When atoms combine to make compounds they bind to each other by electrical forces of attraction. Positive charges attract negative charges and these strong forces create a bond between atoms. These bonds can take different forms: some are covalent bonds in which atoms share valence electrons. The negatively charged electrons occupy the space between the positively charged nuclei and both nuclei are attracted to the electrons. Other bonds are ionic bonds in which atoms gain electrons to become a negatively charged anion or by lose electrons to become a positively charged cation. These ions then attract each other directly with no electrons between them.
|An ionic compound in the act of dissolving in water.
See a full explanation with animations at this page: |
Chemical bonds are formed and broken during chemical reactions that change one substance into another. Chemicals bonds may also be temporarily broken by dissolving a compound in water. Some compounds have bonds that can be broken to create two or more freely moving ions when the compound dissolves in water. This is due to the fact that water is a strongly polar molecule. It has two oppositely charged poles that strongly attract other atoms and molecules. In the illustration at right water molecules are shown dissolving ions from a solid crystal. The water molecules’ positively charged poles attract negative ions away from the crystal and then surround them as the ions then become mobile and move around in the solution. The negative poles of the water molecules do the same for the positive ions. The ions remain surrounded by water molecules just as a famous pop star is always surrounded by an entourage. Chemical compounds which break up into ions when dissolved in water are called electrolytes. There are other compounds which dissolve well in water but which do not break into separate ions. Such compounds are called non-electrolytes. Finally, a third class of compounds, called weak electrolytes, are found to produce a small number of ions when dissolved in water. The majority of the molecules of weak electrolytes remain as whole, uncharged molecules but a small fraction break up into cations and anions. This lab is not concerned with weak electrolytes.
Solutions are homogeneous mixtures of two or more substances. For liquid solutions the solvent is often water. Solvents are materials in which other substances dissolve. Solids, liquids and gases may all dissolve in water and any of them may be called a solute. A solute is any substance that dissolves in a solvent. Solutions may be made to have a variety of different strengths, or concentrations. The concentration
|The image on the left shows dissolved ions at low concentration. |
The image on the right shows dissolved ions at high concentration.
Concentration is higher for solutions containing more of the solute and lower when less material has been dissolved. In the illustration at right there are two boxes showing different concentrations. On the left there are very few particles of the solute and the image represents a solution with a low concentration. On the right there are many more solute particles in a higher-concentration solution.
One interesting consequence of the fact that some compounds produce separate, mobile cations and anions when dissolved in water is that the solution conducts electricity. Imagine a complete circuit containing a light bulb and a battery. The light bulb lights because of the power supplied by the battery. If a wire is cut then the bulb goes out. But if the two cut ends of the wire are placed in an electrolyte solution of high enough concentration the bulb will light again. Very often it is possible to demonstrate this in a classroom. There is also a high-quality simulation available from the PhET project at the University of Colorado. It is called Sugar and Salt Solutions and is available here: http://phet.colorado.edu/en/simulation/sugar-and-salt-solutions.
Electrons in a metal are mobile and can move among the atoms of the metal like water in a pool full of pebbles. When a battery is used to make an electrical current it causes the electrons to move in an organized way, all in one direction, turning the pool into a stream. This motion of the electrons in a piece of metal wire is what we normally think of as an electrical current. The electrical current in an electrolyte solution has a slightly different mechanism. The ions themselves move and carry electrical charge from one wire to the other. Electrons do not leave the metal and travel through the water to the other wire. Instead, cations move toward the negatively charged wire and anions move toward the positively charged wire. Fundamentally, an electrical current is only the movement of charged particles and it doesn’t matter whether the particles are electrons or ions.
The conductivity probes you will use in this lab activity measure how well a solution conducts electricity. It gives a low reading for a low concentration of ions and a higher reading for higher concentrations of ions. In effect, the reading produced by the probe is a measure of the number of ions per liter. The number of ions per liter depends on two things. First, it depends on how much solid is dissolved. The more material that is present in solution, the more conductive the solution will be. Second, it depends on the chemical formula of the dissolved material. Some compounds have more ions in their chemical formulas than others. Consider the following compounds and the number of ions each one makes when they dissolve:
Each equation shows how many ions are produced from each formula when they dissolve in water. Sodium chloride produces two ions, calcium chloride produces three ions, and sodium phosphate produces four ions. Even when these salts are made into solutions with the same concentration, measured as whole formula units per liter, they produce different levels of conductivity. For example, a solution of sodium phosphate will be twice as conductive as a solution of sodium chloride with the same concentration.
The chemicals used in this activity are all of low toxicity and are used in dilute solutions. Even so, it is best to wear standard laboratory safety equipment such as splash goggles, long pants, and nitrile gloves, if desired. Tie back long hair and do not wear dangling jewelry in the lab.
In this part of the lab you will collect data to show the proportion between concentration and conductivity for three strong electrolytes.
Answer the following questions in a typed document on a separate paper. As part of your report you must include the answers to the Pre-lab questions, your formatted and labeled graph, and the answers to the questions below. This lab does not require a formal lab report.