Unit Three: Chemical Bonding and Intermolecular Forces

Lesson 3: Properties of ionic compounds

• Comptencies
• Starter Activities
• Main Activites

Competencies

• Investigate the properties of given samples of ionic compounds
• Test hardness and brittleness of NaCl, CuCl2 and CuSO4
• Test solubility of NaCl, CuCl2 and CuSO4 in water, ethanol, acetone and toluene
• Apply scientific methods

Materials

5Test tubes, 3 beakers, stirrer, mortar and pestle, watch glass, spatula, water, NaCl, CuCl2, CuSO4 , paper clips, tongs, copper foil; table sugar, plastic substances available in the lab or in the surrounding, table salt and other salts available in the laboratory, sulphur, charcoal liquid substances like water, alcohol, oil,

Note for the teacher

Students may memorize the general properties of ionic and covalent compounds listed in their text books. However, they may generalize that all compounds soluble in water are ionic. But some compounds like table sugar are polar covalent.  Similarly they may think that all substances which conduct electricity in solution form are ionic, but polar covalent compounds such as acids can do so. We know that ionic compounds have high melting and boiling points, but there are other covalent substances like diamond silicon carbide which have extremely high melting points. Even regarding brittleness, if you hammer a crystal of table sugar, it breaks into pieces like ionic compounds. So, all this exceptions should not mislead students when identifying compounds. We understand from these facts that a single shot test only cannot be used to identify a compound, but series of tests.

These experiments are qualitative, as a result the outcomes of tests are relative and we usually say high or low. This can be clearly understood if we use other non ionic substances for comparison.
Therefore, in the next series of experiments students should predict the results of the experiments before they conduct and make observations.

Consider two experiments on hardness or brittleness and solubility considering that these activities can be completed in a period. The rest of the experiments on properties of ionic compounds melting point and conductivity can be conducted in the same way. If materials are limited to deliver for groups of students to conduct the activity, you can demonstrate it in such a way that all students can see. Students should write predictions about the results of the activity before they are conducted, observe all the activities, and write their observations and conclusions on their notebooks. The format for data collection can be written on the chalk board.

Starter activity (6minutes)

Keep a collection of different materials and compounds on a table. These can be different metal substances like paper clips, tongs, copper etc; table sugar, plastic substances available in the lab or in the surrounding, table salt and other salts available in the laboratory, sulphur, charcoal liquid substances like water, alcohol, oil, etc.

Tell students to come to the table one after the other to identify ionic substances from the rest of substances. Let them give reasons for their identification.

If the students were introduced to properties of ionic compounds, they will easily skip discard liquid substances, since they also know metals like copper, non-metals like sulphur, they will exclude them. They may comfortably identify table salt as an ionic compound because they are familiar to it.

Generally students may fail to make further identifications.

Evaluation

Students may be asked whether or not it is easier to identify ionic compounds by simple observation. What can we do to identify them?
Obviously students suggest that it is difficult to know whether a compound is ionic or not by observation and they may recommend laboratory activities.

Main activity (28minutes)

1. Hardness and brittleness of ionic compounds.

   Prediction

   1. Write your opinion about the following
      1. What do you feel if you press a grain of ionic compound between your fingers strongly? Is it strong or soft?
      2. What will happen if you hammer a grain of an ionic compound? Will it be shattered or flattened, or doesn’t make any change?
   2. Activity and observation
      1. Hold a grain of sodium chloride between your fingers and press it.
      2. Keep a crystal of sodium chloride on a mortar and press it with a pestle
      3. Repeat the same procedure for copper (II) chloride and copper sulphate

      Write down your observations. Are they the same with your predictions?

   3. What do you generalize from your observations?
   4. Analysis of the experimental findings

   Why did ionic compounds show those properties during the activities?

2. Solubility of ionic compounds
   1. Prediction
      1. If you add some solid ionic compound in water of a beaker and stir, will it dissolve?
      2. How about in solvents like ethanol, toluene or acetone?
   2. Activity and observation
      1. Add a spatula of sodium chloride into a beaker half filled with water and stir.

         Write your observation. Is it in agreement with your prediction?

      2. Repeat the procedure (a) using ethanol, toluene or acetone as a solvent.
      3. Add a spatula full of sodium chloride in a test tube half filled with ethanol. Stir and observe what happens.
      4. Repeat procedure (c) in ethanol and acetone or toluene as a solvent
      5. Now repeat procedures (a) to (d) using copper ( II) chloride and copper (II) sulphate and write your observations
      6. Complete the following table based on your observation by putting a √ mark for soluble and × mark for insoluble

      Ionic substance	Solubility in various solvents
      	Water	Ethanol	Acetone	Toluene	Hexane
      Sodium chloride
      Copper (II) chloride
      Copper (II)sulphate

   3. What do you generalize from your observations?
   4. Analysis of the experimental findings

      Why do ionic compounds show the properties you observed in the activities?

Based on their previous lessons and on results of the activities, students can write down the observations and generalizations. However, they may be unable to analyze the findings. You can assist them to relate their observations to their lessons on formation and nature bonding.
In this case they should come to the conclusion that

• Ionic compounds are hard because they are formed by strong electrostatic forces of attractions between oppositely charged ions
• They are brittle because when external force is applied upon them, it brings like charges close together. As a result, the crystal shatters  due to the repulsive force operating between  oppositely charged ions
• Ionic compounds dissolve in polar solvents like water based on the principle of like dissolves like. Since ionic compounds are made up of ions, they dissolve in solvents which are polar or having dipoles.
• Substances like alcohol and toluene or hexane have no dipole and hence they are called non-polar compounds; as a result, they cannot dissolve polar substances like ionic compounds.

Evaluation

Students can be asked what property of ionic compounds they investigated, what result they found and why these results are obtained.

Concluding activity (6 minutes)

Let the students summarize the lesson in the following table. Write the table on the chalk board and ask students to complete the table.