Wednesday, July 27, 2011

Molecular or Ionic Compounds?

 Molecular or Ionic Compounds?

Mrs. John and Mrs. Hill were cleaning up the back stock room. They found a box of vials containing white solids. Some were powders and some were crystalline in nature; one was small, smooth, pieces, clearly not a powder and not a crystal. The old labels (the ones you lick and stick) on the vials had fallen off and were on the bottom of the box. They were not much help as they either said ionic or molecular. Mrs. John and Mrs. Hill were left trying to figure out which compounds were ionic and which were molecular.  Mrs. John volunteered her students to help identify the compounds.  Here is what we know:

·      There are six different substances. We can tell that by visually looking at the compounds.
·      The compounds are either ionic or molecular.

Characteristics of Ionic Compounds                  Characteristics of Molecular Compounds
High melting and boiling points                             Low melting and boiling points
Solids at room temperature                                     Can be solid, liquid, or gas at room temperature
DO conduct electricity in molten state                    DO NOT conduct electricity in molten state, or
      or when dissolved in water                                       liquid state, or when dissolved in water

To help identify these six compounds you will have to find their crude melting points (i.e. high or low melting point) and their ability to conduct electricity when dissolved in water.

A compound that conducts electricity when dissolved in water is called an electrolyte.
A compound that does NOT conduct electricity when dissolved in water is called a nonelectrolyte.

Here is what we need to do:

1. Cover the top of the hot plate with a piece of clean aluminum foil. Put a small amount of each substance on the foil that is covering the hot plate. Make a diagram of where you put each substance. Turn the hot plate somewhere between  the 8 and 10 setting. Indicate in the chart below which substances have a high melting point and which have a low melting point. (HINT: The ones that melted on the hot plate would have a low melting point. A high melting point would require a temperature much hotter than the hot plate.)

2. Using the spatula provided, put a small amount of each substance in the spot plate. It is helpful to put a “key” under the spot plate so you know which compounds are in each well. You only need a small amount of the substance. Wipe the spatula off with a clean paper towel after each solid. This is important. DO NOT SKIP THIS “CLEANING” BETWEEN SOLIDS. Why?  Add DISTILLED WATER to each well. (DO NOT add tap water. Why? Think about it….) Clean the electrodes off with distilled water only. Make sure the electrodes are NOT touching each other. Turn the conductivity apparatus on. Place the electrodes in one well of the spot plate. Rinse the electrodes off with distilled water after you test the solution. Test the next solution. Rinse the electrodes off with distilled water after you test this second solution as well as after you test each of the remaining solution. Record your results for conductivity for each solid in the table below. 

Data and Observations

High or Low Melting Point?
Conducts electricity or not when dissolved in water?








1. Which of the substances were ionic compounds (ionic bonds)? Which were molecular compounds (covalent)?

2. Which distinguishing characteristics did you use to classify the unknown compounds
as being either ionic or molecular?

3. Why was it important to make sure the spatula was wiped clean after distributing each solid to a well in the spot plate?   

4. Why was it important to clean off the electrodes after each test with distilled water?

5. Why was it important to use distilled water and not tap water in this lab?


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