Composition of Solutions: Solution Composition

Introduction to Solutions

A solution is a homogeneous mixture. That means the components of a solution are so evenly spread throughout the mixture that there are no perceivable differences in composition. Solutions can be formed by mixing two substances together such as sugar and water. If you pour a packet of sugar into a glass of water, initially you have a suspension as the sugar crystals float about in the glass. When you have stirred the sugar and water for long enough, you will eventually get a clear, colorless mixture. Some people, especially young children, can be fooled by such a demonstration into thinking that the sugar has "disappeared". However, as chemists, we know better. The law of conservation of matter states that the sugar can not just disappear, it must have gone somewhere else. That somewhere else is into solution. The sugar has become evenly dispersed. In fact the sugar molecules are so well spread out that we can no longer see a single sugar crystals. However, if you taste the water, you will find it to be sugary--confirming the presence of sugar in the water. The minor component of the solution is called the solute. In the present example, sugar is the solute. The major component of the solution is called the solvent. In this case water is the solvent.

Solutions can also be formed by mixing together many different phases of matter. For instance, air is a solution. The solute gasses oxygen, carbon dioxide, argon, ozone, and others are dissolved in the solvent nitrogen gas. Another example is found in "gold" jewelry. Most of the golden jewelry sold in the world is not 24 karat (i.e. 100% pure gold) but rather it is a solution of other metals, commonly silver and copper, in a gold solvent. Such a solution of metal(s) in another metal is called an amalgam.

The Composition of Solutions

Perhaps the most important property of a solution is its concentration. A dilute acetic acid solution, also called vinegar, is used in cooking while a concentrated solution of acetic acid would kill you if ingested. The only difference between such solutions is the concentration of the solute. In order to quantify the concentrations of solutions, chemists have devised many different units of concentration each of which is useful for different purposes.

Molarity, the number of moles of solute per liter of solution, has the units moles / L which are abbreviated M. This unit is the most commonly used measure of concentration. It is useful when you would like to know the number of moles of solute when you know both the molarity and the volume of a solution. For example, it is easy to calculate the volume of a 1.5 M solution of HCl necessary to completely react with 0.32 moles of NaOH:

Normality, the number of molar equivalents of solute per liter of solution, has the units equivalents / L which are abbreviated N. To illustrate the difference between molarity and normality let's assume that we had used a 1.5 M solution of sulfuric acid, H₂SO₄, instead of a 1.5 M solution of hydrochloric acid, HCl in the above example. Because sulfuric acid can donate two protons to the NaOH, as noted in the , it will only take half as much sulfuric acid as hydrochloric acid to neutralize the sodium hydroxide.

In the present example, the 1.5 M solution of sulfuric acid reacts like a 3.0 M solution of hydrochloric acid because there are two equivalents of H⁺ per mole of sulfuric acid. Therefore, that solution of sulfuric acid is 3.0 N.