Section 3 -Solubility and the Dissolving Process

Solubility and Polarity

Some pairs of liquids form a solution when they are mixed. Solubility is the ability of one substance to dissolve into another at a given temperature and pressure; expressed in terms of the amount of solute that will dissolve in a given amount of solvent to produce a saturated solution

Oils, such as cooking oil, do not mix with water. An oil is nonpolar, andwater is polar. However, paint thinner is soluble with the oil in oil-basedpaints. Both the paint and paint thinner are nonpolar. Polar compoundstend to dissolve in other polar compounds, and nonpolar compounds tendto dissolve in other nonpolar compounds.

The Rule Is “Like Dissolves Like”

When nonpolar molecules are mixed with other nonpolar molecule, the intermolecular forces of the molecules easily match. (They are both very weak)Thus, nonpolar molecules are generally soluble with each other. This is one part of the rule “like dissolves like”: liquids that are completely soluble with each other are described as being miscible in each other.

If molecules are sufficiently polar, there is an additional electricalforce pulling them toward each other. The negative partial charge on oneside of a polar molecule attracts the positive partial charge on the otherside of the next polar molecule. If you add polar molecules to other polarmolecules, such as water, the attraction between the two is strong. This is another part of the rule “like dissolves like”: polar molecules dissolve other polar molecules.

Paint Thinner - NonPolarOil Based Paint - NonPolar

Cooking Oil – NonPolarWater - Polar

Miscible

Imm

iscib

le

“Like Dissolves Like”

Solubilities of Solid Compounds

Even two polar liquids placed in the same container may not dissolve ineach other rapidly. Their strong intermolecular forces can only act onnearby molecules—not between molecules at the top of the container andthose at the bottom. The speed of the process can be increased by shaking the mixture.This action breaks the two liquids into small droplets and therebyincreases the amount of contact between the surfaces of the liquids. Thisprocess works because the only place that dissolving can occur is at thesurface between the two liquids, where the different molecules are neareach other. Similarly, in considering the solubility of solids in liquids, the onlyplace where dissolution can occur is at the surface of the solid particles.The solid must be broken into smaller particles and then into moleculesor ions, which can form a solution with the solvent molecules.

To simplify the above statements: Stirring liquids will hasten the dissolving process as will breaking solids into smaller pieces.

Solubilities of Solids Generally Increase with Temperature

Another way to make most solids dissolve more and faster is to increasethe temperature. Increasing the temperature is effective because, in general,solvent molecules with greater kinetic energy can dissolve moresolute particles.

Gases Can Dissolve in Liquids

Gases can dissolve in liquids, however it is usually not in great amounts. Gases are usually considered to be slightly soluble. You can increase the amount of gas that will dissolve by decreasing temperature of the solvent and by increasing the pressure. This is why it is easier to keep dissolve CO2 in soda by keeping the soda cold and keeping the pressure high by keeping the cap on the bottle.

Solubilities of Ionic Compounds

Solubilities of ionic solids are difficult to predict because of the many factorsinvolved, so they must be measured experimentally. From experimental results of ionic solubilities in water, some patterns emerge, as shown in the chart below from the reference tables.

Determine if the following substances will dissolve in water.

LiClKNO3

PbSNa2S

LiHCO3

CaCO3

NH4OHCaSO4

MgCrO4

PbBr2

Sr(ClO3)2

AlPO4

YESYES

YES

YES

YES

YES

YESNO

NONO

NONO

How Water Polarity Dissolves Ionic Compounds

Water has very strong hydrogen bond poles. As shown below.

The poles will attract the ion of the opposite charge into solution as shown below

OH H+

-

+

Notice the positive dipole surrounds the negative chloride ion and the negative dipole surrounds the positive sodium ion.

Saturation

When the maximum amount of solute is dissolved in a solution, the solution is said to be a saturated. If a solution is saturated, anyadditional solute that is added collects at the bottom of the container. Ifmore solute can be added to a solution and dissolve the solution is considered to be an unsaturated.

Under special conditions, supersaturated solutions can also exist. Supersaturated solutions have more solute dissolved than the solubility indicates would normally be possible

Study the NaNO3 solubility curve.

Any point plotted on the linewill create a saturated solution.

Any point plotted on the graph below the line will create a unsaturated solution.

Any point plotted on the graphAbove the line will create a Supersaturated solution.

Caution: This graph is basedon 100 grams of water.

Based on Reference your reference tables, which salt solution could contain 42 grams of solute per 100 grams of water at 40°C?1. a saturated solution of KClO3 2. a saturated solution of KCl 3. an unsaturated solution of NaCl 4. an unsaturated solution of NH4Cl

A solution containing 60 grams of NaNO3 completely dissolved in 50. grams of water at 50°C is classified as being 1. saturated 2. supersaturated 3. dilute and unsaturated 4. dilute and saturated

One hundred grams of water is saturated with NH4Cl at 50°C. According to Table G, if the temperature is lowered to 10°C, what is the total amount of NH4Cl that will precipitate?1. 5.0 g 2. 17 g 3. 30. g 4. 50. g

According to Reference Table G, how many grams of KNO3 would be needed to saturate 200 grams of water at 70°C?1. 43 g 2. 86 g 3. 134 g 4. 268 g

According to Reference Table G, which of these substances is most soluble at 60°C?1. NaCl 2. KCl 3. KClO3 4. NH4Cl

Which compound forms a saturated solution at 40°C that contains 46 grams per 100 grams of water?1. KNO3 2. NH4Cl 3. NaNO3 4. KCl