# 13: Solutions

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Solutions play a very important role in many biological, laboratory, and industrial applications of chemistry. Of particular importance are solutions involving substances dissolved in water, or aqueous solutions. Solutions represent equilibrium systems, and the lessons learned in our last unit will be of particular importance again. Quantitative measurements of solutions are another key component of this unit. Solutions can involve all physical states - gases dissolved in gases (the air around us), solids dissolved in solids (metal alloys), liquids dissolved in solids (amalgams - liquid mercury dissolved in another metal such as silver, tin or copper). In this unit we will almost exclusively be concerned with aqueous solutions - substances dissolved in water.

• 13.1: Prelude - Tragedy in Cameroon
Lake Nyos is a crater lake in the Northwest Region of Cameroon, and is a deep lake high on the flank of an inactive volcano in the Oku volcanic plain along the Cameroon line of volcanic activity. A volcanic dam impounds the lake waters. A pocket of magma lies beneath the lake and leaks carbon dioxide  into the water, changing it into carbonic acid. Nyos is one of only three known exploding lakes to be saturated with carbon dioxide in this way.
• 13.2: Solutions: Homogeneous Mixtures
There are two types of mixtures: mixtures in which the substances are evenly mixed together (called a solution) and a mixture in which the substances are not evenly mixed (called a heterogeneous mixture). A solution is an even (or homogeneous) mixture of substances. A point should be made here that when a solution is said to have uniform properties throughout, the definition is referring to properties at the particle level.
• 13.3: Solutions of Solids Dissolved in Water: How to Make Rock Candy
solutions can be formed in a variety of combinations using solids, liquids, and gases. We also know that solutions have constant composition and we can also vary this composition up to a point to maintain the homogeneous nature of the solution. The reasons why solutions will form will be explored in this section, along with a discussion of why water is used most frequently to dissolve substances of various types.
• 13.4: Solutions of Gases in Water
Other factors also affect the solubility of a given substance in a given solvent. Temperature is one such factor, with gas solubility typically decreasing as temperature increases. This is one of the major impacts resulting from the thermal pollution of natural bodies of water.
• 13.5: Solution Concentration: Mass Percent
To define a solution precisely, we need to state its concentration: how much solute is dissolved in a certain amount of solvent. Words such as dilute or concentrated are used to describe solutions that have a little or a lot of dissolved solute, respectively, but these are relative terms whose meanings depend on various factors. The mass/mass percent (% m/m) is defined as the mass of a solute divided by the mass of a solution times 100:
• 13.6: Solution Concentration: Molarity
Another way of expressing concentration is to give the number of moles of solute per unit volume of solution. Of all the quantitative measures of concentration, molarity is the one used most frequently by chemists. Molarity is defined as the number of moles of solute per liter of solution. The symbol for molarity is MM or moles/liter. Chemists also use square brackets to indicate a reference to the molarity of a substance.
• 13.7: Solution Dilution
We are often concerned with how much solute is dissolved in a given amount of solution. We will begin our discussion of solution concentration with two related and relative terms - dilute and concentrated.
• 13.8: Solution Stoichiometry
Double replacement reactions involve the reaction between ionic compounds in solution and, in the course of the reaction, the ions in the two reacting compounds are “switched” (they replace each other). Because these reactions occur in aqueous solution, we can use the concept of molarity to directly calculate the number of moles of reactants or products that will be formed, and hence their amounts (i.e. volume of solutions or mass of precipitates).
• 13.9: Freezing Point Depression and Boiling Point Elevation
Freezing Point Depression and Boiling Point Elevation are "colligative properties" that depends on the concentration of solute in a solvent, but not on the type of solute. What this means for the example above is that people in colder climates don't necessarily need salt to get the same effect on the roads - any solute will work. However, the higher the concentration of solute, the more these properties will change.
• 13.10: Osmosis
Osmotic pressure is caused by concentration differences between solutions separated by a semipermeable membrane and is an important biological issue.

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