CHAPTER 8

Solutions, Acids, and Bases

8.1 Formation of Solutions

Dissolving

Recall that a solution is a homogeneous mixture of two or more substances Every solution has two components:

Solute Substance whose particles are dissolved in a

solution Solvent

Substance in which the solute dissolves Ex. Seawater – water is solvent, salt is solute

Dissolving

Solutes and solvents can take the form of solid, liquid, or gas Solution takes state of solvent

Substances can dissolve in water in three ways Dissociation Dispersion Ionization

Dissociation of Ionic Compounds For a solute to dissolve in water

The solute and solvent particles must attract to one another The solute particles are attracted and the

solvent particles are attracted to one another So, before a solution can form, those

attractions must be overcome Process in which an ionic compound

seperates into ions as it dissolves is called dissociation

Dispersion of Molecular Compounds

The water in your saliva dissolves the sugar and flavoring in candy throughout your mouth Sugar dissolves in water by dispersion, or

breaking into small pieces that spread throughout the water Both sugar and water are polar, therefore

attract When enough water molcules have surrounded

the sugar molecule, sugar molecule breaks free and is pulled into solution

Ionization of Molecular Compounds

HCl – molecular compound where hydrogen and chloride share an electron When HCl gas dissolves in water, the H

molecule is transferred to water H30+ and Cl- are produced

When a neutral molecule gain or lose electrons it is known as ionization

This is a chemical change Unlike dispersion and dissociation

Properties of Liquid Solutions Three physical properties of a solution

that can differ from those of its solute and solvent are: Conductivity Freezing point Boiling point

Conductivity

Solid sodium chloride is a poor conductor When it dissociates in water, the sodium

and chloride ions are able to move freely They will then conduct electricity

Freezing Point and Boiling Point MgCl2 is what is sometimes spread on

icy roads When it dissolves in melting ice, it

dissociates into Mg2+ and Cl- These ions are able to interfer with freezing

process Salted roads have a freezing point of -15°C

Freezing Point and Boiling Point Solute can also raise boiling point

ex. Coolant used in most car radiators Adding ethylene glycol to water raises the

boiling point Solution helps prevent the engine from

overheating Also prevents the the liquid from freezing in the

winter

Heat of Solution

During the formation of a solution, energy is either released or absorbed Can be described as either exothermic or

endothermic Dissolving sodium hydroxide in water is

exothermic, releases heat How it Works box explains how cold packs are

used

Heat of Solution

In order for a solution to form, all attractions must be broken That requires energy Formation of solutions, releases energy

The difference between these energies is called the heat of solution

Factors Affecting Rates of Dissolving

Reates of dissolving depend on the frequency and energy of collisions Formation of solutions, collisions occur

between solute and solvent particles Factors that affect the rate of dissoling

include: Surface area Stirring temperature

Factors Affecting Rates of Dissolving

The greater the surface area of a solid solute, the more requent the collision are between the solute and the solvent particles Increase surface area by breaking into

smaller pieces

Stirring Moves dissolved particles away from

surface Allows more collisions between solute and

solvent

Factors Affecting Rates of Dissolving

Increasing temperature is another way to speed up dissolving Increase in temperature causes the

particles to move faster Both the number of collisions and the

energy of these collisions increases Goes into solution more quickly

8.2 Solubility and Concentration

Solubility

Solubility The max amount of a solute that dissolves

in a given amount of solvent at a constant temperature Usually expressed in grams of solute in 100g of

solvent

Solutions are described as: Saturated Unsaturated Supersaturated

Solubility

Saturated Sugar is very soluble in water At 20 degrees C you can dissolve 203.9

grams of sugar in 100g of water What will happen if you try to dissolve more

than that? The extra sugar will not go into solution

The solution is already saturated One that contains as much solute as the solvent

can hold at a given temperature

Solubility

Unsaturated A solution that has less than the max

amount of solute that can be dissolved many beverages are unsaturated

Solubility

Supersaturated If you heat a solvent above the average

temperature it can dissolve more solute If you then carefully cool the solvent back to the

average without jarring it, you may be able to keep the extra solute in the solution

Supersaturated solution One that contains more solute than it can normally

hold at a given temperature Very unstable If a tiny particle falls into a supersaturated solution,

the extra solute may rapidly fall out

Factors Affecting Solubility

Have you ever tried to wash oil or grease off your hands? Will not come off in just water, but in soapy

water it will wash off Not soluble in water, but in soapy water it is

Three factors that affect solubility: Polarity of the solvent Temperature pressure

Factors Affecting Solubility

Polar and Nonpolar Solvents Oil molecules are nonpolar, water is polar

Common guideline for predicting solubility is ‘like dissolves like’

More likely to dissolve if solute and solvent are both polar or both nonpolar Soap molecules have a polar and non polar end Makes it easy to dissolve oil

Factors Affecting Solubility

Temperature In general the solubility of a solute

increases when you increase the temperature of the solvent

When water temp increases, bubbles start to come out of the water These are gas bubbles that are dissolved in

water Unlike most solids, gases usually become less

soluble as the temperature of solvent increases

Factors Affecting Solubility

Pressure Incresing the pressure on a gas increases

its solubility in a liquid Pressure of carbon dioxide in a 12 oz can of

soda at room temp can by two to three times atmospheric pressure