Physical Properties of Ionic and Molecular Compounds Lab
This lab has four stations:
1 – Appearance, Odour
2 – Hardness
3 - Conductivity
4 – Melting Point
Purpose
To study some of the physical properties of two types of solids—ionic and molecular compounds.
Introduction
Some solids consist of molecules in which the atoms are held together by covalent bonds. The molecules are held together by van der Waals’ forces of
attraction. These solids are called molecules.
Other solids consist of an array of positive and negative ions, arranged in such a way that every positive ion has only negative neighbours and
vice versa. The solid is held together because of the attractions between ions of opposite charge. Such solids are called ionic compounds.
In this experiment, you will examine four physical properties of molecular and ionic compounds. You will attempt to relate these properties
to the forces of attraction between the particles in each solid.
Materials
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Sodium chloride
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Sucrose
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Potassium iodide
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Salicylic Acid
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Unknown compound
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Paper towel
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Timer
Well plate
Hot plate
Aluminum foil
Conductivity probe
Computer (Logger Lite)
Procedure
Station 1 – Appearance and Odour
1. Four identified substances and one “unknown” are provided in containers. Place a small pea sized sample of each substance in its own well in
a well-plate. Then perform the following tests for each substance.
2. Describe the appearance of the chemical (“small, same-sized crystals” or “different sized chunks”) and any other obvious physical
characteristics. Write your description into a table in your lab notebook.
3. Observe the odour of each sample (strong, weak or nil).
Station 2 - Hardness
1. Four identified substances and one “unknown” are provided in containers.
2. Obtain a piece of paper towel
3. Arrange rice-grain size amounts of each compound on the paper towel.
4. Carefully press on each sample with the back of your scoopula and try to crush it. Wipe off your scoopula between tests so that you do not
contaminate other samples. If the substance compresses, record it as “soft.” If it crumbles, record it as “medium.” If it does not crush easily
but fragments, record it as “hard.”
5. Dispose of your paper towel and samples.
Stations 3 and 4 – Melting Point (You will go to either of these stations)
1. Cover a hot plate with aluminum foil.
2. Add rice grain sized samples of the various compounds to foil.
3. Plug in and turn on the hot plate by turning the power dial to approximately 80% of the maximum setting. Start the timer.
4. Record the order and time at which the samples melt or undergo other changes. Record all visible changes in the next 10 minutes. If any
samples have not melted or changed by the end of the 10 minutes, record that too and then turn off the plate.
1.
2.
3.
4.
5.
6.
7.
– Conductivity – This will be done as a demonstration
Connect the LabPro and conductivity probe to the computer. Make sure the probe is set to the correct range (0-2000 µS). Launch Logger
Lite. A conductivity reading should be visible.
On your lab sheet, record the conductivity reading that the probe reads before it is placed in any solution (dry air reading). The conductivity is
in µS/cm—this gives a measure of how many ions are able to move through the sample. ______________________
Fill an empty plate well half full with distilled water and record the conductivity of the distilled water.
________________________________
Fill another empty well half full with tap water and record its conductivity. ____________________
Rinse off your probe with distilled water from the squeeze bottle after each reading.
Add your samples and fill the wells half-way with distilled water. Mix (rinse scoopula between mixing) and then measure the conductivity
for 15 seconds. Record the largest reading. (On Data Table)
Repeat until all samples are measured.
Table 1: Physical Properties of Ionic and Covalent Substances
Substance
#
Formula
Appearance
(Write Ionic
or Molecular)
Sodium
1
chloride
Potassium
iodide
2
Salicyclic
Acid
3
Sucrose
4
Unknown
5
Hardness
(soft, medium, hard)
Conductivity
(µS/cm)
(5)
Melting Time
Concluding Questions
1. On the bases of the electronegativities of these elements, which substances are ionic? Justify your answers. (2)
2. What does a strong odour indicate about the ease at which the particles in the solid leave the surface? (1)
3. From your answers to questions 1 and 2 and your observations of odour, which of the substances seem to have stronger forces of attraction?
Explain your answer. (2)
4. From your observations of hardness and melting points, how does the strength of van der Waals’ forces of attraction compare with the
strength of the attractions between ions of opposite charge? (2)