Campbell Biology – Chapters 2 & 3: The Chemical Context of Life and Water & Life (Practice Questions Study Guide)

Chapter 2: The Chemical Context of Life

Elements and Atoms

All matter is composed of elements, which are substances that cannot be broken down into other substances by chemical means. Atoms are the smallest units of elements that retain their properties.

• Element: A substance that cannot be broken down chemically into other substances.

• Atom: The smallest unit of an element, consisting of protons, neutrons, and electrons.

• Atomic Number: The number of protons in an atom, which determines the element.

• Mass Number: The sum of protons and neutrons in the nucleus of an atom.

Atomic Structure and Reactivity

• Electrons are arranged in shells around the nucleus. The chemical reactivity of an atom is primarily determined by the number of electrons in its outermost shell (valence electrons).

• Valence: The bonding capacity of an atom, usually related to the number of unpaired electrons in the outer shell.

Chemical Bonds

• Covalent Bond: A type of bond that involves the sharing of electron pairs between atoms.

• Ionic Bond: A bond formed when one or more electrons are transferred from one atom to another, resulting in oppositely charged ions.

• Hydrogen Bond: A weak bond between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.

Chapter 3: Water and Life

Properties of Water

Water is a polar molecule, meaning it has a partial positive charge on one side and a partial negative charge on the other. This polarity allows water to form hydrogen bonds, which are responsible for many of its unique properties.

• Cohesion: The attraction between water molecules due to hydrogen bonding.

• Adhesion: The attraction between water molecules and other substances.

• Surface Tension: A measure of how difficult it is to stretch or break the surface of a liquid; water has a high surface tension due to hydrogen bonding.

Water as a Solvent

• Water is known as the "universal solvent" because it can dissolve many substances, especially ionic and polar compounds.

Density of Ice vs. Liquid Water

• Ice is less dense than liquid water because the hydrogen bonds in ice keep water molecules farther apart than in liquid water. This is why ice floats.

Acids, Bases, and pH

• pH Scale: Measures the concentration of hydrogen ions ($[H^+]$) in a solution.

• Each decrease of one pH unit represents a tenfold increase in $[H^+]$ concentration.

• Buffer: A substance that minimizes changes in pH by accepting or donating hydrogen ions.

Key Equations

• $\text{pH} = -\log_{10}[H^+]$

• Each pH unit change = tenfold change in $[H^+]$

Sample Multiple Choice Questions (with Answers)

Question	Correct Answer
Which statement best defines an element?	B. A substance that cannot be broken down chemically
The atomic number of an atom is determined by the number of:	C. Protons
Isotopes of an element differ in their number of:	C. Neutrons
Atoms are chemically reactive primarily because of:	C. Number of electrons
Which bond involves the sharing of electrons?	A. Covalent bond
Water is a polar molecule because:	C. Oxygen is more electronegative than hydrogen
Which property explains water's high surface tension?	B. Cohesion between molecules
Ice floats on liquid water because:	C. Ice is less dense than liquid water
A decrease of one pH unit represents a:	B. Tenfold increase in H+ concentration
Buffers work by:	C. Accepting or donating hydrogen ions

Additional info: These questions and explanations cover foundational concepts from Chapters 2 and 3 of a standard General Biology curriculum, focusing on the chemical basis of life and the unique properties of water essential for biological systems.