Temporary hardness in water is primarily caused by the presence of carbonate and bicarbonate salts of calcium and magnesium.
This type of water hardness is characterized by its ability to be removed by simple boiling, a process that precipitates the dissolved mineral salts. These salts originate when water containing dissolved carbon dioxide percolates through rocks such as limestone (calcium carbonate) and dolomite (magnesium carbonate).
Understanding Temporary Hardness
Temporary hardness arises from the dissolution of specific mineral compounds into water. When rainwater, which is slightly acidic due to dissolved carbon dioxide, comes into contact with calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃) found in rocks, it converts them into more soluble bicarbonate forms: calcium bicarbonate [Ca(HCO₃)₂] and magnesium bicarbonate [Mg(HCO₃)₂]. These bicarbonate salts are the main culprits behind temporary hardness.
Key Salts Causing Temporary Hardness
The primary salts responsible for temporary hardness are:
| Mineral Ion | Associated Cation | Chemical Formula | Type of Salt |
|---|---|---|---|
| Bicarbonate | Calcium (Ca²⁺) | Ca(HCO₃)₂ | Calcium Bicarbonate |
| Bicarbonate | Magnesium (Mg²⁺) | Mg(HCO₃)₂ | Magnesium Bicarbonate |
| Carbonate | Calcium (Ca²⁺) | CaCO₃ | Calcium Carbonate |
| Carbonate | Magnesium (Mg²⁺) | MgCO₃ | Magnesium Carbonate |
Note: While temporary hardness is mainly due to the bicarbonate forms dissolved in water, the underlying carbonate minerals (limestone, dolomite) are the source, and these convert to bicarbonates or precipitate as carbonates upon heating.
How Temporary Hardness Forms
Water acquires these salts as it flows through geological formations:
- Carbon Dioxide Dissolution: Rainwater absorbs CO₂ from the atmosphere, forming carbonic acid (H₂CO₃), a weak acid.
- Rock Interaction: This slightly acidic water then permeates through rocks rich in calcium carbonate (limestone) and magnesium carbonate (dolomite).
- Bicarbonate Formation: The carbonic acid reacts with these carbonates, converting them into soluble bicarbonates:
- CaCO₃(s) + H₂CO₃(aq) → Ca(HCO₃)₂(aq)
- MgCO₃(s) + H₂CO₃(aq) → Mg(HCO₃)₂(aq)
These dissolved bicarbonates are what make the water "temporarily hard."
Differentiating Temporary and Permanent Hardness
It's important to distinguish temporary hardness from permanent hardness. While temporary hardness is caused by calcium and magnesium bicarbonates (and carbonates), permanent hardness results from the presence of chloride and sulphate salts of calcium and magnesium, such as calcium chloride (CaCl₂), magnesium chloride (MgCl₂), calcium sulphate (CaSO₄), and magnesium sulphate (MgSO₄). Unlike temporary hardness, permanent hardness cannot be removed by simple boiling.
Impacts of Hard Water
Temporary hard water can lead to several common issues in homes and industries:
- Limescale Buildup: When heated, the bicarbonates decompose, forming insoluble calcium carbonate (limescale) and magnesium carbonate, which precipitate out. This scale can clog pipes, reduce the efficiency of water heaters, and build up on fixtures and appliances.
- Reduced Soap Lathering: The calcium and magnesium ions react with soap, forming an insoluble "scum" instead of a rich lather, which means more soap and detergents are needed.
- Appliance Damage: Limescale accumulation can shorten the lifespan of water-using appliances like washing machines, dishwashers, and kettles.
Solutions for Removing Temporary Hardness
The defining characteristic of temporary hardness is that it can be eliminated through relatively simple methods:
- Boiling: This is the most straightforward and common method. When water containing bicarbonates is boiled, the bicarbonates decompose, releasing carbon dioxide gas and forming insoluble calcium carbonate precipitate (limescale). This precipitate can then be removed, leaving softer water.
- Ca(HCO₃)₂(aq) Heat → CaCO₃(s) + H₂O(l) + CO₂(g)
- Lime Softening (Clark's Process): Adding a calculated amount of calcium hydroxide (slaked lime) to hard water precipitates the calcium and magnesium carbonates, effectively removing the hardness.
- Ca(HCO₃)₂(aq) + Ca(OH)₂(aq) → 2CaCO₃(s) + 2H₂O(l)
- Ion-Exchange Resins: Water softeners using ion-exchange resins can also remove the calcium and magnesium ions responsible for temporary hardness by exchanging them for sodium ions.
Understanding the specific salts that cause temporary hardness helps in choosing appropriate treatment methods for household and industrial water systems. For more detailed information on water hardness, you can consult resources like the U.S. Environmental Protection Agency (EPA).
