solubility

Ionic compounds are solids at room temperature, mainly because the ionic (electrostatic) bond is so strong that the cations and anions are bound in a dense, patterned crystal lattice.



In terms of physical properties, most ionic compounds will have high melting points, meaning it takes lots of energy to disrupt this lattice.

However, some ionic compounds' crystals can be "disassembled" by water, meaning they will dissolve in water. This ability to dissolve in water is called solubility.



Here is another animation of salt being 'dissociated' (dissolved) by water.



A **soluble compound** will readily and completely dissolve in water.

An **insoluble compound** will only partially dissolve in water.

To determine whether an ionic compound is soluble or insoluble in water, use the chart at the bottom of this table.

Try some problems:

When an ionic compound is considered "insoluble", it doesn't necessarily mean the compound is completely untouched by water. All ionic compounds dissolve to some extent. An insoluble compound just doesn't dissolve in any noticeable or appreciable amount.

There are other factors that affect the solubility of ionic compounds:
 * temperature - warmer temperatures will increase a compound's solubility
 * surface area - the act of dissolving is aided by having more surface area exposed to water
 * agitation - mixing or swirling an ionic compound in water will increase the rate of dissolving
 * saturation - if there's already some dissolved ions in the water, the solubility of a solid will decrease
 * pH - some solids will dissolve more rapidly in acidic solutions, albeit by a chemical reaction, not necessarily by dissolution

The amount of dissolved ions in a solution can be measured in many ways, but one of the most common is to test for **conductivity**.

Water that contains aqueous (dissolved) ions is capable of conducting an electric current. The flow of electricity is aided by the presence of cations and anions in the water. This type of solution is referred to as an **electrolyte**.

Obviously, the more concentrated the dissolved ions, the greater the conductivity of the solution.

In environmental science, water quality is quickly assessed by testing its conductivity using a device that measures the solution's ability to conduct electricity. This test is referred to as a "TDS" (total dissolved solids) test. The results of such a test are typically reported in mg/L (milligrams per liter). No details about what kinds of ions are in the water can be made using this technique, though.

More specific tests can be carried out using ion-selective electrodes. These are designed to "look for" certain ions such as chloride, bromide, nitrate, and phosphate, and measure their concentrations.