Class 8 Science Chapter 8: Elements, Compounds and Mixtures

Look Around First

This book, the trees outside, your shoes, the air, the staircase and the water you drink: every one of those is matter, and every one is made of tiny particles. Here is the thing worth noticing. Hardly any of it is made of just one substance.

Poha and Salad

Think about a plate of poha, or a bowl of sprout salad. They look like very different dishes and they have one thing in common: somebody made each of them by putting several things together.

The Definition

That is a mixture. Put two or more substances together so that each one holds on to the properties it had, and what you have is a mixture. Sugar in water is one. So are soup and lemonade.

The Parts Have a Name

The separate substances inside a mixture are its components. That word is worth learning properly, because the everyday word for the parts of a dish is ingredients, and in science the two are not interchangeable.

Nothing Reacts

Here is the condition that makes a mixture a mixture. No component reacts chemically with any other. They sit side by side, unchanged, which is why you can still find each of them afterwards if you know how to look.

When You Can See the Parts

In a sprout salad the green gram, the chickpeas, the onion and the tomato are all there in front of you. A mixture whose components can generally be made out by eye, or with a lens, is called non-uniform. Its parts are not spread evenly through it.

When You Cannot

Now stir sugar into water. Not a grain is visible, and no microscope will pick the sugar out either. A mixture whose components are spread evenly and cannot be told apart is called uniform, and it looks for all the world like a single substance.

That Is the Trap

So the test for a mixture is not whether you can see the parts. It is whether more than one substance is in there, each keeping its own properties. Looking like one thing and being one thing are different questions, and this chapter keeps asking you to separate them.

A Uniform Mixture You Can Hold

Stainless steel is the example to remember. It holds iron, nickel and chromium with a little carbon, blended so evenly that the whole thing looks the same right through and no single metal can be picked out.

Alloys

A mixture of metals like that is called an alloy. Brass is another: copper blended with zinc. Bronze is a third, copper blended with tin. All three are uniform mixtures, and all three are sold and used as though they were single materials.

Mishraloha

This is old knowledge. Indian texts gave the name Mishraloha to a blend of two or more metals whose properties differed from the metals in it. The Charaka Samhita and the Susruta Samhita, among others, describe using such blends medicinally.

Kamsya

One example is worth having. Bronze, called Kamsya, was made from four parts copper to one part tin and was used to help digestion and build resistance to illness. Note the ratio: somebody had worked out that the proportions mattered.

Components Must Be Pure

One more point of strictness, and it is a scientific one. In everyday speech the components of a mixture can themselves be mixtures, as the poha in a plate of poha is. In science, the components of a mixture have to be pure substances.

Soups and Sherbets

It is worth collecting examples, because the word covers more than it looks as though it should. A bowl of soup is a mixture. So is a glass of lemonade, and the sherbet at a wedding, and the air in this room, and the sea. Some of those show their parts and some do not, and every one of them satisfies the definition: more than one substance, each keeping what it had, and nothing reacting.

Why Separate a Mixture?

In earlier grades you separated mixtures to get at the part you wanted and threw the rest away. In science the purpose is different: a mixture is separated in order to obtain pure substances. That difference in purpose is what the next topic begins to need.

What To Carry Forward

A mixture is two or more substances put together with each keeping its own properties, and nothing reacting. Its parts are called components. If you can make the parts out it is non-uniform, and if they are spread evenly and cannot be told apart it is uniform. Alloys are uniform mixtures of metals, and looking like one substance is not the same as being one.

Scenario:

Two friends are asked to bring one uniform mixture and one non-uniform mixture to class. Nadia brings a bowl of sprout salad and a bottle of sherbet. Omar says the sherbet cannot be a mixture, because he can see nothing in it but liquid, and he proposes bringing a glass of plain water instead. Their teacher asks Nadia to pour a little sherbet into three glasses and asks the class to taste each one. All three taste exactly the same. She then asks Omar what he would expect from his glass of plain water.

Analysis:

Omar had the test backwards. Seeing the parts is what makes a mixture non-uniform, not what makes it a mixture. The three glasses tasting alike is the evidence that the sherbet is a mixture spread evenly, with sugar and flavour in every spoonful. Plain water would have been the wrong example because it has nothing mixed into it at all.

Activity:

List three mixtures from your own kitchen and mark each one uniform or non-uniform.

Title:

Try it at home: sort the shelf

Safety:

Do not taste anything, and put every item back where you found it.

What You Notice:

The hardest entries are the ones that look like one substance, such as salt water or ghee. Looking like one thing is not the same as being one thing, and that is the whole point of the uniform case.

Indian texts gave the name Mishraloha to a blend of two or more metals, and describe such blends being used medicinally.

Bronze, called Kamsya, was made from four parts copper to one part tin, so somebody had already found that the proportions mattered.

A magnet will pull iron filings back out of a mixture with sulfur, because mixing changes none of the properties.

LOOKS ARE NOT THE TEST — A uniform mixture looks exactly like one substance. Ask what is in it, not what it looks like.

MIXED, AND STILL THEMSELVES — Every component of a mixture keeps the properties it had, because nothing has reacted.

The Hardest Case

If looking like one substance is not the same as being one, then air is the hardest case in the chapter. You cannot see it, you cannot pick anything out of it, and we talk about it as a single thing all day long. So: is air a mixture?

It Is, and a Uniform One

It is. Several gases are blended into it so evenly that every breath you take is the same as the last. Nitrogen makes up the largest share by a long way, about seventy-eight parts in a hundred. Then comes oxygen, and after that small amounts of argon and carbon dioxide, with water vapour as well.

What Each One Does

Oxygen is the one most living things need in order to stay alive, and it is also what lets things burn. Nitrogen does neither: it takes no part in burning at all, which is worth knowing, because it is the gas there is most of.

The Water in It

Air also carries water in the gaseous state. Let warm air touch a cool surface and that vapour turns back into liquid, which is why droplets appear on the outside of a cold bottle. The water was in the air before it was on the glass.

Proving Carbon Dioxide Is There

Nitrogen and oxygen you have met before. Carbon dioxide is the one worth proving, and there is a neat test for it. It takes an afternoon and a little care.

Making Lime Water

Half fill a glass with water and stir in a little calcium oxide, which is also called quick lime, slowly and carefully. It attacks the water vigorously, gives out heat, and what forms is calcium hydroxide. Keep stirring until it has dissolved, then filter it. You are left with a colourless solution, and that is lime water.

Leave It Out

Pour the lime water into a petri dish and leave it on a ledge for a few hours, stirring it now and then. Come back and look at it. It has turned milky.

Why It Went Milky

Carbon dioxide from the air has met the calcium hydroxide in the dish. The two react to give water and calcium carbonate, and calcium carbonate will not dissolve, so it shows up as a cloud of tiny white particles. Nothing was added to the dish but air. So the air holds carbon dioxide.

Something Else in the Air

Now a different question. Have you watched specks dancing in a beam of sunlight through a gap in a dark room? Those are not any of the gases. What are they, and are they part of the air?

The Black Paper Test

Take a sheet of black paper, make sure there is not a speck on it, and leave it flat and undisturbed near an open window for a few hours. Then look closely, with a lens if you have one. Specks have settled on it.

Dust Is a Passenger

So there is dust suspended in the air. But it is not one of air’s components. Repeat the test in another room or on another day and the number and the kind of specks come out different, while the gases do not. Something that varies like that is carried by the air rather than part of it, and such things count as pollutants.

Pollutants and the Index

The main pollutants in air are particulate matter, meaning dust and soot, along with gases such as sulfur dioxide, nitrogen dioxide, ozone and carbon monoxide. There is a published figure for how bad the air is on a given day, called the air quality index, or AQI.

Mixtures Come in Kinds

Air is a gas mixed into other gases, and that is only one of the possibilities. A mixture can be made from components in any combination of states, and the table in the book runs through six of them.

The Six Kinds

Gas in gas, like air. Gas in liquid, like the fizz in soda water or the oxygen dissolved in a pond. Solid in gas, like carbon particles in smoke. Liquid in liquid, like vinegar, or like oil and water. Solid in liquid, like seawater, or like sand stirred into water. And solid in solid, like baking powder or any alloy.

Uniform or Not, In Each Case

Work down that list asking the question from the last topic and the answers are not all the same. Air and seawater and vinegar are uniform. Sand in water, and oil in water, and carbon in smoke are not. The kind of mixture and the evenness of it are two separate questions.

One Point of Strictness

Finally, the rule the last topic ended on. In ordinary speech a part of a mixture may itself be a mixture. In science every component has to be a pure substance, which raises the obvious question: what exactly is a pure substance? That is the next topic.

What To Carry Forward

Air is a uniform mixture: mostly nitrogen, then oxygen, with argon, carbon dioxide and water vapour. Lime water going milky proves the carbon dioxide is there. Dust is suspended in air and is not a component of it, because it varies from place to place. And mixtures exist in every combination of states.

Scenario:

A group makes lime water by adding quick lime to water, stirring it well and filtering it until it is colourless. They pour some into a petri dish and leave it on a window ledge for the afternoon, stirring now and then. By evening it has gone milky. One pupil writes that the air has made the water dirty. Another writes that the lime water has gone off and should be thrown away. A third says the experiment has worked and asks what the white particles are made of.

Analysis:

The third pupil is right. Carbon dioxide from the air has reacted with the calcium hydroxide in the lime water to make calcium carbonate, which is insoluble, so it shows as tiny white particles. The milkiness is the result the test was set up to produce, and it is the evidence that the air contains carbon dioxide.

Activity:

Find out which gas in the air is used by living things and which takes no part in burning.

Title:

Try it at home: catch the dust

Safety:

Keep the paper away from cooking fires and do not breathe the dust in.

What You Notice:

The two counts come out different, and that is the answer. A component of a mixture is fixed, and anything that changes with the room and the day is a passenger rather than a part.

The lime water has to be filtered colourless before the test, because otherwise there would be no way to say where the cloudiness came from.

The milkiness is not dirt. It is calcium carbonate, which refuses to dissolve and so shows up as a cloud of tiny white particles.

Repeat the black paper test in two rooms and the counts differ, which is exactly why dust cannot be one of air’s components.

MOST NITROGEN, USEFUL OXYGEN — Nitrogen is the biggest share and does nothing in a fire. Oxygen is smaller and does the work.

IT VARIES, SO IT IS A PASSENGER — A component of a mixture is fixed. Anything that changes with the room and the day is carried, not part.

A Word on Two Jobs

The last two topics kept ending on the same requirement: the components of a mixture have to be pure substances. Time to find out what one is. And the first thing to settle is that the word pure is doing two different jobs, in two different places.

Pure, on a Packet

Look at packets of milk, ghee and spices and you will find the word printed on them. There it means unadulterated: nothing cheaper has been put in.

Adulteration

That matters because adulteration is illegal. It means adding something cheaper, or of poorer quality, to a product, usually to increase the quantity or to cut the cost of making it. It lowers the quality, and it can leave the product dangerous to health.

Pure, in Science

In science the word asks a different question. A substance is pure when there is nothing else present in it at all: it is made of a single kind of particle, and every one of those particles behaves the same way as every other.

The Test

There is a sharper way to put it. A pure substance is matter that no physical process can divide into different kinds of matter. Filtering will not do it, nor will boiling, nor will a magnet, nor will settling. That is the test, and it is a strict one.

So Milk Is Not Pure

Which means a scientist will call milk impure even when it is perfectly unadulterated, because water, fat, protein and sugar are all in there. One spelling, two meanings, and both answers are correct in their own sense. Read which question is being asked.

Now a Harder Question

Take water, which looks like one substance and behaves like one. Cool it and it becomes ice. Heat it and it becomes vapour. Cool the vapour and water comes back, so nothing was lost or changed in either direction. Does that make water a pure substance?

Try Electricity Instead

Those were all physical changes, so they prove nothing either way. Here is a different sort of attempt, and it needs doing with a teacher present.

Setting It Up

Two-thirds fill a beaker with water, then let in a few drops of dilute sulfuric acid. Stand a nine-volt battery in the beaker. Fill two small test tubes completely with water from the beaker and, without spilling, stand one upside down over each terminal of the battery.

What Happens

Within minutes bubbles of gas are collecting inside both tubes. Leave it running for ten or fifteen minutes and there is a good deal of gas in each one, and here is a detail worth noting: the two volumes are not equal.

Not Water Vapour

Could the gas be water vapour? No, and the reason is simple. Vapour would have cooled inside the tube and turned back into water. Whatever is collecting has stayed a gas, so it is not water in another state.

Testing the Two Gases

Lift the tubes out one at a time and bring a lighted candle to the mouth of each. One gives a pop, and that pop is how hydrogen announces itself. The other makes the candle flame glow brighter, which is how oxygen does.

What That Means

So an electric current has taken water apart into hydrogen and oxygen. Two different substances have come out of what looked like one. Water is therefore made of those two, which is something no amount of boiling or filtering could have shown.

A Chemical Change

Notice which kind of change that was. Boiling water and freezing it are physical changes, and water comes back from both. Passing a current through it is a chemical change, and it does not come back.

Where This Leads

Water is still a pure substance, by the way. Every particle of it is the same as every other, and no physical process separates it. What the experiment shows is that a pure substance need not be the simplest possible thing: hydrogen and oxygen are simpler, and water is built from them.

Two Kinds of Pure Substance

Which means pure substances come in two sorts. There are the ones that cannot be broken into anything simpler at all, and the ones that can be broken down, though only by chemical means. The next topic gives both of them their names.

What To Carry Forward

On a packet, pure means unadulterated. In science it means one kind of particle only, with nothing else present and no physical process able to divide it. Milk is the first sort and not the second. Passing a current through water gives hydrogen, which pops, and oxygen, which brightens a flame, so water is built from two simpler substances.

Scenario:

A shopkeeper shows a class two packets. One is labelled pure cow ghee and the other pure table salt. He says both are pure, and the class agrees. Their teacher asks a second question: which of the two is a pure substance in the scientific sense? The class says both again. She then asks what each one is made of. Ghee turns out to hold several different substances mixed together. Table salt turns out to be sodium chloride, one kind of particle throughout. One pupil objects that the shopkeeper has not lied about anything.

Analysis:

The pupil is right, and so was the shopkeeper. On a label pure means unadulterated, and nothing cheaper has been added to either packet. In science the word means the substance holds only one kind of particle, so the salt qualifies and the ghee does not. Two tests, one spelling, and both answers are correct in their own sense.

Activity:

Find three packets at home stamped pure and decide which hold a pure substance in the scientific sense.

Title:

Try it at home: read the labels

Safety:

Put every packet back as you found it, and do not open any of them.

What You Notice:

Most lists name several substances, so most of these are mixtures however pure the front of the packet says they are. The word on the front is a claim about adulteration, not about what the contents are made of.

The gases cannot be water vapour, because vapour would have cooled inside the tube and turned straight back into water.

The two volumes of gas come out unequal, which is a first clue to the fixed ratio in which the elements combined.

A few drops of dilute acid are added for one reason only: plain water is a poor path for an electric current.

PACKET SAYS HONEST, SCIENCE SAYS SINGLE — One spelling, two questions. Read which one is being asked before you answer it.

POP IS HYDROGEN, BRIGHTER IS OXYGEN — The two candle tests at the mouths of the tubes, and the whole identification rests on them.

Two Sorts of Pure Substance

The last topic left a loose end. Water is a pure substance and an electric current still took it apart into hydrogen and oxygen. So some pure substances can be broken down and some, presumably, cannot. Both sorts have names, and this topic is about both.

Elements

The hydrogen and the oxygen that came out of the water are the first sort. An element is a substance that cannot be broken down into anything simpler, by any means at all. Elements are what everything else is built from.

Made of Atoms

Each element is made of one kind of particle and nothing else. Those particles are called atoms, and the atoms of one element differ from the atoms of every other. That is what makes an element a single thing rather than a collection.

Molecules

Most elements cannot keep their atoms on their own, so two or more join up into a stable particle called a molecule. Two atoms of hydrogen make one molecule of hydrogen. Two atoms of oxygen make one molecule of oxygen.

Some Examples

Gold, silver, sulfur and carbon are elements. So are iron, aluminium, magnesium, hydrogen and oxygen. There are a hundred and eighteen known at present, and most of them are solids.

Metals and Non-metals

Elements fall into groups you already know. Iron, aluminium, gold, magnesium and silver are metals. Oxygen, hydrogen, sulfur and carbon are non-metals. The division is a useful one, because the two groups behave very differently.

And a Group Between

Some elements will not sit neatly in either camp. Silicon and boron have behaviour that falls between the two main groups, and they are called metalloids. You will meet them properly in a later grade.

A Few Counting Facts

Worth collecting. Eleven elements are gases at room temperature, and every one of those is a non-metal. Only two are liquid: bromine, a non-metal, and mercury, which happens to be a metal. Gallium and caesium are solids that turn liquid at about thirty degrees, which is the temperature of a warm day.

Compounds

Now the second sort of pure substance. When unlike elements join chemically in a proportion that never varies, what results is something entirely new, and that is a compound. Water is the one you have already made twice.

Why Water Will Not Separate

In water the hydrogen and the oxygen are held to each other so tightly that no physical method will part them. That is precisely why the last topic needed a current. A compound hands its elements back only to a chemical change.

Properties of Its Own

Here is the striking thing about a compound. Its properties are not the properties of the elements in it, and they can be the opposite. Hydrogen is a fuel. Oxygen supports burning. Water puts fires out.

The Fixed Ratio

And the proportions are not a matter of choice. In water the number of hydrogen atoms to oxygen atoms is always two to one. That is what makes the unequal gas volumes in the last topic worth noticing: the ratio was visible in the experiment.

Salt, From Two Unlikely Parents

Sodium is a soft metal and chlorine is a hazardous gas. Put them together and they combine into sodium chloride, which is harmless, improves the taste of food and is essential to us. The ratio here is one to one.

Salt Dissolved Is Still Salt

Note the difference between two things that sound alike. Salt dissolved in water can be got back by evaporating the water, because that is a mixture. Splitting sodium chloride into sodium and chlorine is a different matter altogether, and no physical process will do it.

Is Sugar an Element?

Try it and see. Drop a teaspoonful of sugar into a boiling tube and heat it gently, with a teacher present. It turns brown, and then it chars to black. And droplets of water appear near the open end of the tube.

Where That Water Came From

The tube is being heated, not cooled, so nothing is condensing on it from the air, and the sugar came dry out of a jar. The water must have come out of the sugar. Since water is hydrogen and oxygen, and the black residue is carbon, sugar holds three elements.

So Sugar Is a Compound

Which settles it: sugar cannot be an element, because it has been broken into simpler things. It is a compound of carbon, hydrogen and oxygen. Scoop the charcoal onto a watch glass and you can even try burning it like coal.

What To Carry Forward

An element cannot be broken into anything simpler and is made of one kind of atom. Elements are metals, non-metals or metalloids. A compound is unlike elements joined chemically in one unvarying proportion, with properties of its own that can be the opposite of its parts, and no physical process separates it. Heating sugar proves it is a compound of three elements.

Scenario:

A group heats a teaspoon of sugar gently in a boiling tube with their teacher watching. The sugar browns, then chars to a black mass, and droplets of water appear near the open end of the tube. One pupil says the droplets are water vapour from the air condensing on the glass. Another says the sugar must have been damp when they started. A third points out that the black residue scooped onto a watch glass looks exactly like charcoal and asks whether it will burn.

Analysis:

The first two explanations do not hold. The tube is being heated, so nothing is condensing on it, and the sugar was dry from a sealed jar. The water therefore came out of the sugar itself. Since water is made of hydrogen and oxygen, and the residue is carbon, sugar holds three elements and cannot possibly be one.

Activity:

Find out which three elements glucose is made of and compare them with sugar.

Title:

Try it at home: hunt for elements

Safety:

This is a looking exercise only. Do not heat or taste anything.

What You Notice:

The metal list fills up quickly and the compound list is harder, because most compounds are sold under their everyday names. Not one of the compounds can be separated by hand, which is exactly what distinguishes them from mixtures.

Gallium and caesium are solid elements that turn liquid at about thirty degrees Celsius, which is the temperature of a warm day.

More than forty-five different elements go into making a mobile phone, counting the screen, the battery and the rest of its parts.

The charcoal left after heating sugar can be scooped onto a watch glass, and it will burn much as coal does.

FUEL PLUS FIRE-FEEDER MAKES FIRE-KILLER — Hydrogen burns and oxygen supports burning, and the water they build is what puts a fire out.

THE RATIO IS NOT A CHOICE — Water is always two hydrogen atoms to one of oxygen, which is why the two gas volumes were unequal.

The Cleanest Experiment in the Chapter

Everything so far has been definitions. This topic is one experiment that settles the lot, and its design is worth as much as its result: take the SAME two elements, make two different things out of them, and test both the same way.

Weigh Out the Two

Put five point six grams of iron filings and three point two grams of sulfur powder on a watch glass. Look at them first. The filings are dark and the sulfur is yellow, and nobody could mistake one for the other.

Sample A

Mix them thoroughly on the watch glass and call the result Sample A. Look again. You can still see black specks and yellow specks, so this is a mixture, and a non-uniform one at that.

Sample B

Now take half of Sample A into a china dish and heat it gently, stirring all the while, until a black mass forms. Let it cool, grind it in a mortar, and put it on a second watch glass as Sample B.

Two Samples, One Pair of Elements

That is the whole point of the design. Sample A and Sample B contain nothing but iron and sulfur, in the same proportions. Anything the two samples do differently must come from how the elements are joined, and not from what is in them.

Test One: Look

Sample A still shows two colours, because its components are sitting side by side. Sample B is the same colour and texture right through. One test, and already they are behaving differently.

Test Two: A Magnet

Pass a magnet over Sample A and the iron filings leap to it, so the iron and the sulfur can be pulled apart by hand. Pass the same magnet over Sample B and nothing moves at all.

Why the Magnet Fails on B

Because there is no free iron in Sample B to find. The iron has combined chemically with the sulfur, and what resulted is a new substance with properties of its own, one of which is that a magnet ignores it.

Test Three: Acid, and a Smell

Put a little of each sample in a tube and let in a drop or two of dilute hydrochloric acid. Both give off a gas, and this is where many people stop too early, because the two gases are not the same. The iron in the mixture reacts with the acid to give iron chloride and hydrogen. The hydrogen has no colour and no smell, and it burns with a pop at a flame. The sulfur just sits in the tube, yellow and untouched, because sulfur will not react with the acid at all.

Sample B with Acid

The iron sulfide reacts with the acid to give iron chloride and hydrogen sulfide. That gas has no colour either, and it smells strongly of rotten eggs. Waft it towards your nose with a hand; never put your nose to a tube.

What the Three Tests Say Together

Sample A is a mixture of iron and sulfur. Its components kept their own properties, which is why the colours stayed visible, the magnet worked, and the acid found iron to react with. Sample B is a compound, iron sulfide, which has properties none of its parts had and will not be separated by anything physical.

Why Three Tests, Not One

Because one can mislead. A pupil who only used the magnet would be right that Sample B is different and could easily conclude that the iron has somehow gone. The rotten-egg gas proves the iron is still there, since hydrogen sulfide cannot form without it. Three tests agreeing is what a conclusion needs.

Where All of This Shows Up

Elements, compounds and mixtures are not a classroom exercise. The air is a mixture, water is a compound, and iron and aluminium are elements used in bridges, buildings and vehicles. Chemists study how elements combine in order to invent medicines and vaccines, and to make fertilisers that feed a growing population.

Engineering a Mixture on Purpose

Material scientists use the same knowledge to design what they need. Stainless steel is stronger and lasts longer than pure iron, which is why it is an alloy rather than a metal. Wood, steel and concrete are all mixtures, and all three hold up buildings.

Minerals

Most rocks are mixtures of minerals, and you can often see the grains with a lens. Quartz, calcite, mica, pyroxene and olivine are common examples. Cement starts from iron oxide, alumina, quartz and calcite, all of which are minerals or come from them. Talcum powder comes from the mineral talc.

Native Minerals

Most minerals are compounds of more than one element, but a few are not. A native mineral is a pure element: gold, silver and copper among the metals, sulfur and carbon among the non-metals. So the question "is a mineral a compound?" has the answer "usually, but not always".

One Last Distinction

Everything made of matter is built from elements and compounds, and matter means anything with mass that occupies space. Not everything around us qualifies. Light, heat and electricity matter a great deal to our world, and so do thoughts and feelings, and not one of them is made of matter at all.

What To Carry Forward

Take two elements, mix them, and heat half the mixture. The mixture shows two colours, gives up its iron to a magnet, and makes odourless hydrogen with acid. The compound is one colour, ignores a magnet, and makes hydrogen sulfide that smells of rotten eggs. Three tests, not one. And matter is not everything: light and heat are not made of it.

Scenario:

Two groups are given Sample B, the black mass made by heating iron filings with sulfur. Group One passes a magnet over it, sees nothing move, and writes that the iron has been used up and is gone. Group Two does the same magnet test and then adds a little dilute hydrochloric acid, which gives off a gas smelling strongly of rotten eggs. They write that the iron is still present but chemically combined. Their teacher asks which group has the stronger case and why.

Analysis:

Group Two, because they have a second line of evidence. The magnet shows only that free iron is absent, not that iron has left. The hydrogen sulfide proves iron and sulfur are both still there, joined in iron sulfide, because that gas could not form without either of them. One test tells you what has changed; two tell you what it changed into.

Activity:

List three minerals and say for each whether it is a compound or a native element.

Title:

Try it at home: sort the shelf again

Safety:

This is a sorting exercise on paper. Nothing needs to be handled or heated.

What You Notice:

The circled entries are the elements and the compounds together, because a pure substance is either one. A mixture never qualifies, since it holds more than one kind of particle however evenly it is mixed.

Hydrogen sulfide cannot form without both iron and sulfur, which is why its smell proves the iron is still inside Sample B.

Graphene aerogel is made from carbon and is lighter than any other material known, so light that a blade of grass will hold a piece of it up.

Dhokra art from Bihar and Odisha is cast in molten brass or bronze, using a clay mould shaped first in beeswax.

ONE TEST CHANGES, THREE TESTS TELL — The magnet shows only that free iron is absent. The rotten-egg gas shows the iron is still there.

SAME PARTS, DIFFERENT JOINING — Both samples hold the same elements in the same proportions, so every difference comes from the joining.