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Questions and Answers
What are the primary cations responsible for water hardness?
What are the primary cations responsible for water hardness?
Calcium, Magnesium, Strontium, Ferrous iron, and Manganese
How is hardness in water typically measured?
How is hardness in water typically measured?
By the amount of Calcium and Magnesium ions in the water
Which compound in soil acts as a weak acid that dissolves limestone?
Which compound in soil acts as a weak acid that dissolves limestone?
Carbon dioxide
What is the range of hardness for water classified as 'Moderately Hard'?
What is the range of hardness for water classified as 'Moderately Hard'?
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Which titration method is used to measure water hardness accurately?
Which titration method is used to measure water hardness accurately?
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What kind of complex is formed during hardness determination using complexometric titration?
What kind of complex is formed during hardness determination using complexometric titration?
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Write the chemical reaction representing the reaction between metal ions (M2+) and EDTA in complexometric titration.
Write the chemical reaction representing the reaction between metal ions (M2+) and EDTA in complexometric titration.
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Describe the initial state of rainwater before it absorbs hardness.
Describe the initial state of rainwater before it absorbs hardness.
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Name one anion that Calcium (Ca2+) reacts with to cause hardness in water.
Name one anion that Calcium (Ca2+) reacts with to cause hardness in water.
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What is the concentration range for water classified as 'Very Hard'?
What is the concentration range for water classified as 'Very Hard'?
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Study Notes
Hardness in Water
- Hardness in water is caused by divalent cations, namely Calcium, Magnesium, Strontium, Ferrous iron, and Manganese.
Characteristics of Hardness-Causing Cations
- These cations react with soap to form precipitates.
- They also react with some anions to form scale.
- Hardness is measured using the amount of Calcium and Magnesium ions in the water.
Cations That Cause Hardness
- Ca2+ reacts with HCO3-.
- Mg2+ reacts with SO42-.
- Sr2+ reacts with Cl-.
- Fe2+ reacts with NO3-.
- Mn2+ reacts with SiO32-.
How Hardness Occurs
- Rainwater is initially soft.
- It picks up hardness as it flows through the ground.
- Soil contains carbon dioxide, which is released by bacteria.
- The carbon dioxide acts as a weak acid, which dissolves limestone.
- Limestone is made up of Calcium Carbonate (CaCO3) and contains impurities like sulfates, chlorides, and silicates.
- As the limestone dissolves, the dissolved minerals make the water hard.
Classifying Hardness
- Soft water has a concentration of 0-75 mg/L.
- Moderately hard water has a concentration of 75-150 mg/L.
- Hard water has a concentration of 150-300 mg/L.
- Very hard water has a concentration of 300+ mg/L.
Determining Hardness Accurately
- Hardness can be accurately determined by titration with EDTA (ethylenediaminetetraacetic acid) or its disodium salts.
Structure of EDTA
- EDTA has the structure: HOOC-H2C-N-CH2-CH2-N-CH2-COOH.
Determining Hardness by Complexometry Titration
- Hardness can be determined using a standard solution of EDTA and complexometric titration.
- Metal ions (M2+) react with EDTA to form a stable complex.
- The reaction can be represented:
- M2+ + In → [M-In] (wine-red).
- [M-EDTA] + 2Na+ + In → [M-EDTA] + In (blue).
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Description
Understand the causes and effects of hardness in water, including the divalent cations that cause it and how it is measured. Learn about the reactions that occur with soap and anions.
