Chemical compounds are combinations of atoms held together by chemical bonds. These chemical bonds are of two basic types – ionic and covalent. Ionic bonds result when one or more electrons from one atom or group of atoms is/are transferred to another atom. Positive and negative ions are created through this transfer of electrons. In covalent compounds, no electrons are transferred. Instead, electrons are shared by the bonded atoms.
The physical properties of a substance, such as melting point, solubility, and conductivity, can be used to predict the type of bond that binds the atoms of the compound. In this experiment, you will test six compounds to determine these properties. Your compiled data will enable you to classify the substances as either ionic or covalent compounds.
MATERIALS
SUPPLIES / CHEMICALS1. 24-well microplate / 1. Water
2. Bunsen burner / 2. Potassium iodide (KI)
3. Conductivity tester / 3. Sodium chloride (NaCl)
4. Iron ring / 4. Acetaminophen (C8H9NO2)
5. Ring stand / 5. Sucrose (C12H22O11)
6. Aluminum foil square
7. Wire gauze / 6. Unknown #1
7. Unknown #2
8. Thin-stemmed pipet
9. Thermal gloves
10. Lab apron
11. Safety goggles
SAFETY
1. Always wear safety goggles, gloves, and lab apron to protect your eyes and clothing. If you get a chemical in your eyes, immediately flush the chemical out at the eyewash station while calling for your teacher. Know the location of the emergency lab shower and eyewash station and the procedures for using them.
2. Do not touch any chemicals. If you get a chemical on your skin or clothing, wash the chemical off at the sink while calling for your teacher. Make sure you carefully read the labels and follow the precautions on all containers of chemicals that you use. If there are no precautions stated on the label, ask your teacher what precautions to follow. Do not taste any chemicals or items used in the laboratory. Never return leftovers to their original container; take only small amounts to avoid wasting supplies.
3. Do not heat glassware that is broken, chipped, or cracked. Use tongs or hot mitts to handle heated glassware and other equipment. Hot glassware does not always look hot.
4. When using a flame, confine long hair and loose clothing. If your clothing catches on fire, walk to the emergency lab shower and use it to put out the fire.
PROCEDURE
1. Put on safety goggles and lab apron.
2. Fold a piece of aluminum foil around a wire gauze. Place a few crystals of each of the solids (4 known and 2 unknown) in separate locations on the piece of aluminum foil. Do not allow the samples of solids to touch. Use the space below to diagram and label the position of each compound.
3. Before you begin testing melting points, write a brief description of each of the six substances in Table 1.
4. Place an iron ring attached to a ring stand. Position the ring so that it is just above the tip of the Bunsen burner flame. Light the burner for a moment to check that you have the correct height.
5. Place the aluminum foil with the samples on the ring. For this experiment, it is not necessary to determine exact values for the melting point. Heat the substances for two minutes and observe which ones have melted in that time.
6. After 2 minutes, record in Table 1 which substances did and did not melt. Extinguish the flame. Allow the apparatus to cool while completing remainder of lab.
7. Put a few crystals of each of the solids in the top row of your microplate.
8. Add 10 drops of water to each well. Do not stir. Record the solubility of each substance in Table 1.
9. Look at the back of the conductivity tester to fill in the conductivity information below.
Red Light / Green Light / ConductivityOff / Off
Dim / Off
Medium / Off
Bright / Dim
Very Bright / Medium
10. Test the conductivity of each water solution by dipping both electrodes into each well of the microplate. Be sure the rinse the electrodes and dry them with a paper towel after each test. Using the table in step 9, determine the level of conductivity. Record your results in Table 1.
11. Clean the microplate. Use a small brush if any of the wells are difficult to clean.
12. Once the Bunsen burner apparatus is cooled, disassemble and put away all materials.
13. Wash your hand thoroughly before you leave the lab and after all work is finished.
TABLE 1 – CHARACTERISTICS OF COMPOUNDS
Compound / Description / Melting / Solubilityin Water / Conductivity
Acetaminophen
Potassium Iodide
Sodium Chloride
Sucrose
Unknown #1
Unknown #2
ANALYSIS
1. Organize your results by grouping the known and unknown substances into two groups according to their properties.
GROUP 1:
GROUP 2:
2. List the properties of each group.
GROUP 1:
GROUP 2:
CONCLUSIONS
1. Use your class notes/text to determine which group consists of ionic compounds and which group consists of covalent compounds.
GROUP 1: GROUP 2:
2. Answer the following questions about unknown #1:
a. In which group did you place unknown #1?
b. Provide TWO reasons you placed unknown #1 in this group.
c. Were there any properties of unknown #1 that didn’t exactly fit with the properties of the other substances in the group? Explain.
3. Answer the following questions about unknown #2:
a. In which group did you place unknown #2?
b. Provide TWO reasons you placed unknown #2 in this group.
c. Were there any properties of unknown #2 that didn’t exactly fit with the properties of the other substances in the group? Explain.
4. Compare the formulas of the four known substances (found on directions sheet). How are the ionic formulas different than the covalent formulas?
5. Answer the following questions about the compounds C7H7NO3:
- Is this an ionic or covalent compound? How can you tell?
- Predict TWO properties that this compound may have.
6. Answer the following questions about the compounds CaBr2:
- Is this an ionic or covalent compound? How can you tell?
- Predict TWO properties that this compound may have.