Solutions and Concentration Notes

Homogeneous Mixtures (Solutions)

Solutions are homogeneous mixtures containing two or more substances called the solute and the solvent.
The solute is the substance that gets dissolved.
The solvent is the substance that does the dissolving.
In a solution, it is not possible to distinguish the solute from the solvent.

States of Solutions

A solution can exist as a gas, liquid, or solid depending on the state of its solvent; solutions are not always a liquid.
- Air is a gaseous solution containing oxygen gas as the solute and nitrogen gas as the solvent.
- Nitinol, used in braces, is a solid solution of titanium in nickel.
- A mixture of metals is called an alloy; alloys are examples of solid solutions.
Most solutions are liquids, and reactions can take place in aqueous solutions, where the solvent is water.

Forming Solutions

Some combinations of substances readily form solutions, and others do not.
A substance that dissolves in a solvent is said to be soluble in that solvent.
- Sugar is soluble in water.
Two liquids that are soluble in each other in any proportion are said to be miscible.
- Cherry syrup can dissolve in water; therefore, these liquids are miscible.
A substance that does not dissolve in a solvent is said to be insoluble in that solvent.
- Sand is insoluble in water.
Two liquids that are not soluble with each other are called immiscible solutions.
- Oil and water cannot mix; therefore, they are immiscible.

Dissolving Compounds

Substances that dissolve most readily in water include ionic compounds and polar covalent compounds.
Water molecules are polar, with a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms.
Nonpolar covalent compounds, such as methane, and compounds found in oil, grease, and gasoline, do not dissolve in water.
Oil and grease will dissolve in gasoline.
Polar solvents will dissolve only polar solutes, and nonpolar solvents will only dissolve nonpolar solutes. "Like dissolves like."

The Solvation Process

To form a solution, solute particles must separate from one another, and the individual solute particles and solvent particles must mix.
The process of surrounding solute particles with solvent particles to form a solution is called solvation; solvation in water is called hydration.
When a solid solute is placed in a solvent, the solvent particles completely surround the surface of the solid solute.
If the attractive forces between the solvent and solute particles are greater than the attractive forces holding the solute particles together, the solvent particles will pull the solute particles and surround them.
These surrounded solute particles then move away from the solid solute and out into the solution.
"Like dissolves like" is the general rule used to determine whether solvation will occur in a specific solvent.
To determine whether a solvent and solute are alike, you must examine the bonding and polarity of the particles and IM forces.

Factors That Affect Solvation

Solvation only occurs when the solute and solvent particles come in contact with each other.
Three common ways to increase the collisions between solute and solvent particles and thus increase the rate at which the solute dissolves are agitation, surface area, and temperature.

Agitation

Stirring or shaking increases the rate a solute dissolves because it helps move the solvent around the solute and spreads out the dissolved solute particles throughout the solution.
Agitation only affects the rate at which the solid solute dissolves; it does not influence the amount of solute that will dissolve.
An insoluble substance remains undissolved regardless of how vigorously or for how long the solution is agitated.

Surface Area

Breaking the solute into small pieces increases its surface area.
A greater surface area allows more collisions to occur between the solute and solvent particles, causing faster rates of solvation.
A teaspoon of granulated sugar dissolves more quickly than an equal amount of sugar in cube form.
Smaller particles tend to dissolve at faster rates because they have more surface area exposed to the solvent.

Temperature

Hotter solvents can generally dissolve more solid solute.
- Sugar dissolves more quickly and in larger amounts in hot tea rather than iced tea.
At higher temperatures, the kinetic energy of water molecules is greater than at lower temperatures, so the molecules move faster.
The more rapid motion of the solvent molecules leads to an increase in the frequency of the force of collisions between water molecules and the surfaces of the sugar crystals.

Solubility of Liquids

The solubility of a solute, measured in g/L, also depends on the nature of the solute and solvent.
Solvation occurs when solvent particles collide with the solute’s surface particles, and they begin to mix randomly amongst each other.
At first, the solute particles are carried away from the crystal, but as the number of solvated particles increases, the same random mixing results in increasingly frequent collisions between solvated solute particles and the remaining undissolved crystal.
Solvation is a reversible process, and sometimes the solute particles in the liquid can bump back into the solid solute and stick back on, forming crystals again. This process is called crystallization.
There is only so much solvent available to dissolve a certain amount of solute. Once that point is reached and no more solute can be dissolved in the solution, equilibrium occurs between the two processes. If too much solid solute is present in a solution, it will sink to the bottom and is not able to be dissolved.

Types of Solutions

A mixture in which no more solute can dissolve at a given temperature is called a saturated solution. Any extra solute that remains will be present undissolved at the bottom of the container.
When the rate of dissolving equals the rate of crystallizing, equilibrium is reached.
An unsaturated solution is a mixture where the solvent can still dissolve more solute, and no solute will remain at the bottom since there’s room for more to dissolve.
A supersaturated solution contains more dissolved solute than a saturated solution at the same temperature. These solutions are very unstable, and the excess solute will precipitate out, often forming crystals if disturbed or cooled.
- A classic example is dissolving a large amount of sugar in hot water, then carefully cooling the solution. If you then introduce a seed crystal of sugar, the excess sugar will rapidly crystallize out of the solution.
- A seed crystal is a tiny amount of solute that you add to a solution to help start or speed up the process of crystallization.

Solubility of Gases

Gases tend to be less soluble at higher temperatures than at lower temperatures. This is a predictable trend for all gaseous solutes in liquid solvents.
The kinetic energy of gas particles increases at higher temperatures, allowing them to escape from a solution more readily when at higher temps.
This extra energy makes it easier for gas particles that are dissolved in a liquid (like carbon dioxide in soda) to break free from the liquid and escape into the air.
Therefore, when a solution’s temperature increases, the solubility decreases.

Pressure and Henry’s Law

The solubility of a gas in any solvent increases as its external pressure (the pressure above the solution) increases.
Carbonated beverages depend on this fact since they contain carbon dioxide gas dissolved in an aqueous solution. When bottling these beverages, carbon dioxide is dissolved in the solution at a pressure higher than atmospheric pressure. This allows the carbon dioxide to remain in the solution and not escape into the surroundings, which causes it to go flat.
Henry’s law states that at a given temperature, the solubility $(S)$ of a gas in a liquid is directly proportional to the pressure $(P)$ of the gas above the liquid.
When the bottle of soda is closed, the pressure above the solution keeps carbon dioxide from escaping the solution. When it is opened, the pressure in the bottle decreases, and bubbles of carbon dioxide gas form in the solution, rise to the top, and escape.
The relationship can be expressed using the following formula: $\frac{S<em>1}{P</em>1} = \frac{S<em>2}{P</em>2}$
- Solubility $(S) = g/L$
- Pressure $(P) = atm$

Concentration

The concentration of a solution is a measure of how much solute is dissolved in a specific amount of solvent or solution.
A concentrated solution contains large amounts of solute, while a dilute solution contains a small amount of solute.
Commonly used quantitative descriptions are percent by mass, molarity, percent by volume, and molality.
These descriptions express concentration as a ratio of measured amounts of solute and solvent or solution.

Molarity

Molarity $(M)$ is the number of moles of solute dissolved per liter of solution.
Molarity is also known as molar concentration, and the unit M is read as molar.
To calculate a solution’s molarity, you must know the volume of the solution in liters and amount of solute being dissolved in moles.
$Molarity (M) = \frac{moles \, of \, solute}{liters \, of \, solution}$
$M = \frac{mol}{L}$
The unit M is equivalent to moles per liter $(mol/L)$ .

Molality

The volume of a solution changes with temperature as it expands or contracts. This change in volume alters the molarity of the solution. Masses, however, do not change with temperature.
Molality is the ratio of the number of moles of solute dissolved in 1 kg of solvent.
The unit m is read as molal.
$Molality (m) = \frac{moles \, of \, solute}{kg \, of \, solvent}$
$m = \frac{mol}{kg}$
The unit m is equivalent to moles per kilogram of solvent $(mol/kg)$ .