Prescriptive Activities: Facet Cluster 2.2: PF#83
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Prescriptive #83:
83 The student thinks that strong (electrical) bonds lead to low melting and boiling points.
Background:
The strength of bonds holding ions/molecules together determines many of their properties,
including their melting and boiling points. In order to melt or boil a substance the ions or
molecules must have enough energy to overcome the forces holding them together.
Within an ionic crystal there are many bonds holding the ions to each other and all of these
electrostatic interactions must be broken in order to melt the crystal. The forces holding molecules
together in molecular solids are much weaker than forces holding ions together and therefore
require less energy to disrupt, leading to lower melting and boiling points. So, weaker bonds lead
to lower melting and boiling points.
Keep in mind that if a substance exists as a gas, it has already boiled. For example, the nitrogen
(N2) in the air around you boiled at a temperature far below room temperature. That is why it is a
gas!
Here you will explore the difference in melting points using the familiar compounds sugar
(covalent) and salt (ionic).
Materials:
Student handout
Salt (NaCl)
Sugar (C12H22O11)
Baking soda
Aluminum foil
Ring stand and ring
Bunsen burner
Tongs
Safety goggles
Note: If you are doing this activity at home, you can simply put a square of foil on the bottom of a
pot and place the sugar, salt or baking soda on the foil and heat it on the stove.
Directions of Activity:
Safety: Use caution when working with fire.
Melting point of sugar – molecular compound
1. Place a small square of aluminum foil on the ring of the ring stand.
2. Put a small amount of sugar on the aluminum foil.
3. Light the Bunsen burner and place a low flame under the foil.
4. Gently heat the sugar until it you observe changes – notice the time it takes for the change
to begin – as soon as you observe a change REMOVE FROM FLAME.
5. Write down your observations and the approximate time it took for changes to occur.
Prescriptive Activities: Facet Cluster 2.2: PF#83
Teacher Page
Melting point of salt - an ionic compound
1. Place a small square of aluminum foil on the ring of the ring stand
2. Put a small amount of salt on the aluminum foil
3. Light the Bunsen burner and place a low flame under the aluminum foil
4. Gently heat the salt for 5 minutes
5. Write down your observations
Melting point of an unknown solid
1. Place a small square of aluminum foil on the ring of the ring stand
2. Put a small amount of baking soda on the aluminum foil
3. Light the Bunsen burner and place a low flame under the aluminum foil
4. Gently heat the baking soda for a few minutes
5. Write down your observations
6. You can discard the sugar, salt, baking soda and foil in the trash (recycle the foil if there is
no residue stuck to it).
Questions:
1. What changes did you observe upon heating the sugar (covalent compound)?
2. Approximately how long did it take for the sugar to change?
3. What changes did you observe upon heating the salt (ionic compound) for 5 minutes?
4. What changes, if any, did you observe upon heating the baking soda?
5. Based on your observations do you think baking soda is an ionic or covalent compound?
Explain your reasoning.
6. Give an explanation for the differences you observed in heating the sugar and the salt.
7. If you had to make a general rule based on this one example how would you describe the
differences in melting points between ionic and covalent compounds?
Teacher Notes
When students are heating the sugar, remind them to remove the flame as soon as they see a
change. Otherwise the sugar will begin to decompose.
This activity can also be done at home. Remind students to be careful when working with flames
and hot substances.