Difficulty: Introductory | Prerequisites: None. This is the starting point for AP Biology.
Unit 1 of AP Biology introduces the chemistry that underpins everything else in the course: how atoms bond, why water behaves the way it does, and how pH keeps cells alive. If you have never taken chemistry, or it has been a while, this is the unit that fills in the background. Every later topic, from cell membranes to DNA replication, leans on the bonding and polarity concepts covered here. Come in knowing what atoms and elements are, and you will be fine.
Water is a polar molecule held together by covalent bonds, and its hydrogen bonds give it the special properties (cohesion, adhesion, high surface tension, solvent ability) that make life possible. The pH scale measures hydrogen-ion concentration on a 0–14 range, and buffers keep biological systems stable by resisting sharp pH swings.
Biochemistry
The study of matter, its properties, and its role in living things. Think of it as the place where chemistry meets biology: every reaction inside a cell is biochemistry.
Matter
Anything that has mass and volume and cannot be broken down by chemical means into simpler substances. In simple terms, matter is the "stuff" everything is made of.
Compound
A substance made of two or more elements bonded in a fixed ratio. Water (H₂O) is a compound: always two hydrogens to one oxygen.
Covalent bond
A bond in which atoms share electrons. It can be polar (unequal sharing) or non-polar (equal sharing). Think of it as two people holding the same rope: if one pulls harder, the sharing is unequal, and the bond is polar.
Electronegativity
An atom's tendency to attract shared electrons toward itself. Oxygen is more electronegative than hydrogen, which is why water's bonds are polar.
Hydrogen bond
A weak attraction between a slightly positive hydrogen on one molecule and a slightly negative atom (usually oxygen or nitrogen) on another. Individually weak, but collectively they give water its remarkable properties.
Cohesion
Water's ability to form hydrogen bonds with other water molecules. In simple terms, water sticks to itself.
Adhesion
Water's ability to form hydrogen bonds with other surfaces. This is water clinging to the inside of a glass tube or a plant's xylem wall.
Surface tension
The tendency of water at the surface to be pulled inward by cohesion, creating a "skin" that small insects can walk on.
Capillary action
The movement of water against gravity through narrow spaces, driven by cohesion (holding the water column together) and adhesion (pulling it upward along the vessel wall). This is how plants move water from roots to leaves without a pump.
Hydrophilic
"Water-loving." Describes substances that dissolve in or interact readily with water, typically because they are polar or ionic.
Hydrophobic
"Water-fearing." Describes non-polar substances that do not dissolve in water. Oils are the classic example.
pH
The negative logarithm of the hydrogen-ion concentration in a solution. A lower number means more H⁺ ions and a more acidic solution.
Acidic
A solution with a pH between 0 and 7; it donates H⁺ ions. Lemon juice (pH ~2) is a common example.
Basic (alkaline)
A solution with a pH between 7 and 14; it accepts H⁺ ions. The concentration of OH⁻ ions is higher than H⁺.
Buffer
A solution containing a weak acid and its corresponding base that resists changes in pH. In simple terms, a buffer absorbs extra H⁺ or OH⁻ to keep conditions stable, which is critical inside cells.
Hydroxide ion (OH⁻)
The ion left when a water molecule loses a hydrogen. A hallmark of basic solutions.
Hydronium ion (H₃O⁺)
The ion formed when a water molecule gains an extra H⁺. Often written simply as H⁺ in shorthand.
Water (H₂O) is made of two hydrogen atoms and one oxygen atom joined by polar covalent bonds
Oxygen is more electronegative than hydrogen, so shared electrons spend more time near the oxygen end of the molecule
This gives the oxygen end a slight negative charge (δ⁻) and each hydrogen end a slight positive charge (δ⁺)
Because of this charge separation, water is a polar molecule
The slightly positive hydrogen of one water molecule is attracted to the slightly negative oxygen of a neighbouring molecule, forming a hydrogen bond
Hydrogen bonds are individually weak (about 1/20 the strength of a covalent bond), but water forms many of them at once, and their collective effect is enormous
Hydrogen bonds are responsible for nearly every special property of water that matters in biology
All of the properties below trace back to hydrogen bonding. If you remember nothing else, remember that.
Cohesion – water molecules stick to each other through hydrogen bonds
This is why water forms droplets rather than spreading into a thin film
Adhesion – water molecules stick to other polar surfaces
Adhesion to glass, for example, is why water climbs the sides of a graduated cylinder (the meniscus curves upward)
Surface tension – cohesion among surface molecules creates a resilient "skin" at the water's surface
Strong enough for small insects to walk on water without breaking through
Capillary action – the combination of cohesion and adhesion pulls water upward through narrow tubes against gravity
In plants, this is one mechanism (alongside transpiration pull) that moves water from roots to leaves through xylem vessels
Density anomaly of ice – when water freezes, hydrogen bonds lock molecules into a rigid crystal lattice that takes up more volume than liquid water
Ice is therefore less dense than liquid water and floats
This insulates bodies of water in winter, keeping liquid water (and aquatic life) beneath the ice
Universal solvent – water's polarity allows it to dissolve a wide range of substances
Ionic compounds (salts) dissociate in water because polar water molecules surround and pull apart the ions
Polar molecules dissolve readily; non-polar molecules (fats, oils) do not
Substances that dissolve in water are hydrophilic; those that do not are hydrophobic
Nearly every chemical reaction in a cell takes place in an aqueous (water-based) environment
Water molecules occasionally transfer a hydrogen ion (H⁺) to one another, producing a hydroxide ion (OH⁻) and a hydronium ion (H₃O⁺)
The pH scale measures the concentration of H⁺ ions in a solution, running from 0 (most acidic) to 14 (most basic), with 7 as neutral
pH is calculated as the negative log of H⁺ concentration: pH = –log[H⁺]
Each whole-number step on the scale represents a tenfold change in H⁺ concentration
Acidic solutions (pH 0–7) have a higher concentration of H⁺ than OH⁻
Basic (alkaline) solutions (pH 7–14) have a higher concentration of OH⁻ than H⁺
pH and H⁺ concentration have an inverse relationship: more H⁺ ions means a lower pH number
Even small shifts in pH can denature proteins and disrupt cellular processes, which is why organisms rely on buffers
A buffer is a mixture of a weak acid and its conjugate base that absorbs excess H⁺ or OH⁻, keeping pH within a narrow, liveable range
Blood, for instance, is buffered at about pH 7.4
pH formula: pH = –log[H⁺]
Ion product of water: [H⁺] × [OH⁻] = 10⁻¹⁴ (at 25 °C)
If [H⁺] = 10⁻⁴, then pH = 4 and [OH⁻] = 10⁻¹⁰
If [H⁺] = 10⁻⁹, then pH = 9 and [OH⁻] = 10⁻⁵
Water molecule structure: two O–H polar covalent bonds at a ~104.5° angle, giving the molecule a bent shape and a net dipole
Capillary action is the principle behind paper towels soaking up a spill and the way ink moves through blotting paper.
Ice floating on lakes is the reason aquatic ecosystems survive winter: the ice layer insulates the liquid water below, keeping it above freezing.
Buffers are used in medicine (IV fluids are buffered to match blood pH) and in laboratory biology (most biochemical assays run in buffered solutions to prevent pH drift from skewing results).
Students often confuse covalent bonds (within a water molecule) with hydrogen bonds (between water molecules). Covalent bonds are strong and hold atoms together inside the molecule. Hydrogen bonds are weak and form between separate molecules.
A pH of 7 is neutral only at 25 °C. The concept of "neutral" means equal concentrations of H⁺ and OH⁻, which shifts with temperature, but AP Biology treats 7 as the standard.
"Universal solvent" does not mean water dissolves everything. Non-polar substances such as oils and fats are largely insoluble in water.
Students sometimes assume higher pH means more H⁺. The relationship is inverse: a higher pH means fewer H⁺ ions.
⚠️ Be ready to explain how hydrogen bonding accounts for each property of water (cohesion, adhesion, surface tension, capillary action, ice density, solvent properties). This is a perennial free-response question.
⚠️ Know the difference between hydrophilic and hydrophobic, and be able to predict whether a substance will dissolve based on its polarity.
⚠️ Calculate pH from H⁺ concentration and vice versa, using the relationship [H⁺] × [OH⁻] = 10⁻¹⁴.
⚠️ Understand why buffers matter biologically, not just what they are. Exam questions often ask what happens to enzyme function when pH changes.
True or false: Hydrogen bonds are stronger than covalent bonds. (False. Hydrogen bonds are much weaker individually.)
Fill in the blank: Water is called a polar molecule because its oxygen end carries a slight ______ charge and its hydrogen ends carry a slight ______ charge. (negative; positive)
True or false: Ice sinks in liquid water. (False. Ice is less dense and floats.)
Fill in the blank: A solution with a pH of 3 has a ______ concentration of H⁺ than a solution with a pH of 5. (higher, by a factor of 100)
True or false: A buffer works by adding strong acid to a solution. (False. A buffer contains a weak acid and its conjugate base and absorbs excess H⁺ or OH⁻.)
Q: Explain how hydrogen bonding accounts for water's high surface tension.
A: Water molecules at the surface are pulled inward and sideways by hydrogen bonds with neighbouring water molecules, but they have no water above them to balance that pull. This net inward force contracts the surface, creating a resilient "skin" (surface tension) strong enough to support light objects.
Q: A solution has an H⁺ concentration of 10⁻⁶ M. What is its pH, and is it acidic, basic, or neutral?
A: pH = –log(10⁻⁶) = 6. Because 6 is below 7, the solution is acidic.
Q: Why does ice float, and why does this matter for aquatic life?
A: When water freezes, hydrogen bonds hold molecules in a crystalline lattice with more space between them than in liquid water, making ice less dense. Floating ice insulates the water below, preventing lakes and ponds from freezing solid and allowing organisms to survive winter.
Q: A student adds a strong acid to a buffered solution and to pure water. Predict which system shows a larger pH change and explain why.
A: The pure water shows a larger pH change. The buffer's weak acid/conjugate base pair absorbs the added H⁺ ions, minimising the pH shift. Pure water has no such mechanism, so its pH drops sharply.
Q: Describe how cohesion and adhesion work together to move water up a plant's xylem.
A: Adhesion pulls water molecules along the hydrophilic walls of the xylem vessel, while cohesion keeps the water column intact by linking molecules together through hydrogen bonds. Together, these forces draw water upward against gravity (capillary action), supplemented by transpiration pull from the leaves.
Water's solvent properties connect directly to cell membranes (Unit 2): the phospholipid bilayer forms precisely because its hydrophobic tails avoid water while its hydrophilic heads face it. pH and buffers come back in enzyme kinetics (Unit 3), where even small pH changes alter enzyme shape and activity. Hydrogen bonding also holds together the two strands of the DNA double helix (Unit 6), so the same intermolecular force you learn here reappears in genetics.
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