Explanation of what a solution is

Whenever you dissolve something in a liquid, you've made a solution.  Another way of thinking about it is that if you have a liquid and it's not a pure substance, it's a solution.  The liquid part is the "solvent" and the stuff you dissolved (usually a solid) is the "solute".  An example:  If you dissolve salt in water, the salt is the solute and the water is the solvent. 

A handy rule of thumb:  If somebody asks you to tell you what the solvent in a solution is and you have no idea, say "water".  Water is by far the most common solvent for solutions that you're likely to run into.

When we're talking about how much of the solute is dissolved in the liquid, we're talking about the concentration.  There are four terms we can use to describe the concentration of a solution:

• unsaturated:  This means that if you were to add more solute to the liquid, it would keep dissolving.  For example, if you take one teaspoon of salt and put it in a bucket of water, you've made an unsaturated solution.  (In other words, if you added another teaspoon of salt, it would dissolve, too).
• saturated:  This means that the liquid has dissolved all of the solute that is possible.  If any of you have a little brother or sister who adds sugar to iced tea, you know what I'm talking about.  If you add one teaspoon of sugar to iced tea, you've got an unsaturated solution.  If you keep adding sugar to iced tea, you eventually get to the point where the rest of the sugar just sinks to the bottom.  When this happens, it means that the solution is saturated, because no more sugar could dissolve.
• supersaturated:  This means that MORE solute has dissolved than is possible.  How, you might ask, does this happen?  If you have a very hot saturated solution and cool it down, the solubility of the solute decreases as the solution cools.  (In other words, hot solutions can dissolve more solute than cold ones).  What usually happens in this situation is that the solute starts forming crystals at the bottom of the container.  However, under weird circumstances where there are no little grains to start crystal formation, the crystals never form - as a result, the solution is MORE concentrated than possible.  This doesn't happen much, so you'll never run into it in real life, most likely.
• molarity:  This is another unit of concentration.  Keep reading for more...

Molarity:  What it is and how to calculate it

Molarity is one of those terms that people like to talk about a lot when describing solutions.  Unlike "unsaturated", "saturated", and "supersaturated", molarity is a numerical way of saying exactly how much solute is dissolved in a solvent.  As you can probably tell from the section before, the term unsaturated or supersaturated can be applied to a wide variety of solution concentrations.

OK.  Here's the definition of molarity:

Molarity is equal to the moles of solute divided by the liters of solution.  To put it in the form of an equation:

OK.  Quit freaking out.  It's not that hard to figure out.  Let's do an example:

Example:  What's the molarity of a solution that contains 5.5 moles of sodium chloride in 10.5 liters of solution?

Answer:  M = moles / liters, or (5.5 moles) / (10.5 liters) = 0.52 M.

In this case, the unit is M.  M stands for "molarity".  A 0.52 M solution is referred to as being a "0.52 molar" solution.  That's simple enough!
 

Sometimes these problems get a little bit harder.  Instead of giving you moles, they give you grams.  Instead of liters, they give you milliliters.  Fortunately, you know how to do the necessary conversions.  Here's a handy diagram to help you:

Let's do an example.

Example:  If I have 3.50 grams of sodium chloride in 1250 mL of a solution, what's the molarity?

Solution:  To find molarity, we need to convert grams to moles and milliliters to liters.  To convert grams to moles, we first need to divide the number of grams by the molar mass of sodium chloride.  (3.5/58.5) = 0.060 moles.  To convert milliliters to liters, multiply by 0.001.  (1250 x 0.001) = 1.25 liters.  In the final step, divide the number of moles by the number of liters to get the molarity.  Since 0.060 / 1.25 = 0.048, the molarity of the solution is 0.048 M.

You are now a molarity expert!

Colligative properties

Not surprisingly, when you dissolve something in a liquid, it has different properties than the pure liquid.  Any properties that change when the concentration of a solution changes are called colligative properties.

People always list boiling point elevation, melting point depression, and osmotic pressure as the main colligative properties of liquids.  We're going to do it a little differently - after all, I'm not trying to sell you guys a textbook, so I don't have to use the confusing words that school districts like to see.

Think about Kool Aid.  Let's make a weak Kool Aid solution by dissolving one grain of Kool Aid in a glass of water.  Let's also make a strong Kool Aid solution by dissolving a cup of Kool Aid powder in a glass of water.  The properties that are different between the two glasses of Kool Aid are "colligative properties".

In our example, we would find that:

• Strong Kool Aid is darker than weak Kool Aid.  As a result, we would say that "color" is a colligative property of liquids. This is the basis for a field called spectophotometry, where you can figure out how concentrated a solution is by looking at the color.
• Strong Kool Aid is a lot sweeter than weak Kool Aid.  To make it a little broader, we can say that "taste" is a colligative property of liquids.  True, but it's usually not all that handy in the lab.  They say that sodium cyanide tastes like almonds, but I wouldn't test the concentration of a cyanide solution by taste!  (Who figured that out, anyway?)
• Strong Kool Aid is thicker and goopier than weak Kool Aid.  Texture is a colligative property.
• Strong Kool Aid is denser than weak Kool Aid.  Density is a colligative property.
• Strong Kool Aid boils at a higher temperature than weak Kool Aid.  Boiling point is a colligative property.
• Strong Kool Aid freezes at a lower temperature than weak Kool Aid (if you've ever tried to make homemade popsicles, you know this to be true).  Melting / freezing point is a colligative property.

Now that you know what colligative properties are, let's look at the ones that your textbook probably focuses on:

Boiling point:  As the molarity of a solution increases, the boiling point increases.  This is because the solute lowers the vapor pressure of the solution and solutions don't boil until the vapor pressure of the solution is equal to atmospheric pressure.  If this doesn't make any sense, just remember that strong solutions boil at higher temperatures than weak ones.

Melting point:  As the molarity of a solution increases, the melting point decreases.  This is because the solute keeps the solvent molecules from forming a nice solid lattice.  Think of this:  Ocean water doesn't freeze very easily - this is because it's got salt dissolved in it.

Osmotic pressure:  You should have heard about this one in your biology class somewhere along the road.  Basically, if you've got a solution that's separated from a pure solvent by a semi-permeable membrane, the pure solvent likes to move across the membrane to decrease the concentration of the solution.  The pressure that the solvent pushes across the membrane with is called osmotic pressure.  Not surprisingly, the more concentrated the solution, the more the pure solvent likes to push across the membrane and the higher the osmotic pressure.