Hardness of Water Overview

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Questions and Answers

What causes hard water to form a grey insoluble compound when soap is added?

The calcium or magnesium ions in hard water react with stearate ions from soap to form calcium stearate.

How can temporary hardness be removed from water?

Temporary hardness can be removed by boiling the water.

What is the primary difference between temporary and permanent hardness in water?

Temporary hardness can be removed by boiling, while permanent hardness cannot be eliminated through boiling.

Explain the role of carbon dioxide in creating temporary hardness in rainwater.

<p>Carbon dioxide dissolves in rainwater to form carbonic acid, which reacts with limestone to produce calcium hydrogen carbonate.</p> Signup and view all the answers

What reaction occurs when boiling temporarily hard water?

<p>Boiling causes calcium hydrogen carbonate to decompose into calcium carbonate, carbon dioxide, and water.</p> Signup and view all the answers

What is one disadvantage of using distillation to remove hardness from water?

<p>Distillation is too expensive to implement on a large scale.</p> Signup and view all the answers

How does washing soda help in softening hard water?

<p>Washing soda reacts with Ca^2+ ions to form insoluble calcium carbonate.</p> Signup and view all the answers

What is the function of a cation exchange resin in removing hardness from water?

<p>Cation exchange resin exchanges calcium and magnesium ions with sodium ions.</p> Signup and view all the answers

What is the role of Na^+^ ions in cation-exchange resin during water softening?

<p>Na^+^ ions replace the hard water's Ca^2+^ ions, thus softening the water.</p> Signup and view all the answers

How does cation-exchange resin regenerate after exhausting its Na^+^ ions?

<p>It is replenished by passing a concentrated sodium chloride solution through it.</p> Signup and view all the answers

What is the difference between deionised water and distilled water?

<p>Deionised water has impurities removed but may contain dissolved gases, while distilled water is purer with all impurities and gases eliminated.</p> Signup and view all the answers

In the mixed-bed resin used for deionisation, what ions does the cation-exchange resin replace?

<p>The cation-exchange resin replaces Na^+^ ions with H^+^ ions.</p> Signup and view all the answers

List one advantage and one disadvantage of hard water.

<p>Advantage: Provides calcium for teeth and bones; Disadvantage: Wastes soap.</p> Signup and view all the answers

What transactions occur when Cl^-^ ions interact with the anion-exchange resin?

<p>Cl^-^ ions are replaced by OH^-^ ions in the water.</p> Signup and view all the answers

During the experiment to determine total hardness, what should be done with the pipette before use?

<p>The pipette should be washed and rinsed with deionised water.</p> Signup and view all the answers

What type of water does the ion-exchange resin process aim to produce?

<p>The ion-exchange resin process aims to produce deionised water.</p> Signup and view all the answers

How should the burette be prepared before starting the titration?

<p>The burette should be filled with the EDTA solution, ensuring it's vertical, and adjusted to the zero mark without the funnel.</p> Signup and view all the answers

What color indicates that the endpoint of the titration has been reached?

<p>The endpoint of the titration is indicated by a dark blue color with no tinge of wine-red.</p> Signup and view all the answers

Calculate the concentration of total hardness in the water based on the titration result of 15.2 cm^3 of 0.01 M EDTA.

<p>The total hardness is 300 ppm CaCO3.</p> Signup and view all the answers

What is the function of the buffer solution added during the titration of hard water?

<p>The buffer solution maintains the pH at 10, which is essential for the proper functioning of the indicator.</p> Signup and view all the answers

Describe the flocculation process in water treatment.

<p>Flocculation involves the coagulation of small suspended particles into larger particles for better sedimentation.</p> Signup and view all the answers

What is the role of chlorine in the water treatment process?

<p>Chlorine is added to sterilize the water by killing harmful microorganisms.</p> Signup and view all the answers

What are the components remaining in the water after the sedimentation process?

<p>After sedimentation, about 90% of larger particles are removed, leaving smaller suspended particles.</p> Signup and view all the answers

Why is it important to carry out multiple accurate titrations until two titres agree to within 0.1 cm^3?

<p>Performing multiple titrations ensures the reliability and accuracy of the final volume measurement.</p> Signup and view all the answers

What is the purpose of rinsing the reagent bottle with deionised water before filling it with the sample?

<p>To avoid trapped air bubbles that can affect the accuracy of the dissolved oxygen measurement.</p> Signup and view all the answers

Why is it necessary to add sulfuric acid after creating the brown precipitate?

<p>Sulfuric acid is needed to dissolve the precipitate and release iodine, resulting in a golden brown solution.</p> Signup and view all the answers

What is the significance of using sodium thiosulfate in the titration process?

<p>Sodium thiosulfate serves to quantify the amount of free iodine present in the solution by reducing it to iodide.</p> Signup and view all the answers

What is meant by 'taking the average of two accurate titration results'?

<p>It refers to calculating the mean of two titrations where the results differ by no more than 0.1 cm^3.</p> Signup and view all the answers

Why are air bubbles a concern during the initial filling of the reagent bottle?

<p>Air bubbles can lead to incorrect readings of dissolved oxygen concentration as they can alter sample volume.</p> Signup and view all the answers

What role does the starch indicator play during the titration with thiosulfate?

<p>The starch indicator helps visualize the endpoint of the titration by changing color, indicating that all iodine has reacted.</p> Signup and view all the answers

How is dissolved oxygen concentration calculated from the results obtained?

<p>By converting the moles of iodine measured from the titration into moles of oxygen, and subsequently into grams and p.p.m.</p> Signup and view all the answers

In the context of sewage treatment, why is measuring dissolved oxygen important?

<p>Dissolved oxygen levels indicate the water's ability to support aquatic life and the effectiveness of the treatment process.</p> Signup and view all the answers

What volumes of solution are added to the third, fourth, and fifth flasks?

<p>2.0 cm^3, 4.0 cm^3, and 8.0 cm^3.</p> Signup and view all the answers

What is the purpose of using a colorimeter in this experiment?

<p>The colorimeter is used to measure the absorbance of the solutions at a wavelength of 440 nm.</p> Signup and view all the answers

What is the first step in preparing the working standards after adding the solution to the flasks?

<p>Add 5.0 cm^3 of 2% potassium iodide solution to each flask.</p> Signup and view all the answers

Explain how to prepare flask F for measuring chlorine concentration.

<p>Add 5 cm^3 of 5% ethanoic acid and 5.0 cm^3 of 5% potassium iodide, then fill with swimming pool water up to the mark.</p> Signup and view all the answers

What is the formula to convert total suspended solids to parts per million (ppm)?

<p>Total suspended solids in ppm can be calculated using the formula: $0.68 \times 2 \times 1000 = 1360$ ppm.</p> Signup and view all the answers

How do you determine the concentration of free chlorine in the sample from flask F?

<p>Obtain the absorbance from the colorimeter and use the graph to find the concentration of NaOCl, then multiply by $71 / 74.5$.</p> Signup and view all the answers

How do you determine the Biochemical Oxygen Demand (B.O.D) of a water sample?

<p>B.O.D is determined by measuring the difference in dissolved oxygen levels between two samples: one tested immediately and one kept in the dark for five days.</p> Signup and view all the answers

What negative impact does eutrophication have on aquatic life?

<p>Eutrophication leads to rapid algae growth that blocks sunlight, preventing photosynthesis and decreasing oxygen levels, which harms aquatic life.</p> Signup and view all the answers

What is essential to do between measuring different samples in the colorimeter?

<p>Rinse the sample cells with deionised water after each sample.</p> Signup and view all the answers

What type of light should be used when comparing colors in the comparator?

<p>Daylight should be used instead of artificial light.</p> Signup and view all the answers

What methods can be used to detect heavy metals in water?

<p>Heavy metals in water can be detected using Atomic Absorption Spectrometry (AAS).</p> Signup and view all the answers

What is the role of the DPD No.1 tablet in the chlorine test?

<p>The DPD No.1 tablet reacts with chlorine to produce a color that can be compared to standards.</p> Signup and view all the answers

Why is it essential to monitor oxygen levels in water systems?

<p>Monitoring oxygen levels is crucial to ensure the health and survival of aquatic organisms which depend on dissolved oxygen.</p> Signup and view all the answers

What is the significance of conducting a B.O.D test in the dark?

<p>Conducting the B.O.D test in the dark prevents photosynthesis, ensuring that any change in dissolved oxygen is due solely to microbial activity.</p> Signup and view all the answers

What is the relationship between high B.O.D levels and microbial presence in water?

<p>High B.O.D levels indicate high microbial activity, reflecting a low level of dissolved oxygen in the water.</p> Signup and view all the answers

How does precipitation help in the removal of heavy metal ions from water?

<p>Precipitation helps remove heavy metal ions by converting them into solid particulates that can be filtered out of the water.</p> Signup and view all the answers

Flashcards

Cation-exchange resin

A resin that swaps positive ions in water for sodium ions.

Hard water

Water with high levels of dissolved minerals, mainly calcium and magnesium.

Mixed-bed resin

A combination of cation and anion exchange resins used for deionization.

Deionized water

Water with all positive and negative ions removed, but may contain dissolved gases

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Distilled water

Purified water with all dissolved solids and gases removed.

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Ion-exchange process

The process of exchanging ions in water using resins.

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EDTA

A chemical used to measure water hardness.

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Water hardness experiment

A lab experiment to measure the total hardness of water using EDTA.

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Temporary Hardness

Hardness that can be removed by boiling, caused by calcium and magnesium hydrogencarbonates.

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Permanent Hardness

Hardness not removed by boiling, caused by calcium and magnesium sulfates.

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Methods to remove temporary hardness

Boiling, washing soda, or ion-exchange resins can help eliminate temporary hardness.

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Calcium Stearate

Insoluble compound formed when soap reacts with hard water ions, causing the water to become cloudy or cloudy-looking.

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Distillation

A method of removing impurities from water by boiling it and collecting the vapor as pure water.

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Washing Soda

A chemical that reacts with calcium ions to reduce water hardness.

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Dissolved Oxygen

The amount of oxygen gas dissolved in a water sample.

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Redox Titration

A titration method using a redox reaction to determine the concentration of a substance.

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Manganese(II) Sulfate

A chemical used to react with dissolved oxygen in water.

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Alkaline Potassium Iodide

A chemical used to enhance the reaction between dissolved oxygen and manganese(II) sulfate.

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Sulfuric Acid

An acid used to dissolve the brown precipitate formed in the dissolved oxygen experiment.

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Sodium Thiosulfate

A chemical used to titrate the free iodine released in the dissolved oxygen experiment.

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Starch Indicator

Used to identify the end point in the titration, turning the solution blue.

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Sewage Treatment

The process of cleaning wastewater generated by towns and cities.

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Hardness in water

The presence of dissolved minerals (mainly calcium and magnesium) in water.

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Titration

A method to determine the concentration of a solution by reacting it with a solution of known concentration.

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End point

The point in a titration where the reaction is complete, indicated by a color change.

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Buffer solution

A solution that resists changes in pH, ensuring the titration occurs at the desired pH.

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Eriochrome black T

An indicator used in water hardness titration, changing color when all hardness ions are bound by EDTA.

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ppm (parts per million)

A unit of concentration used to express the amount of a substance in a solution, indicating the number of milligrams per liter.

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Flocculation

The process of clumping together small suspended particles in water to form larger, easier-to-settle particles.

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Total Suspended Solids

The weight of solid particles that are suspended in a water sample, expressed in parts per million (ppm).

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Total Dissolved Solids

The weight of dissolved minerals and salts in a water sample, expressed in parts per million (ppm).

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BOD Test

A test to determine the amount of dissolved oxygen consumed by microorganisms in a water sample over a five-day period at 20°C.

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High BOD

Indicates a high level of microorganisms in the water, resulting in low dissolved oxygen levels.

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Eutrophication

The excessive growth of algae and plants in a body of water due to an oversupply of nutrients, usually nitrates and phosphates.

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Heavy Metals

Metals, like lead, that can be harmful to living organisms even in small amounts.

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Atomic Absorption Spectrometry (AAS)

A technique used to detect and measure the concentration of heavy metals in water samples.

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Precipitation

A chemical process used to remove heavy metals from water by forming insoluble compounds that can be removed.

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Colorimeter

An instrument used to measure the absorbance of light through a solution, which can be related to the concentration of a substance.

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Absorbance

The amount of light that is absorbed by a substance, measured using a colorimeter.

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Concentration

The amount of a substance dissolved in a given volume of solution. It's often expressed as a percentage or in units like milligrams per liter (mg/L).

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Standard Solution

A solution of known concentration used for calibration and comparison in analytical methods.

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DPD No.1 Tablet

A tablet used in the comparator method to measure the concentration of free chlorine in water. It reacts with chlorine, producing a color.

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Comparator

A device with multiple compartments that hold water samples and standard solutions. It’s used to compare colors visually and estimate the concentration of a substance visually.

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Free Chlorine

The active form of chlorine in water that is available to kill bacteria and other microorganisms.

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p.p.m.

Parts per million, a unit used to express the concentration of a substance, including free chlorine, in water samples.

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Study Notes

Hardness of Water

  • Hard water is water that does not easily create a lather with soap.
  • Hardness is caused by dissolved calcium (Ca²⁺) or magnesium (Mg²⁺) ions in the water.
  • Temporary hardness can be removed by boiling the water.
  • Temporary hardness is caused by calcium hydrogencarbonate (Ca(HCO₃)₂) and magnesium hydrogencarbonate (Mg(HCO₃)₂).
  • Boiling these compounds decomposes them into calcium carbonate (CaCO₃), carbon dioxide (CO₂), and water (H₂O), removing the hardness.
  • Permanent hardness cannot be removed by boiling.
  • Permanent hardness is caused by calcium sulfate (CaSO₄) or magnesium sulfate (MgSO₄) in the water.
  • Heating water containing these substances does not cause a chemical reaction to precipitate the Ca²⁺ or Mg²⁺ ions, so the hardness remains.

Methods of Removing Hardness

  • Distillation: Water is boiled, and the dissolved solids causing hardness are left behind in the residue. This is not usually practical for large scale use.
  • Using Washing Soda: This reacts with Ca²⁺ ions in the water to form calcium carbonate (insoluble in water)
  • Ion-exchange resin: Exchanges Ca²⁺ and Mg²⁺ ions for Na⁺ ions.
  • Cation-exchange resin: Replaces positive ions in the water with Na⁺ ions.
  • Deionised water: Produced by passing ordinary water through a water deioniser containing a mixture of cation and anion exchange resins to remove all positive and negative ions. This produces water with only H⁺ and OH⁻ ions.

Advantages and Disadvantages of Hard Water

  • Advantages:

    • Provides calcium for teeth and bones.
    • Better taste and flavor.
    • Good for brewing and tanning leather.

  • Disadvantages:

    • Blocks pipes (scales on kettles), impacting plumbing.
    • Wastes soap, producing more suds.
    • Produces scum.

Mandatory Experiment: Total Hardness

  • Instructions are given for a lab experiment to determine total hardness in a water sample using EDTA.

Water Treatment

  • Screening: Removing floating debris like branches and plastic bags.
  • Flocculation: Coagulating small suspended particles into larger ones for easier settling.
  • Sedimentation: Passing water through tanks to allow suspended particles to settle to the bottom.
  • Filtration: Passing water through beds of sand to remove remaining particles.
  • Chlorination: Adding chlorine to sterilize water and kill harmful microorganisms.
  • Fluoridation: Adding fluoride to prevent tooth decay as required by law.
  • pH Adjustment: Maintaining optimal pH (7.2) by adding acids or bases.

Water Pollution

  • Biochemical Oxygen Demand (BOD): Amount of dissolved oxygen consumed by biological action in water over 5 days at 20°C in the dark.
  • Eutrophication: Enrichment of water with nutrients causing excessive growth of algae and other plants, often resulting from fertilizers and sewage, which leads to depletion in oxygen levels.

Pollution Caused by Heavy Metals

  • Lead (Pb²⁺) ions are heavy metal ions which can be removed via precipitation and have harmful consequences for organisms.
  • Atomic absorption spectrometry (AAS) can be used to detect these metals in water.

Mandatory Experiment: Dissolved Oxygen

  • Procedure for measuring dissolved oxygen in a water sample using a redox reaction.

Sewage Treatment

  • Sewage treatment involves removing materials from sewage to maintain a healthy water system.
  • Primary treatment: Removing large solids (screening) and suspended solids (settling).
  • Secondary treatment: Biological oxidation process to reduce suspended and dissolved organic materials using bacteria.
  • Tertiary treatment: Removal of phosphorous (precipitating) and nitrogen (ion-exchange). This prevents eutrophication.

Instrumental Methods of Water Analysis

  • pH Analysis: Using a pH sensor.
  • Atomic Absorption Spectroscopy (AAS): Detects the concentration of heavy metals in water by measuring the amount of light absorbed by the metal.

Mandatory Experiment: Free Chlorine

  • Procedures for determining free chlorine concentration in swimming pool water using a comparator or a colorimeter.

Exam Questions - 2012

  • Questions with explanations to calculate total hardness in a water sample or determine total suspended and dissolved solids, calculate pH, or determine free chlorine in a water sample

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