SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
PROPERTIES OF IONIC SUBSTANCES
Laboratory experiments reveal that, in general, ionic substances are characterized by the following properties:
1. In the solid phase at room temperature they do not conduct appreciable electric current.
2. In the liquid phase they are relatively good conductors of electric current. The conductivity of ionic substances is much smaller than that of metallic substances.
3. They have relatively high melting and boiling points. There is a wide variation in the properties of different ionic compounds. For example, potassium iodide (KI) melts at 686°C and boils at 1,330°C, while magnesium oxide (MgO) melts at 2,800°C and boils at 3,600°C. Both KI and MgO are ionic compounds.
4. They have relatively low volatilities and low vapor pressures. In other words, they do not vaporize readily at room temperature.
5. They are brittle and easily broken when stress is exerted on them.
6. Those that are soluble in water form electrolytic solutions that are good conductors of electricity. There is, however, a wide range in the solubilities of ionic compounds. For example, at 25°C, 92 grams of sodium nitrate (NaNO3) dissolves in 100 grams of water, while only 0.0002 grams of barium sulfate (BaSO4 ) dissolves in the same mass of water.