Chapter 13
Section 1
Solutions is a lead-in chapter to Acids and Bases. We have to know what makes up a solution before we can decide what characteristics the solution has.
First of all, something has to be soluble. That means it has to be able to dissolve. A solution is a homogeneous mixture of two or more substances in a single phase. The "thing" that does the dissolving is the solvent, and the "thing" that gets dissolved is the solute.
There are three different types of solutions.
1. A solution is composed of very tiny particles. The particles can be atoms, ions or molecules. They do not separate, can not be separated, and do not scatter light. That means you can not see the light in the solution
2. Colloids contain slightly bigger particles. They do not separate, can not be separated, but they do scatter light. In fact, you can see the beam of light as it goes through the liquid. This is called the Tyndall effect, and it is pretty neat.
3. Suspensions have the biggest particles, which will settle out, can be separated by filtering, and may scatter light, but may not be transparent.
See the charts on pages 396, 397 and 398 for the examples of different types of solutions.
Some solutions are electrolytes. This means that they will conduct electric current.
Some solutions are not electrolytes, and they will not conduct electric current.
Section 2
Several factors affect the rate at which something will dissolve.
1. Increase the surface area. We do this when we grind up something into a powder.
2. Agitate the solution. This means to stir or shake the solution.
3. Heat the solvent.
Only a scientist would do these three things separately. Most of the time we do all three together, and do not even think about the scientific process going on.
Solution equilibrium is the physical state in which the dissolving and crystallization of the solute occurs at the same rate. We will not be able to see this because it occurs on the atomic level.
A saturated solution has all the solute it can hold. If you put more solute in and stir, it will not dissolve. Before a solution becomes saturated, it is unsaturated. If we use some pretty sneaky tricks, we can even create a supersaturated solution. That means we put more solute into the solvent, and got it to dissolve that we normally can. Scientists have even figured out the solubility values for various chemicals.
Ionic solutions will dissolve in water because the water is a polar molecule. It is not cold, but rather acts like a magnet, pulling the ions away from the solid. This process is called hydration.
Have you ever heard of the saying, "Like dissolves like."? That means that we need to use a solvent that is like the solute in order to get the solute to dissolve.
Some compounds do not respond to polar solvents, like water. These compounds need a nonpolar solvent.
If you want to remove a dirt stain, soap and water will work just fine. If you want to remove a ballpoint pen mark, don't use soap and water. It won't work. You need a solvent made of elements similar to the pen ink. Usually hair spray works on ballpoint pen ink. This ink is nonpolar, and the hair spray is also nonpolar.
If the solute and solvent do not mix, they are said to be immiscible. If they do mix, they are miscible. Water and oil are immiscible. Water and vinegar are miscible.
Pressure will also affect how much we can dissolve. Henry's Law says the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas on the surface of the liquid. One good example of this is in soft drinks. When you open a can of soda, you will hear the gas escaping because you have changed the pressure of the container. The rapid escape of the gas is called effervescence.
Temperature can have an effect on how something dissolves. We have to look up the data for the particular compound. Most of the time there is an energy change when something is dissolved into something else. It can be negative, releasing heat, or positive, absorbing heat. Again, different cpds do different things.
Section 3
Lets make some solutions.
1. The first kind of solution is called a Molarity solution. We get a volumetric flask, add some liquid, put in the correct number of moles of solute, mix, and continue to add liquid to the mark. This type of solution is called a Molar solution. We call it a "M" solution. "M" means moles of solute/ 1 liter solvent.
2. The second kind of solution is called a Molality solution. Watch out for the consonants! To make this solution, we get 1kilogram of liquid, add the moles of solute, mix, and presto, we have a Molal solution. We call it a "m" solution. "m" means moles of solute / 1kg solvent.
Please be in class for the math. Obviously we will convert lots of cpds to moles.