Water Hardness and Removal Methods
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Questions and Answers

What ion does a cation-exchange resin primarily exchange for positive ions in water?

  • H^+
  • Na^+ (correct)
  • Ca^2+
  • Cl^-

Deionised water is produced by passing water through a device that only contains cation exchange resin.

False (B)

What happens to the Na^+ ions in the cation-exchange resin after prolonged use?

They are replaced and the resin needs to be replenished.

What causes temporary hardness in water?

<p>Calcium hydrogencarbonate (C)</p> Signup and view all the answers

The process of producing deionised water combines cation-exchange resin and __________ resin.

<p>anion</p> Signup and view all the answers

Match the type of water with its characteristic:

<p>Deionised water = Removes suspended and dissolved solids Distilled water = Removes all suspended/dissolved solids and gases Hard water = Contains calcium and magnesium ions Soft water = Lacks significant metal ions</p> Signup and view all the answers

Permanent hardness can be removed by boiling the water.

<p>False (B)</p> Signup and view all the answers

Which of the following ions is not exchanged by the cation-exchange resin?

<p>OH^- (B)</p> Signup and view all the answers

What is formed when soap is added to hard water?

<p>Calcium stearate</p> Signup and view all the answers

Hard water has advantages and disadvantages, including providing calcium for teeth and bones.

<p>True (A)</p> Signup and view all the answers

Temporary hardness is caused by Ca(HCO3)2 and Mg(HCO3)2, which are known as ________.

<p>calcium hydrogencarbonate and magnesium hydrogencarbonate</p> Signup and view all the answers

What is the role of ethylenediaminetetraacetic acid (EDTA) in the experiment to determine water hardness?

<p>It acts as a chelating agent to bind calcium and magnesium ions.</p> Signup and view all the answers

Match the following substances with their effects on water hardness:

<p>CaSO4 = Permanent hardness Ca(HCO3)2 = Temporary hardness MgSO4 = Permanent hardness Mg(HCO3)2 = Temporary hardness</p> Signup and view all the answers

Which method of removing water hardness involves boiling?

<p>Boiling (A)</p> Signup and view all the answers

Ion-exchange resins can replace calcium and magnesium ions with sodium ions.

<p>True (A)</p> Signup and view all the answers

What is the role of washing soda in softening hard water?

<p>It reacts with Ca²⁺ ions to form insoluble calcium carbonate.</p> Signup and view all the answers

What is the primary purpose of adding fluorine compounds to water?

<p>To reduce tooth decay (D)</p> Signup and view all the answers

Fluoridation of water is optional in Ireland.

<p>False (B)</p> Signup and view all the answers

What happens if excess chlorine is added to water?

<p>Taste and smell of water affected</p> Signup and view all the answers

The optimum pH for water is _____

<p>7.2</p> Signup and view all the answers

Match the following chemicals with their purpose:

<p>Aluminium Sulfate = Coagulation of small, suspended particles Chlorine = Sterilize water Calcium hydroxide = Raise pH Sodium carbonate = Soften water</p> Signup and view all the answers

What problem is associated with excess Sodium carbonate in water?

<p>Taste of water affected (D)</p> Signup and view all the answers

What is the total suspended solids if the mass of dry filter paper increased by 0.68 g after filtering a water sample?

<p>680 mg/l (ppm)</p> Signup and view all the answers

To determine total dissolved solids, you must _____ the filtered water sample.

<p>evaporate</p> Signup and view all the answers

What is the approximate concentration of total suspended solids in water in ppm?

<p>1360 ppm (A)</p> Signup and view all the answers

Eutrophication is caused only by a decrease in plant nutrients in the water.

<p>False (B)</p> Signup and view all the answers

What is B.O.D. defined as?

<p>The amount of dissolved oxygen consumed by biological action in water over five days at 20°C in the dark.</p> Signup and view all the answers

The process used to measure heavy metal ions in water is called __________.

<p>Atomic absorption spectrometry</p> Signup and view all the answers

Match the following terms with their definitions:

<p>B.O.D. = Amount of dissolved oxygen consumed in 5 days Eutrophication = Enrichment of water with nutrients Total Suspended Solids = Solids suspended in water contributing to turbidity Heavy metals = Toxic metals that can accumulate in organisms</p> Signup and view all the answers

Which method is used to prevent photosynthesis during B.O.D. testing?

<p>Keeping the bottle in complete darkness (C)</p> Signup and view all the answers

Lead ions (Pb²⁺) can have serious consequences for organisms if they accumulate in water.

<p>True (A)</p> Signup and view all the answers

What causes an increase in the growth of algae and other plants in water?

<p>High levels of nitrates and phosphates.</p> Signup and view all the answers

What color indicates the end point of the titration when using EDTA with hard water?

<p>Dark blue (B)</p> Signup and view all the answers

The mole ratio of EDTA to calcium ions is 1:2.

<p>False (B)</p> Signup and view all the answers

What is the total hardness of the water sample in ppm?

<p>300 ppm</p> Signup and view all the answers

The flocculating agent used in water treatment is __________.

<p>Al2(SO4)3</p> Signup and view all the answers

Match the following water treatment processes with their description.

<p>Screening = Removes floating debris Flocculation = Coagulates small particles Sedimentation = Allows particles to settle Filtration = Removes remaining particles</p> Signup and view all the answers

How much solid indicator is added to the conical flask during the titration?

<p>0.03 g (C)</p> Signup and view all the answers

The sedimentation process typically removes 50% of the particles from the water.

<p>False (B)</p> Signup and view all the answers

What is the purpose of chlorination in water treatment?

<p>To sterilize the water and kill harmful microorganisms.</p> Signup and view all the answers

What is the main purpose of adding manganese(II) sulfate and alkaline potassium iodide to the water sample?

<p>To initiate a redox reaction (A)</p> Signup and view all the answers

The free iodine must be estimated using a potassium iodide solution.

<p>False (B)</p> Signup and view all the answers

What color does the iodine solution turn when the excess iodide has been consumed during the titration?

<p>Colorless</p> Signup and view all the answers

The _____ is used to measure the concentration of dissolved oxygen in water samples.

<p>redox reaction</p> Signup and view all the answers

Which step follows the addition of concentrated sulfuric acid to dissolve the precipitate?

<p>Invert repeatedly to redissolve the precipitate (C)</p> Signup and view all the answers

Match the following steps in the procedure with their outcomes:

<p>Adding sulfuric acid = Dissolves brown precipitate Titrating with thiosulfate = Estimates free iodine Adding starch indicator = Determines the end point Rinsing with deionised water = Prevents contamination</p> Signup and view all the answers

What concentration of thiosulfate solution is used during the titration?

<p>0.005 M (A)</p> Signup and view all the answers

Sewage treatment refers to the material delivered to _____ treatment plants.

<p>waste</p> Signup and view all the answers

Flashcards

Hard Water

Water that doesn't easily form a lather with soap due to dissolved calcium (Ca²⁺) or magnesium (Mg²⁺) ions.

Temporary Hardness

Hardness that can be removed by boiling the water, caused by calcium hydrogencarbonate (Ca(HCO₃)₂) and magnesium hydrogencarbonate (Mg(HCO₃)₂).

Permanent Hardness

Hardness that cannot be removed by boiling, caused by calcium sulfate (CaSO₄) or magnesium sulfate (MgSO₄).

Temporary Hardness Removal

Boiling causes calcium hydrogencarbonate and magnesium hydrogencarbonate to decompose into calcium carbonate, carbon dioxide, and water, removing the hardness.

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Distillation

A method of removing hardness from water by boiling and collecting the steam, leaving behind solid impurities.

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Washing Soda

Hydrated sodium carbonate (Na₂CO₃·10H₂O) used to remove hardness caused by calcium ions in water.

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Ion-exchange Resin

A material that exchanges calcium and magnesium ions for sodium ions, softening water.

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Soap Reaction with Hard Water

Soap reacts with calcium or magnesium ions to form insoluble compounds (like calcium stearate), which prevents suds formation.

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Cation Exchange Resin

A material that exchanges positive ions in water for sodium ions (Na+).

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Hard Water Softening

Removing calcium and magnesium ions from water using a cation exchange resin (e.g., RNa).

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Mixed-bed Resin

A combination of cation and anion exchange resins used to produce deionized water.

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Deionized Water

Water with all positive and negative ions removed.

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Distilled Water

The purest form of water, with all solids and gases removed.

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Hard Water Disadvantages

Disadvantages include pipe blockages, soap scum, and less desirable tastes.

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EDTA Hardness Test

A laboratory method to measure total hardness in water samples using a specific chemical.

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Fluoridation

Adding fluorine to water to help prevent tooth decay. It is typically done by law in many countries like Ireland.

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pH adjustment

Adjusting the pH of water to the ideal level (usually around 7.2) by adding acids or bases. This ensures optimal water quality for various uses.

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Aluminium Sulfate

A chemical used to clump together small particles in water for easier removal.

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Chlorine

A chemical used to kill bacteria in water, making it safe to drink.

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Fluorine compound

A chemical used to prevent tooth decay by strengthening tooth enamel.

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Calcium hydroxide

A chemical used to increase the pH of water, making it less acidic.

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Sodium carbonate

A chemical used to soften hard water by removing calcium and magnesium ions.

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Sulfuric acid

A chemical used to decrease the pH of water, making it more acidic.

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Titration of hard water

A process to determine the total hardness of a water sample by reacting it with a solution of EDTA.

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EDTA solution

A solution used as a titrant in the analysis of hard water, reacting one-to-one with calcium and magnesium ions.

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Buffer solution

A solution that resists changes in pH when small amounts of acid or base are added.

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Total hardness (ppm)

A measure of the concentration of calcium and magnesium ions (often expressed as CaCO3) in water, indicates the amount of hardness-causing minerals.

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Conical Flask

A type of laboratory glassware used to hold liquids during titrations and chemical reactions.

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Burette

A graduated glass tube used for delivering precise volumes of liquid during a titration.

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Titrant

A solution of known concentration used to react with the analyte (in this case, hard water) to determine its concentration.

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End point

The point in a titration where the indicator changes color, signaling the completion of the reaction between the titrant and the liquid being analyzed.

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Total Suspended Solids (TSS)

The total amount of solid particles suspended in a water sample. These particles are usually visible and can be removed by filtration.

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Total Dissolved Solids (TDS)

The total amount of dissolved mineral salts and organic compounds in a water sample. These substances cannot be easily removed by filtration.

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Biochemical Oxygen Demand (BOD)

The amount of oxygen consumed by microorganisms in water over a five-day period at 20°C. It indicates the amount of organic matter present in the water which needs to be broken down.

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What does a high BOD indicate?

A high BOD indicates a large amount of organic matter in the water, leading to low dissolved oxygen levels, which can harm aquatic life.

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Eutrophication

The excessive growth of algae and plants in a water body caused by an abundance of nutrients, typically nitrates and phosphates.

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Impact of Eutrophication on Oxygen Levels

Eutrophication leads to decreased dissolved oxygen levels. The algae bloom blocks sunlight, preventing aquatic plants from producing oxygen through photosynthesis.

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Heavy Metal Ions

Metal ions with high atomic weight and density, such as lead (Pb²⁺). They can accumulate in water and pose risks to organisms.

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Atomic Absorption Spectrometry (AAS)

A technique used to detect and measure the concentration of specific metal ions in water samples.

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Dissolved Oxygen

The amount of oxygen gas (O₂) dissolved in water, essential for aquatic life.

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Redox Reaction

A chemical reaction where electrons are transferred between reactants, involving both oxidation (loss of electrons) and reduction (gain of electrons).

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Manganese(II) Sulfate

A chemical compound used in the dissolved oxygen determination method, reacting with dissolved oxygen to form a brown precipitate.

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Alkaline Potassium Iodide

A chemical compound used in the dissolved oxygen determination method, reacting with manganese(II) hydroxide to liberate iodine.

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Sodium Thiosulfate

A chemical compound used to titrate the iodine released in the dissolved oxygen determination, reacting with iodine to form colorless products.

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Sewage Treatment

The process of cleaning and treating wastewater (sewage) from homes and industries to remove contaminants and make it safe for discharge or reuse.

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Waste Treatment Plants

Facilities that process and treat wastewater, removing pollutants and contaminants before discharging it into waterways.

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Study Notes

Hardness of Water

  • Hard water is water that does not easily form a lather with soap. Hardness is caused by Ca²⁺ or Mg²⁺ ions dissolved in the water.
  • Common soap ingredient is sodium stearate (C₁₇H₃₅COONa). In hard water, calcium or magnesium ions react with stearate ions to form insoluble calcium stearate, which floats on the water.
  • Temporary hardness is hardness that can be removed by boiling the water.
  • Source of temporary hardness is calcium hydrogencarbonate (Ca(HCO₃)₂) and magnesium hydrogencarbonate (Mg(HCO₃)₂).
  • Boiling converts these to calcium carbonate (CaCO₃), which precipitates out of solution, and releases carbon dioxide (CO₂) and water (H₂O).
  • Permanent hardness is hardness that cannot be removed by boiling the water.
  • Caused by the presence of calcium sulfate (CaSO₄) or magnesium sulfate (MgSO₄) in the water.
  • Heating does not precipitate these ions, so the hardness is not removed

Methods of Removing Hardness

  • Distillation: Boiled water, removing all dissolved solids. Expensive
  • Using Washing Soda: Washing soda (Na₂CO₃•10H₂O) reacts with Ca²⁺ ions to form calcium carbonate, which is insoluble.
  • Ion-exchange Resin: Exchanges Ca²⁺ and Mg²⁺ for Na⁺ ions.
    • The resin is a cation exchange resin because it exchanges positive ions.
    • Eventually, the resin needs replenishment with a concentrated sodium chloride solution to restore the Na+ ions.
  • Producing Deionised Water: Using a mixture of cation and anion exchange resins (mixed-bed resin) to remove all positive and negative ions, respectively.
    • Cation resin exchanges Na⁺ for H⁺ ions.
    • Anion resin exchanges Cl⁻ for OH⁻ ions.
    • The H⁺ and OH⁻ ions combine to form water (H₂O).
  • Distilled Water: The purest form of water, as all dissolved solids, including gases, are removed.

Advantages and Disadvantages of Hard Water

  • Advantages: Provides calcium for teeth and bones, better taste, good for brewing and tanning.
  • Disadvantages: Blocks pipes, leaves scale, wastes soap, produces scum

Mandatory Experiment: Determining Total Hardness

  • Procedure involves using EDTA solution and an indicator to determine the total hardness in a water sample.

Water Treatment

  • Screening: Removes floating debris.
  • Flocculation: Coagulates small suspended particles for better sedimentation.
  • Sedimentation: Allows larger particles to settle by gravity.
  • Filtration: Removes remaining particles using sand filters.
  • Chlorination: Sterilizes the water by killing harmful microorganisms.
  • Fluoridation: Prevents tooth decay.
  • pH Adjustment: Adjusts the pH level to the optimal level (7.2).

Water Pollution Overview

  • Biochemical Oxygen Demand (BOD): Defined as the amount of dissolved oxygen consumed by biological action in a water sample kept at 20°C in the dark for five days.
  • Eutrophication: Excessive enrichment of water with nutrients, leading to excessive growth of algae or plants—caused by fertilizers and sewage.

Pollution Caused by Heavy Metals

  • Heavy metal ions like lead (Pb²⁺) can have serious consequences if they build up in water bodies.
  • Atomic absorption spectrometry (AAS) can measure heavy metal concentration in water.
  • Precipitation can remove certain metal ions from water.

Mandatory Experiment: Measuring Dissolved Oxygen

  • Procedure involves rinsing a 250 cm³ bottle, filling it with water sample (avoiding air bubbles), adding manganese(II) sulfate, alkaline potassium iodide solution, stoppering (allowing overflow), inverting to allow precipitate to settle, adding sulfuric acid to dissolve precipitate, and then repeating the inversion till precipitate is dissolved.
  • free iodine can be calculated using thiosulfate titration.

Sewage Treatment

  • Primary Treatment: Removes large solids by screening and settlement.
  • Secondary Treatment: Uses biological oxidation to reduce dissolved and organic matter. Activated sludge process.
  • Tertiary Treatment: Removes phosphorous and nitrogen to prevent eutrophication.

Instrumental Methods of Water Analysis

  • pH Analysis: Uses pH sensors.
  • Atomic Absorption Spectroscopy (AAS): Used to detect heavy metals in water.

Mandatory Experiment: Estimating Free Chlorine

  • Uses 5% ethanoic acid and potassium iodide solution for the procedure to estimate free chlorine in swimming pool water using either a comparator or a colorimeter , plotting a graph of absorbance versus concentration of chlorine to find the result.

Exam Questions/Summary

  • Hard water is caused by calcium/magnesium hydrogen carbonate.
  • Hard water causes scum when soap is used—this is due to the reaction of the Ca2+ and Mg2+ ions with soap.
  • Water can be softened by ion exhange to remove positive ions.
  • Flocculating agents are used to remove fine suspended particles.
  • Chlorine is used to kill harmful microorganisms in water.
  • Fluorine helps prevent tooth decay.

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