Life began in water, and every cell is still mostly water. Here is why one small, lopsided molecule clings to itself and to other things, dissolves what cells need, climbs to the tops of trees, and shapes the lives of a diving bird and an Arctic seal.
IB Biology A1.1SL and HL2 hours SL, 3 HL, plus a practical8 chaptersdry lab
Eight chapters, the last two for higher level. Read one, play its figure through or tap its parts, and make each prediction before you open the answer.
The first cells appeared in water more than 3.5 billion years ago. Where exactly, in the sea or in hot pools, is still debated. What is not in doubt is that for about three billion years every living thing stayed in water, and plants and animals reached land only in the last 500 million years or so.
Water suited them because things dissolve in it. Dissolved substances can move about, meet and react, and that is how the first cells could run their chemistry. Water is the medium for life, and the chemistry it carries, metabolism, still happens in it today.
A typical cell is about 70% water by mass. Its cytoplasm is an aqueous solution, and most of the reactions of metabolism take place in it. Blood plasma, about 90% water, and xylem sap carry dissolved substances round bodies and plants.
Land organisms did not leave water behind. They carry it inside them, in their cells, blood and tissue fluid, and a skin or a waxy cuticle slows its loss.
Watch out "Land animals live without water." They do not: they carry it inside them, and every reaction in their cells still happens in water.
In numbers A typical cell is about 70% water by mass, an adult human about 60% and a jellyfish about 95%.
A packet of dry bean seeds sits in a cupboard for three years. What is happening to the metabolism inside each seed?
Almost stopped. A dry seed holds only about a tenth of its mass as water, too little for most reactions. Soak it, and within hours water floods in, its enzymes start working and the seed begins to germinate. Metabolism needs water as its medium, which is what this statement means.
Liquid water appeared very early, over 4 billion years ago.
The first cells lived in it, more than 3.5 billion years ago, and for about three billion years every living thing stayed there.
Plants and animals reached land only in the last 500 million years or so.
Even there, life carries its water inside it: cells, blood plasma and xylem sap are all mostly water.
A1.1.2 Hydrogen bonds
A lopsided molecule makes hydrogen bonds
A water molecule is one oxygen atom joined to two hydrogen atoms by covalent bonds, and it is bent, with about 104.5° between the two bonds.
The sharing is unequal. Oxygen is more electronegative: it pulls on the shared electrons harder than hydrogen does, so they spend more time near the oxygen. That makes each O–H bond a polar covalent bond. The oxygen end gains a small negative charge and each hydrogen a small positive one: partial charges, written δ− and δ+. The molecule is bent, so one side is negative and the other positive, and the whole thing is a polar molecule.
Opposite charges attract. The δ+ hydrogen of one molecule is pulled towards the δ− oxygen of its neighbour, and that attraction is a hydrogen bond. It forms between molecules, never inside one. Each is weak, about a twentieth as strong as a covalent bond, and lasts only a few trillionths of a second. But there are vast numbers, always breaking and re-forming, and one molecule can make up to four.
Draw it the IB way: solid lines for covalent bonds, dashed lines for hydrogen bonds.
Write δ+ and δ−, never a bare + or −: the molecule is neutral overall.
Why it matters: to pull water molecules apart, hydrogen bonds must be broken, and that takes energy. Water boils at 100 °C, but methane, a molecule of about the same size with no hydrogen bonds, boils at −162 °C. When water evaporates, its fastest molecules break their hydrogen bonds and escape, taking energy with them, so what is left behind cools. Each gram that evaporates takes about 2.4 kJ, the latent heat of vaporisation. That is how sweat cools skin and transpiration cools leaves.
Remember it What hydrogen bonds give water
CatsCohesionwater sticks to water (chapter 3)
AvoidAdhesionwater sticks to polar or charged surfaces (chapter 4)
TakingThermalslow to warm, and takes a lot of heat to evaporate (here and chapter 6)
ShowersSolventdissolves polar and charged substances (chapter 5)
Watch out "A hydrogen bond is the bond between O and H inside a water molecule." Those are covalent bonds. A hydrogen bond is between molecules.
Watch out "δ+ and δ− are the charges of ions." They are partial charges: a water molecule is neutral, so write δ, never a bare + or −.
How do we know? No one has watched a hydrogen bond form: each is far too small and lasts a few trillionths of a second. The evidence is indirect. Water boils at 100 °C, where molecules of the same size boil far lower (methane at −162 °C, hydrogen sulfide at −60 °C), and its surface tension and heat capacity are unusually high. One idea, hydrogen bonding, explains all of these at once, and X-ray and neutron studies of ice show each hydrogen pointing at an oxygen on the next molecule. A theory earns trust by explaining many observations and surviving every test so far.
Links Hydrogen bonds hold the two strands of DNA together (A1.2) and help hold proteins in shape (B1.2). The same weak, numerous attractions run through the whole course.
Water boils in a kettle. Which bonds break as it turns to steam?
Only the hydrogen bonds. Boiling and evaporation separate whole molecules from one another, and the covalent bonds, about twenty times stronger, stay as they are. Breaking all those hydrogen bonds is why water needs so much heat to boil or evaporate, and why sweat cools you. The dry lab puts a number on it.
One water molecule: an oxygen with two hydrogens, bent, with about 104.5° between the bonds.
The shared electrons (the pink cloud) crowd round the oxygen and thin out round each hydrogen.
So the oxygen is slightly negative and each hydrogen slightly positive: partial charges, not whole ones.
A second molecule turns its oxygen to face a hydrogen of the first, and a dashed line joins them: a hydrogen bond.
One molecule can make up to four: two through its hydrogens and two to its oxygen.
The notation the exam wants: letters, solid lines for covalent bonds, dashed lines for hydrogen bonds, and δ signs. Draw it like this.
Each water molecule is polar: the oxygen is slightly negative and the hydrogens slightly positive.
A slightly positive hydrogen is attracted to the oxygen of another molecule: a hydrogen bond.
Each bond lasts only a moment. They break and form all the time as the molecules move; in liquid water most molecules are held by several at once.
Raise the temperature: the molecules move faster and each hydrogen bond lasts a shorter time. Even so, most molecules are still held by a couple of bonds at any moment.
A1.1.3 Cohesion
Water holds on to itself
Hydrogen bonds make water molecules stick to one another. That is cohesion.
Inside a drop, each molecule is pulled equally in every direction by its neighbours. A molecule at the surface has neighbours beside it and below it but none above, so it is pulled sideways and inwards. The surface molecules cling together and the surface acts like a stretched film that resists being broken. That is surface tension, and water's is higher than that of almost any other common liquid.
Small animals use the film as a habitat. A pond skater (Gerris lacustris) spreads its weight over long legs, and water-repellent hairs on its feet press dimples into the surface without breaking through. Mosquito larvae hang from the underside and breathe through a tube that pokes up through the film.
Cohesion also lifts water to the top of a tree. In a leaf, water evaporates from the wet walls of cells and leaves as vapour through small pores, which is transpiration. That pulls water out of the xylem. Because the molecules cohere, the whole column of water is pulled up from the roots, like a rope pulled from the top. The water is under tension: it is being pulled, not pushed. Cohesion keeps the column whole however tall the plant, but an air bubble breaks it, and that tube stops carrying water. (The detail of xylem vessels is B3.2.)
Watch out "Capillary action lifts water to the top of a tree." In a xylem vessel, capillary action alone would lift water only a metre or so at most, and far less in wider vessels. The pull comes from evaporation in the leaves, passed down the column by cohesion.
Watch out "Surface tension is a skin of different molecules," or "surface water is denser." It is the same water, held more tightly by hydrogen bonds at the surface.
Example The tallest living tree, a coast redwood in California, is about 116 m tall. Holding up a column of water that tall takes a pull of more than 1 MPa, about eleven times the pressure of the air, and cohesion holds.
Links Processes at surfaces: the surface film is a habitat, and evaporation from wet cell walls drives the pull up the xylem.
Ground pepper floats on a bowl of water. You touch the middle of the surface with a finger dipped in washing-up liquid. What does the pepper do?
It shoots outwards. Soap molecules crowd into the surface where you touch it and weaken the hydrogen bonding there, so the surface tension falls in the middle. The clean surface all round still pulls strongly and drags the water, and the pepper on it, to the edges. Detergent in a pond does the same to a pond skater: with less surface tension to stand on, it sinks.
A pond skater stands on water. Its long legs spread its weight, and each foot makes a small dimple in the surface.
Inside the water, each molecule is pulled by its neighbours equally in every direction, so it stays where it is.
At the surface there are no neighbours above: the pull is sideways and inwards, and the surface holds together like a stretched film. That is surface tension.
Close up, water-repellent hairs on the foot press the film into a dimple without breaking it, and the film pushes back up.
A mosquito larva hangs from the underside of the film and breathes through a tube that pokes through it.
Switch on the air bubble: below the break the water stops, because cohesion cannot pull across a gap.
A1.1.4 Adhesion
Water clings to polar surfaces
Water is attracted to other materials too, as long as they are polar or carry a charge. That is adhesion: hydrogen bonds or ionic attractions pull water on to the surface. Glass does it. So does cellulose, which is covered in OH groups, and so do the charged surfaces of clay and humus in soil. A waxy, non-polar surface gives water nothing to cling to, so water beads up on it instead.
Put a narrow gap between surfaces that water clings to and something useful happens. Water adheres to the walls and creeps up them, and cohesion drags the rest of the water along behind. This is capillary action. The narrower the gap, the higher the water rises.
In soil, rain is held in the narrow spaces between the particles by adhesion and cohesion, so it does not all drain away, and it can creep through the soil towards roots. Clay, with tiny spaces and charged particles, holds the most.
In a plant, a cell wall is a mesh of cellulose fibres with tiny gaps between them. Water soaks through the wall by capillary action and keeps it wet. In a leaf, those wet walls are where the evaporation of chapter 3 happens. (The detail of walls and leaves is B3.2.)
Remember it Cohesion or adhesion?
ClonesCohesionwater clings to its clones: other water molecules
AnythingAdhesionwater clings to anything polar or charged: glass, cellulose, soil
Watch out "Cohesion and adhesion are the same thing." Cohesion is water to water. Adhesion is water to a different material that is polar or charged. Capillary action needs both.
Example A cotton towel dries you because water adheres to its cellulose fibres and creeps along the spaces between them.
Links Much of biology happens at surfaces: water held on soil particles and in cell walls here, gas exchange across moist surfaces in lungs and leaves (B3.1), and reactions on membranes (B2.1).
Three cups in a row: red water, an empty cup, blue water. A strip of paper towel bridges each full cup and the empty one. What is in the middle cup a few hours later?
Purple water. Paper towel is cellulose. Water adheres to its fibres and creeps up and along the strip by capillary action, cohesion pulling the rest behind, and drips into the empty cup until the levels even out. Both colours arrive and mix. The same process keeps the walls of a leaf's cells wet and moves water through soil.
Water is attracted to the polar glass wall and forms bonds with it: adhesion. The water’s edge curves up where it meets the glass.
Cohesion drags more water up behind the edge, and the column rises until its weight balances the pull.
The narrower the tube, the higher the water climbs.
Wax has nothing for water to cling to: with no adhesion the surface curves down and sits below the level outside.
The same pull holds water between soil particles and soaks it through the gaps in a cellulose cell wall.
A1.1.5 Water as a solvent
The solvent cells run in
A solution is a solute dissolved in a solvent. Water dissolves a wide variety of substances: ions such as Na+, K+ and Cl−, and polar molecules such as glucose, amino acids and urea.
It does so because of its own charges. Water molecules crowd round an ion with their δ− oxygens turned towards positive ions and their δ+ hydrogens towards negative ions, and pull the ions apart. Polar molecules dissolve by forming hydrogen bonds with water. Each dissolved particle ends up wrapped in water molecules, which is called solvation. A positive ion such as Na+ is a cation and a negative ion such as Cl− is an anion.
Substances that are polar or charged are hydrophilic. Substances that are non-polar, such as fats, oils and waxes, are hydrophobic: water molecules bond to each other instead, which pushes the non-polar molecules together, as oil forms droplets.
That makes water the medium for metabolism. Most enzymes catalyse reactions in aqueous solution, in the cytoplasm and in the fluids inside organelles, where dissolved enzymes and substrates move about and collide. It also makes water the medium for transport. Blood plasma carries glucose, amino acids, ions, urea and hormones dissolved in it, and xylem sap carries mineral ions such as nitrate and potassium from roots to leaves. Phloem sap carries dissolved sugars. Fats cannot travel dissolved, so in blood they ride inside lipoproteins, packages of lipid and protein that are soluble.
Some molecules work because they are hydrophobic and insoluble. The wax on a leaf stops water escaping. Fat stores energy, and insulates, without dissolving into the cell's water (the seal's insulating fat, in chapter 6). The non-polar tails of phospholipids make membranes a barrier to water-soluble substances (B1.1 and B2.1). Ten substances to sort are below.
Watch out "Water is the universal solvent." It dissolves more substances than any other common liquid, but not everything: fats, waxes, cellulose and bone mineral do not dissolve, and life depends on that.
Watch out "Hydrophilic means soluble." Cellulose is hydrophilic, and it is insoluble.
Fish get all their oxygen from water. Oxygen molecules (O2) are non-polar. How does the oxygen get into the water?
A little dissolves. A litre of water at 20 °C holds only about 9 mg of oxygen, about a thirtieth of the oxygen in a litre of air. Small non-polar molecules such as oxygen slip in between water molecules in small amounts. It is enough for fish, which pass a great deal of water over large gills. Warm water holds even less, which is why fish gasp at the surface in a heatwave.
A grain of salt is a lattice of ions, sodium (Na+) and chloride (Cl−), held together by their opposite charges.
Water molecules turn to face the ions, oxygen towards Na+ and hydrogens towards Cl−, and together they pull ions loose.
Dissolved: every ion is wrapped in its own shell of water molecules and drifts away from the rest.
A sugar is covered in OH groups, which make hydrogen bonds with water, so it dissolves too.
Oil molecules are non-polar. Water bonds to water, not to oil, so the oil is pushed together into a droplet.
Ten substances sorted: five dissolve in water and five do not. The five that do not are useful because of it: wax keeps water in the leaf, fat stores energy and insulates, cholesterol is part of membranes, cellulose makes cell walls that must not dissolve, and calcium phosphate makes bones that last.
A1.1.6 Water as a habitat
Life in water, life in air
Water is a habitat as well as a medium, and living in it is very different from living in air. Four properties decide most of the difference. The values here are for 20 °C.
Buoyancy. Water is over 800 times as dense as air, so it pushes up on a body with almost as much force as the body weighs. Air hardly supports a body at all.
Viscosity. Water is about 55 times as viscous as air, and far denser, so a body moving through it meets much more drag. Streamlined shapes save energy.
Thermal conductivity. Water conducts heat about 23 times as fast as air, so a warm body loses heat far faster in water than in air at the same temperature. Animals in cold water need thick insulation.
Specific heat capacity. It takes about 4.2 J to warm 1 g of water by 1 °C, about four times as much as for 1 g of air, and a litre of water holds about 3,500 times as much heat for each degree as a litre of air. Much of the energy goes into breaking hydrogen bonds before the molecules can move faster. So lakes and seas change temperature slowly, from day to night and season to season, and make a stable habitat.
Two animals show the contrast. The black-throated loon (Gavia arctica) is a diving bird that breeds on large northern lakes, winters at sea and chases fish underwater. The ringed seal (Pusa hispida) is the commonest Arctic seal. It lives on and under the sea ice, keeps breathing holes open with its claws, and is kept warm in the water by a thick layer of blubber. The figure sets water against air for each property, and shows what it means for each animal.
Remember it The four properties A1.1.6 compares
BigBuoyancywater holds a body up
VesselsViscositywater resists movement: more drag
TurnThermal conductivitywater draws heat from a body fast
SlowlySpecific heat capacitywater's temperature changes slowly
Watch out "Thermal conductivity and specific heat capacity are the same." Water is high in both: it drains heat from a warm body quickly, while its own temperature hardly changes.
Watch out "Fur keeps a seal warm in the water." Fur insulates by trapping air, and wet fur loses it. In water, blubber does the job.
Example Same bird, two names: in Britain and Ireland Gavia arctica is the black-throated diver. In an exam, either the common name or the scientific name is accepted.
Ringed seal pups are born in snow caves on the sea ice, with thick woolly fur but almost no blubber. What happens to a young pup that has to stay in the sea for a long time?
It chills fast. The woolly fur keeps a pup warm in the snow cave because it traps air, a poor conductor of heat. Soaked, it holds water instead, and the pup has almost no blubber yet. The sea, at about −1.8 °C, is indeed warmer than the winter air, but water draws heat away so much faster that the pup loses heat far more quickly in it. That is why the snow caves matter.
Property
Black-throated loon
Ringed seal
Buoyancy
In water: Water holds it up so well that it must work to sink: its bones are denser than most birds’, and it squeezes air from its feathers before a dive. In air: Air holds up almost nothing: its heavy body needs a long run across the water to take off, and it cannot take off from land.
In water: Water carries its weight, so its flippers drive and steer instead of holding it up. In air: On the ice nothing holds it up: it hauls its whole weight along on its belly.
Viscosity
In water: Water is thick and heavy to push through: a streamlined body, and big webbed feet set far back, drive it after fish. In air: Air slips past easily, but legs set far back for swimming make it clumsy on land, so it nests right at the water’s edge.
In water: A smooth torpedo shape, no ear flaps and flat fur cut the drag, and side-to-side sweeps of its hind flippers push it along. In air: Air gives almost no drag, but flippers built for water make poor legs: on the ice it is slow.
Heat flow
In water: Water would draw heat from its skin fast: dense, oiled feathers trap a layer of air, which conducts heat poorly. In air: Air draws heat away slowly: the same feathers keep it warm with less effort.
In water: Wet fur loses its trapped air, so a thick layer of blubber under the skin keeps its heat in. In air: Air draws heat away far more slowly than water, even when the air is colder than the sea.
Heat capacity
In water: Its lake changes temperature only slowly, from day to night and from week to week. In air: Air warms and cools quickly: on land the temperature swings between day and night.
In water: Under the ice the sea stays near −1.8 °C all year, winter and summer. In air: On the ice the air may fall below −30 °C in winter and rise above 0 °C in summer.
Water against air for the black-throated loon and the ringed seal: the same four properties, in each medium, and what each means for the animal.
A1.1.7 The origin of Earth's water
Water from space, kept by gravity
Water has been abundant on Earth for billions of years, and that gave life the time it needed to arise and evolve. So where did it come from?
Earth formed about 4.5 billion years ago, and it was hot. Much of its water is thought to have come from space. Asteroids crashed into the young planet, and some of them had water locked inside their minerals. That is the hypothesis this statement asks for.
Bringing water was only half of it. Earth also kept it, for two reasons.
Gravity. Earth is massive enough for its gravity to hold water vapour and the rest of the atmosphere. The Moon, with about a hundredth of Earth's mass, could not, and has almost no water: only a little ice in craters that sunlight never reaches.
Temperatures low enough for water to condense. Earth is far enough from the Sun, and cooled enough, for water vapour to condense into liquid water. It rained out to form oceans that have lasted about four billion years.
Remember it Why Earth kept its water
GripGravityholds water vapour and air to the planet
ChillCool enough to condensetemperatures low enough for vapour to become liquid
Watch out "Earth kept its water because it has an atmosphere." The atmosphere is itself held by gravity. The two reasons to learn are gravity and temperatures low enough to condense water.
How do we know? No one saw it happen, so the hypothesis is tested with evidence. Hydrogen comes in a light form and a heavy form (deuterium), and the ratio of heavy to light hydrogen in water depends on where in the early Solar System the water formed. Water locked in some meteorites, which are fragments of asteroids, has almost the same ratio as Earth's oceans. That supports the asteroid hypothesis.
Venus is almost the same size and mass as Earth, but closer to the Sun. Why are there no oceans on Venus?
Too hot. Venus's gravity is nearly as strong as Earth's, so gravity is not the problem. Its surface is about 460 °C today, so its water stayed as vapour and has since been lost. A planet needs both: enough gravity, and temperatures low enough for water to condense.
The young Earth, about 4.5 billion years ago, is hot, with no oceans.
Asteroids crash into it. Some carry water locked inside their minerals, and they bring it to the planet.
The water turns to vapour and fills the air. Earth’s gravity holds it, and the rest of the atmosphere, in.
Earth cools until the temperature is low enough for the vapour to condense. It rains, and oceans form.
The Moon, at the same distance from the Sun, has too little gravity to hold water, and stays dry. Earth’s oceans have lasted about four billion years.
A1.1.8 The search for life elsewhere
Not too hot, not too cold: the Goldilocks zone
All the life we know needs liquid water as its medium (chapters 1 and 5), so the search for extraterrestrial life starts by looking for liquid water.
The Goldilocks zone, also called the habitable zone, is the band of distances round a star where a planet's surface is warm enough for water not to freeze and cool enough for it not to boil away, so water could stay liquid. Too close and it is too hot. Too far and it is too cold. The zone depends on the star: a brighter, hotter star has its zone farther out, and a dimmer, cooler star has it closer in.
Earth lies inside the Sun's zone. Venus, closer, is far too hot. Mars, just outside the zone and much smaller than Earth, is cold, with a thin atmosphere and its water frozen. Astronomers have now found more than 5,000 planets round other stars, called exoplanets. The rocky ones in their star's zone are the first targets in the search for life, and space agencies share their missions and data to look for them.
The zone is a model, and every model has limits. Being in the zone does not mean a planet has water: it must have received some and kept it (chapter 7). And liquid water can exist outside the zone, under ice warmed from inside, as on Jupiter's moon Europa and Saturn's moon Enceladus. Searching only the zone could miss life, like looking for lost keys only under the street light because the light is better there.
Watch out "Every star's Goldilocks zone is at Earth's distance." It moves with the star's brightness.
Watch out "A planet in the zone has water, so it has life." The zone says where liquid water could exist on the surface, and nothing more.
A star gives out much less energy than the Sun. Where would its Goldilocks zone be?
Much closer. A planet must be near a dim star to receive enough energy to keep water liquid. Round red dwarfs, the commonest stars in our galaxy, the zone is so close that a planet's year there lasts only days or weeks.
A star warms the planets round it. The pale band is its Goldilocks zone: the distances at which a planet’s surface could keep liquid water.
Venus is closer than the zone’s inner edge. It is too hot: water would boil away as vapour.
Mars is beyond the outer edge, and small. It is too cold: its water is frozen and its air is thin.
Earth is inside the zone: just right for liquid oceans.
Move the planet, then change the star. Where is the zone around a dim star?
02Dry lab: Wet wicks: cooling by evaporation
Wrap thermometers in wicks soaked in different liquids, switch a fan on or off, change the humidity, and find out what makes a wet surface cool, and for how long.
Your questionWhat makes a wet surface cool down? Do different liquids cool it by different amounts, at different speeds, or for different lengths of time?
What you can changeThe liquid on each of four wicks (none, cooking oil, water, ethanol), the air (still, or a fan), the room's humidity, the volume of liquid put on each wick and the room temperature.
What to recordEach probe's temperature every 30 seconds for 10 minutes. From each run: the starting temperature, the lowest, the largest fall and when it came. Repeat each set-up at least three times: the runs scatter, as real readings do.
Find out
Is it the cold, or the evaporation? Before you run, write a hypothesis for what each wick will do, and why. Then run all four wicks in still air at 50% humidity, three times. Which wicks cool at all?
Water against ethanol. From the same runs: which cools faster? Which cools further? Which keeps cooling longer? Look at the whole ten minutes, not just the start.
The wet hand again. Switch the fan on and run three times. What changes for each liquid?
A muggy day. Fan off. Run at 20% and at 80% humidity, three times each. Whose cooling depends on the humidity?
Record
For every run: the settings, and each probe's start, lowest, largest fall and when the lowest came.
Change one setting at a time and keep the rest the same.
Repeat each set-up at least three times. Work out the mean, SD and SE of the largest fall for one liquid by hand, then check them against Graph your results.
Draw a graph with error bars, from the lab or from the CSV, and say which error bars you used.
Ethanol cooled the probe faster and further than water, then warmed up again, while water's cooling lasted the whole run. Yet each gram of water that evaporates takes about 2.4 kJ of heat with it, and each gram of ethanol only about 0.9 kJ. How can both be true?
Ethanol escapes more easily. Each ethanol molecule forms fewer hydrogen bonds, and part of it is a non-polar chain, so far more molecules leave each second: a fast, deep fall, until the wick runs dry and warms up again. Water's molecules are held by up to four hydrogen bonds each, so they leave slowly, but every gram that goes takes about two and a half times as much heat. The same few drops of water keep cooling for over an hour. The oil wick shows the liquids were not cold to begin with.
Water's cooling shrank at 80% humidity, but ethanol's hardly changed. Why?
Evaporation depends on how much of that liquid's vapour is already in the air. Humid air is full of water vapour, so many water molecules return to the wick, and the net loss, and the cooling, is small. The room holds no ethanol vapour, so ethanol evaporates almost as fast as ever. It is why sweat cools you poorly on a muggy day.
Blowing on a wet hand makes it feel colder. Why does moving air speed up the cooling?
The probe with no liquid, in the fan's wind, stays at room temperature, so the air itself is no colder. Next to a wet surface the air fills with vapour, and some of it goes back into the liquid. Moving air sweeps it away, so evaporation, and cooling, go faster. That is the wet hand you blew on at the start.
03Check
A fresh paper from the exam bank on this guide's topics each time: past-paper style questions, marked as you go, and saved to your Exam practice progress when you are signed in. It opens on biologybybradford.com, so it needs the internet.
The guide as three one-hour lessons, and one more at higher level. Each has a plan to print (the timings, what to project and where to stop, the activities with what they need, what students get wrong) and slides to teach it from, in the same order, with the teacher's moves in the notes.
Lesson
Covers
Focus
Chapters
Lesson plan
Slides
1Water's charges, and what they do
A1.1.1 to A1.1.360 min
Water as the medium for life, its polar molecules and hydrogen bonds, and cohesion: surface tension and the pull up the xylem
How this was made. The course structure, the activities and the teaching choices come from my own classroom, my presentations and my years of teaching IB and AP Biology. I use Claude, an AI, to help write and build the interactive guides, lesson plans, slides and practice questions, and to draw the diagrams and code the interactives. I direct the work and review it against the IB and AP specifications before it goes live.