Solution Chemistry

Earlier we learned that a homogeneous mixture of two or more substances is called a solution. If one of the substances is present in much greater quantities than all the other substances, then it is called the solvent. The other substances in the solution are known as solutes. For example, when a small amount of NH₄Cl is dissolved in a large quantity of water, we refer to water as the solvent and NH₄Cl as the solute. Another example is Naphthalene (used in mothballs) can be dissolved in benzene. In this example, benzene is the solvent, and naphthalene is the solute.

Solutes dissolved in water (solvent) are called aqueous solutions. Not all substances are soluble in water. Why do some substances dissolve in water and others don't? It has to do with the structure of the water molecule.

Oxygen has a greater attraction for electrons, so the shared electrons (bonding electrons) spend more time close to oxygen than to either of the hydrogens. This gives oxygen a slightly excess negative charge and hydrogen a slightly more positive charge.

This unequal charge distribution makes water a polar molecule and gives water its ability to dissolve compounds. When an ionic solid dissolves in water, the positive ends of the water molecule are attracted to the negatively charged anions, and the negative ends of the water molecule are attracted to the positively charged cations. For example, when NaCl is dissolved in water, we find

So when an ionic substance (salt) dissolves in water, it is broken up into individual cations and anions, which are surrounded by water molecules. For example, when NH₄ NO₃ is dissolved in water, it breaks up into separate ions.

NH₄⁺ and NO₃^- ions are floating around in H₂O essentially independent of each other.

Water also dissolves non-ionic substances. For example, C₂H₅OH (ethanol) is very soluble in H₂O. This is because C₂H₅OH has a polar OH bond that the water molecules like to hang around.

Many substances do not dissolve in water, and that is because they are non-polar and do not interact well with water molecules. A common example is oil and water. Oil contains non-polar molecules; thus, they do not dissolve in water.

How do we know that ionic solids dissolve in water and form cations and anions that float around separately? One clue comes from conductivity experiments. Anions and Cations should act as charge carriers in the solution. Therefore a solution with dissolved ions should conduct electricity. Let's look at a few examples. Pure (distilled) water contains no dissolved ions. Therefore, pure water will not conduct electricity. In a simple conductivity experiment, as shown below, we would not expect the light to be on.

An aqueous NaCl solution, however, will have dissolved ions present and, therefore, will conduct electricity. Therefore, the light in our conductivity experiment will be on if dipped in an aqueous NaCl solution.

NaCl ionizes completely when dissolved in water. It's helpful to think of this process as two steps:

Substances that exist in solution almost completely as ions are called strong electrolytes.

Substances that do not form ions when they dissolve in water are called non-electrolytes. An example of a non-electrolyte is sugar. Sugar will readily dissolve in water but doesn't form cations and anions in solution. That is, there are no charge carriers formed.

Substances that only partially ionize into ions when dissolved in water are called weak electrolytes. For example, Acetic Acid (HC₂H₃O₂) dissolves in water but only partially dissociates into ions.

Be careful not to confuse how soluble a substance is in water with whether it is a weak, strong, or non-electrolyte. For example, sugar dissolves completely in water, but it is a non-electrolyte. Another example is salts that can be very insoluble in water, but the small amount of salt that does dissolve in water is a strong electrolyte.