Difference Between Temporary and Permanent Hardness of Water
The hardness of water is a central concept in JEE Main Chemistry, defined as the measure of dissolved calcium (Ca2+) and magnesium (Mg2+) ions in water. This property is important in both theory and practicals, since it affects industrial processes, household tasks, and water quality standards. Hard water forms less lather with soap and leads to scaling in boilers, pipes, and appliances, making its understanding vital for engineering chemistry applications and numerical problems.
Hardness of water in chemistry is not just about household inconvenience but a chemical phenomenon governed by the presence of certain metal ions. In JEE-style questions, you may be asked to define, classify, calculate, or suggest methods for removal of hardness using chemical reactions or titration principles. Mastery of this topic combines conceptual clarity, formula exactness, and real-life application in engineering contexts.
Types of Hardness of Water
For JEE, it is crucial to distinguish between temporary (carbonate) and permanent (non-carbonate) hardness, as both have distinct causes, removal methods, and impacts. The sum of these is called total hardness.
| Type | Caused By | Removal Method | Example Reaction |
|---|---|---|---|
| Temporary Hardness | Ca(HCO3)2, Mg(HCO3)2 | Boiling, Lime addition | Ca(HCO3)2 → CaCO3↓ + CO2 + H2O |
| Permanent Hardness | CaSO4, MgSO4, CaCl2, MgCl2 | Washing soda, Ion-exchange | CaSO4 + Na2CO3 → CaCO3↓ + Na2SO4 |
| Total Hardness | Sum of above ions | Varies | — |
Remember: temporary hardness is due to bicarbonates and is removed on boiling, while permanent hardness comes from sulfates/chlorides and persists after boiling. This distinction is frequently tested in JEE conceptual and multiple-choice questions.
Chemical Causes and Reactions
The primary chemicals responsible for hardness of water are soluble salts of calcium and magnesium. Temporary hardness arises from Ca(HCO3)2 and Mg(HCO3)2, which decompose upon heating:
- Ca(HCO3)2 + Heat → CaCO3↓ + CO2 + H2O
- Mg(HCO3)2 + Heat → Mg(OH)2↓ + 2CO2
Permanent hardness results from CaSO4, MgSO4, CaCl2, and MgCl2. It cannot be removed by boiling, but reacts with chemicals like sodium carbonate (washing soda) or can be treated using ion-exchange resins. Understanding these chemical reactions allows you to predict results and solve practical removal problems.
Measurement and Units of Hardness
In JEE chemistry practicals, water hardness is measured in mg/L or parts per million (ppm), expressed as CaCO3. The main formula for calculation is:
- Hardness (mg/L as CaCO3) = (Volume of EDTA used × Molarity of EDTA × 1000) / Volume of Sample (mL)
A standard chemistry method uses EDTA titration, with Eriochrome Black T as indicator. At endpoint, a color change from wine red to blue marks the completion. IS 10500 recommends a maximum of 200 mg/L for drinking water in India. Values above 500 mg/L are considered very hard and typically unsuitable without treatment.
Effects and Applications in Engineering Chemistry
Water hardness impacts both real life and engineering. Hard water:
- Prevents proper soap lathering, causing wastage.
- Forms scale in boilers, leading to inefficiency.
- Corrodes pipes and damages appliances.
- Reduces textile dyeing quality and efficiency.
- Requires more energy for heating in industries.
- Can alter chemical process yields and purity.
- Is an important parameter for industrial water treatment plants.
- May slightly affect taste but is not harmful at moderate levels.
In engineering chemistry and JEE numericals, predicting hardness effects helps solve questions involving scaling, material damage, and process optimization. Some industries demand soft water for quality assurance (e.g., in boilers, textiles).
Methods for Removing Hardness
A variety of methods can remove or reduce hardness of water for both domestic and industrial needs:
- Boiling: Removes temporary hardness by precipitating CaCO3 and Mg(OH)2.
- Lime-soda process: Treats permanent and temporary hardness by chemical addition of Ca(OH)2 and Na2CO3.
- Washing soda (Na2CO3): Precipitates Ca2+ and Mg2+ as carbonates.
- Ion exchange (zeolite or resin method): Exchanges Ca2+/Mg2+ with Na+ ions.
- Reverse osmosis and distillation: Physically remove dissolved ions.
Among these, the ion exchange resin method is widely used in municipal water treatment plants and for laboratory purposes, as it produces highly soft water suitable for industrial use.
JEE-Style Example: Calculating Hardness
A 50 mL water sample requires 15 mL of 0.01 M EDTA to reach endpoint during titration. Calculate the hardness in mg/L as CaCO3.
- Hardness = (15 × 0.01 × 1000) / 50 = 3 mg/mL or 300 mg/L.
This value indicates hard water as per the standard classification schemes commonly asked in JEE Main objective questions.
Summary Table: Water Hardness Classifications
| Category | Range (mg/L as CaCO3) |
|---|---|
| Soft | 0–60 |
| Moderately hard | 61–120 |
| Hard | 121–180 |
| Very hard | >180 |
Mastering water hardness chemistry makes it easier to solve JEE pattern questions involving calculation, analysis of reactions, and application to engineering scenarios. For further concept clarity, see solutions, chemical equilibrium, and acid-base topics on Vedantu.
- Solutions
- Acids, Bases, and Salts
- Environmental Chemistry
- Principles Related to Practical Chemistry
- Chemical Bonding
- Principles of Qualitative Analysis
- Purification and Characterisation of Organic Compounds
- Difference Between Toilet Soap and Laundry Soap
- Redox Reactions and Electrochemistry
- Hydrogen
- Some Basic Concepts in Chemistry
- Chemical Equilibrium
- Chemical Bonding and Molecular Structure
- Oxides of Sulphur
- Modern Periodic Table
- Ionisation Constant of Acids and Bases
- Chemical Kinetics
- States of Matter
Consistent practice with units, formulas, and chemical logic is vital for solving hardness of water numericals and applied reasoning in the JEE Main exam. Use Vedantu’s resources for more solved examples and practice sets.
FAQs on Hardness of Water in Chemistry: Definition, Types, and Importance
1. What is hardness of water in chemistry?
Hardness of water in chemistry refers to the presence of dissolved calcium (Ca2+) and magnesium (Mg2+) ions in water, which makes it difficult to form lather with soap.
Key points include:
- Measured in ppm (parts per million) or mg/L
- Caused mainly by salts of calcium and magnesium
- Relevant for exams and practicals
2. What causes hardness of water?
Hardness of water is caused by the presence of calcium and magnesium salts, mostly in the form of chlorides, sulphates, and bicarbonates.
Sources include:
- Limestone, gypsum, and dolomite rocks
- Dissolved Ca2+ and Mg2+ ions in groundwater
- Runoff from mineral-rich soils
3. What is a good water hardness to have?
A good water hardness value for drinking water is generally 60–120 ppm (mg/L) as CaCO3.
According to standards:
- 0-60 ppm: Soft water
- 61-120 ppm: Moderately hard (ideal range)
- 121-180 ppm: Hard
- Above 180 ppm: Very hard
4. Is high water hardness bad?
High water hardness can cause several problems in household and industrial contexts.
Main effects:
- Prevents soap from lathering
- Causes scaling in pipes and boilers
- Reduces efficiency of heating systems
- Can cause taste changes, but is not usually harmful to health
5. What are the types of hardness of water?
There are two main types of hardness in water – temporary and permanent hardness.
- Temporary hardness: Caused by bicarbonates of calcium and magnesium; removable by boiling.
- Permanent hardness: Caused by chlorides and sulphates of calcium and magnesium; removable by chemical treatment.
- Total hardness: Sum of both temporary and permanent hardness.
6. What is the hardness of drinking water standard in ppm?
The BIS standard for total hardness in drinking water is 200 mg/L (ppm) as CaCO3, while the WHO recommends 500 mg/L as the maximum permissible limit.
- Soft: < 60 ppm
- Moderately Hard: 61–120 ppm
- Hard: 121–180 ppm
- Very Hard: >180 ppm
7. How do you measure the hardness of water?
The hardness of water is commonly measured by EDTA titration or by calculating based on standard formulas.
Standard steps:
- EDTA titration to determine Ca2+ and Mg2+ ion concentration
- Results expressed as mg/L or ppm of CaCO3
- Used for practical and board exams
8. How can we remove permanent and temporary hardness of water?
Temporary hardness is removed by boiling water, while permanent hardness requires treatment with chemicals or methods like ion-exchange.
Common removal methods:
- Boiling (for temporary hardness)
- Lime-soda process
- Zeolite or ion-exchange resins
- Using washing soda (sodium carbonate)
9. Which chemicals are responsible for water hardness?
The primary chemicals responsible for water hardness are calcium and magnesium salts.
These include:
- Calcium chloride (CaCl2)
- Calcium sulphate (CaSO4)
- Magnesium chloride (MgCl2)
- Magnesium sulphate (MgSO4)
- Calcium and magnesium bicarbonates
10. What is the difference between temporary and permanent hardness?
The main difference is the cause and removal method:
- Temporary hardness comes from bicarbonates of calcium and magnesium; removed by boiling.
- Permanent hardness results from chlorides and sulphates of these ions; requires chemical treatment.
11. Why does hard water produce less lather with soap?
Hard water produces less lather with soap because calcium and magnesium ions react with soap to form insoluble scum, rather than allowing the soap to foam.
- Soap + Hardness ions → Insoluble precipitate (scum)
- This wastes soap and makes cleaning harder
12. Can water hardness influence taste or health?
Moderate water hardness can affect taste, making it slightly mineral-rich, but it is generally not harmful to health.
- Extremely hard water can cause taste issues and scaling
- Some studies suggest health benefits due to minerals
13. Is EDTA titration the only way to measure water hardness in the lab?
No, although EDTA titration is the most common and accurate method, other methods include soap titration, complexometric titration, and use of colorimetric kits for rapid testing.
