called molecules. The oxygen we breathe travels as pairs of oxygen atoms, written O2, and nitrogen in the air travels as N2. Both are still elements, because both atoms in each pair are of the same kind. This is an important idea: an element can contain molecules, as long as the molecules are built from one kind of atom only.

Something else is worth noticing. The same element can sometimes appear in strikingly different forms. Diamond and graphite are both made purely of carbon atoms, yet diamond is the hardest natural material known while graphite is soft enough to leave a mark on your notebook when you write with a pencil. The difference lies in how the carbon atoms are arranged, not in what they are. It is a lovely reminder that in chemistry, arrangement matters just as much as ingredients.

Based on their properties, elements are sorted broadly into metals and non-metals, with a small in-between group often called metalloids that show a mix of both behaviours. The next section looks closely at that division, because it explains a large part of why we use the materials we do.

If you lay out samples of many elements on a bench, they sort themselves quite naturally into two families by the way they look and behave. The larger family is the metals, which includes iron, copper, aluminium, zinc, gold, silver, sodium and calcium. The smaller family is the non-metals, which includes oxygen, nitrogen, carbon, sulphur, phosphorus, iodine and the gases we call noble gases such as helium and neon.

Metals share a set of physical properties that make them the backbone of construction, transport, cooking and electrical work.

• Lustre. A freshly cut or freshly polished metal surface shines. This is why silver and gold have been used for ornaments for thousands of years.
• Malleability. Metals can be beaten into thin sheets without cracking. Aluminium foil in the kitchen and the silver varak on sweets both depend on this.
• Ductility. Metals can be drawn out into long thin wires. Copper wiring in your home and the fine gold wire used by jewellers are everyday examples.
• Conduction. Metals let heat and electricity pass through them easily, which is why cooking vessels and electrical cables are made of metal.
• Sonority. Metals produce a ringing sound when struck. Temple bells, plates and thalis all take advantage of this.
• State and hardness. Most metals are solid at room temperature and most of them have fairly high melting points.

Non-metals behave roughly in the opposite way. Most of them are dull rather than shiny, and the solid ones break or crumble into powder when hammered instead of flattening out, so they cannot be drawn into wires either. They are poor conductors of heat and electricity. They exist in all three states at room temperature: oxygen and nitrogen are gases, bromine is a liquid, and sulphur, carbon, phosphorus and iodine are solids.

Now for the exceptions, which are as important as the rules themselves, because they show that these categories are useful generalisations rather than rigid laws.

• Mercury is a metal, yet it is a liquid at room temperature. It is the metal used in older thermometers.
• Bromine is a non-metal, yet it too is a liquid at room temperature. It is the one non-metal in that state under ordinary conditions.
• Graphite, a form of carbon and therefore a non-metal, conducts electricity well. This is why graphite rods are used as electrodes.
• Iodine is a non-metal, yet its crystals have a definite shine.
• Sodium and potassium are metals, yet they are so soft that a knife cuts them, and they are less dense than water, so they stay on the surface while reacting with it vigorously.

Between the two families sit a few elements such as silicon that show a mixture of metallic and non-metallic behaviour. Silicon is the material at the heart of computer chips and solar cells, precisely because it conducts electricity somewhat but not freely like copper, and that in-between behaviour can be controlled.

Understanding this sorting is not just classification for the sake of it. When an engineer chooses aluminium for an aircraft body, copper for wiring, tungsten for a lamp filament or graphite for a pencil lead, the choice comes straight out of the properties listed above. Matching the material to the job is one of the most practical skills chemistry gives you.

A compound is a pure substance formed when atoms of two or more different elements join chemically. The word chemically is doing a lot of work in that sentence. The atoms are not merely sitting next to each other as they do in a mixture; they are bonded together into new particles, and those new particles have their own personality.

Two features define a compound, and both are worth learning carefully.

The first is fixed proportion. Every sample of a given compound contains the same elements combined in the same ratio by mass, no matter where the sample came from or who prepared it. Water from a Himalayan stream, water condensed from steam in a kitchen and water made in a laboratory all contain hydrogen and oxygen in the mass ratio 1 to 8. Two atoms of hydrogen are joined to one atom of oxygen, which is why water is written as H2O. You cannot decide to make a batch of water that is a little heavier on hydrogen, the way you can decide to make your tea a little heavier on sugar. This constancy is known as the law of constant proportion, and it is one of the neatest rules in all of chemistry.

The second is that a compound has completely new properties. The elements that went in do not survive as themselves. Consider common salt, whose chemical name is sodium chloride. Sodium on its own is a soft, silvery metal so reactive that it must be stored under kerosene, and it catches fire when dropped into water. Chlorine on its own is a choking, greenish-yellow poisonous gas. Join them chemically and you get white crystals that you sprinkle on your food and could not survive without. Nothing about salt hints at the two dangerous substances that formed it.

Water tells the same story from the other direction. Hydrogen burns readily and oxygen is the gas that supports burning, yet the compound they form is the substance we throw on a fire to put it out. A compound is genuinely a new substance and not a blend of the old ones.

The classic classroom demonstration of the difference uses iron and sulphur. Mix iron filings with sulphur powder and you have a mixture: you can see the grey and yellow specks separately, a magnet held near the mixture pulls the iron filings out, and the ratio of the two can be whatever you decide to make it. Now heat the same mixture strongly. It glows and forms a dark grey solid called iron sulphide. Test it and everything has changed: a magnet no longer pulls anything out, the yellow and grey specks are gone, and the ratio of iron to sulphur in the product is fixed rather than whatever you chose. A chemical change has happened and a compound has been made.

A few more points tie the idea together. A compound can only be separated back into its elements by chemical means, not by filtering, sieving or picking. Because a compound is a pure substance, it melts at one fixed temperature of its own, and that temperature is nothing like the melting points of the elements inside it. And an enormous number of compounds can be built from a small number of elements, just as thousands of words are built from a handful of letters. Carbon dioxide, cane sugar, common salt, baking soda, washing soda, the glucose your body burns for energy and the calcium carbonate that makes up limestone and marble are all compounds.

Once you know a sample is a mixture, the next useful question is whether it is evenly blended or visibly patchy. That single question splits mixtures into two groups.

A homogeneous mixture has the same composition and appearance throughout. Take a spoonful from anywhere in it and you get the same thing. You cannot see the separate parts even with a good hand lens, and there is no boundary between them. Sugar dissolved in water is the standard example: once it dissolves, the sweetness is spread evenly and you cannot point to where the sugar is. Salt solution, clean air, vinegar in water, soda water and the brass in a bell all behave the same way.

A heterogeneous mixture has parts you can distinguish. Sand mixed with salt, oil floating on water, muddy water, a bowl of mixed dal, chalk powder stirred into water, and soil are all heterogeneous. Different portions of the sample are visibly different, and there are boundaries between the components.

When one substance dissolves fully in another, the result is called a solution. The substance that dissolves is the solute and the one doing the dissolving is the solvent. In salt water, salt is the solute and water is the solvent. Water dissolves so many things that it is often called the universal solvent, though that name is a description of its usefulness and not a claim that it dissolves absolutely everything.

Two more ideas about solutions are worth adding here. A solution that has dissolved as much solute as it can hold at that temperature is described as saturated. Below that limit it is unsaturated. Warming the solvent then does two different things that are easy to confuse. For most solids it raises that limit, so hot water can hold more dissolved sugar than the same amount of cold water can. Separately, warming makes the particles move about faster, so whatever does dissolve disappears more quickly. It is the second effect, the speed, that you notice when a spoon of sugar vanishes in hot tea in seconds.

Not everything that gets stirred into water forms a true solution. It is helpful to compare three cases side by side.

• In a solution such as salt water, the particles are far too small to see, they never settle, and they pass straight through filter paper.
• In a suspension such as chalk powder in water or muddy water, the particles are large enough to see, the mixture looks cloudy, the particles settle to the bottom if left standing, and filter paper traps them.
• In between sit mixtures such as milk and fog, where the particles are too small to settle out but large enough to scatter a beam of light. Shine a torch through a glass of milk in a dark room and you can see the path of the beam, which you cannot do through clear salt water.

One family of homogeneous mixtures deserves a special mention because it is everywhere in Indian homes and industry. Alloys are mixtures of a metal with one or more other metals or non-metals, blended while molten so the result is uniform throughout. Brass is copper mixed with zinc. Bronze is copper mixed with tin. Steel is iron mixed with a small amount of carbon, and stainless steel adds chromium and nickel so that it resists rusting. Alloys are counted as mixtures because the proportions can be varied by the maker and the components are not chemically bonded into a compound. Deliberately varying those proportions is exactly how engineers get the hardness, strength or shine they need.

Air is worth one last look. It is a homogeneous mixture of gases, roughly four fifths nitrogen and one fifth oxygen by volume, with small amounts of argon, carbon dioxide and water vapour. It qualifies as a mixture because the proportions shift from place to place, because each gas keeps its own behaviour inside it, and because the gases can be separated by physical means without any chemical reaction.

Because the parts of a mixture keep their own properties, any property in which they differ can be used to pull them apart. That single sentence is the logic behind every technique below. Learn what difference each method exploits and you will never have to memorise the list blindly.

Handpicking works when the pieces differ in size, colour or shape and are few enough to pick out, such as small stones from rice or dal. Sieving separates particles of different sizes, which is what happens when flour is passed through a sieve to hold back bran and lumps. Winnowing works when one part is far lighter than the other for its size, so a breeze pushes the husk sideways and carries it off while the heavier grain falls almost straight down into a heap.

Magnetic separation works when one component is attracted to a magnet and the other is not. Drawing iron filings out of sand or sawdust is the classroom version; large electromagnets pulling scrap iron out of waste is the industrial one.

Sedimentation and decantation handle an insoluble solid that is denser than the liquid it is mixed with. Let the mixture stand and those particles sink to the bottom as a layer called sediment; then pour off the clear liquid carefully without disturbing that layer. This is the everyday method of dealing with muddy water. Adding a substance such as alum makes fine particles clump together and settle faster, a step called loading.

Filtration separates an insoluble solid from a liquid using a filter that has holes too small for the solid to pass. Filter paper in the laboratory and a tea strainer or a cloth at home do the same job. Remember that filtration cannot remove a dissolved substance, because dissolved particles slip through the pores easily. Filtering salt water gives you salt water.

Evaporation recovers a dissolved solid by driving the liquid off as vapour. Salt is obtained from sea water on a very large scale by letting sunlight evaporate the water from shallow pans and leaving the salt behind. The catch is that the liquid is lost to the air.

Condensation turns a vapour back into a liquid on cooling, and it is the partner step that makes the next method possible.

Distillation combines the two. Boil the mixture so the liquid turns to vapour, then cool that vapour so it condenses and is collected in a separate vessel. Distilling salt water gives you pure water in the receiver and salt left behind in the flask, because salt does not turn into vapour at the temperature at which water boils. When two liquids that mix with each other must be separated, the same idea is used but with more care, since the liquid with the lower boiling point vapourises first and is collected first.

A separating funnel handles two liquids that do not mix, such as oil and water. The mixture settles into two layers, the tap at the bottom is opened, the lower liquid runs out and the tap is closed the moment the boundary reaches it.

Sublimation is used when one component turns directly from solid to vapour on heating without becoming a liquid in between. Camphor, naphthalene and ammonium chloride behave this way, so a mixture of camphor and salt can be separated by warming it gently: the camphor leaves as vapour, turns straight back into solid on a cool surface held above the dish, and is scraped off from there, while the salt stays behind unchanged.

Churning and centrifugation spin a mixture fast so that the denser parts are thrown outwards and the less dense parts are left nearer the centre. Churning curd to get butter is the traditional example, and machines that spin blood or milk at high speed use the same principle.

Chromatography separates substances that dissolve in the same solvent but travel through paper at different speeds. Put a dot of ink near the lower edge of a strip of filter paper and stand the strip in a dish holding a little water, taking care that the dot itself is just above the water line so that it is not simply washed away. Water creeps up through the paper, carries the dyes along with it, and because each dye travels at its own speed the single dot spreads out into separate bands of colour.

Crystallisation gives purer solids than plain evaporation. A hot saturated solution is allowed to cool slowly so the pure substance grows as crystals while the impurities stay behind in the liquid.

Knowing the toolkit is half the job. The other half is deciding which tool to reach for. A short chain of questions handles almost every mixture you will be asked about.

1. What states are the components in? Solid with solid, solid with liquid, liquid with liquid, or a gas in the picture? This alone narrows the choice enormously.
2. Is anything dissolved? If the solid has dissolved, filtration is ruled out immediately and you must think about evaporation, distillation or crystallisation.
3. Do the parts differ in some special property? Magnetic or not, sublimes or not, floats or sinks, one colour or many.
4. Do I need to save both parts, or only one? This is the question students forget, and it is often what decides between two otherwise correct methods.

Try that chain on a few mixtures.

Sand and water. Sand is an insoluble solid in a liquid, so it settles on standing. Decant the clear water and then filter to catch the last of the fine particles. Both parts are recovered.

Salt and water. Here the solid is dissolved, so filtering achieves nothing. If you only want the salt, evaporate the water. If you want the water too, as in getting drinking water from sea water, use distillation.

Common salt, sand and iron filings together. Take the special property first: run a magnet over the dry mixture and the iron filings come away. Then add water and stir, which dissolves the salt but not the sand. Filter to hold back the sand, and evaporate the filtrate to recover the salt. Three components, three steps, each one chosen because of a property difference.

Camphor and common salt. Both are solids and both are white, so size and colour give you nothing. Camphor sublimes and salt does not, so warm the mixture gently and collect the camphor vapour on a cool surface.

Kerosene and water. They do not mix and form two clear layers, so a separating funnel is the fastest and cleanest choice.

Different coloured dyes in a single ink. All of them are dissolved in the same solvent, so nothing will filter out or settle. Chromatography is the method built for exactly this situation.

Two cautions are worth carrying with you. First, no separation method breaks up a compound. If you are handed sugar and asked to separate the carbon from it, no amount of filtering, distilling or sieving will help, because the atoms are chemically bonded. Physical separation only ever works on mixtures. Second, a separation is rarely perfect in one go. Salt obtained by evaporating sea water still carries other dissolved substances, which is why it is purified further by dissolving it again and crystallising it.

It is also worth seeing how much of this already happens around you. A water purifier at home filters out suspended dirt and then treats the water further. Salt pans along the coast use nothing but sunlight and evaporation. Farmers winnow grain in the open air. Milk is separated into cream and skimmed milk by spinning. Petroleum refineries separate crude oil into fuels by boiling and condensing at different temperatures. The same handful of ideas you are learning in this chapter runs a surprisingly large part of the world outside your classroom.