Properties of Ionic and Covalent Bonds
By Susan Lustig
Carolina Teaching Partner
Chemical bonding is one of the fundamentals of a chemistry course and the framework for many chemistry concepts, including solubility, melting point, and formula writing. However, while students are able to define covalent bonding (sharing of electrons) and ionic bonding (transfer of electrons) almost by rote, they often have difficulty remembering the properties of these bonds and the explanations for them. To help my students really understand chemical bonds, I’ve developed an introductory lesson that uses common substances and hands-on testing.
Using common substances makes the lesson more engaging and keeps cost low since the materials come from the cupboard or the dollar store. You will use wax, water, and sugar as examples of covalent bonds and sodium chloride (table salt) and calcium chloride (ice melt) as examples of ionic bonds. The properties that students will test for are solubility in water, melting point, and electrical conductivity (in solid and liquid phases).
Before doing any hands-on testing, be sure to have your students put on appropriate personal protective equipment (PPE), including goggles, apron, and gloves. It’s also a good time to review your school’s lab safety regulations. To record their results, students can complete a data table similar to the one below as they perform the tests.
Properties of Ionic and Covalent Substances
|Substance||Formula||Describe How It Melts||Water Soluble?||Electrical Conductivity (solid)||Electrical Conductivity (aqueous)|
To test melting point, students place each substance in an aluminum foil burner protector and place the burner protector on a hot plate set at low heat. It is important to do this test at low heat because after sugar melts it can quickly smoke and burn (think of roasting marshmallows). Students record the rate at which each substance melts (fast or slow). Note: Burner protectors cost about 15¢ each at the dollar store, are recyclable, and have a lip around them which prevents the substances from going onto the hot plate. Some even have 6 sections, which keeps the substances from combining when they melt. I highly recommend them.
To test solubility in water, you can either use a spot plate or small test tube. It is important that only small amounts of substances (e.g., one or 2 small crystals) are used for this test. Remind students that the ice is not dissolving in the water; it is melting.
To test conductivity, use a simple conductivity tester. Students should test solid state conductivity first by placing each substance in its own small beaker or plastic cup and using the conductivity tester. To test aqueous conductivity, simply add distilled water to the beakers or cups and use the conductivity tester. When testing solutions, students should rinse the electrodes in distilled water between tests. Students are surprised that distilled water (and often tap water) is not a conductor. Note: Test your tap water before class; if it conducts, use distilled water instead. You need water that does not conduct.
Usually a student asks “Why can’t I swim during a thunderstorm if water doesn’t conduct electricity?” I explain to the class that as soon as a person enters the water, the water becomes a conductor since the salt on our skin is water soluble. Swimming in the ocean is even riskier because salt water is a much better conductor than freshwater. This is why, I stress, that one must never swim during a thunderstorm.
A more dramatic conductivity test can be done using a large lightbulb-type conductivity tester, such as Carolina’s Conductivity of Water Tester. For safety, I recommend that this test only be done as a teacher demonstration. Use the student procedure given above. You will find that none of the substances are conductors in solid state. Add distilled water to each beaker or cup and retest. The ionic substances cause the bulb to produce a bright light, while the covalent ones do not.
After students have finished their testing, you can help them analyze their data either as a class or in small groups. Experimental data reveal that substances formed by covalent bonds do not conduct electricity and have low melting points (quickly melt). (Their solubility in water depends on the type of covalent bond, but that is another topic.) The data also reveal that substances formed by ionic bonds are water soluble, conduct electricity in aqueous solution (but not when solid), and have high melting points (slowly melt or do not melt in the classroom).
After analyzing the data, students should understand the difference between ionic and covalent bonds. You can introduce electronegativity differences at this time to help explain bond characteristics. Two strong magnets are another way of showing how the opposite charges on the ions are attracted. One more way to expand on this topic is for students to draw Lewis dot diagrams for each of the substances tested.
You can evaluate students’ knowledge by having them do a written laboratory report. It can include a conclusion stating the properties of substances formed by ionic and covalent bonds and how the type of bond results in these properties. A chart similar to the one below can be used to help students organize their conclusions.
|Ionic Bond||Covalent Bond|
|Metal/Nonmetal in Substance|
After completing this activity and analyzing the results, your students should not only be able to define ionic and covalent bonds, but also be able to explain the properties of these bonds as they are observed in the tested substances and the reasons for them.