Class 7 Science Chapter 5: Changes Around Us: Physical and Chemical

Three Things to Try

Start with three small experiments and a question to hold while you do them. Fold some sheets of paper into shapes. Blow up a balloon and let the air out. Crush a small piece of chalk to powder. Then ask whether the thing in front of you is still the same material.

The Paper

Fold a few sheets into boats, planes, whatever you like. The shapes are new and the paper is not. Unfold one and you have your sheet back, perhaps creased. Nothing has become anything other than paper at any point.

The Balloon

Blow up a balloon and grip the neck. It is enormously bigger than it was. Let the air out and it shrinks to what it was before. Now take another, inflate it, and prick it with a pin. Be careful doing that. It bursts, and you cannot make it whole again.

The Burst One Is Interesting

Hold on to the burst balloon, because it is the useful case. You cannot undo what happened. But look at what you are holding: a piece of rubber sheet, exactly the material you started with. Nothing new has been made, even though nothing can be put back.

The Chalk

Crush a small piece of chalk between two stones and you have a heap of powder. Can you get the piece back? No. Is the powder still chalk? Yes, completely. It would still write on a board if you could hold it together.

What the Three Share

In all three cases the material stayed the same. The paper is paper, the rubber is rubber, the chalk is chalk. What altered was the appearance: the shape, or the size. That is the pattern, and it has a name.

Physical Changes

Where the only things to alter are properties such as shape, size or state, we call the change a physical one. The key words are no new substance. You may hardly recognise the object afterwards, and it does not matter, because the material is unchanged.

What a Physical Property Is

A physical property is something you can observe without turning the substance into anything else. Shape is one. Size is one. State is one. Colour is another. A physical change rearranges these and leaves the substance itself alone.

And State Counts Too

You met this in Grade 6. Water exists as a solid, a liquid and a gas, and it can pass from one to another. Ice melts, water boils away, steam condenses on a cold window. At every stage it is still water, so every one of those is a physical change.

The Trap Worth Avoiding Now

Here is where people go wrong, so it is worth naming early. They assume a change that cannot be undone must be a deeper kind of change. That is not what decides it. A burst balloon cannot be undone and is physical. Melting ice can be undone and is also physical.

Ask the Right Question

So train yourself to ask only one thing: has a new substance appeared? If the answer is no, the change is physical, however dramatic it looked and however permanent it is. If the answer is yes, you are looking at something else, which is the next topic.

Weighing It Helps

There is a useful habit here. A crushed piece of chalk weighs what the piece weighed. A folded sheet weighs what the flat sheet weighed. Nothing has left and nothing has arrived, which fits the idea that no new substance has been made.

Everyday Examples

Once you have the idea, physical changes are everywhere. Chopping a vegetable. Tearing a cloth. Dissolving sugar in water, where the sugar is still sugar. Stretching a rubber band. Hammering a metal sheet flatter, which you met in the last chapter as malleability.

Why the Name Is Honest

Notice that the name tells you exactly what it does. Physical properties change; the substance does not. Many scientific terms are harder than that, so it is worth enjoying one that says precisely what it means.

One Last Check You Can Run

Here is a quick way to test yourself on any change. Say out loud what the material was before, and what it is now. If you use the same word twice, the change was physical. Paper and paper. Rubber and rubber. Chalk and chalk. Water and water.

What Comes Next

You now have one of the two main kinds of change, defined by what does not happen in it. The next topic blows air through two glasses of liquid and gets a very different result from one of them, which is where the other kind begins.

Scenario:

Anita aunt makes clay pots. Anita watches her wedge a lump of clay, roll it flat, cut it into strips and coil the strips into a pot. Then the pot dries in the sun and goes into a kiln, and what comes out is hard, ringing and cannot be softened in water again. Anita cousin says every stage was the same kind of change, because the clay never stopped being clay and the pot cannot be turned back into a lump. Anita disagrees. She says all the shaping was one kind of change and the firing was something else, and the test is not whether you can go back.

Analysis:

Anita has the right test and her cousin has the wrong one. Wedging, rolling, cutting and coiling all alter shape and size only; the clay is clay at every stage, so each is a physical change. Drying in the sun removes water, which is also physical, since the water leaving is still water. The firing is different, because the fired pot will no longer soften in water the way raw clay does, which tells you a new substance has formed. Notice why her cousin reasoning fails. He uses irreversibility as his test, and that cannot work: cutting the clay strips is irreversible too, and nobody would call that more than a change of shape. The question is always whether something new has appeared.

Activity:

Watch three changes at home today, such as ice melting in a glass, bread being toasted, and a vegetable being chopped. For each one write down what you had before, what you had after, and whether any new substance appeared. Mark the one you found hardest to judge.

Title:

Try it at home: three changes, one question

Safety:

Take care with the pin, and keep it away from your face and eyes. Ask before using the sugar.

What You Notice:

The answer is no every time, so all four are physical changes. Two of them cannot be undone and two can, which shows that going back is a different question from what kind of change it was.

Water is the easiest substance to watch moving between all three states in one kitchen.

A balloon burst by a pin is still a physical change, because the rubber is unaltered.

Crushed chalk cannot be reassembled, which shows that being undoable is not what makes a change chemical.

Ask one question: is there a new substance? No means physical.

Same stuff, new shape: that is the whole of a physical change.

Two Glasses, One Difference

Take two glasses. Put ordinary tap water in one. In the other put lime water, which is made by stirring a little chuna into water and letting it settle until the liquid above is clear. Now blow gently through a straw into each one for about a minute.

Never Suck

Before you do it, one safety point that matters. Blow out only. Do not suck the liquid up the straw at any point, particularly not the lime water. If you feel yourself needing a breath, take the straw out of your mouth first.

What You See

In the tap water, bubbles rise and burst and the water looks exactly as it did. In the lime water, something else happens. The clear liquid turns cloudy, then milky white. Leave it standing and a white powder settles slowly at the bottom.

Where the White Came From

That white solid was not in the glass before you blew into it. It was not in your breath either. So it must have been made during the minute you were blowing, out of things that were already there.

The Two Ingredients

Your exhaled breath carries carbon dioxide. The lime water is a solution of calcium hydroxide. Those two met in the glass, and between them they produced a substance called calcium carbonate, along with a little water.

Why It Looks Milky

Calcium carbonate is insoluble, which means it does not dissolve in water. So instead of vanishing into the liquid the way sugar would, it stays as countless tiny white particles spread through the lime water. That is what you see as milkiness.

Writing It Down

Scientists write this sort of thing in a short form rather than a sentence. The starting substances go on the left, an arrow shows the direction, and the new substances go on the right.

The Equation

Calcium hydroxide + Carbon dioxide gives Calcium carbonate + Water. That short line is called a chemical equation, and the process it describes is called a chemical reaction.

What Makes It Chemical

Here is the definition the activity has earned. A change that leaves at least one substance which was not there before is a chemical change. Compare that with a physical change, where the whole point was that nothing new appeared.

The Three Words

Keep the three words straight. The change is the chemical change. The process that brought it about is the chemical reaction. The short way you write it is the chemical equation. They are not three names for one thing.

A Test You Can Use

This activity hands you something useful. Since lime water goes milky with carbon dioxide and stays clear otherwise, you can use it to find out whether a gas is carbon dioxide. That is the standard test, and it comes up repeatedly later.

A Second Reaction

Try another one. Pour a little vinegar or lemon juice into a glass and add half a spoon of baking soda. It fizzes energetically. Something is clearly being produced, because gas is coming off a liquid and a powder that produced none on their own.

What the Fizzing Is

That gas is carbon dioxide again, and you can prove it. Collect the gas and pass it into lime water, and the lime water goes milky. So the test you just learned has given you the answer to a new question.

Its Equation

Vinegar + Baking soda gives Carbon dioxide + Other substances. Two reactions, two equations, and in both cases substances that were not there at the start are there at the end.

Why the Tap Water Mattered

Look back at the first glass for a moment. It would have been quicker to skip it and blow straight into the lime water. But then somebody could argue that your breath simply carries a white powder, and you would have no answer. The plain water glass closes that door.

A Comparison Is Part of the Experiment

That is worth taking away as a method, not just as a result. When you want to show that something caused an effect, set up a second case with that something missing. The difference between the two is your evidence. One glass on its own proves far less.

The Habit to Build

From now on, when you watch a change, ask what is present afterwards that was absent before. A new colour, a new solid, a gas coming off, a new smell. Any of those is a hint that you are looking at a chemical change rather than a physical one.

Scenario:

Devi is helping her mother bake. The recipe says to add baking soda to the batter, which has lemon juice in it. Devi notices the batter rise and fill with tiny holes as it bakes. Her brother says the holes are just air that got beaten in. Devi is not convinced, because the batter rose even where nobody beat it. She remembers the lime water test from school and wonders whether she could settle the argument with a glass of chuna water, a spoon of the same batter mixture, and a little patience.

Analysis:

Devi is on the right track and her test would work. Her brother explanation would predict that the rise depends on how hard the batter was beaten, and she has already noticed that it does not. The better explanation is that the baking soda is reacting with the lemon juice and making a gas inside the batter, which pushes it open into holes. To settle it she can mix a little lemon juice with baking soda in a jar. Then she can lead the gas that comes off into clear lime water and watch. Milky lime water would identify the gas as carbon dioxide and show that a new substance is being formed, which makes this a chemical change rather than beaten air. Notice that the holes alone were not enough evidence, and nor was the fizzing. The test is what names the gas.

Activity:

Make lime water at home by stirring half a spoon of chuna into a glass of water and letting it settle overnight. Pour the clear liquid off into a second glass. Blow into it through a straw, blowing out only, and time how many seconds it takes to turn milky. Write down the time and what you saw at the bottom an hour later.

Title:

Try it at home: make a gas and name it

Safety:

Blow out only and never suck liquid up a straw. Ask before using the vinegar, and keep it away from your eyes.

What You Notice:

The tap water only bubbles, while the lime water turns milky and later settles a white layer. The fizzing gas also turns lime water milky, so both reactions are making carbon dioxide, and both are chemical changes.

Lime water has nothing to do with the fruit. It is made from chuna, which is calcium hydroxide.

The milkiness never dissolves away, because calcium carbonate is insoluble in water.

The fizzing of baking soda in lemon juice is carbon dioxide escaping as the reaction proceeds.

Milky means carbon dioxide: that one result is worth memorising.

New substance, new name. If you must give the product a fresh name, the change was chemical.

A Nail Left Out in the Damp

Leave an iron nail in a damp place for some days and a brown flaky coating creeps over it. That coating is rust, and it is not iron. It has a different colour, it crumbles rather than bends, and it has to be given its own name.

So Rusting Is Chemical

Apply the test from the last topic. Is there something present at the end that was absent at the start? Yes, the rust. So rusting is a chemical change, and you met it in the chapter on metals as corrosion.

A Much Faster One

Now a change that takes seconds rather than days. A short strip of magnesium ribbon is cleaned with sandpaper, held in tongs and put into a flame. Your teacher does this, not you, and nobody looks straight at it.

What Burning Magnesium Does

It catches fire and burns with a dazzling white light, giving off plenty of heat. When it stops, a soft white powder is left in the tongs. The silvery ribbon has gone and a white powder has arrived, so again a new substance has been made.

Its Equation

The ribbon reacted with the oxygen in the air, and the powder is magnesium oxide. Magnesium + Oxygen gives Magnesium oxide + Heat + Light. Notice that the heat and the light are written in, because they are part of what the reaction produces.

Combustion

That gives us a name for this sort of reaction. A reaction in which a substance combines with oxygen, giving out heat and often light, is called combustion. In ordinary speech you would call it burning.

Combustible Substances

Not everything will do it. Substances that burn are called combustible: wood, paper, cotton cloth and kerosene are all combustible. A stone will not burn, and nor will a sheet of glass, which is why those are safe to keep near a flame.

Proving Oxygen Is Needed

Light a candle and let it burn steadily. Now lower an upturned glass tumbler over it and watch the flame. It shrinks, flickers and goes out, within a few seconds, long before the wax has run out.

What That Shows

The candle had plenty of wax left, so the wax was not the problem. What ran out was the oxygen inside the tumbler. So combustion needs oxygen, and the supply inside a closed space is limited.

And What Was Left Behind

There is a second result in the same tumbler. Pour a little lime water into it after the flame has died, swirl it round, and it turns milky. So burning the candle also made carbon dioxide, which is exactly the test you learned a topic ago.

Light Is Not Always Burning

Before the definition goes too far, a caution. A firefly glows brightly in the dark and does not get hot, and nothing in it is burning. So light on its own is not proof of combustion. The heat and the oxygen are what matter.

Why a Fire Needs a Match

Paper is combustible and air is all around it, yet a notebook on a desk does not burst into flame. Something is missing, and it is heat. Every substance has a temperature it must reach before it will catch fire at all.

Ignition Temperature

That temperature is called the ignition temperature. You can reach the ignition temperature of paper without a match. Hold a magnifying glass in bright sunlight so that the light gathers to a tiny bright spot on the paper, and wait. The spot browns, smokes and then burns.

The Fire Triangle

So three things are needed together. Something combustible to act as fuel. Oxygen, which comes from the air. And enough heat to bring the fuel to its ignition temperature. These three are drawn as the sides of a triangle, called the fire triangle.

Using It to Stop a Fire

The triangle is useful because it tells you how to put a fire out: remove any one side. Water cools the fuel below its ignition temperature. A lid on a pan cuts off the oxygen. Clearing dry grass away takes the fuel out of reach.

One Habit Worth Having

Before the last point, a habit. Whenever you meet a fire, name its three sides out loud. What is burning here? Where is its air coming from? What heated it in the first place? The answers tell you which side you can take away.

Clothes on Fire

If a person clothes catch fire, wrap them at once in a blanket. That smothers the flame by cutting off the air. It must be a cotton blanket and never a synthetic one, because synthetic cloth melts in heat and sticks to the skin, which makes the injury far worse.

Scenario:

Faiz is heating oil in a pan when it catches fire. His first thought is to carry the pan to the sink and pour water on it. His sister shouts at him to leave it alone and slides a steel plate across the top instead, and the flame dies. A moment later his own sleeve is smoking at the cuff, and his sister pulls a cotton bedsheet off the chair and wraps his arm in it tightly. There is a nylon jacket hanging on the same chair, which was closer to hand.

Analysis:

His sister made two correct decisions, and the fire triangle explains both. The plate across the pan removed the oxygen side. A fire with no air supply stops at once. Faiz water plan was the dangerous one. Carrying a burning pan spreads the fire. And water poured into hot oil turns to steam instantly, which throws the burning oil outwards. For the sleeve she again removed the oxygen, by wrapping the arm so no air could reach the cloth. Her choice of the cotton sheet over the nearer nylon jacket is the point worth remembering. Synthetic cloth melts in heat and sticks to skin, so reaching for the closer jacket would have turned a small burn into a far worse one.

Activity:

Walk round your home with an adult and find three things that could feed a fire and three things that could stop one. For each stopper, write down which side of the fire triangle it removes. Then check where the nearest cotton cloth is kept in the kitchen.

Title:

Try it with an adult: the tumbler test

Safety:

An adult must light and put out the candle. Tie long hair back, keep paper away from the flame, and never carry a lit candle.

What You Notice:

The flame dies in seconds with plenty of wax left, so the oxygen was what ran out. The lime water turns milky, so burning the candle also made carbon dioxide.

A firefly produces light without getting hot, so its glow is not a kind of burning at all.

Paper can be set alight by sunlight alone if a lens concentrates enough of it on one spot.

A tumbler placed over a burning candle puts it out in seconds, and the gas left inside clouds lime water.

Fuel, air, heat: pull out any one leg and the fire falls over.

Oxygen in, heat out. That pair is what makes a reaction a combustion.

Watch One Candle Closely

Light a candle and sit with it for a few minutes, close enough to see properly. Do not touch the flame or the hot wax. There is more happening in front of you than you might expect, and it is all worth naming separately.

The Wax Near the Flame

The first thing you notice is a small pool of liquid wax forming around the wick. The heat of the flame has melted the solid wax there. That is a change of state, from solid to liquid, and the liquid is still wax.

The Wax Running Down

Some of that liquid runs down the side of the candle, away from the heat, and sets hard again into a lumpy ridge. Solid to liquid, then liquid back to solid, and both of them changes of state. So both are physical.

The Wax That Disappears

Something else happens to the wax closest to the wick. It gets hot enough to turn into vapour and rise. You can catch sight of this indirectly: blow the candle out and watch the thin pale trail that drifts up from the wick for a second.

That Trail Is the Clue

The trail is wax vapour that escaped without burning. It tells you that wax really does become a gas near the flame. Evaporation is a change of state as well, so that step too is a physical change.

Now the Flame Itself

Here is the part that is different. The wax vapour, surrounded by air, burns. It reacts with oxygen and gives out heat and light, which is the combustion you met in the last topic. New substances are formed, including carbon dioxide.

So the Candle Does Both

Count what you have. Melting is physical. Solidifying is physical. Evaporating is physical. Burning is chemical. All four are happening at the same time in one small flame, which is why the question of which kind of change a candle shows has no single answer.

The Order Matters Too

Notice how they fit together. The solid wax does not burn directly. The heat melts it, then evaporates it, and only then does it burn. The physical changes are feeding the chemical one, step by step, for as long as the candle lasts.

A Famous Set of Lectures

This is not a small subject. Michael Faraday, one of the great experimenters, gave a celebrated series of lectures called The Chemical History of a Candle, built entirely around what you have just been watching.

A Different Question Now

Put the candle aside and ask something else about changes. Not what kind they are, but whether they can be brought back. These are two separate questions, and keeping them separate is the point of the rest of this topic.

Changes That Come Back

Melt an ice cube and you get water. Put the water in the freezer and you get ice again. Boil water away and it becomes vapour, and the vapour can be condensed back to water on a cold surface. Changes like these are called reversible changes.

Changes That Do Not

Now chop a vegetable into small pieces. No arrangement of the pieces joins them back into a whole vegetable. Or heat maize grains in a covered pan until they pop. Nothing you do to popcorn turns it back into maize grains.

Why the Two Questions Are Separate

Here is the trap, and it is the same one from the first topic. Chopping a vegetable is a physical change, and it cannot be undone. Melting ice is a physical change, and it can. So knowing the kind of change does not tell you whether it is reversible.

And the Other Way Round

It works in the other direction too. Knowing that a change cannot be undone does not tell you its kind. Chopped vegetables and popped maize are both final, but a chopped vegetable is still that vegetable, while popping has made something genuinely new.

Why the Candle Is the Standard Example

There is a reason this one object turns up in every textbook. Most examples show you one kind of change at a time, which makes them easy and a little misleading. A candle refuses to be tidy. It makes you look at each step separately, which is the real skill.

What to Carry Forward

So ask both questions of every change you meet. First, has a new substance been formed? That gives you physical or chemical. Second, can the change be brought back? That gives you reversible or not. The answers are independent, and a candle will keep you honest about both.

Scenario:

The power goes out and Meera lights a candle on a saucer. Her little brother watches the wax pool spread, then run down and harden into a lumpy ridge, and asks whether the candle is being destroyed. Meera says nothing is being destroyed at all, because the ridge proves the wax is still wax. Their father, making popcorn on the gas, says the two things are the same sort of change, since neither the ridge nor the popcorn can be put back. Meera is not satisfied with that, and she wants to separate two things her father has run together.

Analysis:

Meera is right to object, and the ridge is the key to it. Wax that melts and then sets hard is still wax, so that step is a physical change, and it is reversible, because melting it again gives the same liquid back. Popcorn is different in both respects. A popped grain is genuinely a new thing and not simply a maize grain in a new shape, so popping is a chemical change, and no cooling or waiting turns it back. Her father has collapsed two separate questions into one. The first is whether a new substance was formed, which separates physical from chemical. The second is whether the change can be brought back. His own kitchen contains a case that breaks his rule, because a chopped vegetable cannot be put back either, and it is plainly still that vegetable.

Activity:

Light a candle with an adult and watch it for five minutes. Write down every separate change you can see happening to the wax, then mark each one as physical or chemical and as reversible or final. Compare your list with a friend and see which of you found more.

Title:

Try it with an adult: the vapour trail

Safety:

Only the adult handles the match and the flame. Keep paper and hair well away, and never carry a lit candle.

What You Notice:

You should find three physical changes and one chemical one. The pale trail is unburnt wax vapour, which is the proof that wax really does turn into a gas before it burns.

The solid wax in a candle never burns directly; it is the wax vapour near the wick that burns.

Michael Faraday filled a lecture series with nothing but the science of a burning candle.

No amount of cooling will turn popcorn back into maize grains.

Melt, run, set, evaporate, burn: only the last one is chemical.

Two questions, not one: which kind of change, and can it come back?

Changes We Arrange on Purpose

Some changes we set up deliberately because we want the result. A little curd stirred into warm milk and left overnight gives a whole bowl of curd. Green mangoes left in the warm ripen and sweeten. Raw dough in a hot pan becomes a roti.

Changes We Would Rather Avoid

Others arrive without being invited and cost us something. An iron gate left out in the rain grows a brown flaky coat and weakens. Food kept too long goes bad and has to be thrown away. Nobody arranged either of those.

Two Useful Words

So changes can be sorted a second way. The ones we want are called desirable changes, and the ones we do not are called undesirable changes. Notice at once that this has nothing to do with whether they are physical or chemical.

The Interesting Case

Now consider one change twice. Vegetable peel and leftover food decompose. In the kitchen store that is a disaster, because it ruins what you meant to eat and smells unpleasant. In a compost pit it is exactly what you want, because it gives you manure for the garden.

What That Tells You

The process has not altered in the slightest. What altered is our purpose. So desirable and undesirable are not properties of a change the way being chemical is. They describe how the change fits what we were trying to do.

Changes We Cause Without Meaning To

There is a third group worth naming: changes we bring about as a side effect. When we burn fuels in vehicles and stoves and factories, the combustion adds carbon dioxide to the atmosphere. Nobody burns fuel in order to do that, and it happens all the same.

Another Everyday One

Paint drying on a wall seems harmless. As it dries it releases substances into the air that contribute to pollution. The paint was wanted; the release was not, and it came with it.

Why These Matter

These are worth separating out because they are nobody intention and everybody problem. The lesson is not to stop cooking or painting, but to notice that a desirable change can carry an undesirable one along with it.

Now Slow Down a Long Way

Change the time scale completely. Go to the foot of a hill or a mountain and look at the heaps of broken stone, grit and loose soil lying there. None of it was made there. All of it was once part of the solid rock above.

How Rock Comes Apart

Several things work on rock where it stands. Day after day it heats in the sun and cools at night, and the repeated expansion and shrinking cracks it. Plant roots work into those cracks and widen them as they grow. Water that gets into a crack and freezes pushes outwards hard enough to split the stone.

That Is the Physical Half

All three of those are physical. The rock ends up in smaller pieces, and each piece is still the same rock. Nothing new has been made, exactly as with crushed chalk in the first topic of this chapter.

And There Is a Chemical Half

Look at basalt, a hard black rock that contains iron. At an exposed surface it often shows a reddish layer. That red is iron oxide, formed when the iron in the rock reacted with its surroundings. A new substance has appeared, so this part is chemical.

Weathering

Taken together, these physical and chemical processes breaking rock down where it sits are called weathering. It is slow beyond anything in your own experience, and given enough time it is one of the ways soil is formed.

Erosion Is the Other Word

Weathering loosens the material but does not take it anywhere. Wind and flowing water do that, picking up pebbles, grit and soil and carrying them off, sometimes a great distance. That removal and transport is called erosion.

Landslides and Settling

Erosion is mostly physical. A landslide tearing a hillside away is a physical change, and so is a river carrying sand downstream. Where the water slows or the wind drops, the load it was carrying settles out as sediment.

Two Words, Two Jobs

Keep the pair straight with a short phrase. Weathering breaks it; erosion takes it. One works on rock that has not moved. The other moves what the first one loosened.

Rock Being Made Again

Those sediments pile up in layers, and over thousands of years the weight and the time harden them into new rock. That is worth sitting with: rock is being broken down in one place and built up in another, and the building is not something that can be undone.

Scenario:

A new road is cut into a hillside behind Tanvi village, and the cut face is left bare. Within two monsoons the villagers notice three things. Loose grit and small stones collect at the base of the cut after every spell of rain. The black rock of the face has developed red-brown streaks in places. And the stream below the cut now runs muddy and has built a wide fan of sand where it flattens out near the fields. The village council argues about whether to pay for a retaining wall or for planting the slope, and somebody says the two problems are really one problem.

Analysis:

There are two distinct processes at work and the council needs both words. The cracking of the exposed rock and the red-brown streaks are weathering. The rock is being broken down where it stands. That is partly physical, as it heats and cools and as water gets into cracks. It is partly chemical too, because the red-brown colour is iron in the rock becoming iron oxide. The muddy stream and the fan of sand are erosion. Flowing water is carrying the loosened material away and dropping it where the stream slows and spreads out, which is exactly why the fan has formed there. The two are linked but not identical, and the distinction decides the spending. A retaining wall catches material that has already come loose, which addresses the erosion. Planting the slope slows the weathering and binds the surface so there is less loose material to carry. Given enough time the fan of sand will itself harden into rock, and that part cannot be undone.

Activity:

Find a wall, a rock or a paved path near your home that has been exposed to the weather for years. Write down every sign of breaking down you can see, and mark each one as physical or chemical. Then look for somewhere nearby that loose material has collected, and say what carried it there.

Title:

Try it at home: a crack and a bottle of water

Safety:

Use a plastic bottle and never glass, and ask before using the freezer. Do not force the cap back on once the ice has expanded.

What You Notice:

The ice pushes the bottle out of shape, which is the same force that splits rock when water freezes in a crack. Reddish patches on a dark stone are iron oxide, the chemical side of weathering.

The heaps of loose material at the foot of a mountain were once part of the rock above.

Black basalt turns red at the surface because the iron in it becomes iron oxide.

Sediments laid down today may be solid rock in a few thousand years, and that cannot be reversed.

Weathering breaks it; erosion takes it. Two words, two jobs.

Desirable is about us, not about the change. Ask who wanted it.