Monday, January 22, 2018

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Chemistry - Ions in Liquid Solutions

Many liquid substances undergo a process whereby their constituent molecules split into positively and negatively charged ion pairs, the positively-charge ion called a cation and the negatively-charged ion called an anion1. Liquid ionic compounds split into ions completely or nearly completely, while only a small percentage of the molecules in a liquid covalent compound split into ions. The process of neutral molecules separating into ion pairs is called dissociation when it happens to ionic compounds, and ionization when it happens to covalent compounds.

Molten salt (NaCl) is an example of the former, while pure water (H2O) is an example of the latter. The large presence of ions in molten salt explains why it is a good conductor of electricity, while the comparative lack of ions in pure water explains why it is often considered an insulator. In fact, the electrical conductivity of a liquid substance is the definitive test of whether it is an ionic or a covalent (“molecular”) substance.

Pure water ionizes into positive hydrogen ions2 (H+) and negative hydroxyl ions (OH). At room temperature, the concentration of hydrogen and hydroxyl ions in a sample of pure water is quite small: a molarity of 10-7 M (moles per liter) each.

Given the fact that pure water has a mass of 1 kilogram (1000 grams) per liter, and one mole of pure water has a mass of 18 grams, we must conclude that there are approximately 55.56 moles of water molecules in one liter (55.56 M). If only 107 moles of those molecules ionize at room temperature, that represents an extremely small percentage of the total:

 
 
It is not difficult to see why pure water is such a poor conductor of electricity. With so few ions available to act as charge carriers, pure water is practically an insulator. The vast majority of water molecules remain un-ionized and therefore cannot transport electric charges from one point to another.

The molarity of both hydrogen and hydroxyl ions in a pure water sample increases with increasing temperature. For example, at 60o C, the molarity of hydrogen and hydroxyl ions increases to 3.1 × 10-7 M, which is still only 0.0056 parts per million, but definitely larger than the concentration at room temperature (25 oC).

The electrical conductivity of water may be greatly enhanced by dissolving an ionic compound in it, such as salt. When dissolved, the salt molecules (NaCl) immediately dissociate into sodium cations (Na+) and chlorine anions (Cl), becoming available as charge carriers for an electric current. In industry, we may exploit this relationship between electrical conductivity and ionic dissociation to detect the presence of ionic compounds in otherwise pure water.

 

1These names have their origin in the terms used to classify positive and negative electrodes immersed in a liquid solution. The positive electrode is called the “anode” while the negative electrode is called the “cathode.” An anion is an ion attracted to the anode. A cation is an ion attracted to the cathode. Since opposite electrical charges tend to attract, this means “anions” are negatively charged and “cations” are positively charged.

2Actually, the more common form of positive ion in water is hydronium: H3O+, but we often simply refer to the positive half of an ionized water molecule as hydrogen (H+).

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