Chapter 2 — ଆମ ଚାରିପାଖରେ ଥିବା ପଦାର୍ଥ ବିଶୁଦ୍ଧ କି ?

In everyday speech pure means unadulterated: pure milk, pure honey, pure ghee. In chemistry the word is stricter. A pure substance consists of a single kind of particle throughout, and so has a fixed composition and fixed properties — a definite melting point, boiling point and density. Sugar, common salt, distilled water, iron, oxygen and gold are pure substances. Milk, however pure in the dairy sense, is a mixture of water, fat, proteins, sugar and minerals; so is honey; so is the air we breathe, sea water, soil, petrol and a cup of tea.

A mixture contains two or more pure substances (elements or compounds) mixed together in any proportion, and its components keep their own properties; no new substance is formed and the components can be separated by physical means. Sugar dissolved in water is a mixture: the water still freezes and the sugar still tastes sweet, and boiling the water off returns the sugar unchanged. The proportion is variable — a little sugar or a lot — and the mixture has no fixed melting or boiling point.

Pure substances are of two kinds. An element is a substance that cannot be broken down into simpler substances by any chemical means; it is made of only one kind of atom — hydrogen, oxygen, carbon, iron, copper, gold, sulphur. About 118 elements are known, of which about 92 occur naturally. A compound is a substance made of two or more elements chemically combined in a fixed proportion by mass; water (hydrogen and oxygen in the ratio 1 to 8 by mass), common salt (sodium and chlorine), carbon dioxide, sugar and ammonia are compounds. A compound has properties entirely different from its elements — water puts out fire though hydrogen burns and oxygen supports burning — and its components can be separated only by chemical means.

So the matter around us is classified as follows: matter is either a pure substance or a mixture; a pure substance is an element or a compound; a mixture is homogeneous or heterogeneous. This scheme is the backbone of the chapter and should be drawn as a tree. The chapter's question — is the matter around us pure? — is answered no: almost everything is a mixture, and the chemist's task is first to recognise mixtures and then to separate them into pure substances, which is the work of the later topics.

A caution about a word that changes meaning: substance in chemistry means a pure substance; a chemist does not call milk a substance but a mixture.

Mixtures are of two types according to whether their composition is uniform.

A homogeneous mixture has a uniform composition throughout: every part of it is the same as every other part, and the components cannot be seen separately even under a microscope. Salt or sugar dissolved in water, copper sulphate solution, alcohol in water, air (a mixture of nitrogen, oxygen, argon and carbon dioxide), soft drinks (carbon dioxide dissolved in flavoured water), vinegar (acetic acid in water), brass (copper and zinc) and steel (iron with carbon) are homogeneous. Such mixtures are also called solutions, and they may be liquid (sugar in water), gaseous (air) or solid (alloys). A homogeneous mixture has a single phase — one cannot draw a boundary between components. Note that a homogeneous mixture can still have a variable proportion: two sugar solutions may differ in sweetness, but each is uniform within itself.

A heterogeneous mixture has a non-uniform composition: different parts have different composition, and the components are visibly separate or can be seen under a microscope, with boundaries between them. Sand in water, oil in water, a mixture of salt and sulphur, iron filings and sand, soil, a chana-and-rice mixture, muddy water, smoke in air, and granite (visible grains of different minerals) are heterogeneous. A heterogeneous mixture has two or more phases.

How does one decide? Take a spoon of sugar and dissolve it in a glass of water: the sugar disappears, every drop tastes equally sweet, and no particles are seen — homogeneous. Take a spoon of sand and stir it into water: the sand stays visible, settles at the bottom, and the water at the top is different from the layer at the bottom — heterogeneous. Take oil and water: two layers form — heterogeneous. Take milk: it looks uniform to the eye, but under a microscope droplets of fat are seen floating in water, so milk is strictly heterogeneous, though it is usually treated as a colloid, a category described later.

The distinction matters for separation. The components of a heterogeneous mixture can often be separated by simple physical methods — picking, sieving, filtering, using a magnet, letting the mixture settle — because the components are in separate phases. The components of a homogeneous mixture need methods such as evaporation, distillation or chromatography that use differences in boiling point, solubility or adsorption, because filtration cannot separate what has dissolved.

In the examination the student should be ready to classify a list of mixtures and to give reasons: uniform composition and no visible boundaries means homogeneous; non-uniform composition and visible boundaries or layers means heterogeneous.

A solution is a homogeneous mixture of two or more substances. Lemonade, soda water, sugar syrup, salt water, tincture of iodine and air are solutions. The component of a solution present in the larger amount, which dissolves the other, is the solvent; the component present in the smaller amount, which is dissolved, is the solute. In sugar solution, water is the solvent and sugar the solute; in tincture of iodine, alcohol is the solvent and iodine the solute; in soda water, water is the solvent and carbon dioxide gas the solute. Water dissolves so many substances that it is called the universal solvent, but alcohol, kerosene, petrol and acetone are also common solvents.

Solutions need not be liquid. In a gaseous solution such as air, nitrogen is the solvent and oxygen and other gases are solutes. In a solid solution — an alloy — the metal in larger amount is the solvent: in brass (about 70 percent copper, 30 percent zinc) copper is the solvent and zinc the solute. An alloy is a homogeneous mixture of metals, or of a metal and a non-metal, that cannot be separated into its components by physical methods but is still a mixture because its composition can vary and its components keep their properties; steel, brass, bronze and solder are alloys.

A solution has these properties, and the examination often asks for them. 1. It is a homogeneous mixture. 2. Its particles are extremely small — less than 1 nanometre (10⁻⁹ m) in diameter — so they cannot be seen even with a microscope; they are individual molecules or ions. 3. Because the particles are so small, they do not scatter a beam of light; the path of light through a solution is invisible, and a solution does not show the Tyndall effect. 4. The solute particles do not settle when the solution is left undisturbed; a solution is stable. 5. The solute particles cannot be separated by filtration; they pass through filter paper with the solvent.

Compare this with the sand-in-water mixture of the previous topic: there the particles are visible, settle on standing, can be filtered and scatter light. A true solution is at the opposite extreme, and between the two lies the colloid, treated later.

Finally a word on the language of mixing. When a solute dissolves it does so because its particles are pulled apart by the solvent particles and spread into the spaces among them — the particle picture of the previous chapter. Stirring, heating and powdering the solute speed up dissolving because they bring more solvent particles into contact with the solute more quickly, but they do not change how much can dissolve; that is the subject of the next topic.

Two sugar solutions can be equally homogeneous yet one may be much sweeter than the other. The concentration of a solution is the amount of solute present in a given amount of solution or of solvent. A solution with a small amount of solute is dilute; one with a large amount is concentrated. These words are relative; for exact work concentration is expressed as a number in one of the following ways.

Mass by mass percentage = (mass of solute ÷ mass of solution) × 100. Here the mass of the solution is the mass of solute plus the mass of solvent. This is the most common form at this level.

Mass by volume percentage = (mass of solute ÷ volume of solution) × 100. It is used when the solute is a solid and the solution is measured by volume, as in medicines: a 5 percent (mass by volume) glucose solution contains 5 g of glucose in 100 mL of solution.

Volume by volume percentage = (volume of solute ÷ volume of solution) × 100, used when both are liquids, as in the alcohol content of a spirit.

Worked example 1. A solution contains 40 g of common salt in 320 g of water. Find its concentration in mass by mass percentage. Mass of solute = 40 g; mass of solvent = 320 g; mass of solution = 40 + 320 = 360 g. Concentration = (40 ÷ 360) × 100 = 11.1 percent. Note the trap: the denominator is the mass of solution, 360 g, not the mass of water.

Worked example 2. How much sugar and water are needed to prepare 500 g of a 20 percent sugar solution? Mass of sugar = 20 percent of 500 = 100 g; mass of water = 500 − 100 = 400 g.

Worked example 3. 15 g of a solute is dissolved in 100 mL of solution. Mass by volume concentration = (15 ÷ 100) × 100 = 15 percent (mass by volume).

Worked example 4. A solution is made by dissolving 5.6 g of potassium chloride in 44.4 g of water. Mass of solution = 50 g; concentration = (5.6 ÷ 50) × 100 = 11.2 percent.

Two practical notes. First, ppm (parts per million) is used for very dilute solutions, such as fluoride in drinking water: 1 ppm means 1 g of solute in a million grams of solution, roughly 1 mg per litre. Second, in a numerical problem always write the three lines — mass of solute, mass of solution, formula with numbers substituted — before the answer; the examiner awards marks for each step.

Keep adding salt to a glass of water, stirring after each spoon. At first every spoonful dissolves; after a while a spoonful no longer dissolves however long you stir, and the undissolved salt lies at the bottom. The solution has become saturated: it contains the maximum amount of solute that can be dissolved in that amount of solvent at that temperature. Before that point it was unsaturated: it could dissolve more solute. Now warm the saturated solution: the undissolved salt dissolves, and more can be added, because the solution is unsaturated at the higher temperature. Cool it again and the excess solute separates out as crystals. Saturation therefore depends on temperature, and a saturated solution must always be described as saturated at a stated temperature.

The solubility of a substance is the maximum amount of it that can be dissolved in 100 g of a solvent at a given temperature to form a saturated solution. Solubility is a characteristic property of the solute-solvent pair: at 20 degrees Celsius about 36 g of sodium chloride, 32 g of potassium nitrate, 21 g of copper sulphate and 34 g of potassium chloride dissolve in 100 g of water; at 60 degrees Celsius the figures are about 37, 106, 40 and 46 g. For most solids solubility increases with temperature, sharply for potassium nitrate and hardly at all for sodium chloride. A few substances, such as calcium hydroxide (lime), become less soluble on heating. For gases the rule is reversed: the solubility of a gas in a liquid decreases with rising temperature — which is why warm soda goes flat and why fish in warm water gasp for oxygen — and increases with pressure, which is why carbon dioxide is forced into soft drinks under pressure and fizzes out when the bottle is opened.

Worked example. At 20 degrees Celsius, 36 g of salt dissolves in 100 g of water to give a saturated solution. What is the concentration of this saturated solution? Mass of solution = 136 g; concentration = (36 ÷ 136) × 100 = 26.5 percent. If 25 g of salt is stirred into 100 g of water at 20 degrees Celsius, the solution is unsaturated, since 25 is less than 36; 11 g more can be dissolved.

A solution that holds more solute than a saturated one at that temperature — made by cooling a hot saturated solution very carefully — is called supersaturated; it is unstable, and a single crystal dropped in makes the excess crystallise at once. This is the principle behind crystallisation as a method of purification, which appears later in the chapter, and behind the traditional making of sugar candy (misri) by slowly cooling a hot concentrated sugar solution.

A solubility curve, a graph of solubility against temperature, lets one read the solubility at any temperature and predict how much solute will crystallise out when a hot solution cools.

Stir a spoon of chalk powder, or of fine sand, or of flour, into a glass of water. The water turns cloudy; the particles remain visible to the naked eye; and if the glass is left standing for a few minutes the particles sink to the bottom and the water above clears. This is a suspension: a heterogeneous mixture in which the solute-like particles do not dissolve but remain suspended throughout the bulk of the liquid, and are large enough to be seen.

The properties of a suspension follow from the size of its particles, which is greater than about 100 nanometres (10⁻⁷ m) and often large enough to be seen without a microscope. 1. A suspension is a heterogeneous mixture. 2. The particles are visible to the naked eye. 3. The particles scatter a beam of light, so the path of light through a suspension is visible — a suspension shows the Tyndall effect. 4. A suspension is unstable: when left undisturbed the particles settle down under gravity, and once they have settled the mixture no longer scatters light. 5. The particles can be separated by filtration: they are held back by filter paper while the clear liquid passes through.

Examples are everywhere. Muddy river water in the monsoon is a suspension of soil in water, which is why it clears when left in a bucket and why municipal water plants use settling tanks before filtration. Chalk in water, sand in water, flour in water, and dust in air are suspensions. Many medicines — cough syrups, antacid suspensions, certain antibiotics for children — are suspensions, and the label says shake well before use precisely because the solid settles on standing. Paints are suspensions of pigment in a liquid that must be stirred before painting.

The contrast with a true solution is sharp: a solution has particles smaller than 1 nm, invisible, non-settling, non-filterable and showing no Tyndall effect; a suspension has particles larger than 100 nm, visible, settling, filterable and showing the Tyndall effect. Between these two sizes lies a third category, the colloid, whose particles are big enough to scatter light but too small to settle or be filtered; it is the subject of the next topic. The three categories together are best remembered as a table of five properties, which the following topic completes.

One further point that examiners ask: why does a suspension become clear when it settles but a colloid does not? Because the particles of a suspension are heavy enough for gravity to pull them down through the liquid, while colloidal particles are so small that the constant bombardment of solvent particles — Brownian motion — keeps them from settling.

Milk looks like a solution — it is uniform, nothing settles, it passes through a cloth — yet under a microscope it is seen to contain tiny droplets of fat floating in water, and a beam of light through diluted milk is visible. Milk is a colloid (or colloidal solution): a heterogeneous mixture whose particles are intermediate in size between those of a true solution and a suspension — between 1 and 100 nanometres — so that they are too small to be seen or filtered but large enough to scatter light.

The properties of a colloid: 1. It is heterogeneous, though it appears homogeneous. 2. The particles cannot be seen with the naked eye. 3. The particles are large enough to scatter a beam of light, so a colloid shows the Tyndall effect. 4. It is quite stable: the particles do not settle on standing, because they are kept moving by collisions with the solvent particles. 5. The particles pass through ordinary filter paper, so a colloid cannot be separated by filtration, but it can be separated by centrifugation.

The Tyndall effect is the scattering of light by colloidal particles, which makes the path of the beam visible. It is seen when sunlight enters a dark room through a small hole and the beam shows up as a bright path because of dust and smoke particles in the air; when the headlights of a vehicle show as cones in fog; when sunbeams stream through the gaps in a forest canopy, made visible by mist; and when a torch shines through a glass of milk-and-water. A true solution does not show it because its particles are too small to scatter light; the effect was described by the physicist John Tyndall.

The particles of a colloid are called the dispersed phase and the medium in which they are spread is the dispersion medium. Colloids are classified by the states of these two.

Colloids are important in life and industry: blood, the protoplasm of cells, milk, ink, paint, gum, starch solution and most foods are colloids; the blue of the sky is the scattering of sunlight by fine particles and molecules of the air; and the cleansing action of soap depends on forming an emulsion of oil in water.

The components of a mixture keep their own properties, and every method of separation exploits a difference in some property — size, density, solubility, boiling point, volatility, magnetism or adsorption. Simple heterogeneous mixtures are separated by hand-picking (stones from rice), winnowing (chaff from grain, by density in a stream of air), sieving (bran from flour, by particle size), a magnet (iron filings from sand), sedimentation and decantation (mud from water), and filtration (sand from water). The chapter concentrates on the methods needed for finer mixtures.

Evaporation separates a dissolved non-volatile solid from a liquid, or recovers the solid. Take a china dish with a few millilitres of ink or of salt solution, place it over a beaker of water and heat the beaker so that the water boils and its steam warms the dish; the liquid in the dish evaporates, and a residue of dye or salt is left. This is a water bath, used so that the solid does not char. The method shows that ink is a mixture of a dye in water, and it is how common salt is obtained from sea water in the salt pans of Ganjam and Balasore, where sea water is spread in shallow beds and the Sun does the evaporating. Evaporation cannot recover the liquid; for that distillation is needed.

Centrifugation separates the components of a colloid or a fine suspension whose particles are too small to be filtered. The mixture is spun at high speed in a centrifuge; the heavier particles are thrown outward and settle at the bottom of the tube while the lighter liquid stays above. The principle is that denser particles are forced to the bottom and lighter ones stay at the top when rotated rapidly. It is used to separate cream (fat) from milk in dairies, to separate blood cells from plasma in pathology laboratories, to dry clothes in a washing machine's spin drier, and to separate the sediment from a urine sample for microscopic examination. The traditional churning of curd to get butter is a slow domestic centrifugation.

The separating funnel separates two immiscible liquids — liquids that do not mix and form two layers, such as oil and water, or kerosene and water. The mixture is poured into a pear-shaped glass funnel with a stopcock at the bottom and left to stand until the two layers separate, the denser liquid at the bottom. The stopcock is opened and the lower layer is run out into a beaker; it is closed just as the boundary reaches the stopcock, leaving the upper layer in the funnel. The principle is that immiscible liquids separate into layers according to their densities. It is used to separate oil from water and, in the extraction of iron, to run slag off from molten iron in the blast furnace. Miscible liquids such as alcohol and water form no layers and need distillation instead.

Sublimation separates a mixture in which one component sublimes — changes directly from solid to vapour on heating — while the other does not. Ammonium chloride, camphor, naphthalene, anthracene and iodine sublime; common salt, sand and most other solids do not. To separate ammonium chloride from common salt, the mixture is placed in a china dish and covered with an inverted funnel whose stem is plugged with cotton wool; on gentle heating the ammonium chloride sublimes, rises as vapour, and solidifies as pure white crystals on the cool inner wall of the funnel, while the salt remains in the dish. The cotton plug prevents the vapour escaping. The principle is the difference in volatility: one solid sublimes and the other does not.

Chromatography separates the components of a mixture that are present in very small amounts and are all dissolved in the same solvent — the different dyes in an ink, the pigments in a leaf, the drugs in blood, the colours in a sweet. The word comes from the Greek chroma, colour, because the technique was first used to separate coloured substances, though it now separates colourless ones too. The simplest form is paper chromatography.

Take a strip of filter paper and draw a pencil line about 3 cm from one end; put a small drop of black ink on the centre of the line and let it dry. Hang the strip in a jar containing a little water so that the bottom edge of the paper dips into the water but the ink spot stays above it. Water rises up the paper by capillary action, and as it passes the spot it carries the dye particles along. The ink is a mixture of dyes; the dye that is more soluble in water and less strongly adsorbed on the paper travels faster and farther, and the less soluble, more strongly adsorbed dye lags behind. After some time the single black spot has separated into two or more coloured spots at different heights. The paper with the separated spots is called a chromatogram. The stationary paper is the stationary phase and the moving solvent the mobile phase; the principle is the difference in the solubility of the components in the solvent and in their adsorption on the paper.

Chromatography is used to separate the colours in a dye, the pigments (chlorophyll a, chlorophyll b, carotene, xanthophyll) from natural colours such as a leaf extract, drugs from blood in forensic and sports testing, and the components of a complex mixture in research laboratories. Its advantages are that it works on tiny amounts, needs no heat, and separates substances so similar that no other method would. Higher classes meet thin-layer, column and gas chromatography, all on the same principle.

A question the examination likes: why is the starting line drawn in pencil and not in pen? Because pen ink would itself dissolve and run up the paper, confusing the result; graphite does not dissolve.

Evaporation separates a solid from a liquid but loses the liquid; when the liquid itself is wanted pure, or when two miscible liquids must be separated, we use distillation: the liquid is boiled and its vapour is condensed back to liquid in a separate vessel. The principle is the difference in boiling points of the components.

Simple distillation separates a liquid from a dissolved non-volatile solid, or two miscible liquids whose boiling points differ by a large amount (more than about 25 degrees Celsius) and which boil without decomposing. The apparatus is a distillation flask fitted with a thermometer at the neck, connected through a water-cooled condenser to a receiving flask. To separate a mixture of acetone (boiling point 56 degrees Celsius) and water (100 degrees Celsius), the mixture is heated gently; the acetone, being more volatile, boils first, its vapour passes into the condenser, is cooled by the cold water flowing through the outer jacket, and drips as pure acetone into the receiver; the thermometer stays at 56 degrees Celsius while this happens. Only when all the acetone has gone does the temperature rise, and the water is left in the flask. Distilled water used in laboratories, batteries and injections is made by distilling ordinary water, leaving all dissolved salts behind. Note that the cooling water enters the condenser at the lower end nearest the receiver and leaves at the upper end, so that it flows against the vapour and cools most efficiently.

Fractional distillation is needed when the boiling points of the miscible liquids are close together (less than 25 degrees Celsius apart), such as ethanol (78 degrees Celsius) and water (100 degrees Celsius), because the vapour of the more volatile liquid would carry some of the other with it. The apparatus is the same except that a fractionating column — a long vertical tube packed with glass beads — is fitted between the flask and the condenser. The beads provide a large surface on which the rising vapour condenses and re-evaporates many times; at each step the vapour becomes richer in the more volatile component, and the less volatile one runs back into the flask. The vapour reaching the top and passing to the condenser is nearly pure ethanol. Fractional distillation is used to separate the components of petroleum in a refinery — petrol, kerosene, diesel, lubricating oil — and to separate the gases of air, described in the next topic.

Distillation is thus the universal method for purifying liquids and for separating mixtures of liquids; its limits are that the components must not decompose on heating and must have different boiling points. Where the boiling points are far apart, simple distillation suffices; where they are close, a fractionating column is added. The examination frequently asks for a labelled diagram of one or both, and for the reasoning behind choosing one over the other.

Crystallisation is a method of purifying a solid by obtaining it as pure crystals from its solution. Take about 5 g of impure copper sulphate in a china dish, dissolve it in the minimum amount of water, and filter to remove the insoluble impurities. Heat the filtrate to evaporate part of the water until a saturated solution is obtained — a drop that crystallises on a cold glass rod shows the point has been reached. Cover the dish with filter paper and leave it to cool slowly, undisturbed. As the solution cools, the solubility of copper sulphate falls and it separates as large, pure blue crystals, while the small amount of soluble impurity stays dissolved in the remaining liquid, called the mother liquor. The crystals are filtered off and dried.

Why is crystallisation better than simple evaporation? Because on evaporating to dryness some solids decompose or char on heating — sugar, for instance — and because the impurities that are soluble remain in the residue and contaminate it; on cooling a saturated solution, only the main substance crystallises out and the impurities stay in the mother liquor. Crystallisation is used to purify salt obtained from sea water, to obtain pure crystals of alum, potassium nitrate and sugar, and in the manufacture of most crystalline chemicals and drugs.

Separation of the gases of air. Air is a homogeneous mixture, and its components — nitrogen (78 percent), oxygen (21 percent), argon (0.9 percent) and carbon dioxide (0.04 percent) — are all gases with very low but different boiling points. They are separated by fractional distillation of liquid air. First the air is filtered to remove dust, and carbon dioxide and water vapour are removed (they would freeze and block the pipes). The air is then compressed to a high pressure and cooled; when it is allowed to expand suddenly it cools further, and by repeating this it is liquefied at about minus 200 degrees Celsius. The liquid air is allowed to warm slowly in a fractional distillation column. The component with the lowest boiling point boils off first: nitrogen at minus 196 degrees Celsius, then argon at minus 186, and oxygen last at minus 183 degrees Celsius. Each is collected separately at a different height in the column. The oxygen is compressed into cylinders for hospitals and steel plants, nitrogen for fertiliser manufacture and food packaging, argon for welding and electric bulbs. The medical oxygen supplied from the steel plants of Rourkela and the aluminium smelters of Odisha during the 2021 pandemic came from exactly such air-separation units.

The flow chart that the examination asks for is: air → filter (remove dust) → remove CO₂ and water vapour → compress and cool to liquefy → fractional distillation → nitrogen, argon, oxygen collected in order of increasing boiling point.

Every substance has two kinds of properties. Physical properties — colour, hardness, rigidity, fluidity, density, melting point, boiling point, solubility, conductivity — can be observed or measured without changing the identity of the substance. Chemical properties describe how a substance reacts to form new substances — whether it burns, rusts, reacts with acids.

A physical change is a change in the physical properties of a substance in which no new substance is formed; the chemical composition stays the same and the change is usually reversible. Melting of ice, boiling of water, condensation of steam, sublimation of camphor, dissolving sugar in water, magnetising an iron rod, breaking a glass, cutting paper, stretching a rubber band and the glowing of a bulb filament are physical changes. Ice, water and steam are all H₂O; the sugar can be recovered from the solution; the broken glass is still glass. All the changes of state in Chapter 1 and all the separations in this chapter are physical.

A chemical change (or chemical reaction) produces one or more new substances with different properties, and it is usually not reversible by simple physical means. Burning of paper, wood or LPG, rusting of iron, souring of milk, ripening of a fruit, digestion of food, cooking of rice, the setting of cement, and the explosion of a firecracker are chemical changes. Burning paper gives carbon dioxide, water vapour and ash, and no cooling brings the paper back. Signs that a chemical change may have happened are a change of colour, evolution of a gas, formation of a precipitate, a change of temperature, or the emission of light or sound — though a single sign is not proof, since dissolving some salts also warms or cools water.

The energy aspect deserves a note. Physical changes usually involve small amounts of energy — the latent heat of melting or boiling, for instance — while chemical changes often involve large amounts: burning releases much heat, and a chemical change may absorb or release energy as heat, light or electricity.

A table for revision:

A pair of contrasts fixes the idea: the melting of wax is physical, but the burning of the candle is chemical, and both happen at once in a burning candle; the dissolving of salt in water is physical, but the dissolving of a zinc granule in acid, which gives off hydrogen, is chemical. The student should be able to classify a list of changes and to justify each answer by asking one question — was a new substance formed?

The clearest way to understand the difference between a mixture and a compound is an experiment that every Class 9 student should be able to describe.

Take about 7 g of iron filings and 4 g of sulphur powder. Divide the mixture into two halves. Leave one half as it is — grind it in a mortar to mix it well: this is the mixture of iron and sulphur, grey-yellow in colour. Heat the other half strongly in a china dish or boiling tube until it glows red and then cool it: the iron and sulphur have combined to form a black brittle solid, iron sulphide (FeS) — a compound. Now test both samples.

Test 1: appearance. In the mixture the grey iron particles and yellow sulphur particles can be distinguished with a lens. The compound is uniformly black. Test 2: magnet. A magnet moved over the mixture pulls out the iron filings, leaving the sulphur. The magnet has no effect on the compound; the iron has lost its magnetic property. Test 3: carbon disulphide. Shaking the mixture with carbon disulphide dissolves the sulphur, and filtering leaves the iron; the compound does not dissolve at all. Test 4: dilute sulphuric acid. The mixture gives off hydrogen, a colourless, odourless gas that burns with a pop sound, because the iron reacts with the acid. The compound gives off hydrogen sulphide, a colourless gas with the smell of rotten eggs. A new substance with new properties has been formed by heating.

From this experiment the differences follow.

The ratio 7 to 4 by mass in the experiment is chosen because iron (atomic mass 56) and sulphur (atomic mass 32) combine one atom to one atom in FeS, and 56 to 32 simplifies to 7 to 4; if extra sulphur is used, the excess simply remains as unreacted sulphur — a compound never takes more than its fixed proportion. This fixed proportion is the law of constant proportions, which the next chapter states formally.

The same distinction can be made with other pairs: hydrogen and oxygen gases mixed in a jar are a mixture that can be separated, but once a spark makes them combine they form water, whose properties — a liquid that puts out fire — resemble neither gas. Air is a mixture (its composition varies from place to place, its gases are separated by physical fractional distillation, and its properties are those of its gases), while carbon dioxide is a compound. The examination often asks: give three reasons why air is a mixture and not a compound; the table above supplies them.

The pure substances of which everything is finally made are the elements — substances that cannot be broken into simpler substances by any chemical reaction. Robert Boyle first used the word element in this sense in 1661, and Antoine Lavoisier, the French chemist, defined it as the basic form of matter that cannot be broken down by chemical reactions. About 118 elements are known today; about 92 occur in nature and the rest have been made in laboratories. At room temperature most elements are solids, eleven are gases (hydrogen, oxygen, nitrogen, chlorine, fluorine and the noble gases helium, neon, argon, krypton, xenon and radon), and two are liquids (mercury and bromine); gallium and caesium melt on a warm day, at about 30 degrees Celsius.

Elements are classified by their properties into metals, non-metals and metalloids.

Metals — iron, copper, aluminium, gold, silver, zinc, sodium, magnesium, mercury — have these properties: they have a lustre (shine) when freshly cut or polished; they are usually silvery-grey or golden-yellow in colour; they are good conductors of heat and electricity; they are malleable — can be hammered into thin sheets, as gold leaf or aluminium foil; they are ductile — can be drawn into wires, as copper wire; they are sonorous — ring when struck, which is why bells are made of metal; they are generally hard, dense and have high melting points; and they are solids at room temperature, mercury being the one liquid metal. Sodium and potassium are soft enough to cut with a knife; these exceptions are noted in later chapters.

Non-metals — hydrogen, oxygen, nitrogen, chlorine, carbon, sulphur, phosphorus, iodine — show the opposite properties: they have no lustre (except iodine, which is shiny), are dull in appearance and of various colours; they are poor conductors of heat and electricity (except graphite, a form of carbon, which conducts); they are neither malleable nor ductile but brittle when solid — sulphur crumbles; they are not sonorous; and they are mostly gases or soft solids with low melting points, bromine being a liquid. Diamond, another form of carbon, is the hardest known substance — another exception to note.

Metalloids show some properties of metals and some of non-metals: boron, silicon, germanium, arsenic, antimony and tellurium. Silicon, for example, is shiny like a metal but brittle and a poor conductor like a non-metal; it conducts electricity moderately as a semiconductor, which is why silicon and germanium are the material of transistors, computer chips and solar cells.

Of the 118 elements, about 22 are non-metals, about 6 are metalloids and the rest are metals. Elements are represented by symbols — a topic of the next chapter — and are arranged by their properties in the periodic table, which later classes study. For now the student should be able to name five metals and five non-metals, list the distinguishing properties, and explain why copper is used for wires (ductile, conductor), aluminium for foil (malleable), and silicon for chips (metalloid semiconductor).