Is Matter Around Us Pure?

Almost everything we see around us — sea water, air, soil, milk, most rocks — is a mixture, not a pure substance. A mixture is matter made up of two or more substances (elements or compounds) that are mixed together in any proportion and are not chemically combined, so each one keeps its own properties.

Mixtures are of two kinds. In a homogeneous mixture the composition is uniform throughout and there is only one phase visible — for example salt dissolved in water, or air. In a heterogeneous mixture the composition is not uniform; you can see the different components, or there is more than one phase — for example a mixture of salt and iron filings, or oil and water. Whether a mixture is homogeneous or heterogeneous depends on how well the particles of one substance are distributed in the other, which in turn depends on the size of the particles — this is exactly what separates solutions, suspensions and colloids from each other in the next three sections.

A solution is a homogeneous mixture of two or more substances. The substance present in the larger amount is the solvent, and the substance present in the smaller amount, which gets dissolved, is the solute — in a sugar solution, sugar is the solute and water is the solvent. A solution can involve any state of matter: solid in liquid (sugar in water), gas in liquid (soda water), gas in gas (air), liquid in liquid (alcohol and water) or solid in solid (alloys such as brass, zinc dissolved in copper).

Solutions have three properties that come up repeatedly in exams: (i) a solution is homogeneous — a spoon of it taken from anywhere has the same composition; (ii) its particles are too small to be seen even under a microscope, and never settle on standing; (iii) they cannot be separated by filtration, and a beam of light passing through does not scatter — the path of light is not visible. A solution in which no more solute can dissolve at a given temperature is a saturated solution; if more can still dissolve, it is unsaturated. The mass of solute that can dissolve in 100 g of a solvent at a given temperature to form a saturated solution is its solubility, and it changes with temperature.

A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the medium — chalk powder in water, or muddy river water, are common examples. Because the particles are large enough to be seen with the naked eye, a suspension has properties opposite to a true solution: the particles settle down under gravity when the suspension is left undisturbed, and they scatter a beam of light passing through the mixture, so the path of light becomes visible. The particles of a suspension can be separated from the mixture by the ordinary process of filtration.

A colloid (or colloidal solution) is a heterogeneous mixture whose particle size lies between that of a true solution and a suspension, so it behaves in a mixed way. Milk, ink, blood, shaving cream, smoke and fog are all colloids. Because the particle size is small (too small to be seen with the naked eye, but large enough to scatter light), a colloid appears homogeneous to the eye but is heterogeneous on close inspection; the particles do not settle down on standing, and they cannot be separated by ordinary filtration — separation needs a technique called centrifugation.

The most important named effect here is the Tyndall effect: when a beam of light is passed through a colloid, the path of the light becomes visible because the colloidal particles scatter the light. You see this when sunlight passing through a canopy of trees is scattered by tiny dust or water particles in the air, or when a beam of light shone into a dark room reveals dust particles floating in it.

Every colloid has two phases. The dispersed phase is the solute-like component present in a smaller amount (the particles that are spread through the medium), and the dispersion medium is the component in which the dispersed phase is suspended, playing the role of the solvent. In milk, tiny fat globules (dispersed phase) are spread through water (dispersion medium).

Since the components of a mixture keep their own physical properties, a mixture can always be separated back into its components by physical methods that exploit those differences. NCERT lists several methods, one at a time, and exams routinely ask 'name the method and give one use'.

Evaporation separates a solid dissolved in a liquid, when we want the dissolved solid and are willing to lose the liquid as vapour — for example, recovering common salt from sea water, or dye from an ink sample (the solid colour left behind after the liquid part of ink evaporates). The solution is heated until all the solvent turns to vapour and escapes, leaving the solid solute behind.

Centrifugation is used when the particles of a mixture are so fine that they pass through a filter paper — for instance, separating the fat (cream) from milk, or separating fine clay particles from water, or separating blood components in the diagnostic lab. The mixture is spun at very high speed in a centrifuge; the denser component experiences a stronger outward force and settles at the bottom (or moves outward), while the lighter component stays near the centre/top, so the two can be drawn off separately. The principle is: denser particles are forced to the bottom and lighter particles remain at the top when spun rapidly.

A separating funnel is used to separate two immiscible liquids (liquids that do not mix and form two visible layers), such as kerosene and water — the denser liquid settles below the lighter one, the stopcock is opened to run off the lower liquid, and it is closed the instant the upper liquid reaches the stopcock. This method is also used industrially, in the extraction of iron, to separate lighter molten slag from heavier molten iron.

Sublimation separates a component that sublimes (changes directly from solid to gas on heating, without passing through the liquid state) from one that does not — for example, separating ammonium chloride, camphor, naphthalene or iodine from sand. On heating, the sublimable solid turns to vapour and is cooled and collected back as a solid on a cold surface, leaving the non-sublimable solid behind.

Chromatography (Greek for 'colour writing') separates components of a mixture that dissolve in the same solvent but to different extents, so it is the method of choice for dyes, pigments and drugs extracted from a biological fluid. In paper chromatography, a spot of the mixture is placed near the bottom edge of a filter-paper strip, dipped (below the spot) into a solvent; the solvent rises by capillary action, carrying components at different speeds because each has a different solubility in it — the fastest, farthest-travelling component is the most soluble, so the mixture separates into distinct spots as the solvent front moves up.

Distillation is used to separate a solvent and a dissolved solid, or two miscible liquids, whose boiling points are sufficiently different (a difference of more than about 25 K is preferred for simple distillation) — for example, separating acetone from a solution, or separating a pure sample of water from sea water. In simple distillation the mixture is heated in a distillation flask; the liquid with the lower boiling point vaporises first, the vapour passes through a delivery tube into a condenser where it is cooled back into a liquid (the distillate), which is collected in a separate receiver, while the higher-boiling (or non-volatile) component stays behind in the flask.

Fractional distillation is used to separate two or more miscible liquids whose boiling points are close to each other (less than about 25 K apart) — the industrial example examiners love is separating the different fractions (petrol, kerosene, diesel and so on) of petroleum, and separating the different gases of air after it has been liquefied. The apparatus is the same as for simple distillation but with a fractionating column fitted between the flask and the condenser. The column is usually packed with glass beads, which offer a large surface area, so that as the mixed vapour rises up the column, the component with the higher boiling point condenses on the beads first (and drips back into the flask to be re-vaporised), while the component with the lower boiling point passes through and reaches the condenser first — the repeated condensation-and-vaporisation along the column sharpens the separation far better than a single distillation could.

Crystallisation is a technique used to purify a solid that has small amounts of impurity mixed in it, and it is better than simple evaporation whenever the solute (i) decomposes, or (ii) picks up impurities from the air, on strong or continued heating, or when we specifically want well-formed crystals rather than a shapeless solid residue — for example, purifying copper sulphate crystals that contain sodium chloride and other insoluble impurities, or obtaining pure alum crystals from an impure sample, or purifying the salt obtained from sea water (which contains sodium chloride mixed with other salts).

The impure solid is dissolved in a minimum quantity of water to make a nearly saturated solution at a warm temperature; the hot solution is filtered to remove any insoluble impurity; the filtrate is then allowed to cool slowly and undisturbed. As it cools, the solubility of the solute drops and it separates out as crystals, leaving the more soluble impurities dissolved behind in the remaining liquid (the mother liquor), which is drained off. This gives a purer, better-formed solid than evaporating the solution all the way to dryness would.

Matter can undergo two broad kinds of change. In a physical change, no new substance is formed — only physical properties like state, shape or size change, and the change is generally reversible; melting ice, dissolving sugar in water, and the separation techniques above are all physical changes, because the substances involved can be recovered chemically unchanged. In a chemical change, one or more new substances with different properties are formed, and the change is generally not reversible by simple physical means; rusting of iron, burning of magnesium ribbon, and digestion of food are chemical changes.

This distinction matters directly for this chapter: a mixture is defined as a physical combination of substances (formed by a physical change, so it can be separated by physical methods), while a compound is formed by a chemical change (a chemical combination), so it cannot be broken back into its elements by ordinary physical methods — only by a further chemical process.

A pure substance is matter that has a fixed, definite composition throughout, made up of only one kind of particle — unlike a mixture, all the constituent particles of a pure substance have the same chemical nature, so it shows a fixed set of properties such as a sharp melting or boiling point. There are two types of pure substances: elements and compounds.

An element is a basic form of matter that cannot be broken down into simpler substances by any chemical reaction; it is made of only one kind of atom. Elements are classified into three groups. Metals are lustrous (shiny), malleable (can be hammered into sheets), ductile (can be drawn into wires), sonorous, and are generally good conductors of heat and electricity — examples are gold, silver, copper, iron. Non-metals are generally non-lustrous (with the exception of iodine, which is lustrous, and graphite, which conducts electricity — both are important exceptions to remember), brittle if solid, and poor conductors of heat and electricity — examples are hydrogen, oxygen, carbon, sulphur, iodine. Metalloids (such as boron, silicon, germanium, arsenic) show intermediate or mixed properties of metals and non-metals — for example, silicon looks metallic (lustrous) but is a poor conductor, closer to a non-metal in that respect.

A compound is a substance made up of two or more elements, chemically combined with one another in a fixed proportion by mass. Because the combination is chemical, not physical, the properties of a compound are entirely different from the properties of its constituent elements — for example, hydrogen is a highly combustible gas and oxygen supports combustion, but water (H₂O), the compound they form, is used to put out fires. Similarly, iron is attracted to a magnet and sulphur is not, but iron sulphide (FeS), the compound formed when they are heated together, shows neither property.

A compound cannot be separated into its constituent elements by any physical method — only a chemical reaction can decompose it. This is the sharpest exam-tested distinction between a compound and a mixture, summarised in the table below and worth memorising word for word, since 'difference between mixture and compound' is one of the most repeated short-answer questions in this chapter.

• A mixture has a variable composition; a compound has a fixed composition by mass.
• A mixture's components keep their individual properties; a compound has properties different from its constituent elements.
• A mixture can be separated into its components by physical methods; a compound needs a chemical method to be broken down.
• A mixture may or may not involve energy change on formation; forming a compound usually involves energy change (heat, light, and so on).
• A mixture can be homogeneous or heterogeneous; a compound is always homogeneous.