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Pollution and hard water treatment by precipitation

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Intros
Lessons
  1. What is 'pollution' and hard water?
  2. Using precipitation to clean water.
  3. Worked calculation: Using Ksp and equilibrium to treat water.
  4. Hard water.
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Examples
Lessons
  1. Use the Ksp expression to calculate concentrations of aqueous ions in treating water pollution.

    A sample of industrial waste water has a Pb2+ concentration of 7.9*10-4 M, and 'acceptable levels' of lead ions in water have been stated as a maximum of 7.25*10-10 M.

    What amount of OH- ions need to be added to precipitate lead ions, so that [Pb2+] drops to the acceptable level at most? The Ksp for Pb(OH)2 is 1.43*10-20 M.1
    1. Use the Ksp expression to calculate concentrations of aqueous ions in treating water pollution.

      A sample of industrial waste water has a Pb2+ concentration of 6.5*10-3 M, and 'acceptable levels' of lead ions in water have been stated as a maximum of 7.25*10-10 M.

      What amount of CO32- ions need to be added to precipitate lead ions, so that [Pb2+] drops to the acceptable level at most? The Ksp for PbCO3 is 7.4*10-14 M.1

      1 Reference for solubility constant data: http://www4.ncsu.edu/~franzen/public_html/CH201/data/Solubility_Product_Constants.pdf
      Topic Notes
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      Introduction: Pollution and Hard Water Treatment by Precipitation

      Pollution and hard water are significant environmental challenges that affect water quality worldwide. This section explores the crucial role of precipitation in addressing these issues through effective water treatment processes. The accompanying introduction video provides a comprehensive overview of pollution sources and the impact of hard water on daily life and industrial operations. Precipitation, a key chemical process, is extensively used in water treatment to remove contaminants and soften hard water. This method involves adding chemicals to water, causing dissolved pollutants to form solid particles that can be easily filtered out. Understanding precipitation is essential for developing efficient water treatment strategies to combat pollution and mitigate the effects of hard water. The video demonstrates various precipitation techniques and their applications in real-world scenarios, highlighting the importance of this process in ensuring clean, safe water for both domestic and industrial use. By mastering precipitation methods, we can significantly improve water quality and contribute to environmental conservation efforts.

      Understanding Pollution and Its Impact on Water

      Water pollution is a critical environmental issue that affects ecosystems and human health worldwide. One significant aspect of water pollution is the presence of harmful ions, particularly heavy metals, which can have devastating effects on living organisms. To understand this concept, it's essential to differentiate between safe and dangerous ions in water systems.

      Not all ions present in water are harmful. In fact, some are necessary for life and are considered safe in appropriate concentrations. For example, sodium and potassium ions are vital for cellular functions and are naturally present in many water sources. These ions play crucial roles in maintaining proper hydration, nerve function, and muscle contraction. However, when the balance of these ions is disrupted or when harmful ions are introduced, water pollution occurs.

      Dangerous ions, particularly heavy metal ions like lead and cadmium, pose significant threats to water quality and ecosystem health. Unlike beneficial ions, these harmful substances can accumulate in living organisms over time, leading to a range of adverse effects. Lead, for instance, is a notorious water pollutant that can cause severe neurological damage, especially in children. Cadmium, another toxic heavy metal, can lead to kidney damage and bone fragility when ingested over long periods.

      The accumulation of heavy metal ions in water systems occurs through various pathways. Industrial discharge, mining activities, and improper waste disposal are common sources of these pollutants. Once released into water bodies, these ions can persist for extended periods, making their removal challenging and costly. As they move through the food chain, heavy metals become more concentrated in a process called biomagnification. This means that organisms at higher trophic levels, including humans, are at greater risk of exposure to dangerous concentrations of these pollutants.

      The effects of heavy metal accumulation on living organisms are far-reaching and often severe. Aquatic life, such as fish and shellfish, are particularly vulnerable to these pollutants. Exposure can lead to reduced growth rates, reproductive issues, and even death in extreme cases. In humans, chronic exposure to heavy metals through contaminated water can result in a wide array of health problems, including cancer, organ damage, and developmental issues in children.

      Addressing water pollution caused by harmful ions requires a multifaceted approach. This includes implementing stricter regulations on industrial discharge, improving wastewater treatment technologies, and promoting sustainable practices in agriculture and manufacturing. Additionally, regular monitoring of water quality and public awareness campaigns are crucial in mitigating the risks associated with water pollution.

      In conclusion, understanding the difference between safe and dangerous ions in water is crucial for addressing water pollution effectively. While some ions are essential for life, the presence of harmful ions, especially heavy metals, poses significant risks to ecosystems and human health. By recognizing the sources and impacts of these pollutants, we can work towards developing more effective strategies for protecting our water resources and the life they support.

      Precipitation as a Water Treatment Method

      Precipitation is a crucial process in water treatment that effectively removes harmful substances from water, ensuring it's safe for consumption and various uses. This method involves transforming dissolved contaminants into solid particles that can be easily separated from the water. The precipitation process in water treatment consists of three main steps: the dissolved state, suspension, and precipitate formation.

      In the first step, contaminants exist in a dissolved state within the water. These dissolved substances can include heavy metals, minerals, and other pollutants that are not visible to the naked eye. At this stage, the water appears clear, but it may contain potentially harmful elements that need to be removed.

      The second step involves creating a suspension. This is achieved by adding specific chemicals, known as coagulants or flocculants, to the water. These chemicals interact with the dissolved contaminants, causing them to form tiny particles. As these particles begin to form, they create a cloudy appearance in the water, known as a suspension.

      The final step is precipitate formation. As the suspension continues to develop, the tiny particles grow larger and heavier. Eventually, they become too heavy to remain suspended in the water and begin to settle at the bottom of the treatment tank. This settled material is called the precipitate, and it contains the harmful substances that were originally dissolved in the water.

      Two important terms in this process are supernatant and sedimentation. The supernatant refers to the clear liquid that remains above the precipitate. This water is now largely free of the contaminants that have been removed through precipitation. Sedimentation is the process by which the precipitate settles to the bottom of the tank due to gravity, effectively separating it from the clean water above.

      Precipitation plays a vital role in removing a wide range of harmful substances from water. It is particularly effective in eliminating heavy metals such as lead, mercury, and arsenic, which can be extremely dangerous to human health even in small quantities. The process is also useful for removing excess minerals that can cause water hardness, as well as certain organic compounds and phosphates that can contribute to water pollution.

      One of the key advantages of precipitation in water treatment is its versatility. By adjusting the types and amounts of chemicals used, water treatment facilities can target specific contaminants based on the composition of the source water. This allows for a customized approach to water purification that can address various water quality issues effectively.

      Moreover, precipitation is often used in conjunction with other water treatment methods to achieve optimal results. For example, after the precipitation process, the water may undergo filtration to remove any remaining fine particles. This multi-step approach ensures that the final treated water meets strict quality standards and is safe for distribution.

      In industrial applications, precipitation is also valuable for treating wastewater before it's released back into the environment. This helps prevent the discharge of harmful pollutants into natural water bodies, protecting ecosystems and maintaining environmental balance.

      While precipitation is highly effective, it's important to note that the process requires careful management. The chemicals used in precipitation must be added in precise amounts to achieve the desired results without introducing new contaminants to the water. Additionally, the precipitate that forms during the process must be properly disposed of to prevent environmental contamination.

      In conclusion, precipitation is a fundamental and highly effective method in water treatment. By transforming dissolved contaminants into solid particles that can be easily removed, this process plays a crucial role in providing clean, safe water for communities worldwide. Understanding the steps of dissolution, suspension, and precipitate formation, as well as key concepts like supernatant and sedimentation, is essential for appreciating the complexity and importance of this water treatment technique.

      Using KSP and Ion Concentration in Pollution Treatment

      Understanding how to use the solubility product constant (KSP) and ion concentration is crucial in addressing pollution problems, particularly in water treatment. Let's explore a detailed example of how these concepts can be applied to reduce lead contamination in water, a significant environmental concern.

      Consider a scenario where lead pollution has been detected in a water source. The goal is to reduce the lead concentration to safe levels using hydroxide ions. This process involves calculating the required hydroxide ion concentration to precipitate lead out of the solution.

      Step 1: Identify the problem and given information
      - Initial lead concentration: 1.0 x 10^-5 M
      - Safe lead concentration: 1.0 x 10^-8 M
      - KSP of lead(II) hydroxide [Pb(OH)2]: 1.2 x 10^-15

      Step 2: Write the chemical equation for lead(II) hydroxide precipitation
      Pb^2+ + 2OH^- Pb(OH)2 (s)

      Step 3: Express the KSP equation
      KSP = [Pb^2+][OH^-]^2 = 1.2 x 10^-15

      Step 4: Calculate the required lead ion concentration
      To reach the safe level, we need to reduce [Pb^2+] to 1.0 x 10^-8 M

      Step 5: Substitute the values into the KSP equation
      1.2 x 10^-15 = (1.0 x 10^-8)[OH^-]^2

      Step 6: Solve for [OH^-]
      [OH^-]^2 = (1.2 x 10^-15) / (1.0 x 10^-8)
      [OH^-]^2 = 1.2 x 10^-7
      [OH^-] = (1.2 x 10^-7) = 3.46 x 10^-4 M

      Step 7: Interpret the result
      To reduce the lead concentration from 1.0 x 10^-5 M to 1.0 x 10^-8 M, we need to add hydroxide ions to achieve a concentration of 3.46 x 10^-4 M.

      This example demonstrates how KSP and ion concentration calculations can be used to determine the amount of treatment needed in pollution control. By understanding the solubility product constant of lead(II) hydroxide and manipulating ion concentrations, we can effectively precipitate excess lead from the water.

      The process can be applied to other metal pollutants as well. For instance, similar calculations could be used to treat water contaminated with copper, cadmium, or mercury. The key is to know the KSP value of the metal hydroxide (or other insoluble compound) and the initial concentration of the pollutant.

      In practice, water treatment plants might use this information to determine the amount of lime (Ca(OH)2) or sodium hydroxide (NaOH) to add to contaminated water. These compounds dissociate to provide the necessary hydroxide ions for precipitation.

      It's important to note that while this method is effective, it's not the only approach to water treatment. Advanced techniques like ion exchange, reverse osmosis, and activated carbon filtration are also used in combination with chemical precipitation for comprehensive water purification.

      Understanding and applying these chemical principles is crucial for environmental scientists, water treatment specialists, and chemical engineers working on pollution control. By leveraging KSP and ion concentration calculations, they can develop efficient and targeted solutions to reduce harmful pollutants in our water systems, ensuring safer and cleaner water for communities.

      Hard Water: Causes and Consequences

      Hard water is a common household issue that affects millions of people worldwide. This phenomenon occurs when water contains high levels of dissolved minerals, primarily calcium and magnesium ions. Understanding the causes and consequences of hard water is essential for homeowners and businesses alike.

      The formation of hard water is a natural process that begins with rainwater. As rain falls, it absorbs carbon dioxide from the atmosphere, forming a weak carbonic acid. When this slightly acidic rainwater comes into contact with limestone or other calcium-rich rocks, it dissolves small amounts of calcium and magnesium. These dissolved minerals then travel with the water as it moves through the ground and into our water supply.

      Acid rain, which is more acidic than normal rainwater due to pollutants, can accelerate this process. The increased acidity of acid rain allows it to dissolve more minerals from rocks and soil, potentially leading to higher concentrations of calcium and magnesium ions in the water.

      The presence of these minerals in water results in several noticeable effects. One of the most immediate impacts is on the taste of the water. Hard water often has a distinct mineral taste that some people find unpleasant. While not harmful to health, this taste can affect the flavor of beverages and food prepared with hard water.

      Perhaps the most significant consequence of hard water is the formation of lime scale. As hard water flows through pipes and appliances, it leaves behind mineral deposits. Over time, these deposits build up, forming a hard, chalky substance known as lime scale. This buildup can reduce the efficiency of water heaters, clog pipes, and shorten the lifespan of appliances like dishwashers and washing machines.

      Another notable effect of hard water is its impact on soap and detergent effectiveness. The calcium and magnesium ions in hard water react with soap molecules, forming a sticky substance called soap scum. This reaction reduces the cleaning power of soaps and detergents, often requiring more product to achieve the same level of cleanliness. It can also leave a residue on clothes, dishes, and even skin and hair after washing.

      In addition to these issues, hard water can cause other problems in households and industries. It can lead to spotting on glassware and dishes, leave mineral stains on clothing, and reduce the lather of shampoos and soaps. In industrial settings, hard water can cause scaling in boilers and cooling towers, reducing their efficiency and increasing maintenance costs.

      While hard water is generally not a health concern, its effects can be costly and inconvenient. Many homeowners choose to address hard water issues through water softening systems, which remove calcium and magnesium ions through an ion exchange process. These systems can effectively mitigate the problems associated with hard water, improving water quality for household use.

      Understanding the causes and consequences of hard water is crucial for effective water management. By recognizing the signs of hard water and taking appropriate measures, homeowners and businesses can protect their plumbing systems, extend the life of their appliances, and enjoy better-quality water for daily use. Whether through water softening systems or other treatment methods, addressing hard water issues can lead to significant improvements in water quality and overall household efficiency.

      Treatment Methods for Hard Water

      Hard water treatment is an essential process for improving water quality in many households and industries. The presence of dissolved calcium and magnesium ions in water leads to its hardness, which can cause various problems such as scale buildup in pipes and reduced efficiency of soap and detergents. There are two types of hard water: temporary and permanent, each requiring different treatment methods.

      Temporary hard water contains dissolved calcium hydrogen carbonate and magnesium hydrogen carbonate. This type of hardness can be treated relatively easily through heating or by adding chemicals like sodium carbonate (Na2CO3), also known as washing soda. When temporary hard water is heated, the following reaction occurs:

      Ca(HCO3)2 CaCO3 + H2O + CO2

      This process, known as boiling out, causes the calcium carbonate to precipitate, effectively removing the hardness from the water. The heat treatment method is particularly useful for small-scale applications, such as in households.

      On the other hand, permanent hard water contains dissolved calcium and magnesium sulfates or chlorides. Unlike temporary hardness, permanent hardness cannot be removed by simple heating. Instead, it requires chemical treatment methods, with sodium carbonate being one of the most effective and commonly used substances.

      When sodium carbonate is added to permanent hard water, it reacts with the calcium and magnesium ions to form insoluble carbonates, which precipitate out of the solution. The reactions can be represented as follows:

      CaSO4 + Na2CO3 CaCO3 + Na2SO4

      MgCl2 + Na2CO3 MgCO3 + 2NaCl

      The addition of sodium carbonate effectively removes both calcium and magnesium ions from the water, treating the permanent hardness. This method is widely used in industrial settings and large-scale water treatment plants due to its efficiency and cost-effectiveness.

      The use of sodium carbonate in hard water treatment is an excellent example of Le Chatelier's principle in action. This principle states that when a system at equilibrium is subjected to a change, the system will adjust to counteract that change. In the case of hard water treatment, the addition of carbonate ions shifts the equilibrium, causing the precipitation of calcium and magnesium carbonates.

      It's worth noting that while sodium carbonate is highly effective for treating both temporary and permanent hard water, there are other methods available. Ion exchange systems, such as water softeners, use resin beads to replace calcium and magnesium ions with sodium ions. Reverse osmosis is another advanced technique that can remove various dissolved solids, including those responsible for water hardness.

      In conclusion, the treatment of hard water is crucial for maintaining the efficiency of water systems and improving overall water quality. While temporary hard water can be treated through simple heating, permanent hard water requires chemical intervention. Sodium carbonate stands out as a versatile and effective treatment method for both types of hard water, making it a popular choice in various applications. Understanding the differences between temporary and permanent hard water and their respective treatment methods is essential for selecting the most appropriate and efficient water softening solution.

      Conclusion: The Importance of Precipitation in Water Treatment

      In this article, we've explored the crucial role of precipitation in water treatment, addressing both pollution and hard water issues. The introduction video provided a visual foundation for understanding these complex processes. Precipitation emerges as a powerful tool in removing contaminants and softening water, making it safer for consumption and industrial use. This method's effectiveness in tackling various pollutants and minerals underscores its significance in environmental protection and public health. By grasping these concepts, readers can better appreciate the intricacies of water treatment and its impact on our daily lives. We encourage you to apply this knowledge in real-world scenarios, whether it's understanding your local water treatment facilities or making informed decisions about home water systems. Continue exploring water treatment processes to enhance your environmental awareness and contribute to sustainable water management practices. Remember, clean water is not just a necessity; it's a responsibility we all share in preserving our planet's most precious resource.

      What is 'pollution' and hard water? Using precipitation to clean water.

      Step 1: Understanding Pollution and Hard Water

      Pollution refers to the presence of harmful substances in the environment, which can adversely affect the health of living organisms and the quality of natural resources. In the context of water, pollution often involves the introduction of contaminants such as heavy metals, chemicals, and biological agents that can make the water unsafe for consumption and use.

      Hard water, on the other hand, is water that contains high concentrations of dissolved minerals, particularly calcium (Ca2+) and magnesium (Mg2+) ions. While these minerals are not necessarily harmful to health, they can cause problems such as scaling in pipes and appliances, reducing the effectiveness of soaps and detergents, and leaving residues on surfaces.

      Step 2: The Role of Precipitation in Water Treatment

      Precipitation is a chemical process used to remove dissolved ions from water by converting them into solid particles that can be easily filtered out. This process is particularly useful for treating water contaminated with heavy metals and other pollutants that are harmful to health.

      In the context of water treatment, precipitation involves adding a reagent to the water that reacts with the dissolved ions to form an insoluble compound. This compound then precipitates out of the solution as a solid, which can be removed through filtration or sedimentation.

      Step 3: The Process of Precipitation

      The process of precipitation can be broken down into three main steps:

      • Initial State: In the initial state, the water contains dissolved ions that are individually dispersed and too small to see. This could be water from a natural source like a river or ocean, or water that has been contaminated by industrial processes.
      • Formation of Suspension: When a reagent is added to the water, it reacts with the dissolved ions to form an insoluble compound. This compound forms tiny particles that create a suspension in the water. At this stage, the particles are visible but have not yet settled out of the solution.
      • Formation of Precipitate: Over time, the particles in the suspension settle out of the solution to form a precipitate. This process, known as sedimentation, results in the formation of a solid layer at the bottom of the container, with the remaining liquid (supernatant) above it. The supernatant is now free of the harmful ions that were removed through precipitation.

      Step 4: Practical Applications and Considerations

      Precipitation is a widely used technique in water treatment for removing heavy metals and other contaminants. It is particularly effective for treating water with high concentrations of ions that have low solubility, such as lead (Pb2+) and cadmium (Cd2+).

      In practical applications, the choice of reagent and the conditions under which precipitation is carried out can significantly affect the efficiency of the process. Factors such as pH, temperature, and the presence of other ions can influence the solubility of the target ions and the formation of the precipitate.

      Once the precipitate has formed and settled, it can be removed through filtration or other separation techniques. The remaining water, now free of the harmful ions, can be safely used for drinking, irrigation, or other purposes.

      Step 5: Conclusion

      Using precipitation to treat pollution and hard water is an effective method for improving water quality. By converting dissolved ions into solid particles that can be easily removed, precipitation helps to eliminate harmful contaminants and reduce the negative effects of hard water. Understanding the principles and steps involved in this process is essential for developing efficient water treatment strategies and ensuring access to clean, safe water.

      FAQs

      1. What is precipitation in water treatment?

        Precipitation in water treatment is a chemical process where dissolved contaminants are converted into solid particles that can be easily removed from water. This process involves adding specific chemicals to the water, which react with the dissolved pollutants, causing them to form insoluble compounds that settle out of the solution. This method is particularly effective for removing heavy metals, excess minerals, and certain organic compounds from water.

      2. How does precipitation help in treating hard water?

        Precipitation is crucial in treating hard water by removing excess calcium and magnesium ions. Chemicals like sodium carbonate (Na2CO3) are added to the water, which react with these ions to form insoluble carbonates. These carbonates precipitate out of the solution, effectively softening the water. This process is particularly useful for treating both temporary and permanent hardness, making water more suitable for domestic and industrial use.

      3. What is the difference between temporary and permanent hard water?

        Temporary hard water contains dissolved calcium and magnesium hydrogen carbonates, which can be treated by simple heating or boiling. Permanent hard water, on the other hand, contains calcium and magnesium sulfates or chlorides, which cannot be removed by heating alone and require chemical treatment methods like the addition of sodium carbonate.

      4. How does KSP (solubility product constant) relate to water treatment?

        The solubility product constant (KSP) is crucial in water treatment as it helps determine the concentration of ions needed to precipitate a specific compound. By understanding the KSP of various contaminants, water treatment specialists can calculate the exact amount of chemicals needed to effectively remove pollutants through precipitation. This knowledge allows for more efficient and targeted water treatment processes.

      5. What are the environmental impacts of water hardness and its treatment?

        While hard water itself is not typically harmful to the environment, its treatment can have both positive and negative impacts. Softening water reduces scale buildup in pipes and appliances, potentially saving energy and extending equipment lifespan. However, some water softening methods, like ion exchange, can increase sodium levels in water, which may affect soil and plant health if used for irrigation. It's important to balance the benefits of water treatment with potential environmental considerations.

      Prerequisite Topics

      Understanding the fundamental concepts that underpin complex environmental issues is crucial for developing effective solutions. When it comes to "Pollution and hard water treatment by precipitation," one of the most essential prerequisite topics is the solubility constant, also known as the solubility product constant. This concept is vital for grasping the intricacies of water treatment processes and pollution control mechanisms.

      The solubility product constant plays a pivotal role in understanding how substances dissolve in water and, more importantly, how they can be removed from water through precipitation. This knowledge is directly applicable to hard water treatment and pollution control strategies. Hard water, which contains high levels of dissolved minerals like calcium and magnesium, can be treated effectively using precipitation methods that rely on the principles of solubility.

      By mastering the solubility product, students can better comprehend how pollutants interact with water and other substances in the environment. This understanding is crucial for developing innovative techniques to remove contaminants from water sources. For instance, when dealing with heavy metal pollution in water bodies, knowing how to manipulate solubility can lead to more efficient precipitation-based treatment methods.

      Furthermore, the concept of solubility constant is essential for predicting the behavior of pollutants in different environmental conditions. This knowledge allows environmental scientists and engineers to design more effective treatment systems and develop strategies for preventing pollution at its source. By understanding how various factors affect solubility, professionals can optimize water treatment processes and create more sustainable solutions for managing water quality.

      In the context of hard water treatment, the solubility product constant is particularly relevant. It helps explain why certain minerals precipitate out of solution under specific conditions, which is the basis for many water softening techniques. By applying this knowledge, water treatment specialists can design more efficient systems to remove hardness-causing ions from water, improving its quality for both domestic and industrial use.

      Students who have a solid grasp of the solubility product are better equipped to tackle complex environmental challenges. They can more easily understand the chemical processes involved in water treatment and pollution control, leading to more innovative and effective solutions. This prerequisite knowledge serves as a foundation for advanced studies in environmental science, chemistry, and engineering, enabling students to contribute meaningfully to the field of water quality management and pollution prevention.

      In conclusion, the importance of understanding prerequisite topics like the solubility constant cannot be overstated when studying pollution and hard water treatment by precipitation. It provides the necessary framework for analyzing complex environmental issues and developing practical solutions. By investing time in mastering these fundamental concepts, students set themselves up for success in addressing some of the most pressing environmental challenges of our time.

      In this lesson, we will learn:

      • How precipitation is used to treat water pollution and hard water.
      • To recall the meaning of the terms suspension and supernatant.
      • How to use Ksp and ion concentrations to solve problems related to hard water and pollution.

      Notes:

      • Being able to precipitate ions out of solution is very important in chemistry. Removing impurities or ‘treating’ a water sample is similar to cleaning a work space.
        • Some dirt/stains are very light and don’t interfere with general working. Dust builds up but can be removed quickly too, so a general dry wipe across a surface is enough.
          In chemistry, light metal ions such as Na+ or K+ are fairly safe in living environments. They are very soluble in water but are carried around quite easily too, and they do not build up to dangerous levels very often. Many living organisms actually need these lighter ions for living processes.
        • On the other hand, some dirt/stains build up slowly but they are harder to remove and can make workspaces harder to use. Sometimes we need to use chemicals specifically designed to remove dirt.
          Transition metal ions and heavy metal ions such as Pb2+ and Cd2+ have high mass, build up where they are produced and interfere with living processes. These properties make them harmful to living organisms in rivers or other aquatic environments.

      • Although they are harmful, heavy metals can be precipitated out of solution. When we precipitate the harmful heavy metal ions, they are converted from the aqueous state (where they cause harm), to the solid state which can be trapped and filtered out of the solution. There are three stages we need to describe in this process:
        • The solution, as we already know is when the ions are still dissolved.
        • The suspension, where the aqueous ions have been precipitated but have not settled in the container/system.
        • The precipitate, where the ions have settled at the bottom of the container. The remaining solution without the precipitate is called the supernatant.
        Precipitating pollutant ions out of solution can be done using the principles from Predicting a precipitate and Solubility product.

      • Worked example:

        A sample of waste water has a Pb2+ concentration of 5*10-4 M, and ‘safe levels’ of lead ions in water can be said to be a maximum of 7.25*10-8 M.

        What amount of OH- ions need to be added to precipitate lead ions, so that [Pb2+] drops to the safe level at most? The Ksp for Pb(OH)2 is 1.43*10-20 M.1

        First, write the equilibrium that will occur.

        Pb(OH)2 (s)  ⇌\, \rightleftharpoons \enspace Pb2+(aq) + 2OH-(aq)

        Now write the Ksp expression for this using the value above:

        Ksp = 1.43 * 10-20 = [Pb2+][OH-]2

        [OH-]2 = 1.43∗10−207.25∗10−8\large \frac{1.43 * 10^{-20}}{7.25 * 10^{-8}} = 1.97 * 10-13 M

        [OH-] = 1.97∗10−13\sqrt{1.97 * 10^{-13}} = 4.44 * 10-7 M


        The calculation above is finding the hydroxide ion concentration that causes the equilibrium with Pb(OH)2 (s) to be established, so if [Pb2+] greater than the 7.25*10-8 M safe level was reached, the equilibrium would simply shift to make more precipitate.

      • Treating hard water involves precipitation as well. Hard water is caused by calcium (Ca2+) and magnesium (Mg2+) ions dissolved in water when limestone (CaCO3) and MgCO3 in the ground reacts with rain water (which is slightly acidic) and gets into the water supply.

        CaCo3 (s) ⇌\enspace \rightleftharpoons \enspace Ca2+(aq) + CO2 (g) + H2O(l)


        Hard water has consequences in daily life:
        • It gives drinking water a bad smell/taste.
        • The dissolved ions can precipitate in piping and other places where it is heated, which can reduce the flow of pipes and make some appliances less useful. 
        • It makes soap less effective because the active ingredient in soap forms a precipitate with the Ca2+ and Mg2+ ions instead of cleaning the dirt you are using the soap to clean!

      • Hard water is treated by using Na2CO3. This will precipitate CaCO3 again by the original equilibrium. Both CaCO3 and MgCO3 have low solubility.
        • Permanent hard water is when the water does not contain any HCO3- \, so it must be added.
        • Temporary hard water already contains HCO3-  \, so heating the water can remove hardness.