Hard water is a common term describing water that possesses a high concentration of dissolved minerals, specifically certain metal cations. These minerals are naturally picked up by water as it travels through the environment, fundamentally changing its chemical makeup. Understanding how to control and manipulate this mineral content is useful for various applications, including specialized laboratory experiments, home brewing, and maintaining aquatic environments like aquariums. Since naturally soft water may not meet the requirements for these uses, the ability to intentionally increase water hardness becomes a practical necessity.
Defining the Mineral Components
The property of water hardness is caused by the presence of divalent metal ions, primarily calcium and magnesium. These ions originate from various mineral salts that dissolve in water. Hardness is typically classified into two main types based on the associated anion, which determines how the hardness can be treated.
Temporary hardness is caused by calcium and magnesium bicarbonates. It is called “temporary” because it can be largely removed simply by boiling the water, which causes the bicarbonates to decompose and precipitate out as limescale. Permanent hardness is attributed to calcium and magnesium salts of sulfates or chlorides. These compounds do not precipitate when the water is boiled, meaning the mineral content remains stable even after heating. The total hardness of a water sample is the sum of both the temporary and permanent components.
Natural Acquisition of Water Hardness
The process of water becoming hard begins with precipitation, as rainwater is naturally soft. As this water falls through the atmosphere, it absorbs carbon dioxide, which creates a dilute solution of carbonic acid. This slightly acidic water interacts with the earth’s geology.
When the rainwater percolates through soil and rock layers, it encounters mineral-rich formations. Geological deposits such as limestone, chalk, and dolomite are composed of calcium carbonate and magnesium carbonate. The carbonic acid dissolves these compounds, freeing the calcium and magnesium ions into the water supply.
Groundwater tends to be significantly harder than surface water found in rivers and lakes. This is because groundwater moves slowly, allowing for extended contact time with mineral-rich rocks. Surface water flows more quickly, reducing the opportunity for extensive mineral dissolution.
Experimental Methods for Increasing Hardness
For specific applications requiring controlled water hardness, such as setting up a freshwater aquarium or conducting chemical testing, it is often necessary to manually increase the mineral content. This controlled process typically begins with distilled or reverse osmosis (RO) water, which has virtually no mineral content, and involves the precise addition of soluble salts.
To increase General Hardness (GH), which includes both calcium and magnesium ions, effective choices include Calcium Chloride (\(CaCl_2\)) and Magnesium Sulfate (\(MgSO_4\)), commonly known as Epsom salt. These salts dissolve easily, introducing the desired cations. A common practice is to create a stock solution by dissolving a measured amount of these salts in a small volume of water, then adding that solution incrementally.
If the aim is to increase temporary hardness (Carbonate Hardness or KH), which helps stabilize the water’s pH, sodium bicarbonate or potassium bicarbonate can be used. However, for a balanced approach that mimics natural water, the combination of calcium chloride and magnesium sulfate is generally preferred. Since the exact amount of salt needed depends on the starting volume and the target hardness level, additions should be based on careful measurement and followed by testing with a reliable hardness test kit. Use small, incremental doses to avoid overshoot, as removing excess hardness is more complex than adding it.