Intermediate Topic 614 of 1102 Acids, Bases & Electrochemistry
Water Hardness
Dissolved calcium and magnesium ions that form scum with soap and scale in kettles.
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In-depth explanation of Water Hardness
Water hardness is caused by dissolved calcium and magnesium ions. Temporary hardness comes from their bicarbonates and can be removed by boiling, which converts the bicarbonate to insoluble carbonate that precipitates. Permanent hardness comes from sulfates and chlorides and must be removed by softening with washing soda or by ion exchange. Hard water reduces the lathering of soap because the calcium and magnesium ions form insoluble soap scum, and it causes scale in kettles and boilers. Water softening is an important industrial and household treatment.
Going deeper: Water Hardness
Key ideas: Hardness comes from dissolved Ca2+ and Mg2+, expressed as mg/L CaCO3 equivalent: soft under 60, moderate 60 to 120, hard 120 to 180, very hard above 180. Temporary hardness (bicarbonates) is removed by boiling: Ca(HCO3)2 gives CaCO3 + CO2 + H2O; permanent hardness (sulfates, chlorides) needs washing soda, ion exchange, or lime-soda treatment. EDTA titration measures it with Eriochrome Black T.
Common mistakes and connections: Mistakes: stating that boiling removes all hardness, and computing hardness in the wrong units: 1 mg/L Ca2+ is 2.50 mg/L as CaCO3. Hard water forms scale, wastes soap (Ca stearate precipitate) but is not harmful for health. Ion exchange resins swap Ca2+ for Na+. Related to Ksp and complex formation.
Visualization of Water Hardness
Hardness measures dissolved Ca2+ and Mg2+ as mg/L of CaCO3; water above about 120 mg/L is hard and leaves scale and curdles soap. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Limescale in kettles is mostly calcium carbonate, which forms when calcium bicarbonate is heated.
Frequently asked questions about Water Hardness
What is Water Hardness?
Dissolved calcium and magnesium ions that form scum with soap and scale in kettles.
Is Water Hardness a beginner, intermediate or advanced topic?
Water Hardness is a intermediate topic in Acids, Bases & Electrochemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Water Hardness?
Study these earlier topics in Acids, Bases & Electrochemistry first: Hard and Soft Acids and Bases, Acid Anhydrides, Basic Anhydrides.
What comes after Water Hardness?
The next topics in Acids, Bases & Electrochemistry are Alkalinity.
Can I practise Water Hardness with quizzes?
Yes. The interactive lesson includes a self-check question on Water Hardness, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Where Water Hardness fits in Acids, Bases & Electrochemistry
Water Hardness is topic 104 of 105 in Acids, Bases & Electrochemistry.