Properties of Ionic Compounds
Ximena is setting up a homemade battery to light a small bulb. With tap water the bulb glows, but with distilled water it does not turn on. What does tap water have that distilled water lacks? The answer lies in what ionic compounds are like on the inside, and you will find it at the end of this page.
The Lattice of Ions
Table salt (NaCl) is an ionic compound: a metal (sodium) gave up an electron to a nonmetal (chlorine). The result is ions with opposite charges: $\text{Na}^+$ and $\text{Cl}^-$.
A salt is not made of "NaCl molecules." The ions arrange themselves in a crystal lattice: each positive ion is surrounded by negative ions, and each negative ion by positive ones. Opposite charges attract each other very strongly in all directions.
Ionic or Not? Kitchen Substances
To tell whether a substance is ionic, look at which elements it is made of on the periodic table: if it has a metal + a nonmetal, it is ionic. If it has only nonmetals, it is molecular.
| Substance | Formula | Elements | Type |
|---|---|---|---|
| Table salt | NaCl | Na (metal) + Cl (nonmetal) | ionic |
| Lime | CaO | Ca (metal) + O (nonmetal) | ionic |
| Baking soda | $\text{NaHCO}_3$ | Na (metal) + H, C, O (nonmetals) | ionic |
| Sugar | $\text{C}_{12}\text{H}_{22}\text{O}_{11}$ | only C, H, O | molecular |
| Water | $\text{H}_2\text{O}$ | only H and O | molecular |
Baking soda has a metal (sodium) and a group of nonmetals that form an ion with a negative charge; that is why it is also a lattice of ions.
High Melting Points and Hardness
To melt a solid, you have to supply enough energy for its particles to move and break free from their fixed positions. In an ionic lattice, each ion is held by many strong attractions at once. A lot of energy is needed, that is, a high temperature.
| Substance | Type | Approximate melting point |
|---|---|---|
| Ice | molecular | 0 °C |
| Sugar | molecular | 186 °C |
| Sodium chloride (table salt) | ionic | 801 °C |
| Calcium oxide (lime) | ionic | 2,570 °C |
For the same reason, ionic crystals are hard: they are difficult to scratch because you have to pull apart ions that are strongly bonded.
But they have a weakness: they are brittle. A blow shifts a layer of ions by half a unit, and then ions of the same sign end up facing each other. They repel, and the crystal splits cleanly apart.
Ionic compounds have high melting points, are hard but brittle, and conduct electric current when molten or dissolved in water, but not in the solid state. Choose two of…
Ionic compounds have high melting points, are hard but brittle, and conduct electric current when molten or dissolved in water, but not in the solid state. Choose two of these properties and explain them based on their structure: ions arranged in a lattice with strong attractions.
Conducting Electricity
Electric current in an ionic compound is carried by the ions, as long as they are able to move.
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Solid: the ions are fixed in the lattice. They vibrate, but they do not move from place to place. It does not conduct.
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Molten: when heated, the lattice breaks down and the ions flow freely. It does conduct.
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Dissolved in water: the water pulls the ions out of the lattice, and they end up surrounded by water molecules, free to move. It does conduct.
From Tap Water to Distilled Water
Pure water is made of uncharged $\text{H}_2\text{O}$ molecules. It has almost no ions, so it can hardly carry current. Distilled water is very pure and conducts very little.
Tap water, well water, or unpurified jug water has mineral salts dissolved in it (ionic compounds such as sodium chloride, calcium sulfate, or magnesium bicarbonate). These salts separate into ions that move and carry the charge.
Back to the beginning: Ximena's bulb lights up with tap water because the dissolved salts provide mobile ions that complete the circuit. With distilled water there are almost no ions, and the bulb does not turn on.
A homemade battery built with tap water lights up a small bulb, but with pure distilled water it does not. Using what you know about dissolved ionic compounds and the…
A homemade battery built with tap water lights up a small bulb, but with pure distilled water it does not. Using what you know about dissolved ionic compounds and the movement of charges, explain why ordinary water (with dissolved salts) conducts electricity and distilled water barely does.