Unit 1 Part 1: Chemistry & Water

Hydrophobic

AP Bio: Having little affinity for water; nonpolar substances generally interact poorly with water and tend to cluster in it. | Plain English: Does not mix well with water. | Picture: Oil droplets joining together in salad dressing. They do not actively push water away.

Hydrophilic

AP Bio: Having an affinity for water through polar or charged regions that interact favorably with water molecules. | Plain English: Interacts well with water. | Picture: Sugar surrounded by water as it dissolves. Some hydrophilic structures interact with water without dissolving completely.

Amphipathic

AP Bio: Having both hydrophilic and hydrophobic regions within the same molecule. | Plain English: One part mixes well with water; another part does not. | Picture: A phospholipid with a water-friendly head and oily tails.

Atom

AP Bio: The smallest unit of an element that retains that element's chemical identity. | Plain English: One tiny building unit of an element. | Picture: One carbon atom as a single construction piece.

Element

AP Bio: A substance consisting of atoms with the same number of protons. | Plain English: A basic type of matter, such as carbon or oxygen. | Picture: A bin containing only one kind of building piece.

Carbon

AP Bio: An element with four valence electrons that can form four covalent bonds, making diverse biological molecular skeletons possible. | Plain English: A versatile connector used to build life's molecules. | Picture: A four-connection building piece that can form chains, branches, and rings.

CHNOPS

AP Bio: A mnemonic for six major elements in biological molecules: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. | Plain English: Six especially important ingredients of living things. | Picture: Six labeled ingredient jars; life also needs other elements.

Molecule

AP Bio: Two or more atoms held together by covalent bonds. | Plain English: A group of connected atoms. | Picture: H₂O as one oxygen connected to two hydrogens.

Ion

AP Bio: An atom or group of atoms with a net electrical charge from losing or gaining electrons. | Plain English: A particle with an overall positive or negative charge. | Picture: Na⁺ and Cl⁻ wearing plus and minus badges.

Covalent bond

AP Bio: A chemical bond in which atoms share one or more pairs of electrons. | Plain English: Atoms stay connected by sharing electrons. | Picture: Two people holding the same rope; sharing need not be equal.

Electronegativity

AP Bio: An atom's attraction for electrons shared in a covalent bond. | Plain English: How strongly an atom pulls on shared electrons. | Picture: A tug-of-war in which oxygen usually pulls harder than hydrogen.

Polar covalent bond

AP Bio: A covalent bond with unequal electron sharing, creating partial positive and partial negative charges. | Plain English: One atom gets more of the shared electrons' attention. | Picture: An uneven tug-of-war, such as an O–H bond in water.

Nonpolar covalent bond

AP Bio: A covalent bond in which electrons are shared equally or approximately equally. | Plain English: Neither end gains a strong partial charge from the bond. | Picture: An evenly matched tug-of-war, such as the bond in O₂.

Polar molecule

AP Bio: A molecule with an uneven charge distribution that produces partial positive and negative regions. | Plain English: Different parts of the molecule have slightly different charges. | Picture: Water has a partly negative oxygen end and partly positive hydrogen ends; it is not a full magnet.

Nonpolar molecule

AP Bio: A molecule without a substantial overall separation of partial positive and negative charge. | Plain English: It lacks distinct charged ends and often mixes poorly with water. | Picture: An oily hydrocarbon chain hiding among other oily chains.

Ionic attraction

AP Bio: Electrostatic attraction between oppositely charged ions; water can weaken these interactions by surrounding the ions. | Plain English: Plus and minus ions attract. | Picture: Opposite charge badges drawing together, then water surrounding and separating them as salt dissolves.

Hydrogen bond

AP Bio: An attraction involving a partially positive hydrogen covalently bonded to an electronegative atom and an electronegative atom nearby. | Plain English: A relatively weak attraction between suitable parts of molecules. | Picture: Temporary links between neighboring water molecules; the O–H bonds inside each water molecule are covalent.

Cohesion

AP Bio: Attraction between molecules of the same substance; hydrogen bonding produces strong cohesion among water molecules. | Plain English: Water sticks to other water molecules. | Picture: Water beads joining into one larger droplet.

Adhesion

AP Bio: Attraction between molecules of different substances. | Plain English: Water sticks to another material. | Picture: Water clinging to the inside of a glass tube.

Surface tension

AP Bio: Resistance of a liquid's surface to deformation caused by cohesive attractions among its molecules. | Plain English: Water's surface resists being stretched or broken. | Picture: A water strider standing on a pond; the surface is not an actual solid skin.

Specific heat capacity

AP Bio: The energy needed to raise the temperature of a unit mass of a substance by one degree. | Plain English: How much heat it takes to warm something up. | Picture: A large pot of water warming slowly; water's high specific heat helps stabilize temperatures.

Evaporative cooling

AP Bio: Cooling that occurs when higher-energy molecules leave a liquid as vapor, lowering the average kinetic energy of the remaining liquid. | Plain English: Escaping water molecules carry energy away. | Picture: Sweat cooling skin as it evaporates, rather than simply sitting on the skin.

Ice's lower density

AP Bio: Hydrogen bonds hold water molecules in an open crystal arrangement in ice, making ice less dense than liquid water. | Plain English: Frozen water takes up more room for the same amount of water. | Picture: People spreading out into a roomy pattern; floating ice can insulate water below.

Solvent

AP Bio: The component of a solution that dissolves a solute; water is the main solvent in cells. | Plain English: The liquid doing the dissolving. | Picture: Water in a glass of sugar water.

Solute

AP Bio: A substance dissolved in a solvent. | Plain English: The stuff dissolved in a liquid. | Picture: Sugar dispersed throughout a glass of water.

Solution

AP Bio: A homogeneous mixture of a solvent and one or more dissolved solutes. | Plain English: A mixture that is evenly combined at the molecular or ionic level. | Picture: Clear salt water rather than sand settling at the bottom.

Concentration

AP Bio: The amount of a substance in a specified amount or volume of mixture. | Plain English: How much of something is packed into a given space. | Picture: One spoonful versus five spoonfuls of sugar in the same-sized glass.

Molarity (M)

AP Bio: Concentration expressed as moles of solute per liter of solution. | Plain English: A way to count dissolved particles in each liter. | Picture: A label saying how many standard counting batches are dissolved per liter.

Mole (mol)

AP Bio: An amount containing approximately 6.022 × 10²³ specified entities, such as molecules or ions. | Plain English: A huge counting unit for tiny particles. | Picture: A dozen means 12; a mole means about 602 sextillion.

pH

AP Bio: A logarithmic measure of hydrogen ion concentration, approximately pH = −log₁₀[H⁺]. | Plain English: A scale of acidity; lower pH means more H⁺. | Picture: Each one-step drop means about ten times more H⁺, not just one more unit of acidity.

Acid

AP Bio: A substance that donates H⁺; in water, acids increase the concentration of hydrogen ions. | Plain English: A substance that can release H⁺ into water. | Picture: A dispenser adding H⁺ to a solution.

Base

AP Bio: A substance that accepts H⁺; in water, bases decrease hydrogen ion concentration. | Plain English: A substance that can remove H⁺ from a solution. | Picture: A catcher picking up H⁺; some bases supply OH⁻ that combines with H⁺.

Buffer

AP Bio: A system that resists pH changes by accepting or releasing H⁺ when acids or bases are added. | Plain English: A chemical shock absorber that keeps acidity from changing too much. | Picture: A sponge that absorbs or releases H⁺; its capacity is limited.

Organic molecule

AP Bio: A carbon-based molecule, typically containing carbon–hydrogen bonds, such as a carbohydrate, lipid, protein, or nucleic acid. | Plain English: A molecule built around a carbon framework. | Picture: A carbon skeleton with different chemical parts attached; not every carbon-containing substance is classified as organic.

Functional group

AP Bio: A group of atoms that gives an organic molecule characteristic chemical properties and reactivity. | Plain English: A small attached part that changes how a molecule behaves. | Picture: Different attachments turning the same basic tool into something with a different job.

Hydroxyl group (–OH)

AP Bio: A polar functional group containing oxygen bonded to hydrogen; it can form hydrogen bonds with water. | Plain English: An attached O–H group that often helps a molecule interact with water. | Picture: A water-friendly handle on a sugar molecule.

Carboxyl group (–COOH)

AP Bio: A functional group with a carbonyl and hydroxyl on the same carbon; it can donate H⁺ and become –COO⁻. | Plain English: An acidic attachment that can release H⁺. | Picture: A small H⁺ dispenser at one end of an amino acid.

Amino group (–NH₂)

AP Bio: A nitrogen-containing functional group that can accept H⁺, often becoming –NH₃⁺ in biological conditions. | Plain English: An attachment that can pick up H⁺. | Picture: An H⁺ catcher at one end of an amino acid.

Phosphate group

AP Bio: A phosphorus atom bonded to oxygen atoms; this group commonly carries negative charge in biological molecules. | Plain English: A charged chemical attachment found in DNA, phospholipids, and ATP. | Picture: A negatively charged connector or tag; its exact job depends on the molecule.