Hard water is rich in dissolved minerals, which it collects as it passes through soil and rock formations. The question of whether boiling water makes it softer has a nuanced answer that depends entirely on these specific dissolved minerals. Boiling water can soften it, but this effect is limited to only one specific form of water hardness.
Defining Water Hardness
Water hardness measures the concentration of multivalent metal cations, primarily calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)) ions, dissolved in the water. These dissolved metal ions are responsible for common issues like the formation of soap scum and scale buildup. This mineral content is categorized into two distinct types based on the chemical compounds involved and their reaction to heat.
Temporary Hardness is caused by calcium bicarbonate (\(\text{Ca}(\text{HCO}_3)_2\)) and magnesium bicarbonate (\(\text{Mg}(\text{HCO}_3)_2\)). The presence of these bicarbonate ions allows this type of hardness to be removed relatively easily. Permanent Hardness, conversely, is caused by calcium and magnesium salts of sulfate and chloride, such as calcium sulfate (\(\text{CaSO}_4\)) and magnesium chloride (\(\text{MgCl}_2\)). Only temporary hardness responds to the application of heat.
How Boiling Reduces Temporary Hardness
Boiling is effective for removing temporary hardness because heat triggers a chemical decomposition reaction involving bicarbonate ions. When the water temperature rises, soluble calcium and magnesium bicarbonates break down into more stable compounds. This reaction softens the water, transforming the dissolved ions into an insoluble solid.
Soluble calcium bicarbonate decomposes into insoluble calcium carbonate (\(\text{CaCO}_3\)), carbon dioxide gas (\(\text{CO}_2\)), and water (\(\text{H}_2\text{O}\)). The chemical process is \(\text{Ca}(\text{HCO}_3)_2 \rightarrow \text{CaCO}_3 (\downarrow) + \text{CO}_2 (\uparrow) + \text{H}_2\text{O}\). Similarly, magnesium bicarbonate breaks down to form insoluble magnesium hydroxide (\(\text{Mg}(\text{OH})_2\)).
The newly formed calcium carbonate and magnesium hydroxide are solids that are no longer dissolved. These solids precipitate out of the solution, forming the white, chalky residue known as limescale, which collects on the bottom and sides of kettles and pots. By removing these mineral compounds, the concentration of \(\text{Ca}^{2+}\) and \(\text{Mg}^{2+}\) ions is reduced, resulting in a measurable softening effect.
Why Boiling Fails to Address Permanent Hardness
Boiling does not affect permanent hardness because the chemical nature of these compounds differs fundamentally from bicarbonates. Permanent hardness is caused by salts like calcium sulfate and magnesium chloride, which are highly stable even when heated to the boiling point. These stable salts remain dissolved, meaning they do not undergo the decomposition and precipitation reaction that bicarbonates do.
Because these sulfate and chloride salts do not break down, the hardness-causing \(\text{Ca}^{2+}\) and \(\text{Mg}^{2+}\) ions stay in the solution. Consequently, boiling only addresses the temporary component of water hardness, leaving the permanent hardness untouched. Boiling water is therefore not a comprehensive solution for total water softening, and water with high permanent hardness requires alternative treatment methods like ion exchange or chemical additives.