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Chemistry LibreTexts

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
      The major component of a solution is called the solvent. The minor component of a solution is called the solute. By major and minor we mean whichever component has the greater presence by mass or by moles. Sometimes this becomes confusing, especially with substances with very different molar masses. However, here we will confine the discussion to solutions for which the major component and the minor component are obvious.
    • 13.3: Solutions of Solids Dissolved in Water: How to Make Rock Candy
      Whether or not solutions are formed depends on the similarity of polarity or the "like dissolves like" rule. Polar molecules dissolve in polar solvents, nonpolar molecules dissolve in nonpolar solvents. Ionic compounds dissolve in polar solvents, especially water. This occurs when the positive cation from the ionic solid is attracted to the negative end of the water molecule (oxygen) and the negative anion of the ionic solid is attracted to the positive end of the water molecule (hydrogen).
    • 13.4: Solutions of Gases in Water: How Soda Pop Gets Its Fizz
      The dissolution in a liquid, also known as fizz usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which causes the solution to become effervescent, or fizzy. A common example is the dissolving of carbon dioxide in water, resulting in carbonated 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
      For ionic solutes, the calculation of colligative properties must include the fact that the solutes separate into multiple particles when they dissolve. The equations for calculating colligative properties of solutions of ionic solvents include the van’t Hoff factor, i.
    • 13.9: Freezing Point Depression and Boiling Point Elevation: Making Water Freeze Colder and Boil Hotter
      Colligative properties depend only on the number of dissolved particles (that is, the concentration), not their identity. Raoult’s law is concerned with the vapor pressure depression of solutions. The boiling points of solutions are always higher, and the freezing points of solutions are always lower, than those of the pure solvent.
    • 13.10: Osmosis: Why Drinking Salt Water Causes Dehydration
      In osmosis, the solute remains in its original side of the system; only solvent molecules move through the semipermeable membrane. In the end, the two sides of the system will have different volumes. Because a column of liquid exerts a pressure, there is a pressure difference Π on the two sides of the system that is proportional to the height of the taller column. This pressure difference is called the osmotic pressure, which is a colligative property.