Grade 9 → Water and its importance ↓
Hardness of water (temporary and permanent hardness)
Water is one of the most important natural resources on our planet. It supports life, maintains ecosystems, and plays a vital role in a variety of chemical processes. However, not all water is the same. One way to classify water is by its hardness. Hardness in water refers to the concentration of dissolved minerals, primarily calcium and magnesium ions. Understanding water hardness is important because it affects domestic, industrial, and chemical applications.
What is water hardness?
Water hardness is a measure of the concentration of specific minerals in water. These minerals are usually calcium (Ca^2+) and magnesium (Mg^2+) ions. Water that has high levels of these ions is called 'hard water', while water with low levels is called 'soft water'.
Chemical representation
Ca^2+ + 2HCO3^- → CaCO3 + H2O + CO2 Mg^2+ + 2HCO3^- → MgCO3 + H2O + CO2
The chemical formulas above show how calcium and magnesium ions react with bicarbonates in water, contributing to its hardness.
Why is water hardness important?
Water hardness affects daily life in several ways:
- Household uses: Hard water can form mineral deposits in plumbing, reducing water flow and causing blockages. It also makes soaps and detergents less effective, leading to soap scum on dishes and clothing.
- Health: Hard water is generally safe to drink and can contribute essential minerals to the diet. However, excessive hardness can sometimes be problematic.
- Industrial uses: In industries, hard water can cause breakdowns in boilers, cooling towers and other equipment, reducing efficiency and increasing maintenance costs.
Types of water hardness
Water hardness is usually classified into two types: temporary and permanent hardness, based on how the hardness can be removed.
Temporary stiffness
Temporary hardness is due to the presence of dissolved bicarbonate minerals, mainly calcium bicarbonate (Ca(HCO3)2) and magnesium bicarbonate (Mg(HCO3)2).
The defining characteristic of temporary hardness is that it can be removed by boiling the water. When water is boiled, the bicarbonates dissociate into carbonates, which precipitate out of the water because they are not very soluble.
Chemical equation:Ca(HCO3)2 (aq) → CaCO3 (s) + H2O (l) + CO2 (g) Mg(HCO3)2 (aq) → MgCO3 (s) + H2O (l) + CO2 (g)
Visual example of temporary stiffness removal
Boiling water results in the precipitation of calcium carbonate or magnesium carbonate as a solid precipitate, which can be separated by filtration.
Permanent hardness
Permanent hardness is due to the presence of chlorides, sulphates or nitrates of calcium and magnesium, which do not precipitate on boiling.
Chemical salts:CaCl2, MgCl2, CaSO4, MgSO4
Persistent hardness requires a different approach to remove, such as using chemical methods or water softening agents.
Visual summary of permanent hardness
Ways to remove stiffness
Methods for removing water hardness vary depending on whether the hardness is temporary or permanent.
Temporary hardness removal
As mentioned, temporary hardness can be removed by boiling the water, which causes the bicarbonates to precipitate. Once the precipitate has formed, the precipitate can be removed by filtration.
- Boiling: Heating water until the minerals settle to the bottom as solids.
- Decanting: After boiling, allowing water to sit to separate solids from clear water.
Permanent hardness removal
Permanent hardness requires chemical treatments or mechanical processes to remove, such as:
- Ion exchange resins: Water is passed through an ion exchange column, where calcium and magnesium ions are exchanged with sodium ions.
- Calgon or sodium hexametaphosphate: These chemicals are added to precipitate the calcium as a more soluble compound.
2Na^+ + Ca^2+ (from resin) → 2Ca^2+ + 2Na^+
Conclusion
Understanding water hardness is important as it affects many aspects of life, from household chores to industrial processes. Although it poses some challenges in use, knowing the difference between temporary and permanent hardness gives us ways to manage it effectively. Resolving water hardness issues can prevent damage to appliances, save energy, and save on detergent use. Ultimately, knowing about water hardness helps make better decisions about water treatment and use.
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