Most Things Biology

2.2 Water, pH and the Buffer

Case home  ·  Start at the beginning  ·  The Reveal

Time To Read

27–41 minutes

Date Last Modified

Just Enough Chemistry · Chapter 2 of 7

1985 to 1986, ages ten and eleven. The family moves for Dad’s job; there is no Dr. Gonzalez in the new town, and “my fever calendar is in a box somewhere.” At eleven, the third emergency room of the year: pain that “starts under my belly button and slides down low on the right,” a fever of 103, “they took blood,” and “a stomach bug” — “That’s what you said last time,” Mom tells the doctor. After Mom’s second surgery, Mom sits in front of a box fan with a wet washcloth on her neck, even in March, and Stina handles her own fevers: the Tylenol, the bucket, and the bathroom floor where the tile is cool. This chapter builds the molecule all of that happens in — the shared bond, water’s lopsided charge, why a wet cloth cools, pH, and the buffer that almost certainly held her blood near 7.4 at 103 °F.

The question you will answer

Her temperature could climb four degrees in an afternoon, and nothing anyone wrote down says her blood chemistry moved at all. Before you can read that, you need the bond that holds water together, the lopsided charge that lets water stick, dissolve and soak up heat, and the scale that measures acid. The question for the chapter: when a body’s temperature is off by four degrees, what keeps its blood’s pH inside a window a tenth of a unit wide?

Opener · Sharing a Pair · about 1¾ minutes

The opener is the same narrated slide you will watch in Scene 1.

Watch it in Scene 1 ↓

Watch the opener first. It sets the question; the chapter answers it.

1

Read the chart

Patient chart · Stina

1985–1986 · ages 10–11 · the move, the third ER of the year, Mom’s second surgery

1985–86 · 10–11
The move, for Dad’s job. “Nobody here knows about the fevers.” No Dr. Gonzalez; the three-month visit never happened — “first Mom was too tired, then we were packing.” “My fever calendar is in a box somewhere.” Mom’s headaches are back.

1986 · 11
The third ER this year. The pain “starts under my belly button and slides down low on the right”; she can’t stand up straight; fever 103. A doctor pushes on her stomach and “they took blood”; not her appendix — “a stomach bug.” By Sunday, “totally fine, like it never happened.”

1986 · 11
Mom’s second surgery. Afterward Mom can’t take the heat: a box fan and a wet washcloth on her neck, “even in March.” No doctor unless it’s the ER, and no ER unless she can’t stand up; when the fever comes she gets herself “the Tylenol and the bucket” and lies on the bathroom floor “where the tile is cool.”

Told
“A stomach bug.” Mom: “That’s what you said last time.” “Mrs. Lindqvist,” in the voice grown-ups use on dogs. Grandma: “a funny tummy.”

Chart clues this module: 2 · this module (logged in Chapter 7) — undated, “across these years”; no lab values exist for 1985–86

Before you go on · your prediction

Eleven, a fever of 103, a belly attack, a bucket on the bathroom floor, and blood drawn at the ER — and nothing came of it. What was her blood’s pH doing through all of that?

Open the one you lean toward. Nothing is recorded and nothing is revealed — you come back to it at the end.

Sliding toward acid — a fever that high must be souring the blood

Watch how big a step on the pH scale really is, and what stands between an acid load and the blood within the first second.

Nothing worth asking about — “a stomach bug”

Watch which numbers a body defends during an illness, and whether a steady number means nothing happened or that something worked.

Holding almost exactly where it always sits

Watch for what does the holding, how fast each defender works, and which way vomited stomach acid would push.

From Stina’s diary

Age 11 · “A sensitive tummy”

2

The story

Three short movements: the case, the anatomy and physiology, then the tie back to Stina. Each links to the scene where you’ll see it move.

I · Three times this year

They moved when she was ten or eleven, for Dad’s job, and Mom said it was a fresh start because nobody in the new town knew about the fevers. Stina heard it differently: nobody here was going to believe her either, and they hadn’t even had the chance not to yet. There was no Dr. Gonzalez in the new town. They had never gone back for the three-month visit — first Mom was too tired, then they were packing — and the fever calendar was in a box somewhere. Mom’s headaches came back. At eleven the stomach came for the third time that year. It started under her belly button and slid down low on the right until she couldn’t stand up straight, and her fever was 103. Mom drove her to the emergency room. A doctor pushed on her stomach and they took blood, and then they said it wasn’t her appendix, it was a stomach bug. “That’s what you said last time,” Mom said, and he said “Mrs. Lindqvist” in the voice grown-ups use on dogs. Mom gripped the steering wheel with both hands the whole way home and didn’t turn on the radio. Then the tumor came back and they had to go in again, and afterward Mom couldn’t take the heat at all: she sat in front of the box fan with a wet washcloth on her neck, even in March. Mom slept in the afternoons and Dad worked late, so when the fever came, Stina got herself the Tylenol and the bucket and lay on the bathroom floor, where the tile was cool. They didn’t go to the doctor anymore unless it was the emergency room, and they didn’t go to the emergency room unless she couldn’t stand up.

See it move: Scene 1 · Sharing a Pair · Scene 3 · A Washcloth in March

II · Shared pairs, a lopsided molecule, a tenfold scale

Ionic bonds hand an electron over; covalent bonds share — the strong, generous bond. One shared pair is a single bond, two (O₂) a double, three (N₂) a triple, and the more two atoms share, the stronger the bond; strength and stability are the same question wearing different clothes. Breaking a bond takes energy and forming one releases it — Chapter 3 keeps that ledger. Sharing has fine print. Oxygen has eight protons pulling on the shared pairs and hydrogen has one, so in water the pairs sit closer to oxygen: a polar covalent bond. Water is bent, so the pulls don’t cancel — which is why it looks like Mickey Mouse to me, a slightly negative oxygen chin and two slightly positive hydrogen ears. (Carbon dioxide’s bonds are polar too, but the molecule is straight, so its pulls cancel.) One water’s positive ear reaches for the next one’s negative chin: a hydrogen bond — weak alone, mighty in numbers. Those bonds make water stubborn about temperature, a high specific heat that keeps a body or a coastline steady, and stubborn about leaving, a high heat of vaporization: each gram that evaporates off skin carries away about 2,400 joules. They make water the universal solvent — like dissolves like; plasma is about 92% water — and sticky, cohesion and adhesion. A few water molecules come apart into H⁺ and OH⁻. pH is the negative logarithm of the H⁺ concentration: pure water, with equal H⁺ and OH⁻, is 7, and every step down is ten times more H⁺. Blood is held at 7.35–7.45, just on the basic side; the threat is lopsided, so the defense is too. The defense starts with a buffer: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻.

See it move: Scene 2 · Lopsided · Scene 3 · A Washcloth in March · Scene 4 · The Universal Solvent · Scene 5 · Ten Times Per Step

III · The thermostat’s chemical partner

Here is what was holding steady that afternoon, and what you learn now. A fever of 103 °F is 39.4 °C, about two and a half degrees above her usual — Module 1’s thermostat, deliberately turned up by her own white cells’ alarm. Her blood’s pH did not follow it. Nothing in the diary or the chart says it moved, and the chemistry tells you why it almost certainly didn’t: the bicarbonate buffer is negative feedback written in molecules. Add H⁺ and bicarbonate takes it up as carbonic acid within a fraction of a second; carbonic anhydrase in her red cells turns that into CO₂ and water; her lungs breathe the extra CO₂ away within minutes; her kidneys excrete acid and rebuild bicarbonate over hours to days. Albumin in the plasma and hemoglobin inside the red cells buffer too. The same afternoon’s other chemistry pushed the opposite way. What goes into a bucket is stomach acid, so vomiting takes H⁺ and Cl⁻ out and leaves bicarbonate behind — a push toward alkalosis that Module 20 explains — and a child breathing fast with fever and pain blows off CO₂, which also tips pH up and can bring tingling fingers and light-headedness, symptoms easily read as nerves. Honesty flags: canon keeps no pH or blood-gas values from these years; the diary records no fast breathing; and the idea that inflammation keeps “pushing acid” into the blood is not established, so no tidy story here claims it. What the chemistry does show is a body competently defending its numbers while the chart said “a stomach bug.” One line forward: at forty-nine to fifty, with her kidneys leaking protein, her bicarbonate will read 20 mEq/L, just under the textbook 22–28 (Module 20).

See it move: Scene 6 · The Buffer

3

Watch · explore · think · check

6 scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.

How to read the scenes

  • Animation (dark teal) — a short animated explainer, one idea each
  • Slide (dark teal) — a still to study: a micrograph, a diagram or a map
  • Explore (purple) — something to push on while you watch
  • Game (purple) — a quick challenge
  • Think (gold) — reason it out, then spot-check yourself with a few true/false questions; not graded
  • Check (gold) — a quick H5P that scores itself

Scene 1 of 6

Sharing a Pair

The strong, generous bond: two atoms share a pair — and the sharing is rarely fair.

Narrated slide · in production · 1 min 43 s

Sharing a Pair

Amy’s narrated slide, Sharing: the covalent bond is “the strong one, and also the generous one — instead of one atom robbing another blind, the two atoms share a pair,” a kind of joint super-shell that fills both outer shells — true hand-holding (“you get one, I get one”). Very strong, the hardest to break. Oxygen gas shares two pairs — a double bond, “tougher still than a single… part of why it’s stable enough to ride around in your blood.” The rule: the more electrons two atoms share, the stronger the bond; and “sharing turns out to have a lot of fine print, and that fine print writes the rest of physiology.” Figure: Types of Chemical Bonds — sodium handing an electron to chlorine and magnesium two to oxygen; O–O and H–O–H sharing pairs; copper ions in a shared electron cloud; partial charges drawing two water molecules together. Caption line under the player: the narration’s “look at the figure: carbon dioxide and water” points to the oxygen and water molecules in this figure — carbon dioxide is drawn in Scene 2.

Prefer the full lecture? Covalent Ionic and Hydrogen Bonding (original video on the site)

Explore · in production

Snap the model

A ball-and-stick model kit on a tray: atom balls with their bonding holes drilled in — hydrogen (white, 1 hole), oxygen (red, 2), nitrogen (blue, 3), carbon (black, 4) — and a cup of sticks: short single sticks, and pairs and triples of bendy springs for double and triple bonds. Seven build cards: H₂ · O₂ · N₂ · H₂O · CO₂ · CH₄ (methane) · NH₃ (ammonia). Before each build the student predicts how many shared pairs the finished molecule will hold. Snapping a stick into a hole fills both atoms’ valence shells in an inset Lewis view — dots pairing into shared pairs — and a molecule counts as complete only when every hole is filled. Counters show the shared pairs (H₂ 1; O₂ 2, as one double bond; N₂ 3, as one triple bond; H₂O 2; CO₂ 4, as two double bonds; CH₄ 4; NH₃ 3, plus one lone pair) and a strength meter climbs from single to double to triple: the more pairs shared, the stronger the bond, and the more energy it takes to break it. A sorting tray asks of each build: MOLECULE OF ONE ELEMENT (H₂, O₂, N₂) or COMPOUND (H₂O, CO₂, CH₄, NH₃) — every compound is a molecule, but not every molecule is a compound (site slip N74). A side card on N₂: about 78% of every breath, and almost none of it reacts in you, because that triple bond is so hard to break. The trap card: “Build HO.” One oxygen, one hydrogen: a hole stays open, the model won’t close, and the kit flashes “not a stable molecule — one hole unfilled” (it exists as the hydroxide ion, OH⁻, only by taking an extra electron — Scene 5). Last card: the finished water model, and one question to carry into Scene 2 — “Is the sharing in O–H fair?” Keyboard path: Tab to an atom, Enter places it on the tray, arrow keys pick a hole, Space snaps a stick; after every snap the Lewis inset is described in text (“oxygen: 2 shared pairs, 2 lone pairs, shell full”). A <details> text version lists each molecule’s atoms, bonds and shared pairs.

Think · not graded

Nitrogen gas, N₂, is about 78% of every breath you take, and almost none of it reacts with anything in your body. Using what you know about shared pairs, explain why — and why it takes special enzymes in soil bacteria, not in you, to break it apart.

The true/false spot-check for this prompt is in production.

Check · Fill in the Blanks

Covalent bonds share electron pairs and fill both atoms’ outer shells. Fill in each blank.

This self-scoring check is in production.

Evidence card 1

Add it to your evidence board

“My fever calendar is in a box somewhere”: after the move, the one record of her fever days is packed away, and no doctor in the new town has seen it.

Scene 2 of 6

Lopsided

Oxygen pulls harder than hydrogen, and that one unfair share is why water looks like Mickey Mouse — and why it sticks to itself.

Animation · in production · 2 min 05 s

Lopsided

Chapter 1’s tug-of-war rope reappears between an oxygen and a hydrogen; the shared pair slides toward oxygen and stops short of crossing. Partial-charge symbols bloom — δ− on oxygen, δ+ on each hydrogen — as a red-to-blue shading. The molecule bends to 104.5°, and two round hydrogen ears pop up on an oxygen head: Mickey Mouse. Beside it, carbon dioxide in a straight line: both bonds shaded, the two pulls drawn as equal arrows pointing opposite ways, and they cancel — POLAR BONDS, NONPOLAR MOLECULE. Then hundreds of Mickey heads: each positive ear reaches for a neighbor’s negative chin, and dotted lines flicker in and out — a hydrogen-bond network snapping and re-forming, WEAK ALONE, MIGHTY IN NUMBERS. A label names the only partners that make one: a hydrogen already bonded to oxygen or nitrogen, reaching for another oxygen or nitrogen. A last zoom: a long protein chain, the same dotted lines holding a coil in place — Chapters 5 and 7.

Prefer the full lecture? Covalent Ionic and Hydrogen Bonding (original video on the site)

Explore · in production

Turn on the charge view

Five molecules on a turntable: water (H₂O), carbon dioxide (CO₂), methane (CH₄), ammonia (NH₃) and ethanol (C₂H₅OH), each a plain gray ball-and-stick model. Before toggling anything, the student predicts POLAR or NONPOLAR for each molecule, and EQUAL or UNEQUAL for each kind of bond in it. Toggle CHARGE VIEW and an electron-cloud heat map spreads over the model — red for partial negative, blue for partial positive, white for even — with an electronegativity ladder at the side (oxygen and nitrogen high; carbon and hydrogen close together). Water: red chin, blue ears — polar. CO₂: each oxygen red, the carbon blue, so every bond is polar — but drop the arrow tool on it and the two equal pulls cancel along a straight line: a nonpolar molecule made of polar bonds (the old “shares evenly” line, site slip 22, corrected on screen). Methane: nearly white all over, because C–H sharing is nearly even — nonpolar. Ammonia: a pyramid with a red nitrogen — polar. Ethanol: a nonpolar tail and a polar O–H head — both, which is why it mixes with water and with oil (a bridge to phospholipids, Chapter 4). A misconception card checks the vocabulary: polar doesn’t mean positive, nonpolar doesn’t mean negative, and nonpolar doesn’t mean uncharged — water is polar and uncharged overall. Round 2, POUR: tip a beaker of sixty animated water molecules into a box; dotted hydrogen bonds form between δ+ hydrogens and δ− oxygens, with a live counter of bonds per molecule. A temperature slider runs from −10 to 110 °C: in ice a rigid, open lattice (about four bonds each — the reason ice floats); at 37 °C the bonds break and re-form constantly; near 100 °C molecules tear loose into vapor and the counter falls toward zero. A trap card asks the student to draw a hydrogen bond between two methane molecules; the tool refuses — no hydrogen bonded to O or N, no partner to reach. The reveal: a hydrogen bond is all about hydrogen — a hydrogen already bonded to oxygen or nitrogen, attracted to another oxygen or nitrogen (site slip 24 corrected). Keyboard path: Tab between molecules, C toggles charge view, arrow keys rotate the model; the temperature slider is a native range input, and the bond counter is announced on change. A <details> text version gives each molecule’s polarity reasoning and the bond count by temperature.

Think · not graded

Oil and water never mix, yet ethanol mixes with both. Using partial charges and hydrogen bonds, explain what about ethanol lets it make friends on both sides — and predict what kind of molecule a cell would need to build a wall between two watery compartments.

The true/false spot-check for this prompt is in production.

Check · Mark the Words

Mark the words that describe unequal sharing and the attraction it creates. Leave the equal-sharing words unmarked.

This self-scoring check is in production.

Evidence card 2

Add it to your evidence board

“It starts under my belly button and slides down low on the right”: the third ER visit that year, the same route each time, and the same verdict — “a stomach bug.”

Scene 3 of 6

A Washcloth in March

A wet cloth cools because water has to break its hydrogen bonds to leave — and every gram that leaves takes heat with it.

Animation · in production · 2 min 10 s

A Washcloth in March

A box fan turning in a March kitchen; a wet washcloth folded across the back of a neck. Zoom into the cloth: water molecules held by flickering hydrogen bonds; heat arriving from the skin as faster jiggling; the fastest molecules tearing free into the air, each leaving with a packet of heat — a counter reads ABOUT 2,400 JOULES PER GRAM, then ≈ 580 KCAL PER LITER. A split screen: a pot of water and a pot of oil on identical burners; the oil’s thermometer races and the water’s creeps — HIGH SPECIFIC HEAT, 1 CALORIE PER GRAM PER DEGREE — and a coastal town’s thermometer barely moves while a desert’s swings. Two labels, HEAT OF VAPORIZATION and SPECIFIC HEAT, are pinned to their own pictures and never cross. Then a feverish outline at 103 °F (39.4 °C) on a bathroom floor, with three routes leaving it: evaporation from a wet cloth, conduction into the cool tile, convection carried off by a fan’s moving air — and radiation glowing faintly as the fourth. Last line on screen: “Module 5 has the sweat glands. This chapter has the molecule.”

Prefer the full lecture? Water’s Life Supporting Properties (original video on the site)

Explore · in production

Race to cool

A feverish body outline at 39.4 °C (103 °F) lies on a cut-away floor, with three cooling stations the student switches on one at a time: a WET WASHCLOTH on the neck (evaporation), a COOL TILE floor under the back (conduction), a BOX FAN blowing across the skin (convection — and, with the cloth on, faster evaporation). A fourth route, RADIATION, is always on and shown as a faint glow. Before the race the student ranks the three by how much heat each will carry off in ten minutes. Run each alone, then any pair, then all three: a heat-flow meter (kilojoules) and a skin-temperature readout update, with a standing note that the fever’s set point is unchanged — cooling the skin doesn’t turn the thermostat down, which is why a feverish child shivers under too cold a cloth (Module 1). The reveal: alone, the wet cloth carries off the most heat per minute, because every gram that evaporates takes about 2,400 J; the tile helps until it warms to skin temperature, because conduction needs a difference; paired with the cloth, the fan adds the most by speeding evaporation. Round 2, SWAP THE LIQUID: soak the cloth in water, rubbing alcohol or vegetable oil and race again. Alcohol evaporates fastest and feels coldest at first, but carries off only about a third as much heat per gram and is gone in minutes (a side card: pediatric guidance warns against sponging children with alcohol, which is absorbed through the skin); oil barely evaporates and cools almost nothing. Round 3, HEAT A POT: the same burner under a pot of water and a pot of oil — oil’s temperature races, water’s creeps — high specific heat, 4.18 J per gram per °C, the reason a body that is about 60% water, and an ocean that Amy’s marine-biology page calls the planet’s thermostat, change temperature slowly. Each result must be labeled with the property that explains it — HEAT OF VAPORIZATION (evaporation cools) or SPECIFIC HEAT (temperature resists change) — and a mismatch opens a card explaining the difference. Keyboard path: stations toggle with Tab and Space; the liquids are a radio group; every meter reading is mirrored in a live region. A <details> text version lists each run and its result.

Think · not graded

Mom’s wet washcloth works even in March in a cool room, and it works better with the box fan on. Explain what each one is doing at the level of water molecules — and why a bowl of cool oil on the neck would do almost nothing.

The true/false spot-check for this prompt is in production.

Check · Drag the Words

Build water’s two temperature tricks — and keep them apart. Two distractors are on the bank.

This self-scoring check is in production.

Evidence card 3

Add it to your evidence board

After Mom’s second surgery: a wet washcloth on her neck “even in March”; for Stina’s own fevers, the bathroom floor “where the tile is cool.”

Scene 4 of 6

The Universal Solvent

Like dissolves like: plasma is about 92% water, so every molecule in the cast either dissolves, disperses, or needs a ride.

Narrated slide · in production · 1 min 58 s

The Universal Solvent

Two of Amy’s narrated slides. Slide 4, Blood is a solution: water dissolves more substances than anything else; a solute dissolved in a solvent makes a solution; “seltzer is just carbon dioxide dissolved in water; add a little salt and you’ve made club soda”; hydrophilic sugar and salt, hydrophobic oil and wax; “seawater and blood are fundamentally the same kind of thing,” and “every cell you own is basically a small, complicated packet of saltwater”; whether a molecule dissolves decides “whether it rides freely or needs to be smuggled across by a protein escort.” Figure: a centrifuged tube — about 55% plasma on top, a thin layer of white cells and platelets, about 45% red cells. Slide 2, Ice floats: almost every substance gets denser when it freezes, but water “spreads its molecules into an open, roomy lattice,” so the solid is lighter than the liquid; on an upstate New York lake in January the floating layer “behaves exactly like a blanket” — “sun’s not getting in, but heat’s not getting out.” Caption lines under the player: (1) Pasta and rice don’t dissolve; their starch soaks up water and swells. (2) Seawater is about 3.5% salt and plasma about 0.9%: the same kind of solution, not the same strength. (3) Plasma itself is about 92% water.

Prefer the full lecture? Water’s Life Supporting Properties (original video on the site)

Explore · in production

Stock the beaker shelf

A shelf above a large beaker labeled PLASMA (about 92% water), with six jars: table salt (NaCl) · glucose · olive oil · cholesterol · albumin (a protein) · a steroid hormone (cortisol). Four outcome trays wait under the beaker: DISSOLVES (a solution — particles too small to see, never settle), DISPERSES (a colloid — large particles that stay spread out and scatter a flashlight beam, but don’t settle), FLOATS OR SEPARATES (won’t mix) and NEEDS A CARRIER (rides on or inside a protein). Before each drag the student predicts the tray. Drop a jar in. Salt vanishes into Na⁺ and Cl⁻, each wrapped in a ring of water molecules turned the right way — hydration spheres, δ− oxygens toward Na⁺, δ+ hydrogens toward Cl⁻. Glucose vanishes as whole molecules, hydrogen-bonded to the water. Olive oil beads up and floats — hydrophobic; like dissolves like. Cholesterol settles as a waxy film and the tray reads NEEDS A CARRIER, with a locked card pointing to Chapter 4’s lipoprotein fleet. Albumin disperses: a flashlight beam shows its path through the beaker (a colloid), and a tag notes that albumin is itself a carrier (Chapter 5). Cortisol mostly binds a carrier protein, leaving a small free fraction that does the work — “most,” not “cannot” (notes-digest SCI 7). A seventh jar arrives last, whole blood: it swirls, then stands, and the red cells settle to the bottom — a suspension. The reveal table sorts solution, colloid and suspension by particle size, under the rule like dissolves like. A side panel lists water’s other jobs from OpenStax: cushion and lubricant (the serous fluid between Module 1’s membranes), heat sink (Scene 3) and reactant (it splits molecules in hydrolysis, Chapter 3). A second side panel covers cohesion and adhesion — surface tension holding up a water strider; water wicking up a paper towel or the cuff of your jeans — and makes one honest correction: blood moves through capillaries on the heart’s pressure (“capillary” means hair-thin), and trees lift water by cohesion and tension from evaporating leaves, not by capillary action. Keyboard path: Tab to a jar, Enter adds it, F shines the flashlight; outcomes are announced (“Olive oil: floats — hydrophobic”). A <details> text version gives every jar’s result and the reason.

Think · not graded

Plasma is about 92% water. Salt and glucose dissolve in it; olive oil won’t. If cholesterol and fats can’t dissolve in plasma, how do they get from your gut to your liver to every cell in your body — and what would their carrier have to look like on the outside?

The true/false spot-check for this prompt is in production.

Check · Fill in the Blanks

Water is the solvent every member of the cast travels in. Fill in each blank.

This self-scoring check is in production.

Evidence card 4

Add it to your evidence board

“They took blood” at eleven — and no line in the diary or the chart says what any tube of it was tested for.

Scene 5 of 6

Ten Times Per Step

pH is a count of loose hydrogen ions on a scale where every step is ten times the last.

Slide · in production

Ten Times Per Step

A vertical pH scale from 0 to 14, every step labeled ×10. Acids donate H⁺; bases accept it (or release OH⁻). pH = −log₁₀[H⁺]: pure water at 25 °C has equal H⁺ and OH⁻ and a pH of 7 — not a ratio, not “twice as many,” and not seven of anything. Each step down is ten times more H⁺: pH 6 has ten times the H⁺ of pH 7, pH 5 a hundred times. On the scale: stomach acid about 1.5–3.5 (what goes into a bucket when a child vomits) · lemon juice about 2 · orange juice about 3.5 · black coffee about 5 · urine about 6 (range 4.5–8) · blood 7.35–7.45, a band just on the basic side, “so narrow it borders on obstinate” · seawater about 8.1 · ammonia about 11 · household bleach about 12.5. A red edge marks acidosis below 7.35 and alkalosis above 7.45; beyond about 6.8 or 7.8, life fails — the danger is a few tenths, not a few hundredths. The body makes acid all day, so “the threat is lopsided, and the defense is too.” Last line, from Amy’s marine-biology page: “pH is just a count of loose hydrogen ions.”

Explore · in production

Climb the staircase

A staircase of fifteen steps, pH 0 at the top left down to 14 at the bottom right, every riser printed ×10; a stick figure stands on 7. A tray of seven cards: stomach acid · lemon juice · black coffee · urine · blood · seawater · household bleach. Before placing each card the student predicts its step; the card snaps to the nearest one and shows its value (stomach 1.5–3.5, lemon about 2, coffee about 5, urine about 6 within a 4.5–8 range, blood 7.35–7.45, seawater about 8.1, bleach about 12.5). Every step the figure walks plays a ×10 animation: a jar of H⁺ beads multiplies tenfold per step down and divides tenfold per step up. Blood gets a magnified tread showing its narrow band, the acidosis and alkalosis edges, and a warning that the limits of survival sit only a few tenths away (about 6.8 and 7.8). A calculator panel, HOW MANY TIMES MORE H⁺?, takes any two cards and computes ten to the power of their difference: stomach acid at 1.5 against blood at 7.4 is about 10⁵·⁹ — nearly a million times more H⁺. A seawater card adds the ocean’s own number from Amy’s marine page: surface seawater has dropped about 0.1 pH unit since the 1800s, which is about 26% more H⁺ (10^0.1 ≈ 1.26). Two trap prompts, each with a teaching answer: “At pH 7, are there 7 H⁺ and 7 OH⁻?” (no — equal amounts, about one ten-millionth of a mole of each per liter); “Is water twice as many H⁺ as OH⁻?” (no — that’s the two hydrogens in H₂O, a different thing entirely). Last card: Amy’s student who asked for the most acidic thing on Earth — “0!” — and the correction: very strong acids go below 0; the scale isn’t capped and is never redefined. Keyboard path: Tab to a card, arrow keys move it step by step, Enter places it; the calculator’s two dropdowns are native selects and the answer is announced. A <details> text version lists each substance, its pH and its H⁺ relative to blood.

Prefer the full lecture? pH (original video on the site)

Think · not graded

Normal blood sits between pH 7.35 and 7.45 — a window a tenth of a unit wide. Using the tenfold rule, work out roughly how much the H⁺ concentration changes from the top of that window to the bottom, and then explain why a blood pH of 7.0, which sounds “neutral,” is a medical emergency.

The true/false spot-check for this prompt is in production.

Check · Single Choice

What does a lower number on the pH scale mean?

This self-scoring check is in production.

Evidence card 5

Add it to your evidence board

“I get myself the Tylenol and the bucket”: vomiting is expected with the fever — a routine she runs alone on the bathroom floor.

Scene 6 of 6

The Buffer

A fever of 103 and a bucket on the floor, and her blood held near 7.4 the whole time — chemical negative feedback, the thermostat’s partner.

Animation · in production · 2 min 15 s

The Buffer

An eleven-year-old’s blood drawn as a calm river under a pH gauge at 7.4. A drop of extra H⁺ falls in — STIMULUS. Bicarbonate ions grab it at once, H⁺ + HCO₃⁻ → H₂CO₃ — the RESPONDER, in a fraction of a second. Inside a red cell, carbonic anhydrase splits the carbonic acid into CO₂ and water, and the whole equation writes itself across the screen with arrows running both ways: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. The lungs take the CO₂ and breathe a little faster, under a clock labeled MINUTES; the kidneys excrete H⁺ and rebuild bicarbonate, under a clock labeled HOURS TO DAYS. Albumin in the plasma and hemoglobin in the red cells hold a few H⁺ of their own. Module 1’s thermostat loop slides in beside it — stimulus, receptor, control center, effector, response — same shape, different molecules. Then the opposite push: a bucket, stomach acid leaving as H⁺ and Cl⁻, and a breath counter speeding with pain and fever as CO₂ blows off; both nudge the river up toward ALKALOSIS, a label that locks with MODULE 20. A crossed-out card, CRP AND SAA, floats past: not buffers. Last frame: the river still at 7.4, and a thermometer beside it at 103 °F.

Prefer the full lecture? Bicarbonate Buffer System (original video on the site)

Explore · in production

Pass the buckets

A river of plasma at pH 7.40 runs left to right under a large gauge (7.35–7.45 shaded). Along the bank stands a bucket brigade: a row of HCO₃⁻ (bicarbonate) figures and a row of H₂CO₃ (carbonic acid) figures, with a carbonic anhydrase worker at the red-cell station turning carbonic acid into CO₂ and water. Two buttons drop trouble into the river. ADD H⁺ is labeled with real sources from Amy’s pH page — CO₂ from every cell’s respiration, lactic acid from hard-working muscle, ketone bodies in starvation, phosphoric acid from breaking down proteins and nucleic acids. REMOVE H⁺ is labeled with vomited stomach acid. Before each drop the student predicts how far the gauge will move. With only the buffer on, an H⁺ is passed to a bicarbonate, becomes carbonic acid and then CO₂: the gauge dips a hair and steadies — a chemical shock-absorber that soaks up acid, until it can’t. Drop twenty at once and the bicarbonate row thins (the “alkaline reserve” draining) and the gauge slips toward 7.35. Now two switches, each with its own clock. LUNGS (minutes): breath rate rises, CO₂ bubbles off the river, the gauge comes back — and stops just inside the normal range rather than in the middle, as Amy’s bicarbonate page notes. KIDNEYS (hours to days): a slow tap that excretes H⁺ into urine, partly as ammonium (NH₄⁺), and refills the bicarbonate row; a note says the kidneys work every day, not only when the others are exhausted (site slips 34 and N12). Turn the lungs off and acid piles up as CO₂ (toward respiratory acidosis); press REMOVE H⁺ for vomiting and the brigade hands bicarbonate the other way — the gauge drifts above 7.45 toward alkalosis, and a lock marks Module 20. A third button, FAST BREATHING (fever, pain, fear), blows off CO₂ and tips pH up, with a side note on how that can feel — tingling fingers, light-headedness — and an honesty tag: the diary doesn’t record it. A sidebar sorts the other buffers by where they work — albumin in plasma, hemoglobin in red cells, phosphate inside cells and in urine — beside a crossed-out card, CRP AND SAA, explaining that acute-phase proteins at milligrams per liter do essentially no buffering (the old slide’s slip). The last screen redraws the loop in Module 1’s words: stimulus (added H⁺) · responder (the buffer pair, seconds) · effectors (lungs in minutes, kidneys over hours to days) · response (pH back toward 7.4). Keyboard path: every button and switch is a native control in one Tab order; the gauge value is announced after each action. A <details> text version gives the equation, the three timescales and each scenario’s outcome.

Think · not graded

Module 1’s thermostat senses a temperature, compares it with a set point and sends orders to sweat glands and muscles. Map the bicarbonate buffer system onto the same five words — stimulus, receptor, control center, effector, response — and say which part works in seconds, which in minutes and which over days. Then explain how her temperature could climb more than four degrees Fahrenheit while her pH stayed put.

The true/false spot-check for this prompt is in production.

Check · Single Choice

A buffer resists changes in pH. What does the blood’s main buffer pair do when extra H⁺ arrives?

This self-scoring check is in production.

Evidence card 6

Add it to your evidence board

Fever 103 in the emergency room, blood drawn, and by Sunday “totally fine, like it never happened” — no line anywhere says her blood chemistry was off.

4

Your evidence board

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: A body defending its chemistry, Neither on its own, or A stomach bug. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.

Evidence board · drag and drop · in production

The sorting board for this chapter’s cards is being built. Until it arrives, sort your evidence cards on paper into the three columns above.

5

Your verdict

Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.

Look back at your prediction

A fever of 103, vomiting into a bucket, blood drawn, and “a stomach bug” — read through water and pH, the reading that fits best is…

Find the one you chose at the top. Does the evidence support it, refute it, or revise it?

  • Sliding toward acid — a fever that high must be souring the blood
  • Nothing worth asking about — “a stomach bug”
  • Holding almost exactly where it always sits

Build your verdict · no writing

Choose the statement that holds in each set. The ones you keep become your argument.

This activity is in production.

6

The finding

Finding · water and pH

Held Near 7.4 at 103 °F

1985–86, ages ten and eleven: the move and the lost fever calendar; the third ER of the year — pain from under the belly button sliding low on the right, a fever of 103, blood drawn, “a stomach bug,” fine by Sunday; fevers handled at home with “the Tylenol and the bucket” on a cool tile floor, beside a mother with a wet washcloth in March. Chart entry: water’s polar, hydrogen-bonded structure cools by evaporation and resists temperature change; blood pH is held at 7.35–7.45 by the bicarbonate buffer in seconds, the lungs in minutes and the kidneys over hours to days — chemical negative feedback, the thermostat’s partner — so a child at 103 °F can hold her pH steady while vomited acid pushes the other way. Held open: what an attack costs in fuel and why it switches off (Chapter 3), and what does rise in her blood across these years (Chapters 6 and 7).

Next · Chapter 3

Carbohydrates and Energy

“Everybody else’s stomach bug lasts a week. Mine blows in like a storm and leaves like somebody switched off a light.” An attack is expensive chemistry — and then, by Sunday, it stops.

Continue to Chapter 3 →

Reliable information on rare diseases

NORD · NIH GARD · Orphanet · MedlinePlus Genetics: FMF · FMF & AID Global Association · Autoinflammatory Alliance · Global Genes · Amyloidosis Foundation

Stina has one rare disease. In the United States, about 1 in 10 people live with one of the more than 7,000 known rare diseases. These organizations are where patients, families and clinicians go for trustworthy information.

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← Familial Mediterranean Fever — case home

All pages for Module 2 — Just Enough Chemistry

2.0 Module Overview
2.1 Atoms, Ions, Electrolytes
2.2 Water, pH and the Buffer
2.3 Carbohydrates and Energy
2.4 Lipids: Membranes, Steroids and the Lipoprotein Fleet
2.5 Proteins: Shape Is Everything
2.6 The FMF Molecule Cast
2.7 Misfolding: From Pyrin to Amyloid
2.8 Conclusion and Assessment
2.9 Evidence Behind the Case

All Modules
  1. Anatomical Language, Membranes & Homeostasis
  2. Just Enough Chemistry
  3. The Cell & Its Transport
  4. Making Cells & Proteins
  5. The Integumentary System
  6. The Skeletal System
  7. The Muscular System
  8. Nervous Tissue & the Senses
  9. The Spinal Cord
  10. The Brain & the Blood–Brain Barrier
  11. The Autonomic Nervous System
  12. Special Senses (in development)
  13. The Endocrine System
  14. Blood
  15. The Heart
  16. Blood Vessels
  17. The Digestive System
  18. The Respiratory System
  19. The Urinary System
  20. Fluids, Electrolytes & Acid–Base Balance
  21. The Reproductive System
  22. The Immune System

List of terms