Year 9 Science · Unit 2 · Lesson 8
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Learning Goals
1. Describe how the ions are arranged in a sodium chloride lattice. How many chloride ions surround each sodium ion?
2. Draw two rows of ions in a salt crystal, using + and −. Then draw the same rows after a hammer blow has pushed the top row along by one ion. Label where like charges now line up.
3. Use your drawing to explain why the crystal shatters instead of bending.
A short investigation
Question: does the mass of salt dissolved in water affect the current that flows through it? A class dissolved different masses of salt in 100 mL of water, dipped in two carbon electrodes 3 cm apart, connected a 6 V supply and read the current.
Variables
Results
| Salt dissolved (g) | Trial 1 (mA) | Trial 2 (mA) | Average (mA) |
|---|---|---|---|
| 0.0 | 0 | 0 | 0 |
| 0.5 | 8 | 10 | 9 |
| 1.0 | 16 | 18 | |
| 2.0 | 31 | 33 | 32 |
| 4.0 | 57 | 59 |
Source: sample class results. Each average is rounded to the nearest milliamp.
4. Complete the two missing averages. Then plot the average current against the mass of salt, and draw a smooth curve of best fit.
5. Describe the trend and use your graph to predict the current for 3.0 g of salt. Explain the trend in terms of ions.
6. A shop sells blocks of rock salt (sodium chloride) for building a feature wall inside a steamy sauna, which heats up to about 90 °C. The advertisement says: “Salt is as tough as brick and will last forever.” Evaluate this claim using at least three properties of ionic compounds.