Water Softening

Water is sometimes called "the universal solvent" because it can dissolve so many things. As surface or underground waters pass over rocks and minerals in the soil some of the ions in the solids dissolve into the water. Water that contains magnesium (Mg²⁺) and calcium (Ca²⁺) ions (and also iron or manganese ions) is known as hard water. These ions combine with soap to create a "scum" that does not readily dissolve. This scum – the notorious "bath tub ring" – prevents soap from working well. The net result is the requirement to use more soap, and clothes or skin that do not wash as cleanly.

Soap is a molecule that contains a hydrophilic or water-loving end (usually an Na⁺ ion), and a oleophilic or oil-loving end (an organic fatty acid). The hydrophilic end attaches to water molecules, while the oleophilic end dissolves into fats and oils. Thus, the soap molecules make oils dissolve in water.

However, if the water contains a lot of ions like Mg²⁺ or Ca²⁺, then these ions can replace the Na⁺ ion in the soap. These molecules don't have nearly as strong an attraction to water, so form an insoluble scum.

If ion exchange resins are used to replace the hardness causing Mg²⁺ or Ca²⁺ ions, with Na⁺ ions, soap will do a better job of cleaning. This is an example of an equilibrium at work.

In an ion exchange resin, a polymer bead is used which has many sites to which ions can attach. In a freshly regenerated resin, these sites will be almost all occupied by Na⁺ ions, which do not have a very strong attraction to the resin.

Ion Exchange Animation
Click the green arrow to play the animation

Water flows through the ion exchange resin beads. Ca²⁺ (or Mg²⁺) ions have a stronger attraction for the resin than do the Na⁺ ions. As they come in contact with the resin, they will trade places with the Na⁺ ions (since the ions have a ²⁺ charge, one Ca²⁺ ion will displace two Na⁺ ions). This process could be represented by the following equation:
Na⁺{Resin} + 2 Ca²⁺

Ca²⁺{Resin} + 2 Na⁺
The equilibrium is normally far to the right side, because the stronger attraction of the Ca²⁺ ions causes a high forward rate.

Eventually, the resin will become depleted. This means that all the available ion exchange sites have been replaced by Ca²⁺ ions. The resin then needs to be regenerated. This is done by passing a concentrated brine (saturated NaCl solution) over the resin. The high concentration of Na⁺ ions forces the equilibrium back to the left hand side, regenerating the resin.

From le Chatelier's principle we can predict how the equilibrium will respond to the stress. Increasing the [Na⁺] ions will cause the equilibrium to respond by trying to decrease their concentration (shown in red). Then, everything on the same side of the equation will respond in the same way, while the other side does the opposite.

Applied Stress	Le Chatelier's Principle Prediction of Response to Stress
	Na⁺{Resin}	+ 2 Ca²⁺	Ca²⁺{Resin}	+ 2 Na⁺
Increase [Na^+]	Increase	Increase	Decrease	Decrease

The process of washing the resin with brine must be done every few days, depending on the concentration of ions in the hard water (harder water will deplete the resin more quickly). In homes, it is usually done at night, since there is little demand for water at this time.

Drinking softened water will not cause you to lose minerals (our food is a much more important source of minerals than is our water). However, because softened water contains the Na⁺ ions that have been displaced from the ion exchange resin, the water does have a different taste, and should not be drunk by persons with high blood pressure.

	In an ion exchange resin, a polymer bead is used which has many sites to which ions can attach. In a freshly regenerated resin, these sites will be almost all occupied by Na⁺ ions, which do not have a very strong attraction to the resin.
Click the green arrow to play the animation	Water flows through the ion exchange resin beads. Ca²⁺ (or Mg²⁺) ions have a stronger attraction for the resin than do the Na⁺ ions. As they come in contact with the resin, they will trade places with the Na⁺ ions (since the ions have a ²⁺ charge, one Ca²⁺ ion will displace two Na⁺ ions). This process could be represented by the following equation: Na⁺{Resin} + 2 Ca²⁺ Ca²⁺{Resin} + 2 Na⁺ The equilibrium is normally far to the right side, because the stronger attraction of the Ca²⁺ ions causes a high forward rate.
	Eventually, the resin will become depleted. This means that all the available ion exchange sites have been replaced by Ca²⁺ ions. The resin then needs to be regenerated. This is done by passing a concentrated brine (saturated NaCl solution) over the resin. The high concentration of Na⁺ ions forces the equilibrium back to the left hand side, regenerating the resin.