Chapter 15 Solutions. 15.1 What are solutions?. You might remember from the beginning of the year that solutions are homogenous mixtures. This means solutions contain two or
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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 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.
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.
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.
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-
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).
