Properties of Water

Specific Heat of Water

Water has a high specific heat, which is the amount of heat that must be absorbed or lost for 1 gram of a substance to change its temperature by 1°C. This property is primarily due to hydrogen bonding between water molecules, allowing water to moderate environmental temperatures.

• Definition: Specific heat is the energy required to raise the temperature of 1 g of a substance by 1°C.

• Example: Coastal climates are more moderate than inland climates because of water's high specific heat.

• Formula: (where q is heat energy, m is mass, c is specific heat, and is temperature change)

Heat of Vaporization

The heat of vaporization is the amount of energy required to convert 1 g of a liquid to the gaseous state. Water's high heat of vaporization is also due to hydrogen bonding.

• Definition: The quantity of heat a liquid must absorb for 1 g of it to be converted from liquid to gas.

• Application: Sweating cools the body because evaporation of water requires heat energy, which is taken from the skin.

Cohesion and Surface Tension

Cohesion refers to the tendency of like substances to stick together, such as water molecules sticking due to hydrogen bonds. Surface tension is a measure of how difficult it is to stretch or break the surface of a liquid.

• Cohesion: Responsible for water transport in plants and formation of droplets.

• Surface tension: Allows small insects to walk on water and affects water's behavior at interfaces.

Density and States of Water

Water's density is higher than that of most other liquids, but this is not related to its ability to moderate climate. Ice is less dense than liquid water due to the arrangement of hydrogen bonds.

• Application: Ice floats on water, insulating aquatic life in cold climates.

Chemical Bonds and Interactions

Covalent Bonds

Covalent bonds are formed when atoms share electrons. If the atoms have similar electronegativities, the bond is nonpolar; if they differ, the bond is polar.

• Nonpolar covalent bond: Equal sharing of electrons (e.g., H2, O2).

• Polar covalent bond: Unequal sharing of electrons (e.g., H2O).

• Electronegativity: The tendency of an atom to attract electrons in a bond.

Ionic Bonds

Ionic bonds result from the attraction between ions that have opposite charges. One atom donates an electron to another, forming a cation (+) and an anion (−).

• Example: NaCl (sodium chloride) forms when sodium donates an electron to chlorine.

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom covalently bonded to an electronegative atom (like oxygen or nitrogen) and another electronegative atom. These are responsible for many of water's unique properties.

• Example: Hydrogen bonding between water molecules.

Van der Waals Interactions

Van der Waals interactions are weak attractions due to transient local partial charges, allowing molecules and atoms to stick together.

Acids, Bases, and pH

Acids and Bases

An acid is a substance that donates hydrogen ions (H+) to a solution. A base is a substance that accepts hydrogen ions or donates hydroxide ions (OH−).

• Acidic solutions: pH less than 7; higher concentration of H+.

• Basic solutions: pH greater than 7; lower concentration of H+.

• Neutral solution: pH = 7.

pH Scale

The pH scale is logarithmic and measures the concentration of hydrogen ions in a solution.

• Formula:

• Each unit change in pH represents a tenfold change in H+ concentration.

• Example: Solution at pH 6 has 100 times more H+ than solution at pH 8.

Buffers

Buffers are substances that minimize changes in the concentrations of H+ and OH− in a solution. They help maintain stable pH in biological systems.

• Example: Carbonic acid-bicarbonate buffer system in blood.

Ocean Acidification

Increasing atmospheric CO2 leads to ocean acidification as CO2 dissolves in seawater, forming carbonic acid (H2CO3), which lowers pH and decreases carbonate ion (CO32−) concentration.

• Reaction:

• Carbonic acid dissociates:

• Decreased carbonate ions affect marine organisms' ability to form shells and skeletons.

Atomic Structure and Elements

Atoms and Subatomic Particles

An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of protons (positive charge), neutrons (neutral), and electrons (negative charge).

• Atomic number: Number of protons in the nucleus.

• Mass number: Number of protons plus neutrons.

• Electron shells: Electrons are arranged in shells around the nucleus; the outermost shell is the valence shell.

Atomic Mass

• Proton mass: About 1,200 times that of an electron.

• Neutron mass: About 1,200 times that of an electron.

• Electron mass: Negligible compared to protons and neutrons.

• Dalton: Standard unit of atomic mass (1 dalton ≈ mass of 1 proton or neutron).

Valence Electrons and Reactivity

The valence electrons (electrons in the outermost shell) determine an atom's chemical behavior. Atoms with the same number of valence electrons exhibit similar chemical properties.

• Example: Sulfur (atomic number 16) has 6 valence electrons, similar to oxygen.

• The number of electron shells is not an important determinant of reactivity; the number of valence electrons is.

Electronegativity

Electronegativity is the ability of an atom to attract electrons in a covalent bond. Oxygen is highly electronegative, making O-H bonds polar.

• Polar covalent bond: Electrons spend more time near the more electronegative atom.

• Nonpolar covalent bond: Electrons are shared equally.

Functional Groups in Organic Molecules

Common Functional Groups

Functional groups are specific groups of atoms within molecules that confer particular chemical properties.

• Hydroxyl group: –OH

• Amino group: –NH2

• Phosphate group: –PO4

• Carboxyl group: –COOH

These groups are important in the structure and function of biological molecules.

Table: Comparison of Bond Types

Additional info:

• Some explanations and examples have been expanded for clarity and completeness.

• Functional groups and their importance in organic chemistry have been inferred from context.

• Table entries are logically grouped and expanded for academic clarity.