You might remember from the beginning of the year that solutions are homogenous mixtures. This means solutions contain two or • more substances, and are mixed enough to “look all the same”. Solutions do NOT have to be liquids, bronze is a solid solution and air is a gaseous solution.
All solutions have two parts. The substance doing the dissolving is the solvent and the substances being dissolved are the solutes. • If you don’t know which is which, the substance you have the most of overall is the solvent
The most common solvent is water. It is so common and dissolves so many things that it is often called “the universal solvent”. This is • not entirely true, as only polar solutes will dissolve in water (we say they are soluble). Nonpolar substances will not dissolve in water (we say they are insoluble.)
Even when things are soluble in water, or any other solvent, different solutes will dissolve to different amount. The actual amount (in grams) • that will dissolve in 100 grams of a solvent is called the solubility. When discussing the solubility of solutes, solutions are classified into 3 types: saturated, unsaturated, and supersaturated.
A saturated solution has dissolved as much solute as it can. If you add sugar to iced tea and sugar is just sitting at the bottom of the • glass your iced tea is saturated for sugar.
A supersaturated solution has been “tricked” into dissolving more solute than it should be able to. If any more solute is added to a • supersaturated solution, all the extra solute will undissolve (precipitate). This is how sugar crystal candy (rock candy) is made.
Because the solubility of solutions is dependent on temperature, chemists represent the amounts that can be dissolved with a • solubility curve:
Temperature is the independent variable (the one you control). • This is shown on the bottom of the graph. You can warm up the solution with a stove or cool it down in a freezer.
The amount (in grams) of how much will dissolve at a particular • temperature is the dependent variable (it depends on the temperature). This is shown on the left side of the graph. Notice this is for all the solutes in 100 grams of water.
If the temperature and amount of dissolved solute is • on the line shown for that solute, then the solution is saturated. For example, 70 grams of NH3 in 100 grams of water will be a saturated solution at 10 °C.
If the temperature and amount of dissolved solute is above the line, then the • solution is supersaturated. If the saturated solution of 70 grams of NH3 is very carefully heated, it will still have 70 grams of NH3 at 20 °C. As this amount is above the line, this would be a supersaturated solution.
If the temperature and amount of dissolved solute is • below the line, then the solution is unsaturated. An unsaturated solution could still dissolve more solute if it is added.
Temperature is not the only factor that will affect how much solute can dissolve. Particle size, pressure, and agitation also have an • effect.
Dissolving happens as the solvent touches the solute and starts to pull it into pieces. This happens only on the outside of the particles of the solute.
One big chunk of solute has less outside surface area than a lot of small pieces of solute. Thus, if the solute is crushed or ground it will dissolve faster. This only works for dissolving solids in liquids.
Gases are affected by pressure. With high pressure a lot of gas can be squeezed into a little bit of space. High pressures will help • gases dissolve in a liquid, and low pressure will help remove the gas from a liquid. This only works for dissolving a gas in a liquid, and is why a carbonated beverage goes “flat” after it is opened.
C) When a solid dissolves in a liquid, it takes time for the liquid molecules to grab pieces of the solid and float away. If the liquid is stirred, it speeds up the floating away processes, so more liquid can grab more solid in the same amount of time. Thus stirring or shaking will help a solid dissolve faster in a liquid.
When a gas dissolves in a liquid, it is basically trapped by the liquid molecules. If the liquid is stirred, it removes the water • molecules that are trapping the gas, and the gas will form bubbles and eventually leave the liquid. This is why shaking a carbonated beverage makes so many bubbles.
Just knowing that a solution is unsaturated does not indicate much about the solution. Is there only a little bit of solute, or is there a lot • and it is almost saturated? The only way to tell is to know the solutions concentration.
In general if there is a lot of solute in the solvent, the solution is called concentrated, and if there is only a little bit of solute in the • solvent it is called dilute. But chemists need a more exact way to know how concentrated or dilute the solution is.
Chemists have 5 different ways to determine numbers for concentration. Each way is different and useful in different situations.
A) Percent by mass = mass of solute divided by the mass of solution × 100%
Percent by mass = mass of solute divided by the mass of solution × 100% • B) Percent by volume = volume of solute divided by the volume of solution × 100%
Percent by mass = mass of solute divided by the mass of solution × 100% • Percent by volume = volume of solute divided by the volume of solution × 100% • C) Molarity = moles of solute divided by the liters of solution
Molality = moles of solute divided by the kilograms of solvent • E) Mole fraction = moles of solute divided by the total moles of solute and solvent
For this class you will only use molarity and molality. Review Molarity and Molality
Physical properties of solutions that are affected by how much solute is dissolved are called colligative properties. (Colligative means “depending on the collection”.) There are three colligative properties that we’ll be concerned with: electrolytes, boiling point elevation, and freezing point depression.
If a solute dissolved in solutions ionizes (breaks apart into positive and negative parts) these parts will allow electricity to travel through the solution. For example: pure water will not conduct electricity, but if table salt is added, then the sodium and chlorine that break apart will conduct electricity. • NaCl → Na+ + Cl-
Solutions that conduct electricity are called electrolytes. The more of an ionizing solute that is dissolved, the better the conduction of electricity will be.
On the other hand, solutes that don’t ionize (like sugar) will not conduct electricity in water. So sugar water will not conduct electricity. Solutions that don’t conduct electricity are called nonelectrolytes.
Boiling happens when the molecules of a liquid get enough energy to break through the liquid-gas barrier and escape as a gas. If a solute is added, it gets in the way of the liquid trying to escape, and thus it takes more energy (more heat) to get the liquid to boil. This is called boiling point elevation.
If pure water boils at 100 °C, and salt is added, the solution will no longer boil at 100 °C, but will boil at a higher temperature. Often the directions for cooking pasta require the addition of salt to the water. This causes the water to boil at a hotter temperature and thus the pasta cooks quicker (and the salt adds some flavor to the pasta).
The more solute that is added, the higher the boiling point will be. Thus the boiling point elevation is directly proportional to the molality of the solution.