Class 6 Science Chapter 8: A Journey through States of Water

Ice in the Shikanji

It is a bright afternoon in summer, and Aavi and Thirav are enjoying their shikanji. Thirav looks at the ice floating in the glass and begins to wonder about it. Is that ice the same thing as the lemonade around it, or something else altogether?

Two Views at One Table

Thirav argues that they must be different substances. Ice feels hard and can be held in your hands, and water cannot be held that way at all. Aavi disagrees and says they are the same substance. Thirav then suggests that the freezer may have put something into the ice.

One Ice Cube, One Cup

Aavi proposes a test: put water in a freezer and see whether it turns into ice. Try the reverse too. Set a cube of ice in a cup, stand it on the table, and watch. The ice gets converted into water.

So They Are One Substance

Ice and water are two forms of the same substance. Such forms are also called states. Nothing was added in the freezer and nothing was taken away on the table. Only the form changed.

The Same Thing Behaving Differently

Two states of one substance can still behave nothing alike. Water flows and ice does not. Water splashes and ice does not. That difference in behaviour is what made Thirav suspect two substances in the first place.

Puddles in the Playground

It is a rainy morning, and on the way to school the two of them notice how many water puddles the playground is holding. Coming back to play that evening, they get a surprise. Some of the water in those puddles is no longer there.

The Same Thing on the Utensils

You have probably seen it closer to home. Water left on washed utensils dries up after a while. Aavi wonders whether the utensils have let the water through their surface. Thirav thinks nothing of the sort happened.

Settle It With a Plate

Two guesses and no way to choose between them, so design something that decides. Put one tablespoonful of water onto a steel plate. Watch whether any of it comes through to the far side. Keep checking at even intervals until it has all gone.

What the Plate Shows

The water goes, and the other side of the plate stays dry throughout. So the water did not travel downwards through the steel. One plate is not quite proof for every surface, and it is fair to ask whether one trial settles the matter. Still, for this plate the answer is plain. The question that remains is a better one than the one you started with: where has it gone instead?

Upward, as Vapour

It has gone into the air. Water converted in this way enters its gaseous state, which is called water vapour. That is another state of the same substance, alongside ice and liquid water. So the count stands at three, and you had already met two of them in a glass of shikanji.

The Dosa Pan

Here is the same thing happening fast enough to watch. Water sprinkled on a hot pan before a dosa is gone almost at once. It gets converted into steam, and steam is really water vapour, some part of which turns into water droplets.

Why Steam Can Be Seen

Water vapour by itself is invisible. What your eye catches in a cloud of steam is the tiny droplets of water carried in it. So the water that left the puddles is genuinely in the air around you, unseen.

The Name for the Process

All of this has one name. Water turning into vapour has a name of its own, and that name is evaporation. Naming it is useful, because once you have the name you start noticing how often it happens. The plate, the puddles and the pan were three instances of a single process wearing three disguises.

It Never Stops

Evaporation does not need a hot pan. It takes place continuously, at ordinary room temperature as much as anywhere else. The water on the steel plate had no stove under it and went just the same. Nor did the puddles, and nor does a floor you have just mopped on a cool day.

Now Look Around You

Wet clothes drying on a line, a mopped floor going dry, sweat leaving your skin, sanitiser vanishing as you rub your palms together. Each one is evaporation. And the puddles? Some water soaked into the ground and some evaporated, so the honest answer there is both.

Scenario:

A class leaves an open dish of water on a windowsill and marks the level each morning. By the fourth day the level has dropped by a third and by the seventh the dish is nearly dry. Rohit says the water has vanished. Sana asks where to, and nobody can point. Their teacher sets up a second dish beside the first with a lid on it. After seven days the covered dish has barely changed. The class compares the two. The only difference was the lid, so whatever left the open dish went into the air, and a lid is enough to stop it.

Analysis:

The covered dish is what turns vanishing into leaving. One difference between two dishes names where the water went.

Activity:

Leave two dishes of water out for a week, one covered. Mark the level daily and write what the lid changed.

Title:

Empty a spoonful of water without touching it

Safety:

Use water at room temperature only, and keep the plate away from the edge of the table. Leave hot pans to an adult.

What You Notice:

The underside stays dry from the first check to the last, which is the whole point of running a finger under it. The water on top shrinks from the edges inwards rather than all over, and the last of it sits in the lowest part of the plate. On a warm or breezy day the whole thing finishes noticeably sooner.

Water vapour is invisible, and the tiny droplets of water riding in steam are the only part of it you can see.

Evaporation does not pause for the weather; it carries on at ordinary room temperature, day and night.

Hand sanitiser rubbed on your palms disappears the same way a puddle does.

Where the water went — Nothing underneath, so it went upward — The dry underside of the plate is what rules seeping out.

Why steam is visible — Vapour invisible, droplets visible — What your eye catches in steam is the droplets, never the vapour itself.

Lemonade, and Something Unexpected

A day later Aavi, Thirav and their friends settle on making lemonade. Into a glass tumbler go cold water and a few ice cubes. A few minutes later something has happened, not inside the tumbler but on the outside of it.

Set It Up Yourself

Pour cold water into a glass tumbler and drop in a few ice cubes. Leave it alone for five minutes and look carefully. Touch the outer surface of the tumbler too, and notice whether it feels any different.

What Appears Outside

Water droplets, tiny at first, show up on the outer surface. They are deposited all over it, and then they combine together to form bigger drops. That is why the outside looks misted over before it looks wet. Try the same thing with a metal container and you get the same result, so nothing about glass is special here.

Observe, and Also Wonder

Keep two columns as you watch. In one, write what you observe. In the other, write what you wonder. Both are worth recording, because the questions are what the rest of the topic answers. A question you have written down is harder to forget and harder to answer carelessly.

Thirav Has a Reason

His first idea is that a little of the water may have leaked out through the wall of the tumbler. It is a reasonable thought. Water has appeared outside, and there is water inside.

Aavi Has an Answer, and Thirav a Reply

Aavi points out that the level of water inside has not decreased, so nothing can have leaked out. Thirav is not satisfied. It might have decreased, he says, by too little to be seen.

How to Settle That

Aavi has a neat answer to the objection. Use a tall and narrow bottle instead. In a tall, narrow container the smallest drop in the water level shows up plainly, so a small loss could no longer hide.

A Third Reason on the Table

Somebody suggests that ice might have come out over the rim and melted on the outside. Another suggestion is to keep water at room temperature in a second tumbler and see whether anything appears on that. Each reason can be tested.

You Have Seen This Before

Dew drops sit on plants, and they are most plentiful in the morning. Half-fill a utensil and bring it to the boil, then lay a steel plate over the top; droplets gather on the underside of the plate. The same thing keeps happening.

The Answer

Vapour already in the air, meeting a surface cold enough, comes out on it as droplets of liquid water. Vapour turning back into liquid in this way has a name of its own, and that name is condensation. The tumbler was cold, and the air around it was not empty. So the drops were built on the spot, out of something that had been there all along.

Put It on a Balance

Half-fill a glass tumbler with water, add a few ice cubes and cover it with a small steel plate. Stand the whole thing on a digital weighing balance. Before reading anything, predict: will the mass rise, fall, or stay put?

Record It Every Five Minutes

Note the reading at the start and then every five minutes for half an hour. Keeping the times even matters, because a table of readings shows a trend that a single reading cannot. Thirty minutes gives you seven readings to compare, which is enough to see a direction rather than a wobble.

The Reading Climbs

The figure on the digital balance goes up. Droplets appear outside while the tumbler grows heavier. Water escaping from a covered tumbler could never add mass, so this water came from the air outside it.

One Doubt Still Standing

That does not yet settle everything, and it is worth being honest about why. The balance weighs the whole arrangement at once, so it cannot show whether a little water also crept out through the glass. Mark the water level with a permanent marker or a strip of visible tape and run the activity again.

Now It Is Settled

The water level in the tumbler does not go down, and the extra water collects on the outer surface instead. So nothing is seeping through the glass, and condensation put those drops there. How much vapour the air carries even has its own name, humidity, and machines exist that chill damp air to collect drinking water from it.

Scenario:

A class puts ice cubes and cold water in a steel tumbler and watches drops form outside it. Imran says the tumbler leaks. Their teacher asks the class to test that. They dry the outside, wrap the tumbler in a dry cloth, and wait. The cloth becomes damp on the outside too, which a leak through steel could not manage. Then they add a drop of ink to the water inside. The drops outside stay clear. Nita states the conclusion. The water outside never came from inside, because it carries none of the ink, so it must have come out of the air.

Analysis:

The ink settles it. A leak would carry the colour with it, and the clear drops rule the inside out entirely.

Activity:

Put ice water in a metal glass and add a drop of ink. Watch the outside for ten minutes and write whether the drops are coloured.

Title:

Catch water out of the air

Safety:

Handle the glass tumbler carefully and keep it away from the edge of the table. Do not put ice straight from a freezer against bare skin.

What You Notice:

Both the glass and the metal container collect drops, so the material is not what matters. The drops start as a fine mist and only later run together into drops big enough to slide down. The water stays level with the tape throughout. The third container, at room temperature, stays dry outside, which tells you what the cold was doing.

Atmospheric Water Generator machines cool humid air and collect drinkable water from it by condensation.

Dew drops on plants are condensation, and morning is when you find the most of them.

How much vapour the air is carrying has a name of its own, humidity, and newspapers print it daily.

What condensation needs — Vapour meets cold, vapour turns to drops — A cold surface is the one thing the vapour has to run into.

The two pieces of evidence — Balance says heavier, mark says nothing left — Between them they rule out leaking and point to the air.

Three States in Daily Life

Water is a substance you can observe in three different states without leaving the kitchen. As a solid it is ice. Heat that ice and it melts, giving you the liquid. Heat the liquid in turn and you get the gas.

Carry an Ice Cube Around

Here is a way to compare the three properly. Put an ice cube in one container, then transfer it to a second container of a different shape. Notice what happens to the shape of the cube itself. Nothing does.

Now Pour Some Water

Pour water from one container into another of a different shape and watch how differently it behaves. It flows out, and it settles into whatever shape the new container gives it. Compare that with the cube, which simply sat there.

And Water on a Flat Surface

Pour a little water on a clean flat surface and watch it go outwards in every direction. That is a third thing to test for, separate from flowing: the ability to spread. A cube of ice on the same surface would have stayed a cube, sitting exactly where you left it.

And the Vapour?

Now think about water that has turned into water vapour. It does not pool and it does not run along the table. Instead it reaches into every part of the space open to it, wherever that space happens to go. So the three states answer the same three questions quite differently, which is why a table is the right way to set them out.

Ice, in One Line

Ice holds on to its own shape, whichever container you put it in. It does not flow and it does not spread. That is exactly why a cube of it can be picked up and held in your hand.

Water, in One Line

Water flows and changes its shape. It does not have a fixed shape of its own and takes up the shape of whatever it is kept in. It spreads as well. Through every bit of that, the volume of water remains constant.

Water Vapour, in One Line

Water vapour shows a property of its own: it reaches every part of the space open to it. No gas holds a fixed shape. This state is present at room temperature too, in the air around you, invisible the whole time.

Where the Vapour in the Air Came From

That invisible vapour has a history. Water that evaporates while clothes dry or a floor is mopped adds to the vapour the air around us carries. So the air in your room holds water that used to be somewhere you could see.

It Is Not Only Water

Several other substances exhibit these states as well. Wax, oil and ghee are three of them, and a bottle of coconut oil going solid in winter is the easiest one to notice at home.

Examples to Collect

Look around and the examples come quickly. Stones, wood and glass are solids. Milk and oil are liquids. Oxygen and carbon dioxide are gases. Try adding five more liquids of your own to that list.

Smelling Dinner From the Next Room

Here is the spreading property doing something useful. You can smell food being cooked without entering the kitchen. The smell spreads through the air and reaches your nostrils, because that is what a gas does with the space open to it. No cook has to carry the smell to you.

How to Change a State

Changing state turns out to need only two things. Ice into water, and water into vapour, both need heat supplied. To change water into ice, put it somewhere cold, such as the freezer of a refrigerator. Heat one way, cold the other, and that is the whole trick.

Two Names for Two Changes

A solid turning into its liquid state is what we call melting. A liquid turning into its solid state is what we call freezing. Take ice out of a freezer and it melts; put water back in and it freezes.

A Candle Makes the Point

A candle is made of wax, and wax can be turned liquid and then cooled back into a solid. So water, and other substances too, shift between states when heated or cooled. That gives the full loop: melting and freezing between solid and liquid, and evaporation and condensation between liquid and gas.

Scenario:

A class is asked to sort ice, water and steam by whether each keeps its shape, fills its container, and can be poured. They build a table of three rows. Ice keeps its own shape and cannot be poured. Water takes the shape of the vessel and pours. Steam spreads to fill whatever holds it. Meera then asks which row the substance itself belongs in. Their teacher points out that all three rows describe the same substance. The class sees that the table is not sorting three materials but one material in three conditions, and that heating or cooling moves it between the rows.

Analysis:

Three very different rows and one substance throughout. What changes between them is the state, not the material.

Activity:

Make a three-row table for ice, water and steam with columns for shape, pouring and spreading. Mark which rows you have seen yourself.

Title:

Fill in the table yourself

Safety:

Keep the water at room temperature and mop up spills at once so nobody slips. Ask before taking ice from the freezer.

What You Notice:

The ice column fills up with a row of noes, which is the point of it. The water on the tray spreads to a thin sheet and stops, because the quantity has not changed even though the shape has. The vapour column cannot be filled by looking at anything, which is itself worth noticing about the gaseous state.

The shape of water keeps changing but its volume remains constant, whether it sits in a glass or spreads on a floor.

Coconut oil in a bottle turns solid during winter and becomes liquid again when the weather warms.

You can smell food cooking from another room because a gas spreads out into all the space open to it.

The three states in order — Holds its shape, takes a shape, fills the space — Solid, then liquid, then gas, in one line each.

Melting against freezing — Melt with heat, freeze with cold — Heat goes one way round the loop and cooling comes back.

What Makes Water Go Sooner?

You already know that water evaporates. The next question is what makes it happen faster or slower, and your own surroundings will suggest answers. Think about a cold day against a hot one, and about a fan, a windy day, a rainy day and sweating.

Same Water, Two Shapes

Here is the first test. Put a little water in the small cap of a bottle, and an equal amount on a plate. How much of the water lies open to the air differs sharply between the two. Keep them near each other and record the time each takes to evaporate completely.

Three Questions of a Fair Test

Before reading the result, ask three questions about the test itself. Which conditions stayed fixed? Which single one was altered? And what, exactly, was measured? Here the amount of water and the surroundings stayed fixed, the exposed area was altered, and the time was measured.

What the Plate Tells You

Spread water out on a plate and its area exposed to air is larger. Evaporation is therefore faster, and the plate empties first. So the same quantity of water can take quite different times to go, depending only on its shape.

Sunlight Against Shade

Now change something else. Take identical caps of two bottles and pour an equal amount of water into each. Stand one of them in sunlight and leave the other in shade. Look at both every fifteen minutes and record how long each takes.

And That Result

The cap standing in sunlight empties sooner than the one left in shade. It is the same thing you have noticed without measuring anything, when washing dries quickly on a hot, sunny day.

Now Add Wind

Does washing dry quicker or slower when the wind blows? Quicker, as everybody has noticed. Set the air moving more and water leaves a wet surface faster. You can repeat the two-cap activity on a windy day and see it for yourself.

And Rainy Days Are Different

Everybody has noticed the opposite too. Washing hangs out all day in the rain and stays damp. Where the air is already loaded with water, which is what more humidity means, the process runs slowly. Rainy days are the humid ones.

The Conditions, Collected

So the conditions which make evaporation faster or slower are the exposed area, the humidity, the movement of air, and others besides. Knowing them is useful: it tells you what to change when you actually want something dry.

Why a Fan Feels Cool

Sitting under a fan feels cooler, and now you can say why. Moving air helps the sweat on your skin leave, and its leaving is what cools you. The fan is not making cold air; it is moving air, which is a different thing.

A New Pot at Home

Aavi's mother brings home a new matka, an earthen pot, and the stainless steel one they had kept drinking water in is retired. Coming home from school, Aavi drinks from it and is surprised. Why has the water in this earthen pot turned so cold? The steel one never did that.

The Answer Is Evaporation

A little water passes out through the wall of the earthen pot and evaporates from its outer face. That leaves the water still inside cooler than before. Steel lets nothing through, so no evaporation happens at its surface and nothing gets cooled.

The Same Effect Elsewhere

Once you know to look for it, you find it everywhere. Sprinkling water on a floor or a roof during summer cools it. Rubbing sanitiser on your hands feels cold. A two-wheeler seat left in the sun can be cooled the same way.

Build a Cooler Without Electricity

Find two earthen pots, one smaller than the other. Lay a bed of sand in the bottom of the bigger one. Stand the smaller pot in the centre of it, and pack the gap between the two with more sand. Pour water into that sand, and cover the top of the inner pot with a lid or a wet jute sack.

Then Give It Time

Allow four to five hours for the cooler to cool down, and remember that the time depends on many conditions. Keep vegetables and fruits inside and check their freshness daily for a week. Keep adding water so the sand stays moist. The surahi, a clay pot of its own shape, works on the same idea and has kept water cold in summer for a very long time.

Scenario:

A class tests what makes water evaporate faster. They put equal amounts in a bottle cap and in a flat plate and time how long each takes to dry. The plate wins easily. Rohit then says a fan would be faster still and switches one on, pointing it at the plate only. Their teacher stops the trial. Two things now differ between the dishes, the shape and the moving air, so a difference in the result would not say which caused it. The class runs it again with the fan on both. Sana writes the rule down: change one condition, hold the rest the same, and time the outcome.

Analysis:

Rohit's fan was a good idea and a spoiled trial. Two differences at once leave the result unexplainable whichever way it falls.

Activity:

Dry equal water in a cap and a plate, then repeat with a fan on both. Record all four times and say which condition mattered more.

Title:

Race two spoonfuls of water

Safety:

Keep the water at room temperature and wipe up spills so nobody slips. Do not lean out of a window to reach a sunny ledge.

What You Notice:

The plate is dry long before the cap, although exactly the same amount of water went into each. In the second race the sunlit cap finishes first, and on a breezy day both finish sooner than they did indoors. If you try this during the monsoon, every single time gets longer, which is humidity making itself felt.

A pot-in-pot cooler needs no electricity at all, only two earthen pots, damp sand and a wet cover.

Rainy days are more humid, because the air is carrying more water vapour than usual.

The surahi, a clay pot of its own particular shape, has long been used to keep water cold in summer.

The conditions that speed evaporation up — Wide, warm and windy go fast; damp air goes slow — Exposed area, sunshine and moving air against humidity.

A fair test — Change one, keep the rest, measure the time — Change two things at once and neither result means anything.

Bringing the Water Back

Evaporation has been sending water upwards for the whole chapter. Something has to bring it back, and condensation is what does it. It plays a significant role in returning evaporated water to the surface of the Earth.

Why It Goes Up at All

First, why does air carrying water vapour rise? A gas balloon filled with a lighter gas goes up in the air, and water vapour behaves the same way, being lighter than air. Up means into the atmosphere, which is the thin skin of air wrapped round the Earth.

Higher Is Colder

Here is the fact that everything else follows from. When air moves higher above the surface of the Earth, it becomes cooler and cooler. Rising vapour is therefore heading somewhere steadily colder than where it started. Nobody has to cool it on purpose; going up is enough.

Where the Droplets Appear

Go high enough and the air is cold enough to bring the vapour out as droplets. Those droplets are generally built around specks of dust, which give the vapour something to gather on. It is condensation, high up instead of on a tumbler.

That Is What a Cloud Is

Those droplets hang in the air rather than falling, and in enormous numbers they are what a cloud is. So a cloud is not smoke and not vapour. It is liquid water, in pieces small enough to stay up.

From Droplets to Rain

Droplets run into one another and merge into larger drops. A drop that grows too heavy to hang there drops out of the cloud. A drop coming down is what we call rain, which makes rain the last step of a journey rather than an event on its own. Every drop that lands on you was vapour a while ago.

Hail and Snow

Rain is the usual ending but not the only one. Under special conditions the water might also fall as hail or as snow. That is also the answer to how water is found in the solid state in nature.

Make a Cloud in a Bottle

Find an empty one litre plastic bottle that was going to be thrown away, and pour roughly a cupful of water in. Screw the lid down tight. Now squeeze the bottle and let it go again, over and over, for two or three minutes, watching the space above the water.

Now Add Some Dust

Run it again, this time with a small scrap of burnt newspaper dropped into the water. Handle the burning paper carefully. This time a haziness shows up above the water in the bottle, and that haziness is a cloud.

Why the Burnt Paper Mattered

What the burnt newspaper supplies is dust, in specks far too small to see. Vapour condenses around them, and that is a cloud. So the activity is not a trick with smoke; it supplies exactly the thing the sky supplies for free.

The Whole Journey, in One Sentence

Water leaves the ocean and the land as vapour and rises into the atmosphere. It comes back down as rain, or as hail or snow, and in the end it finds its way to the sea again. Nothing is added at any stage and nothing is lost.

The Name for It

That circulation has a name of its own: the water cycle. Drawing it is the best way to learn it. Put in the ocean, a lake, a river and the groundwater, and mark the cloud. Then label the arrows for evaporation, condensation, rain and snow. A picture you have labelled yourself is very much harder to forget than one you have only looked at.

How Much of It Can We Use?

Only a small share of what the Earth holds can actually serve plants, animals and people. The greater part of it sits in the oceans, where it is no use to anybody straight off. So a full planet of water is not the same as a full glass of it.

And the Demand Keeps Growing

We drink it, and we use it for a great many other things besides. As the population grows, so does the count of people drawing on the supply, and that growing demand leaves many parts of the world short.

So It Has to Be Used Wisely

Two things follow, and both are practical. Using water carefully, and wasting none of it, matters a great deal. And the water bodies we depend on should be kept free from pollution, because a polluted lake takes a small usable portion and makes it smaller.

Scenario:

The pupils of a village school keep a chart for one year. Each month they copy the rainfall and the humidity for their district out of the newspaper, and note the level of the village tank. The chart shows heavy rain in June, July and August, and almost none from January onwards. The tank falls steadily from November and is empty by the middle of April. At the panchayat meeting three things are said. Devanshi says the rain simply failed this year. Harshil says the village has more households than it had ten years ago. An elderly farmer says nothing has changed about the rain, only about how much is drawn from the tank and how much of it is wasted.

Analysis:

The chart records two quite different things: how much water arrived, and how much was taken out. Only one of the three speakers is talking about the arriving side, and the chart itself can be checked against the rest.

Activity:

Copy the rainfall and humidity figures for your own district out of a newspaper every day for a month, and rule a chart of them. Mark the wettest week and the most humid week, and write two sentences on whether washing dried faster or slower during each.

Title:

Make a cloud you can look at

Safety:

An adult must handle the burning paper, and it must be out and cool before it goes anywhere near the bottle. Never light paper indoors on your own, and keep it clear of curtains and clothes.

What You Notice:

In the first round the space above the water stays clear, or nearly so, however hard you work at it. In the second round a faint haziness appears the moment you release the bottle and clears again when you squeeze it, which is worth noticing on its own. The haziness is thickest right at the top of the bottle.

Water vapour rises because it is lighter than air, in the same way a balloon of a lighter gas goes up.

The droplets in a cloud are generally built around specks of dust far too small to see.

The greater part of the Earth's water sits in the oceans, where plants, animals and people cannot use it straight off.

The four steps from puddle to puddle — Rise, cool, gather, fall — Evaporation, then cooling, then droplets on dust, then rain.

Why a cloud needs dust — No speck, no droplet — The burnt paper in the bottle supplies the specks.