Water Hardness Quiz

Study Notes

Water hardness is a measure of the dissolved calcium and magnesium salts in water.
It is typically expressed in milligrams per liter (mg/l) or parts per million (ppm).
Water hardness can be categorized into temporary and permanent hardness depending on the type of salts and their solubility.

Temporary hardness is caused by the presence of bicarbonate salts of calcium and magnesium (Ca(HCO₃)₂ and Mg(HCO₃)₂).
It can be removed by boiling the water which converts the bicarbonate ions to carbonate ions, which precipitate out as insoluble calcium and magnesium carbonates (CaCO₃ and MgCO₃).
This type of hardness can also be removed by adding lime (Ca(OH)₂) to the water.

Permanent hardness is caused by the presence of chloride, sulfate, and nitrate salts of calcium and magnesium (CaCl₂, CaSO₄, Ca(NO₃)₂, MgCl₂, MgSO₄, Mg(NO₃)₂).
It cannot be removed by boiling the water.
It can be removed by using water softeners which exchange calcium and magnesium ions with sodium ions.

Problem 1 demonstrates the calculation of temporary, permanent, and total hardness based on knowing the concentrations of dissolved ions.
- The amount of each salt dissolved in water is converted to an equivalent amount of CaCO₃ using their molecular weights.
- Temporary hardness is calculated based on the amount of bicarbonate salts.
- Permanent hardness is calculated based on the amounts of chloride, sulfate, and nitrate salts.
- Total hardness is the sum of temporary and permanent hardness
Problem 2 demonstrates the calculation of temporary and permanent hardness using a titration method with EDTA (Ethylenediaminetetraaceticacid). EDTA is a chelating agent that binds to calcium and magnesium ions, allowing for their quantification using titration.
- A standard hard water solution with a known concentration of CaCO₃ (0.28 g/L) was used to standardize the EDTA solution.
- The total hardness was determined by titrating a 100 ml water sample with EDTA and using the equivalent amount of CaCO₃ calculated through the standardized EDTA solution.
- The permanent hardness was determined by titrating a 100 ml water sample after boiling and filtering. Boiling the water removes temporary hardness due to the precipitation of calcium and magnesium carbonates.
- The difference between the total hardness and permanent hardness gives the temporary hardness.