Class 9 Science Chapter 5: Exploring Mixtures and their Separation

A mixture whose make-up is the same throughout, so that any sample taken from it matches any other. Another name for it is a solution.

A mixture that is not uniform. Its parts can often be seen, and some of them may drop to the bottom if the mixture is left standing.

The substance that gets dissolved when a solution is made. Sugar plays this part in sugar water.

The substance that does the dissolving. Water plays this part in sugar water, and in a great many other solutions.

Pouring a mixture through filter paper to find out whether anything is held back on it. What stays behind is called the residue.

It is a mixture whose composition is uniform throughout, so that any part of it matches any other. Vinegar, which is acetic acid in water, and aerated drinks such as soda, which hold carbon dioxide in water, are two further examples.

Because it is equally sweet in the first sip and in the last. That sameness of taste from one end of the glass to the other shows that the composition does not vary from one part of the mixture to another.

Because it is not the same throughout. You can pick out the grains by eye, and left alone they drop to the bottom. Stirring does not change either of these behaviours.

The solute is the substance that gets dissolved and the solvent is the substance that dissolves it. Stir sugar into water and the sugar is the solute, the water the solvent.

It means the uniformity of a solution does not break down as it stands. Unlike a mixture whose particles settle to the bottom, a solution keeps the same composition throughout however long it is left undisturbed.

Group A stirs one spatula of common salt into 50 mL of water, Group B one spatula of chalk powder, and Group C a few drops of milk, each in a colourless transparent tumbler or 100 mL beaker. The warning is never to look directly into the laser beam, since it can cause irreversible eye damage.

Looking at each mixture shows whether any particles are visible. Shining a laser through the beaker and observing from the side, perpendicular to the beam, shows how each mixture handles light. Leaving the beakers undisturbed shows which mixtures settle. Filtering each one shows whether a residue is left on the filter paper.

A solution is not something separate from a mixture: it is a mixture, of the homogeneous kind. So every solution is a mixture, while a mixture is only a solution if its composition is uniform throughout. Sand in water is a mixture but not a solution.

Everything turns on one question: is the mixture the same all the way through?

A mixture of sugar and water has a uniform composition throughout. Stirred well, it is equally sweet in the first sip and in the last, so the make-up does not vary from the top of the glass to the bottom. Such a mixture is called a homogeneous mixture, and another name for it is a solution. Vinegar, which is acetic acid in water, and aerated drinks such as soda, which hold carbon dioxide in water, belong to the same family.

One further point holds for all of them: a solution always remains homogeneous, so its uniformity does not break down as it stands.

A stirred mixture of sand and water behaves quite differently. It is not uniform at all. The sand particles are easily visible in the water, and they settle with time. We call such a mixture a heterogeneous mixture, and no amount of stirring will change that.

Oil poured into water raises the same question and is answered the same way, since the two sit in separate layers rather than blending into one uniform liquid.

The evidence to quote is therefore of three kinds: whether the composition is the same everywhere, whether the particles can be seen, and whether anything settles on standing.

Split the class three ways; each group makes up its mixture in something clear and colourless, a plain tumbler or a 100 mL beaker. Group A stirs one spatula of common salt into 50 mL of water, Group B one spatula of chalk powder, and Group C a few drops of milk. Each beaker is labelled with its group letter.

The first test is simply to look. Are the particles visible in each mixture? What is seen is recorded before anything else is done.

The second test uses light. The beam from a laser pointer is directed through each beaker, and observed from the side, perpendicular to the beam. Here the safety warning applies: never look directly into the laser beam, since it can cause irreversible eye damage.

The third test is patience. Each group predicts what would be seen in its beaker if it were left undisturbed for a few minutes, which brings out whether anything settles.

The fourth test uses a filtration apparatus. Each mixture is filtered separately and the filter paper examined for any residue left behind.

Taken together the four tests answer the question the activity was set up to ask. The three mixtures behave differently, so they are not all of one kind but belong to different types, and the rest of the chapter works out what those types are.

Solutions are homogeneous mixtures, which means their composition is uniform throughout.

They are prepared when a solute is mixed with a solvent. The solute is the substance that gets dissolved, and the solvent is the substance that dissolves the solute. Stir sugar into water and the sugar is the solute, the water the solvent.

The parts need not both be solids or both liquids. In vinegar the solute is acetic acid, a liquid, and in an aerated drink such as soda the solute is carbon dioxide, a gas, while water serves as the solvent in each case.

The word solution names a family within mixtures rather than an alternative to them. Every solution is a mixture, since it is made of more than one substance put together without either losing its identity. But not every mixture is a solution, because a mixture only qualifies if its composition is uniform throughout. Sand in water is a mixture and not a solution.

This is why the terms solute and solvent can only be applied once a mixture has been classified as homogeneous. Asking which part of a sand and water mixture is the solute has no sensible answer.

The chapter opens with two puzzles: how a tall green cane yields white sugar crystals, and how a doctor can find malaria in a couple of drops of blood.

Both are separation problems. Many everyday activities are made possible by techniques based on the science of separating mixtures, and from industrial work such as sugar production to life-saving medical tests, that separation plays a real part in daily life.

The chapter builds on what is already known about mixtures and a few simple ways of separating them, going further into their properties, their behaviour, and the range of techniques used on them.

Three further questions are set at the start to be carried through. Why does muddy water clear from the top down if left standing, while milk never does? How is evaporation different from boiling? And why do shafts of light show up where sun comes through gaps in thick foliage?

Each of those is answered before the chapter is finished, and each depends on understanding what kind of mixture is involved. That is why the first thing the chapter does is classify mixtures into the homogeneous and the heterogeneous.

The amount of solute dissolved in a stated amount of solvent or of solution. It is what fixes the proportion between the two, and getting it right matters in medicine, farming, food and cosmetics alike.

The grams of solute carried in every 100 grams of finished solution. Written % m/m or % w/w, and calculated as the mass of solute divided by the mass of solution, times 100.

How many grams of solute are present in 100 millilitres of solution. Written % m/v or % w/v, and used where measuring out a liquid is easier than weighing it.

How many millilitres of solute are present in 100 millilitres of solution. Written % v/v, and used when two miscible liquids are mixed together.

Two liquids that mix into one another to give a uniform solution. Their proportions are expressed by volume by volume percentage.

It is how much solute is carried by a stated quantity of solvent, or of the solution as a whole. It matters because the right proportion is essential when preparing a solution, whether in medicine, agriculture, food, cosmetics or even in making a cup of tea.

ORS is a measured thing: set weights of salt and sugar into a set volume of water. Alter the weights while keeping the water the same, or alter the water while keeping the weights, and what you end with is still salt and sugar dissolved in water, but it is no longer ORS. This is also why not all sugary drinks made at home or sold in shops count as ORS.

Too little pesticide may not protect the crops at all. Too much can damage the crops, the soil and the environment. So the right amount has to be mixed with a fixed amount of water, and more is not better.

$\text{% m/m} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100$. It reports the grams of solute carried in every 100 grams of finished solution. It is the figure quoted on packaged food, on milk powder and on spice mixes as much as on solutions.

The mass of solute is 10 g and the mass of solvent is 90 g, so the total mass of solution is $10 + 90 = 100$ g. The percentage is therefore $\frac{10\ \text{g}}{100\ \text{g}} \times 100 = 10%$ m/m.

Mass by volume percentage gives the grams of solute in 100 millilitres of solution. It is used where measuring a liquid's volume is easier than weighing it, as in a 5% glucose solution. Volume by volume percentage gives the millilitres of solute in 100 millilitres of solution and is used when two miscible liquids are mixed, as in vinegar, perfumes and cosmetics.

Four grams of zinc oxide are present in every 100 g of the powder, so in 300 g there are $\frac{4}{100} \times 300 = 12$ g of zinc oxide.

Vinegar contains 5% v/v acetic acid, and glacial acetic acid is the liquid at 100% strength. Since 5% v/v means 5 millilitres of solute in every 100 millilitres of solution, you would take 5 mL of glacial acetic acid and add enough water to bring the total volume of the solution to 100 mL.

Concentration is how much solute a stated quantity of solvent, or of the whole solution, carries. It is what turns a vague description into a precise one.

The first example is Oral Rehydration Solution. It takes set weights of salt and sugar into a set volume of water. Alter the weights while keeping the water the same, or alter the water while keeping the weights, and you still have salt and sugar dissolved in water, but you do not have ORS. The point is worth pressing, because a sweet drink mixed at home, or bought in a shop, is not thereby ORS.

The second is a pesticide spray. A farmer has the same problem: so much pesticide to so much water, and the proportion has to be right. Too little may not protect the crops, while too much can damage the crops, the soil and the environment, so the error runs both ways.

What both cases show is that getting the proportion right always matters when preparing a solution. The idea is not confined to laboratories either: it is at work in medicine, agriculture, food, cosmetics, and even while making a simple cup of tea.

Concentration can be expressed in several ways, and percentage is the one taken up at this level.

Mass by mass percentage is written $\text{% m/m} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100$, and it reports the grams of solute carried in every 100 grams of finished solution. It is used for homogeneous mixtures, but also for heterogeneous ones such as milk powder and spice mixtures, and for the labels on packaged foods that declare salt, sugar or protein.

Mass by volume percentage is written $\text{% m/v} = \frac{\text{Mass of solute}}{\text{Volume of solution}} \times 100$, and it gives the grams of solute in 100 millilitres of solution. It is the figure to reach for when a liquid is easier to measure out than to weigh, as in medicine and in the laboratory; the 5% glucose solution is the familiar case.

Volume by volume percentage is written $\text{% v/v} = \frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100$, and it gives the millilitres of solute in 100 millilitres of solution. It is the method for two miscible liquids, as in perfumes, cosmetics and vinegar.

One point holds across all three: the quantity on the bottom is the whole solution, never the solvent alone.

The first concerns a mass by mass percentage. Salt weighing 10 g into 90 g of water gives 10 g of solute against 90 g of solvent, so the solution itself weighs $10 + 90 = 100$ g. The percentage is then $\frac{10\ \text{g}}{100\ \text{g}} \times 100 = 10%$ m/m.

The step that matters here is the addition. The denominator has to be the mass of the whole solution, and dividing by the 90 g of solvent instead would give the wrong answer.

The second concerns a mass by volume percentage. Glucose weighing 5 g made up with water to 100 mL gives 5 g of solute in 100 mL of solution, giving $\frac{5\ \text{g}}{100\ \text{mL}} \times 100 = 5%$ m/v. This is the familiar strength of a glucose solution.

The third concerns a volume by volume percentage. If 1 mL of a liquid pesticide is mixed with enough water to form 100 mL of spray for a rice crop, then $\frac{1\ \text{mL}}{100\ \text{mL}} \times 100 = 1%$ v/v.

In each case the method was chosen to match how the two quantities were measured.

The choice follows from how the two quantities have been measured. Weigh both the solute and the solution and the method is mass by mass. Weigh the solute but make the solution up to a volume and it is mass by volume. Measure both as volumes, which is what happens with two miscible liquids, and it is volume by volume.

For the talcum powder, 4% m/m means 4 g of zinc oxide in every 100 g of powder, so a 300 g tin holds $\frac{4}{100} \times 300 = 12$ g.

For the orange juice, two tablespoons at 15 mL each give $2 \times 15 = 30$ mL of concentrate, made up to 150 mL of juice. Since both quantities are volumes the method is volume by volume, giving $\frac{30}{150} \times 100 = 20%$ v/v.

For the vinegar, glacial acetic acid is the liquid at 100% strength and vinegar carries 5% v/v acetic acid. Since 5% v/v means 5 mL of solute in every 100 mL of solution, you would take 5 mL of glacial acetic acid and add water until the total volume reaches 100 mL.

One last point: industry commonly writes weight by weight percentage instead of mass by mass, since weight and mass are used interchangeably, and the two figures are numerically equal.

The largest quantity of a solute that a fixed amount of solvent, taken as 100 mL or 100 g, will dissolve at a stated temperature. The temperature must be given, since the figure changes with it.

A solution that will take up no further solute at that temperature. Adding more simply leaves it undissolved.

A graph of solubility against temperature, with degrees Celsius along the horizontal axis and grams of solute per 100 g of water up the vertical one. Comparing two curves compares two solutes at a glance.

A solid whose particles are arranged in a regular geometric pattern. Rock salt, candy sugar, snowflakes and window frost are all examples.

The laboratory process of forming crystals from a saturated solution. It separates two solids dissolved in the same solvent where one is present in small quantity, and it purifies solids.

Solubility is the largest quantity of solute that a fixed amount of solvent, taken as 100 mL or 100 g, will dissolve at a stated temperature. A saturated solution is one that will take up no further solute at that temperature, so anything added beyond that point stays undissolved.

Because the figure shifts as the temperature shifts. For a solid solute in a liquid solvent the solubility generally increases as things warm up, while for a gas dissolved in a liquid it generally decreases. Without a temperature the number does not say anything definite.

It is a graph of solubility against temperature, with degrees Celsius along the horizontal axis and grams of solute per 100 g of water up the vertical one. Of two curves on the same axes, the steeper one belongs to the solute whose solubility gains more from a rise in temperature.

Only 241 g can stay dissolved at the lower temperature, so the surplus of $287 - 241 = 46$ g must leave the solution. It separates out as a pure solid, commonly in the form of crystals.

A crystal is a solid whose particles are arranged in a regular geometric pattern. Rock salt, snowflakes formed when water vapour freezes in the air, and frost on window panes are three that occur without any laboratory involved.

It is the laboratory process of forming crystals from a saturated solution. It can separate two solids where one is present in small quantity and both dissolve in the same solvent, and it can also purify solids.

Filtering while hot removes insoluble impurities before any crystals have formed, so they are not collected along with the product. Cooling slowly and without disturbance gives the particles in the solution enough time to come together, which yields larger, shiny, well-shaped crystals rather than the small poorly formed ones a rapid chill produces.

It turns on one fact: a substance does not dissolve equally well hot and cold. On cooling, the solution can no longer hold as much of the wanted substance, so the excess separates out as pure solid. The impurities, being present in far smaller quantity, have not exceeded their own solubility and so stay dissolved in the liquid.

Solubility is the point at which a set quantity of solvent — 100 mL or 100 g — will take no more solute, at whatever temperature you are working at. Once a solution cannot dissolve any more solute at that temperature it is described as saturated.

The temperature has to be quoted every time, and the reason is that the figure moves with it.

For a solid solute in a liquid solvent, the solubility generally increases as the temperature rises. More of the solid will go into the same amount of liquid simply because the liquid is warmer.

For a gas dissolved in a liquid, the behaviour runs the other way. Solubility generally decreases as the temperature rises, so a warm liquid holds less dissolved gas than a cold one.

Because of this, a solubility figure quoted without a temperature says nothing definite. The same compound may carry 287 g in 100 g of water at one temperature and only 241 g at another.

Solubility matters beyond definitions because it is an important property used to separate substances from mixtures. Changing the temperature changes what a solution is able to hold, and whatever it can no longer hold has to leave it.

A solubility curve is a graph of solubility against temperature, with degrees Celsius along the horizontal axis and grams of solute per 100 g of water up the vertical one. Reading two values off it is enough to predict what cooling will do.

Take a saturated solution of compound B at 60 °C, made by dissolving 287 g of it in 100 g of water. Now cool it gradually to 40 °C.

At 40 °C the curve gives the solubility of B as 241 g per 100 g of water. That is all the solution is able to hold at the lower temperature.

It follows that only 241 g can remain in the solution, and the rest of it, which is 46 g, has nowhere to stay. It separates out as a pure solid, often in the form of crystals, a crystal being a solid whose particles are arranged in a regular geometric pattern.

The same reasoning explains a related result. If equal masses of hot saturated solutions of two compounds are cooled over the same range, the one whose curve is steeper will deposit more solid, because its solubility has fallen further over that range.

Two safety points come first. Copper sulfate is toxic, so the experiment is performed under adult supervision and it is not touched with bare hands. The drop of dilute sulfuric acid should be added by the teacher, and the acid is handled very carefully. Common salt may be used instead where copper sulfate is unavailable.

Take 1 g of copper sulfate in a 100 mL beaker, add 25 mL of water and a drop of dilute sulfuric acid, and heat the mixture gently in a water bath while stirring constantly. The acid helps in making pure crystals by preventing unwanted reactions, though it is required for only some salts.

Gradually add more copper sulfate until the solution becomes saturated.

Filter the hot solution to remove insoluble impurities, collect the filtrate in a clean beaker, and lay over it a watch glass. Filtering at this stage matters, because doing it later would gather the impurities in with the crystals.

Allow the solution to cool slowly without disturbing it. This gives the particles in the solution enough time to come together, so that larger, shiny, well-shaped and blue crystals result. Chilling it rapidly in ice-cold water would give smaller and less well-formed ones instead.

Last, put them through a filter, wash them down with cold water and leave them on a watch glass to dry.

Crystallization is a common technique for separating pure substances, in the form of crystals, from homogeneous mixtures. It does two jobs. Two solids can be parted this way when one is only a minor component and the same solvent takes up both; and it will clean up a solid besides.

The reason it works is stated simply enough: purification by this route works because a substance does not dissolve equally well at every temperature. On cooling, the solution can no longer hold as much of the wanted substance, so the surplus leaves it as pure solid, while the impurities, present in far smaller quantity, have not reached their own limit and stay dissolved.

This is why the method matters in practice. A freshly made compound rarely comes clean, and crystallization is how what is wanted is parted from what is not.

The same process happens without any apparatus at all. Rock salt crystals, the sugar crystals that grow while candy sugar or mishri is made, snowflakes formed when water vapour freezes in the air, and frost on windows are all crystals formed naturally.

On a larger scale still, crystal deposits occur in mines, in caves such as the Mawsmai Cave at Sohra or Cherrapunji, and within the Earth's crust, quartz being one of the most beautiful.

Warming a mixture until the liquid of lower boiling point turns to vapour, then chilling that vapour back to liquid. It recovers the solvent as well as the solute, which evaporation cannot do.

The part of the apparatus where the vapour is chilled, usually by circulating water or air, so that it returns as a pure liquid and can be collected separately.

Simple distillation works where the two liquids' boiling points stand at least about 25 °C apart. A gap that wide lets one vaporise before the other produces much vapour.

The method for mixtures whose boiling points differ by less than 25 °C. Crude petroleum is split this way into petroleum gas, petrol, kerosene, diesel and the rest.

Separating a mixture by letting a solvent climb a paper strip, carrying the components with it at different speeds according to how each interacts with the solvent and the paper.

Evaporation returns the solute but the solvent disappears into the air. Distillation catches the vapour and cools it back to liquid, so the solvent is recovered too. It also handles two miscible liquids, which evaporation cannot separate at all.

It is the heating of a mixture until the liquid with the lower boiling point vaporises, followed by cooling that vapour so it turns back into liquid. It allows the recovery of the solvent, the separation of liquids differing in boiling point by at least about 25 °C, and the separation of a liquid from a solution containing dissolved solids.

The vapour of the lower-boiling one is led through the condenser, chilled there by water or air kept moving around it, and runs off clean. That liquid gathers in a separate vessel, while the solid or the other liquid from the mixture remains behind in the distillation flask.

Their boiling points lie far enough apart. Acetone turns to vapour at about 56 °C while water needs 100 °C, and a gap that wide lets the acetone boil away before water is producing vapour in any quantity.

Simple distillation separates liquids whose boiling points differ by at least about 25 °C, while fractional distillation deals with relatively small differences of less than 25 °C. Crude petroleum, extracted from the Earth's crust, is split by fractional distillation into fractions such as petroleum gas, petrol, kerosene and diesel.

Cut a strip about 3 cm across from chromatographic paper, or from filter paper if none is to hand, and rule a straight horizontal line across it 2 cm up from the lower edge, using a pencil. Put a single mark from a black sketch pen at the middle of that line. Then stand the strip upright in a thin layer of water, with the level staying under the mark.

It turns on how each component gets on with the solvent and with the paper. The liquid draws the substances up the strip and sorts them by how quickly each one travels. As the water climbs, the mark begins to part into separate coloured spots, which shows it was a mixture.

Distillation cannot separate two liquids of the same boiling point, since the boiling points must differ by at least about 25 °C. The solvent level should start below the sample spot, not above it, or the ink would dissolve away into the liquid. And evaporation and crystallization are not the same process, since crystallization forms crystals from a saturated solution and yields a pure, well-shaped solid.

Evaporation recovers a solute such as salt but loses the solvent, and it cannot touch a mixture of two liquids that mix into one another. Distillation answers both problems.

Two liquids that mix freely can be parted by warming them until whichever boils lower goes off as vapour. That vapour is then cooled, turning back into liquid. This is distillation, and it allows the recovery of the solvent, the separation of liquids differing in boiling point by at least about 25 °C, and the separation of a liquid from a solution containing dissolved solids.

In the apparatus, the mixture sits in a distillation flask on wire gauze above a burner, with a thermometer reading the temperature of the vapour leaving it. The vapour of the lower-boiling one is led through a condenser, chilled there by water or air kept moving around it, and runs off clean. It is caught in a vessel of its own, and whatever else was in the mixture stays behind in the flask.

Acetone and water can be separated this way because their boiling points differ sufficiently, acetone boiling at about 56 °C and water at 100 °C. A large difference lets one liquid vaporise before the vapours of the other are formed in significant amount, so what comes over is essentially pure acetone.

Simple distillation needs the two boiling points to stand at least about 25 °C apart. Where they lie closer than that, another method is required.

Where the boiling points sit close together, under 25 °C apart, the method needed is fractional distillation. It is the method industry relies on for mixtures that simple distillation cannot resolve.

The standard case is petroleum. A refinery is the works in which crude oil is broken down into the various things made from it, and what comes off is a set of useful gases and liquids.

Crude petroleum is drawn out of the Earth's crust and separated into its different fractions by fractional distillation. Among them are petroleum gas, petrol, kerosene which serves as aviation fuel, diesel, lubricating oil and bitumen.

The gaseous fraction is dealt with separately. It is filled into steel cylinders under high pressure, where it gets liquefied, and in that form it is used as domestic fuel under the name Liquefied Petroleum Gas, or LPG. That is how the same process that yields road bitumen at one end also supplies the gas used for cooking at the other.

Begin with a 3 cm wide strip of chromatographic paper, or a strip of filter paper as an alternative, and rule one straight horizontal line 2 cm from the bottom using a pencil. A pencil is specified because ink would itself be carried up by the solvent.

Put a single dot of black sketch pen on the middle of that line. Pour water into a gas jar, a measuring cylinder or a beaker — just enough to cover the base.

Place the paper strip vertically in the container so that its lower end dips into the water, with the water level below the spot. This matters: if the level were above the spot, the ink would simply dissolve into the liquid instead of being carried up the paper.

Observe the strip as the water rises through it. The ink starts to separate into different colour spots, which shows that the single black mark was never one substance.

The method is called paper chromatography, and it works because each component clings to the paper, and travels with the liquid, to a different degree. The liquid draws them all upward, and they sort themselves out by how quickly each is carried. A component held more tightly by the paper lags behind, while one carried readily by the solvent travels further.

Two traditions are named here, and both rest on techniques this chapter teaches.

The first is salt. Drawing salt out as crystals is an old craft of the communities living along India's coasts. Boiling down concentrated sea brines yielded what was called panga salt, while letting seawater dry off in the open yielded karkatch, and the two routes gave crystals of differing sizes.

The second is perfume. Distillation has served many ends over a long stretch of time, among them drawing fragrance out of flowers.

Kannauj, a town in Uttar Pradesh, carries the name of India's perfume capital. The pleasing earthy scent that lifts off dry ground once the first rain falls is caught there and bottled as a natural perfume, Mitti ka Ittar. It is produced by the traditional Deg-Bhapka distillation, handed down through generations in the town, and buyers want it both within India and well beyond.

The Fragrance and Flavour Development Centre there is equipped to draw flavour and fragrance components out of flowers, leaves and other plant parts. It also backs farmers growing fragrance-bearing plants, and advises anyone hoping to start a perfume business of their own.

Two liquids that will not blend, settling instead into separate layers. Oil with water is the standard pair, and a separating funnel parts them by draining the lower layer off first.

Sublimation is a solid passing straight to vapour below its melting point without ever being liquid. Deposition is the reverse, vapour returning to solid without a liquid stage in between.

A heterogeneous mixture whose solid particles stay undissolved and hang throughout the liquid. Its particles exceed 1000 nm, are visible to the naked eye, and settle on standing.

A mixture whose particles measure between 1 and 1000 nm. They escape the naked eye and do not settle out, yet they scatter light, which is what separates a colloid from a true solution.

The scattering of light by particles, making a light beam's path visible. Colloids and suspensions show it; a transparent solution does not.

The mixture is poured into the funnel and left undisturbed until two layers form, with the less dense liquid above. The stopcock is opened slowly to run the lower layer off into a container and closed when it has nearly all drained. The small portion that follows may hold both liquids and is discarded, after which the stopcock is opened again to collect the upper layer separately.

Nothing restrains a gas particle from going any way it likes, so one gas runs evenly through another without trouble — hydrogen and oxygen in rocket fuel, for instance. Smoke is solid particles suspended in air, and fog is tiny liquid water droplets in air. Smoke, fog and dust in the air are all heterogeneous, even though a gas is one of their components.

Sublimation is a solid changing directly to vapour on heating below its melting point, without passing through the liquid state, and deposition is the vapour cooling back to solid without becoming a liquid. Camphor does both, so it rises as vapour and deposits as white solid on the inner wall of the inverted funnel. Sand does not sublime on heating and remains in the china dish.

It is an uneven mixture whose solid particles will not dissolve but hang scattered right through the liquid. Its particles are larger than those in a solution and are visible to the naked eye, whereas the solute particles of a solution cannot be seen at all however closely the mixture is examined.

The mixture is spun in a tube at high speed. The tubes become horizontal and the centrifugal force, which is the outward force acting on a body in circular motion, drives the heavier particles outwards so they settle at the bottom, while the lighter liquid stays on top. Laboratories use it to part blood into its fractions, the red cells from the plasma.

Coagulation is the clumping together of fine suspended particles so they can be removed. Powdered alum, a white crystalline substance used in water purification, acts as the coagulant for muddy water, and the clumps then settle by gravity to be removed by decantation or filtration. For paneer, an acid such as lemon juice or vinegar acts as the coagulant on the milk proteins.

Solution particles measure less than 1 nm, colloid particles between 1 and 1000 nm, and suspension particles more than 1000 nm across. A suspension settles on standing, while the particles of a colloid stay uniformly dispersed throughout the mixture and do not settle over time, behaving in that respect like a solution.

It is the scattering of light by particles, named after John Tyndall who first explained it. In the salt solution the path of the laser beam was not visible, in the chalk suspension the particles scattered the beam so its path showed clearly, and in the milk mixture the path was visible too even though the mixture looked uniform. Light is scattered by a colloid or a suspension but not by a transparent solution.

Liquids that will not blend, such as oil and water, settle into separate layers and are called immiscible. A separating funnel exploits exactly that.

A mixture made of 5 mL mustard oil with 20 mL water is poured into a 50 mL separating funnel and left undisturbed. Two layers form, the yellow mustard oil above and the water below, the lighter liquid taking the upper position. The stopcock is opened slowly and the lower layer of water collected carefully into a container, then closed once the water has almost fully drained. The next small portion may contain both liquids and is discarded, and the stopcock is opened once more to collect the oil separately.

Sublimation handles a different case, a mixture of two solids. A spatula of crushed camphor and sand goes into a clean dry china dish on wire gauze, with a clean dry glass funnel inverted over it and its nozzle plugged with cotton. The dish is heated gently for a few minutes.

White solid camphor is found deposited on the inner wall of the funnel while the sand remains in the dish. Camphor, heated below its melting point, changes directly from solid to vapour without passing through the liquid state, which is sublimation, and on cooling those vapours return to solid without becoming liquid, which is deposition. Sand does not sublime on heating, so the two part company.

Leaving muddy water undisturbed lets the heavier particles settle, but the water may still appear cloudy. Filtering it through cotton cloth or filter paper removes some larger particles, and yet the water often remains cloudy. That shows filtration is not always enough to separate tiny particles, so two further techniques are used.

The first is centrifugation. The mixture is spun in a tube at speed; the tubes swing out flat, and the outward push on anything travelling in a circle throws the denser matter to the far end, where it collects at the tube's bottom, leaving the thinner liquid above it. Laboratories use it constantly to part blood into its fractions, the red cells from the plasma, and it is standard in chemical works besides.

The second is coagulation. Powdered alum, called fitkari, is added to the muddy water. Alum is a white crystalline substance often used in water purification, and it causes the fine suspended particles to clump together, so it acts as a coagulant. The larger clumps then settle by gravity, a process called sedimentation, and can be separated from the water by decantation or filtration.

The same idea appears in the kitchen, where making paneer from milk uses an acid such as lemon juice or vinegar to coagulate the milk proteins.

The three differ mainly in the size of the dissolved or suspended particles. In a solution the particle size is the smallest, less than 1 nm in diameter. Colloids have larger particles, between 1 and 1000 nm. Suspensions have much larger ones, more than 1000 nm across.

That difference in size shows itself in behaviour. The particles of a suspension are visible to the naked eye and settle out on standing, as sand in water does. The particles of a solution cannot be seen at all and never settle. Colloid particles, unlike those of a suspension, do not settle over time either, but stay uniformly dispersed throughout the mixture as in a solution.

A further test separates the last two cases. A colloid scatters light and so does a suspension; a clear solution does not. So the beam shows its track through the first two and leaves none through the third.

Blood sits awkwardly between the categories. It can be separated by centrifugation and it coagulates, which suggests a suspension, yet blood cells cannot be seen with the naked eye, which suggests a solution. It is in fact neither a solution nor a true suspension but a colloid, as milk, tomato sauce and ice cream also are.

When particles throw light about in this way the phenomenon carries the name of John Tyndall, who worked on precisely that and first gave the explanation, and so it is called the Tyndall effect.

Activity 5.1 demonstrated it with a laser and three beakers. In solution A, the salt and water mixture, the path of the light beam was not visible at all. In suspension B, the chalk powder in water, the particles scattered the beam and made its path visible. In mixture C, the milk in water, the path of light was visible as well, indicating that the particles scattered the light even though the mixture appeared homogeneous.

That third result is the interesting one, because it shows a mixture can look uniform to the eye and still carry particles large enough to scatter light. The general rule follows: a colloid scatters light and so does a suspension, while a clear solution does not.

The effect is easy to observe outside a laboratory. A fine beam of light entering a dark room through a small hole becomes visible because it is scattered by the dust and smoke particles in the air. The same thing makes the beams of floodlights visible in a sports stadium, and it is why cities carrying a great deal of smoke and dust in the air often look hazy.

It is a mixture whose composition is uniform throughout, so that any part of it matches any other. Vinegar, which is acetic acid in water, and aerated drinks such as soda, which hold carbon dioxide in water, are two further examples.

Because it is equally sweet in the first sip and in the last. That sameness of taste from one end of the glass to the other shows that the composition does not vary from one part of the mixture to another.

Because it is not the same throughout. You can pick out the grains by eye, and left alone they drop to the bottom. Stirring does not change either of these behaviours.

It is how much solute is carried by a stated quantity of solvent, or of the solution as a whole. It matters because the right proportion is essential when preparing a solution, whether in medicine, agriculture, food, cosmetics or even in making a cup of tea.

ORS is a measured thing: set weights of salt and sugar into a set volume of water. Alter the weights while keeping the water the same, or alter the water while keeping the weights, and what you end with is still salt and sugar dissolved in water, but it is no longer ORS. This is also why not all sugary drinks made at home or sold in shops count as ORS.

Too little pesticide may not protect the crops at all. Too much can damage the crops, the soil and the environment. So the right amount has to be mixed with a fixed amount of water, and more is not better.

Solubility is the largest quantity of solute that a fixed amount of solvent, taken as 100 mL or 100 g, will dissolve at a stated temperature. A saturated solution is one that will take up no further solute at that temperature, so anything added beyond that point stays undissolved.

Because the figure shifts as the temperature shifts. For a solid solute in a liquid solvent the solubility generally increases as things warm up, while for a gas dissolved in a liquid it generally decreases. Without a temperature the number does not say anything definite.

It is a graph of solubility against temperature, with degrees Celsius along the horizontal axis and grams of solute per 100 g of water up the vertical one. Of two curves on the same axes, the steeper one belongs to the solute whose solubility gains more from a rise in temperature.

Evaporation returns the solute but the solvent disappears into the air. Distillation catches the vapour and cools it back to liquid, so the solvent is recovered too. It also handles two miscible liquids, which evaporation cannot separate at all.