Year 9 Science · Unit 2 · Lesson 8
Challenge Worksheet
Learning Goals
1. State two properties shared by almost all ionic compounds, and link each one to the ionic lattice.
2. In a crystal of calcium fluoride (fluorite), each Ca²⁺ ion touches 8 F⁻ ions, but each F⁻ ion touches only 4 Ca²⁺ ions. Explain how these two numbers fit the formula CaF₂ and the charges on the ions.
Scenario: a flawed test
A student put some blue copper(II) sulfate crystals in a dish that had just been rinsed with tap water. She pressed two electrodes onto the crystals, and a bulb in the circuit glowed dimly. She concluded: “Solid copper sulfate conducts electricity, so ionic solids can conduct.”
3. Identify the flaw in her method and explain how it produced the result. Then describe how you would test the solid fairly.
Read the data
The lesson says that MgO dissolves much less than NaCl because the doubly charged ions form a stronger lattice. Table 1 lets you test that idea.
| Compound | Ions | Grams that dissolve in 100 mL of water |
|---|---|---|
| Calcium chloride | Ca²⁺, Cl⁻ | 75 |
| Sodium fluoride | Na⁺, F⁻ | 4.0 |
| Magnesium oxide | Mg²⁺, O²⁻ | 0.009 |
| Sodium chloride | Na⁺, Cl⁻ | 36 |
| Calcium fluoride | Ca²⁺, F⁻ | 0.0016 |
| Magnesium chloride | Mg²⁺, Cl⁻ | 54 |
Source: data book solubilities at 20 °C (magnesium oxide at 30 °C), rounded to two significant figures.
4. Which compounds would fully dissolve if you stirred 10 g of each into 100 mL of water? Name one that would barely dissolve at all.
5. Does Table 1 support the rule “compounds with a 2+ ion dissolve less than compounds with only 1+ and 1− ions”? Give one piece of evidence for the rule and two against it, and suggest what else might affect solubility.
6. Sodium chloride has a high melting point, is brittle, does not conduct as a solid, and conducts when molten. Explain each property using its structure. Then predict and justify how magnesium oxide would compare on each property.