Chapter 2 of 17 · Water and the Weather
Chapter 1. Water changes state
The three states of water, the changes that take it from one to another, and the difference between a puddle drying and a pan boiling.
A puddle that disappears without anyone emptying it
A rain puddle sitting on a pavement disappears within a few hours. Nobody wiped it away, nobody drank it, and the pavement is not leaking. Yet it is no longer there. That is the question this chapter deals with, and it goes deeper than it looks: where has that water gone, and in what form did it leave?
The answer holds in one sentence, and everything else in this volume follows from it. The water has not disappeared: it has changed state. It has gone from a form the eye can see, the liquid, to a form the eye cannot see, the gas. The quantity of matter has not moved by a single gramme; what has changed is its arrangement. A water cycle is nothing other than the sequence of those changes, repeated at the scale of the planet.
This chapter begins with that physics, before clouds and before rain, because none of the chapters that follow can be understood without it. A cloud is a change of state. A downpour is a change of state. Mist on a window, frost on a car, snow, morning dew: six different scenes and a single mechanism.
The objectives of this chapter
By the end of this chapter you must be able to do each of these things. They are observable: either you do them, or you know which section to read again.
- Name the three states in which water is met, and give an example of each taken from outside the book.
- Name the changes of state, and say from which state to which state each one leads.
- Tell evaporation apart from boiling by citing three observable differences.
- Work out which change of state is at work in an everyday scene described in one sentence.
- Measure the disappearance of water left in the open air, and record the result in a dated table.
Prior knowledge: none. This chapter is the first of the volume and it assumes nothing about its subject. It is enough to be able to read a continuous text and to copy a number into a table.
Three states, a single substance
Take an ice cube, a glass of water and the invisible vapour floating above a hot pan. They are three very different things to look at and to touch, and yet they are exactly the same substance. What separates them is not their nature: it is the way their smallest constituents are arranged and the freedom they have to move.
In the solid state, ice, the constituents are packed and ordered: they vibrate on the spot without leaving one another. A solid therefore keeps both its shape and its volume. An ice cube left in a dish stays an ice cube shaped like an ice cube.
In the liquid state, water, the constituents are still packed but they slide over one another. A liquid keeps its volume and loses its shape: pour a litre of water into a bucket, a bottle or a basin, you still have a litre, and each time it takes the shape of the container. Its surface, for its part, stays horizontal.
In the gaseous state, water vapour, the constituents are very far apart and move freely through all the space available. A gas keeps neither its shape nor its volume: it occupies whatever it is given. That is why the water vapour of a room spreads through the whole room, and not only above the pan.
Above all, remember this, which will be used throughout the volume: water vapour is perfectly transparent. It cannot be seen, it cannot be photographed, and there is no such thing as a white cloud of water vapour. Everything that is white and visible in the air has already become liquid or solid again.
The words to remember
These three words will come back in every chapter. Using them accurately avoids half the confusions of the subject.
- The solid state
- The state in which water keeps both its shape and its volume. It is ice, snow, hail, hoar frost. Water in the solid state can be picked up in the hand.
- The liquid state
- The state in which water keeps its volume but takes the shape of whatever contains it. It is the water of rain, of the tap, of rivers, and it is also the droplets in suspension that make up a cloud.
- The gaseous state, or water vapour
- The state in which water occupies all the space available. It is invisible and odourless. Water vapour makes up between 0.1 % and about 5 % of the surrounding air according to Météo-France, and yet nobody has ever seen it.
The changes of state, and their names
Going from one state to another has a name, and there are six possible passages since there are three states. These names are not luxury vocabulary: they are what allows one to say what is happening without writing a ten-line sentence. Météo-France names all six in a single sentence, and sorts them into two halves: "for sublimation, melting and evaporation to take place, energy must be supplied to the system. Conversely, deposition, freezing and condensation release energy." Remember that division, it will serve often: the three passages that go towards more freedom demand energy, the three that go towards more order give it back.
From solid to liquid is melting. An ice cube melting in a glass performs a melting. From liquid to solid is freezing, which some books call solidification. Water setting into ice in a freezer performs one.
From liquid to gas is vaporisation. It takes two very different forms, which the next section separates: evaporation and boiling. From gas to liquid is condensation, also called liquefaction. Météo-France defines it exactly like this, about clouds: there is condensation when water passes "from the gaseous state to the liquid state". Mist on a cold window is a condensation.
There remain the two passages that skip the liquid state. From solid straight to gas is sublimation: snow that dwindles without ever wetting the ground, in hard frost and bright sun, sublimates. From gas straight to solid is deposition, which some books call solid condensation: the hoar frost that grows on a windscreen on a clear night does not come from water that would have frozen after settling, it forms directly out of the vapour in the air.
The vocabulary varies a little from one book to another: some write liquefaction where others write condensation, solidification where others write freezing, and solid condensation where others write deposition. These plays on words denote exactly the same passages. This volume uses condensation, freezing and deposition, which are the words the weather services use, and it flags the alternatives when they first appear.
| The change | From which state | To which state | Where it can be seen |
|---|---|---|---|
| Melting | Solid | Liquid | An ice cube melting in a glass; snow turning to slush on a road. |
| Freezing | Liquid | Solid | An ice-cube tray forgotten in the freezer; a puddle turned to ice in the early morning. |
| Vaporisation | Liquid | Gaseous | A puddle drying; washing drying on a line; a pan boiling. |
| Condensation | Gaseous | Liquid | Mist on a cold window; morning dew on the grass; a cloud forming. |
| Sublimation | Solid | Gaseous | Snow that dwindles in hard frost without wetting the ground. |
| Deposition | Gaseous | Solid | Hoar frost on a windscreen after a clear cold night. |
Evaporation and boiling: two vaporisations, and they are not alike
Here is the most useful distinction in the whole volume, and the one most often missed. Water passes into the gaseous state in two ways, and only one of the two calls for strong heat.
Evaporation takes place at the free surface of the liquid, and it takes place at any temperature. A puddle evaporates at fifteen degrees as well as at thirty, a wet towel dries in a cool bathroom, washing dries outdoors in winter. It is slow, silent, and it makes no bubbles at all. What speeds it up is a larger surface, warmer air, drier air, and wind to carry the vapour away as it forms. That is exactly the mechanism Météo-France describes at the scale of the planet: "the heat of the Sun allows part of the water contained in the oceans, the rivers and the lakes to evaporate".
Boiling takes place throughout the whole body of the liquid, and it takes place at a fixed temperature. Under the usual pressure at sea level, pure water boils at one hundred degrees Celsius, and its temperature then stops rising for as long as there is water left to boil. It is fast, noisy, and it makes bubbles: those bubbles are water vapour forming at the bottom of the pan and rising up through the liquid.
Remember the consequence, because it runs against intuition and because it runs through the whole water cycle: the vast majority of the water that rises into the atmosphere has never boiled. Oceans do not boil. Puddles do not boil. Washing does not boil. All of that evaporates, at ordinary temperature, continuously, and it is that slow, steady flow that feeds the clouds of the entire world.
The trap of this chapter: the white plume above a pan or a kettle is NOT water vapour. Water vapour is invisible, and it occupies the perfectly transparent zone just above the spout, before the plume. The white plume, for its part, is vapour that has already met the cooler air of the room and has condensed back into tiny liquid droplets. In other words: what you see has already become liquid again. The same mistake is repeated with clouds, with the mist of a breath in winter, and with fog.
Worked example 1. Naming the change of state in a scene
Question: "A rain puddle as wide as a table has disappeared from the pavement in the course of a day. It was fourteen degrees and there was neither direct sunshine nor anyone to wipe it up." Which change of state took place, and how can it be shown? This example is guided all the way through: every step is carried out and explained.
- Identify the starting state
The scene begins with a puddle. A puddle takes the shape of the pavement and keeps its volume: it is water in the liquid state. Starting state: liquid. - Identify the arrival state
At the end, the water is no longer visible and the pavement is dry. It has not been absorbed, since the question says the pavement is not leaking, and nobody removed it. The only state in which water can be present without being seen is the gaseous state. Arrival state: gaseous. - Name the passage
The passage from the liquid state to the gaseous state is called vaporisation. The name of the change of state is therefore: vaporisation. - Say which of the two vaporisations it is
The question gives fourteen degrees. Boiling water calls for one hundred degrees under the usual pressure: it is therefore impossible here. That leaves evaporation, which takes place at any temperature, slowly, at the free surface. Precise answer: an evaporation. - Check that the reasoning holds
A good check consists in looking for what would contradict the answer. If the water had evaporated, it should have left slowly and through the surface: a whole day for a puddle is consistent with that. If it had boiled, bubbles would have been visible and a source of heat would have been needed. Nothing in the question argues against evaporation. The reasoning holds. - The result
The puddle underwent a vaporisation, and more precisely an evaporation: the liquid water passed into the gaseous state at ordinary temperature, through its free surface, and it is now scattered through the air of the street as invisible water vapour.
Experiment 1. Measuring an evaporation
What you need: two identical soup plates, tap water, a ruler, a notebook. No source of heat. SAFETY: this experiment is carried out with an adult's agreement, and the adult is present. The adult chooses the two locations, out of reach of a small child and away from any ledge from which a plate could fall.
- Prepare two identical plates
Pour the same depth of tap water into both plates. A finger's depth is enough. Check with the ruler held upright in the water that the two depths really are the same, and write that starting depth in the notebook, with the date and the time. - Put the plates in two different places
With the adult, choose two places in the house that differ by one single thing if possible: for instance a heated room and a cool room, or a spot in a draught and a sheltered spot. Put neither of them in direct sunlight for this first attempt. - Take a reading every day at the same time
Every day, at the same time, measure the depth of water left in each plate and write it down. Do this for at least four days. Also note, in one line, what you observe: is the water cloudy, is there dust at the bottom? - Add nothing, touch nothing
This is the rule that makes the measurement honest. Do not top the water up, do not move the plates, do not cover them. A measurement whose conditions change along the way no longer says anything. - Read the result
After four days, compare the two columns of the notebook. The plate placed in the more favourable condition will have lost more. You have just measured an evaporation, and shown that the factor you varied, and it alone, speeds it up, without ever going above room temperature. If you varied two things at once, write that down: you will know that an evaporation did take place, but not what speeded it up. - Let one plate go all the way
Take one of the two plates and leave it untouched until there is not a drop of water left. Then look at the bottom, in the light, from an angle. There is a faint whitish deposit, or dry marks. Note what you see: that deposit will be used in chapter 7, and it answers in advance a question many people ask. - What the experiment shows, and what it does not
It shows that liquid water leaves into the open air without boiling, that the speed depends on the conditions, and that whatever was dissolved in the water did not leave. It does not show where the water went: for that, it has to be recovered, and that is what the next experiment is for.
Experiment 2. Recovering the water that left into the air
What you need: a glass, ice cubes, a dry plate, a tea towel. SAFETY: this experiment is carried out with an adult's agreement, and the adult is present. The glass is made of glass and becomes slippery once wet: set it down in the middle of a table, never at the edge.
- Dry the outside of the glass
Wipe the outside of the glass carefully with the tea towel, and have the adult confirm it. This is not fussiness: it rules out in advance the objection that the water which appears was already there. - Fill with ice cubes, with no water
Put ice cubes only into the glass, without adding any liquid water. If there is no liquid water in the glass, none can come out of it through a hole or over the rim. - Set the glass down and wait
Put the glass in the middle of the dry plate, in the middle of the table, and wait a few minutes without touching it. - Watch the wall of the glass
The outside of the glass covers itself in fine droplets, then the droplets join up and run down into the plate. After a quarter of an hour, there is liquid water in the plate. - Find where that water comes from
It does not come from the glass, which was dry and contains only ice. It does not come from the plate, which was dry. It therefore comes from the air of the room, which contained invisible water vapour. On contact with the cold wall, that vapour passed back into the liquid state. - Name the change of state
The passage from the gaseous state to the liquid state is a condensation. You have just recovered, in liquid and visible form, water that was in the room in gaseous and invisible form. It is the exact mechanism by which a cloud forms, and chapter 3 will come back to it.
Naming a change of state, in three moves
- Step 1
Say which state the water is in at the start of the scene: does it keep its shape, only its volume, or neither?
- Step 2
Say which state it is in at the end. If it has become invisible without going anywhere else, it is in the gaseous state.
- Step 3
Read the name of the passage in the table of the six changes of state, then check that no detail of the scene contradicts it, the temperature in particular.
Worked example 2. The windscreen in the morning
Question: "After a clear cold night, a windscreen is covered with a thin white layer that scratches under the fingernail. It did not rain during the night." Which change of state? This example is guided up to the next-to-last step: the last is left to the reader.
- Identify the arrival state
The layer is white and it scratches under the fingernail: it keeps its shape, it does not run. It is water in the solid state. Arrival state: solid. - Rule out the most obvious starting state
One is tempted to say that liquid water settled and then froze, which would make a condensation followed by a freezing. But the question states that it did not rain, and a dry windscreen does not manufacture liquid water on its own. - Look for where the water can come from
One single source remains: the air, which always contains invisible water vapour. The clear night let the windscreen cool sharply, down to a temperature below zero degrees. - Recognise the direct passage
When the surface is already below zero, water vapour does not go through the liquid state: it settles directly in the solid state, in small crystals. It is that direct passage which explains the needle-like look of hoar frost, very different from a smooth sheet of ice. - Over to you: name the change of state
You have the starting state, the arrival state and the reason the liquid step is skipped. The name of the passage appears in the table of the six changes of state in this chapter. Write it down, then check it in the answer to the self-check, question 4.
What to remember from this chapter
- Water is met in three states: solid, liquid, gaseous. It is the same substance in all three cases; only the arrangement changes.
- Water vapour is invisible. Everything that is white and visible in the air has already become liquid or solid again.
- Six passages connect these three states: melting, freezing, vaporisation, condensation, sublimation, deposition.
- Vaporisation takes two forms: evaporation, slow, at any temperature, at the free surface; boiling, fast, at one hundred degrees under the usual pressure, throughout the whole body of the liquid.
- The water that feeds the clouds of the entire world evaporates, it never boils.
- A change of state makes no matter appear and none disappear: the quantity of water stays the same from one end to the other.
Self-check for chapter 1
Answer in writing, using only the knowledge given by this chapter. Do not look at the answer, gathered at the end of the volume, until you have written all five of your answers.
- A wet towel hung up in a bathroom at eighteen degrees is dry the next morning. Name the change of state, and say why it is not a boiling.
- A friend claims to have photographed a cloud of water vapour above his kettle. What do you answer, and why?
- Cite three observable differences between an evaporation and a boiling.
- After a clear cold night, a dry windscreen covers itself with a thin white layer that scratches under the fingernail, without any rain having fallen. Name the change of state and justify your answer in one sentence.
- A plate of water is put in the sun and an identical plate in the shade, in the same room. Which will empty first, and what is the only thing this experiment allows you to conclude?