Lesson 7 – Properties of Ionic/Molecular Compounds

Objective:

  • to classify ionic compounds by their properties
  • to classify molecular compounds by their properties

Timeline:

  • 1 – 1.5 classes

Prelab:

This lab is modified from one that I received from a co-worker. Unfortunately, the paper copy I have has no information about where it came from, so I cannot give credit where it is due. My apologies.

Complete the following questions prior to your labwork in class.

1. Before you test the properties of various ionic and molecular substances (most of which are dissolved in water), you will examine the properties of water at room temperature. Why is it important to do this?

2. By looking at the chemical formula, how can you tell if a substance is ionic or molecular?

3. Complete the “prelab” section of the table below.

PRELAB EXPERIMENT
Name Formula Ionic or Molecular Phase at Room Temp. Soluble in Water? Colour of solution Conductivity?
Water
Salt
Potassium Dichromate
CaSO4
Methanol
Cobalt (II) Nitrate
K2CrO4
Ethanol
C6H12O6
Ammonium Sulfate
S8
KMnO4
Copper (II) Sulfate
CaCO3
Wax C25H52
Unknown Unknown

Experiment:

You will examine the substances listed in the table. You will examine their state, their solubility, their appearance in solution and their conductivity. Write a procedure to help you best accomplish these steps. Have it approved by your teacher before you begin.

Results:

By looking at your data in the experiment part of the table above, make generalizations about ionic and molecular compounds (this means the statement will hold true for most substances).

  1. What phase are most ionic compounds in, at room temperature? What phase are most molecular compounds in?
  2. Are most ionic compounds soluble in water? Are most molecular compounds?
  3. What colour are most ionic solutions? What colour are most molecular solutions?
  4. Are ionic solutions conductive? Are molecular solutions conductive?

Conclusions:

  1. Can you identify a substance as ionic or molecular based on its state? Explain.
  2. Can you identify a substance as ionic or molecular based on its solubility in water? Explain.
  3. Can you identify a substance as ionic or molecular based on its colour in solution? Explain.
  4. Can you identify a substance as ionic or molecular based on its conductivity? Explain.

More Information:

Chemical properties tell us how the atoms have bonded together. This allows us to predict how other substances will act, by studying what properties they have. An example of this is Buckyballs – an arrangement of Carbon atoms that have interesting properties and unique applications for technology.

Ionic compounds have very rigid, regular crystalline shapes. This shows that they have a repeating sequence (remember that ionic formulas show the lowest ratio of atoms, not the total number). These are called a crystal lattice.

Ionic compounds also resist breaking apart. This means they have very high melting points. (Salt has a melting point of 800 ºC!)

As a solid, an ionic compound wont conduct electricity, but they do when dissolved! This means that the compound breaks apart in water and forms free ions that float around.

On the other hand, molecular compounds have properties that are almost opposite of ionic. They have low melting points, because the attractive force between the molecules is weaker.

Even though the attraction between separate molecules is weak, the force holding together an individual molecule is very strong! It’s very hard to separate a molecular compound into separate atoms. For example, you need electricity to separate water into oxygen and hydrogen.

Molecular compounds do not conduct electricity. further proving that they don’t break down when dissolved and are completely neutral.

Wait, don’t they say to be careful with electronics around water because you may be electrocuted? Water is a molecular substance and shouldn’t conduct electricity! However, water is also an excellent solvent. This means it dissolves a huge number of other chemicals, including ions. Any water that has ions dissolved in it, is an ionic solution and will conduct!

Self-Check

For each statement, try to identify whether it is an ionic or molecular substance. If you are unable to tell, identify it as unknown.

  1. A white solid that you dissolve in water. It does not conduct.
  2. A clear liquid that you dissolve in water to form a solution.
  3. A solid that you dissolve in water and forms a blue solution.
  4. A colourful solid that does not dissolve.
  5. A substance that is a liquid at room temperature.
  6. You see a clear solution and find that it is conductive.

7. Identify what the mystery substance was, based on your observations of its properties.

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