"DEMAND AND BALANCE": When discussing water chemistry, the terms "Demand" and "Balance" are readily used. To explain "demand," keep in mind that water is a
SOLVENT
(it is often referred to as the "Universal Solvent"). As a solvent, when water chemistry is out of balance, water will satisfy its own needs by dissolving any metallic material that it comes into contact with to satisfy its need for certain minerals. The water will dissolve these metallic materials until it can no longer store what it had previously dissolved (this is called the saturation point of water). After achieving this saturation point, water will then rid itself of the excess dissolved material (this is called the precipitation point of water). Instead of engaging in a prolonged Chemistry 101 lecture, just let it be understood that water has the distinct potential to dissolve, corrode, stain, scale, or calcify any metallic materials in your pool in which it comes into contact (these metallic materials include your pool walls, floor, hand rails, ladders, light fixtures, and the pool equipment-primarily the internals of the pump and heater). You must realize that water will make demands on these materials in your pool until those demands are met. This demand is met by adding chemicals in order to achieve "Water Chemistry" (which is also referred to as "balancing the water"). "Balance" is simply maintaining all chemicals within their desired ideal range.
As long as pool owners are taught about each chemical and its role in achieving and maintaining water chemistry, as well as how to test and adjust each of these chemicals, then meeting the "demands" of water, and creating water "balance" will be a very easy task. "Demand and Balance" is a confusing topic. Do not worry about the concept in a scientific manner. Rather, just understand that water is extremely volatile and must be kept under close supervision.