Chapter 15 - Water and Aqueous Solutions

15.1 - Water and its Properties

• pole creates a sigma bond

• triatomic molecule

• bond angle of 105 degrees

high surface tension and low vapor pressure result from Hydrogen bonding

surface tension - inward force, or pull, that tends to minimize the surface area of a liquid

Water’s surface tension is higher than most — tends to hold drop of liquid in spherical shape

surfactant - any substance that interferes with H bonding

• lowers surface tension

• causes a bead of water to spread out

• EX soaps and detergents

WATER IN THE SOLID STATE

• ice cubes float in water bc solid water has a lower density than liquid water

  • the structure of ice is a regular open framework of water molecules arranged like a honeycomb — when it melts the structure collapses and the molecules are closer together

• this is opposite for other liquids

  • most solids become denser and sink

• ice is one of the only few solids that floats in its own liquid

• MAX density of water - 1g/cm³ @ 4 degrees C

• MIN density of water - 0.9168g/cm³ @ 0 degrees C

Ice melting at 0 degrees C is a high melting temp for a molecule with such low molar mass

The heat absorbed when 1g of water change from a solid to a liquid is 334J — same amt of energy is needed raise the temp of 1g of water from 0 degrees C to 80 degrees C

15.2 - Homogeneous Aqueous Systems

aqueous solution - water that contains dissolved substances

solute - what’s being dissolved

solvent - what’s doing the dissolving

water is a universal solvent for biological systems

homogenous

• mixtures

• can be separated but hard to do so

• equally distributed

• can’t be filtered by filter paper because the particles are so small

ionic compounds and polar covalent bonds dissolves most readily in water

non polar covalent bonds (methane, oil, grease, gas) cannot be dissolved by water but can be by alcohol, chloroform, and ether

“like dissolves like”

• non polar solute and a non polar solvent

• polar solute and polar solvent

solvation - process by which the positive and negative ions of an ionic solid become surrounded by solvent molecules

electrolytes - compound that conducts and electric current when it is in an aqueous solution or in molten state

• EX HCl

all ionic compounds are electrolytes bc they dissociate into ions

non electrolytes

• EX carbon, sugar, rubbing alcohol

strong electrolyte - solution in which a large portion of the solute exerts as ions

• EX most soluble salts, inorganic acids, inorganic bases

weak electrolyte - conducts electricity poorly bc only a fraction of the solute in the solution exists as ions

• EX ammonia, organic acids, organic bases

hydrate - a compound that contains water or hydration

• when writing the formula — use a dot to connect the formula of the compound and the number of water molecules per formula unit

water of hydration and water of crystallization are contained in a crystal

efflorescent hydrate - if the hydrate vapor pressure is greater than the pressure of water vapor then the hydrate will lose its water of hydration

hygroscopic hydrate - low vapor pressure removes water from moist air

desicant - substance used to absorb moisture from the air to create a dry atmosphere

%water = mass of water/mass of hydrate x 100%

15.3 - heterogeneous aqueous systems

suspensions - a heterogeneous mixture from which particles settle out upon standing

• EX blood, oj, milk, cream, salad dressing

colloid - a heterogenous mixture that contains particles that range in size from 1nm to 1000nm and can be any state of matter

• EX smoke, paint, aerosol spray, fog/mist

solution particles < colloid particles < suspension particles

Tyndall effect - scattering of light due to colloid particles — a solution’s particles are too small to scatter light

Brownian movement - chaotic movement of colloidal particles which was first observed by Scottish botanist Robert Brown

coagulation - dumping of particles together in a colloid that fall out of a solution

emulsion - colloidal dispersion of a liquid in a liquid