1. The stuff of rocks: crystals of ionic compound

© 2020 Google

A granite rock sits atop Bukit Timah Hill, marking the highest point of Singapore. It also marks the history of the surrounding area, which used to be home to several granite quarries.

Like the history of the area, granite is a colourful rock decorated by a mixture of pink and white crystals. The pink crystals are called K-feldspar, which is an ionic compound comprising potassium and aluminium cations, as well as, polyatomic silicate anions.

2. Made in the furnace of the Earth: high melting and boiling point

The large pink crystals of K-feldspar we find in granite are formed deep underground, in a hot magma chamber. In other words, K-feldspar can form at and put up with extremely high temperature of thousand odd degrees Celsius.

This physical property of high melting point and boiling point is a hallmark of ionic compound.

3. Holding giant ionic lattice structure together: strong electrostatic forces of attraction

To explain the high melting point of K-feldspar and other ionic compounds, we have to think small but deep: what is holding the crystal together?

This diagram shows a small section of a giant ionic lattice. The white and black spheres represent cations and anions. As they are oppositely charged, they are held in the lattice by strong ionic bond, represented by the lines joining the spheres.

Ionic compounds have giant ionic lattice structure. A long phrase that is hard to swallow? Let me break it down for you.

  • Giant: a big structure formed by countless ions
  • Ionic: ionic bonds hold the oppositely charged ions in fixed positions
  • Lattice: a regular, repeated 3D arrangement of ions

To melt the solid ionic compound is to overcome the ionic bond holding the crystal together. Since the ionic bond is so very strong, temperature must be high af to supply sufficient energy to overcome the strong electrostatic forces of attraction between oppositely charged ions.

4. From pretty rocks to tasty rocks for your cooking wok

Rock salt, which Earth scientists call halite

Ever bought Himalayan salt from NTUC? Geologists call it by a cool name, halite. It is a naturally occurring mineral found in rocks that are mined in the Himalayas. Chemically speaking, halite is sodium chloride, a crystalline ionic compound.

The presence of halite tells us something very important about the past. A long, long time ago, seawater evaporated, leaving behind crystals of sodium chloride as halite.

When sodium chloride dissolves in water, the crystal lattice breaks down into free-moving, aqueous ions (© 2020 CrashCourse)

Now, when we add Himalayan salt to our soup, we are doing the opposite: re-dissolving sodium chloride in water.

Most ionic compounds are soluble in water, but insoluble in organic solvent.

5. Giant ionic lattice structures conduct electricity only in the liquid or aqueous states

Current electricity requires the flow of charges carried by free-moving ions or electrons.

While there are ions in giant ionic lattice structures, in the solid state, strong ionic bond holds the ions in fixed positions. Thus, solid ionic compounds do not have free-moving ions to conduct electricity.

However, molten ionic compounds in the liquid state do conduct electricity. When we heat and melt ionic compound, we overcome the strong electrostatic forces of attraction between oppositely charged ions. Therefore, the ions become free moving to conduct electricity.

All thanks to the dissolved ionic compound in potatoes, they can be wired up to conduct electricity and power the light bulb (© Shutterstock)

Likewise, when we dissolve ionic compounds in water, the ions separate and become free-moving as the mix with water. Therefore, in the aqueous state, the dissolved ions are free-moving to conduct electricity.

Ionic compounds can conduct electricity in the molten or aqueous state, but not the solid state.