2.
2: Properties of Water (hydrogen bonds, dissociation, pH)
Vocab:
Solvent: agent for dissolving
Solution: Substances mixed together
Solutes: when substances are dissolved in a solvent like water
Polar: when molecules carry a partial positive charge on one side and a partial negative
charge on the other
Hydrogen bond: attraction between a hydrogen atom with partial positive charge and
another atom (usually O or N) with a partial negative charge
Hydrophilic: water loving
Hydrophobic: water fearing
Hydrophobic interactions: interactions between nonpolar molecules that result from
being brought together in an aqueous solution
Van der Waals interactions: Once hydrophobic molecules are close to one another, their
association is further stabilized by weak electrical attractions
Cohesion: attraction between like molecules
Adhesion: attraction between unlike molecules
Surface tension: cohesive force caused by attraction between the molecules at the
surface of a liquid
Specific Heat: amount of energy required to raise the temp of 1 gram of a substance by
1 degree Celsius
Heat of vaporization: the energy required to change 1 gram of water from a liquid to
gas-is higher than that of most molecules that are liquid at room temp
Chemical reaction: one substance is combined with others or broken down into another
substance. Atoms
may also be rearranged; in most cases, chemical bonds are broken and new bonds
form
Reactant: initial molecules
Product: final molecules
Hydrogen ion (H+)/hydroxide ion: products
Chemical equilibrium: When the forward and reverse reactions proceed at the same
rate, the quantities of reactants and products remain constant, although not necessarily
equal
Acids: substances that give up protons during chemical reactions and raise the
hydronium ion concentration of water
Bases: molecules or ions that acquire protons during chemical reactions and lower the
hydronium ion concentration of water
Mole: the mass of one mole of an atom is the same as atomic weight
Molecular weight: mass of one mole of a molecule
Molarity: number of moles of the solute present per liter of solution
pH: logarithmic notation to express concentration of protons in solution
Buffers: compounds that minimize changes in pH
Homeostasis: relatively constant conditions
Water
o Life is based on water
o Water is the most abundant molecule in organisms
o Vital due to its nature of being an excellent solvent
What Properties are Correlated with Water’s Structure?
o Water is an efficient solvent
o Water molecule is polar in nature
▪ Any ionic or polar compound will be able to dissolve in water
▪ Hydrogen bonds and other similar interactions between water and
hydrophilic solutes are extremely important due to the constant
dissolving of compounds in aqueous solution
o Van der waals attractions
▪ Can significantly increase the stability of clustered hydrophobic
molecules
o Cohesion, Adhesion, and Surface Tension
o Water adheres to surfaces that have any polar or charged components
o Cohesion and adhesion are important in explaining how water can move
from the roots of plants to their leaves against the force of gravity
o Water resists any force that increases its surface area
▪ Water acts as an elastic membrane
o Water is denser as a liquid than as a solid
o More molecules of water in a given volume of liquid water than are in the
same volume of solid water
▪ Lattice structure
• Why heating ice causes hydrogen bonds to break and the
lattice structure to collapse
o If water did have these properties earth’s oceans
would have frozen before life could begin
o Water has a high capacity for absorbing energy
o Due to hydrogen bonding
▪ Water has a high specific heat because hydrogen bonds must be
broken before heat can be transferred
▪ Takes large amount of energy to break hydrogen bonds in liquid.
Water and transfer to gas phase
• Reason why sweating is effective way to cool off on a hot
day
o Water molecules in sweat absorb a great deal of
energy from body before they evaporate…lose heat
Role of Water in Acid-Base reactions
o Water is not completely stable as a molecule… water continually undergoes
chemical reactions within itself
o Concentration of hydrogen ions is always equal to the concentration of hydroxide
ions
o Water is an extremely weak acid and an extremely weak base at any moment
o Water provided the physical environment for key chemical reactions to take place
in early chemical evolution
