Question

Design an Experiment You suspect the water in a lake close to your school might contain lead in the form of \(\mathrm{Pb}^{2+}(\mathrm{aq})\) ions. Formulate your suspicion as a hypothesis and design an experiment to test your theory. Write the net ionic equations for the reactions of your experiment. \(\left(\text { Hint: In aqueous solution, } P b^{2+} \text { forms com- }\right.\) pounds that are solids with \(C l^{-}, B r^{-}, I^{-},\) and \(S O_{4}^{2-}\) ions.)

Short answer

The hypothesis for this experiment is: "The water in the lake close to the school contains lead in the form of \(\mathrm{Pb}^{2+}(\mathrm{aq})\) ions." To test this hypothesis, collect a water sample from the lake and prepare four separate solutions containing \(C l^{-}, B r^{-}, I^{-}\), and \(S O_{4}^{2-}\) ions. Add a few drops of each prepared solution sequentially to separate portions of the collected lake water sample while observing any changes. Record which solutions, if any, produced a solid precipitate. The possible net ionic equations for the reactions are: 1. \(\mathrm{Pb}^{2+}(\mathrm{aq}) + 2C l^{-}(\mathrm{aq}) \rightarrow \mathrm{PbCl}_2(\mathrm{s})\) 2. \(\mathrm{Pb}^{2+}(\mathrm{aq}) + 2B r^{-}(\mathrm{aq}) \rightarrow \mathrm{PbBr}_2(\mathrm{s})\) 3. \(\mathrm{Pb}^{2+}(\mathrm{aq}) + 2I^{-}(\mathrm{aq}) \rightarrow \mathrm{PbI}_2(\mathrm{s})\) 4. \(\mathrm{Pb}^{2+}(\mathrm{aq}) + S O_{4}^{2-}(\mathrm{aq}) \rightarrow \mathrm{PbSO}_4(\mathrm{s})\) These equations may be used to interpret the results of the experiment and confirm the presence of lead ions in the water.

Step-by-step solution

Formulate the Hypothesis

The hypothesis for this experiment is: "The water in the lake close to the school contains lead in the form of \(\mathrm{Pb}^{2+}(\mathrm{aq})\) ions."

Collect Water Sample

Collect a water sample from the lake in a clean glass beaker or container.

Prepare Reaction Solutions

Prepare four separate solutions containing \(C l^{-}, B r^{-}, I^{-}\), and \(S O_{4}^{2-}\) ions. These solutions can be prepared by dissolving a soluble salt containing the respective anion like sodium chloride (NaCl), sodium bromide (NaBr), potassium iodide (KI), and sodium sulfate (Na2SO4) in distilled water.

Add Reaction Solutions to the Water Sample

Add a few drops of each prepared solution sequentially to separate portions of the collected lake water sample while observing any changes. If a precipitate forms after adding any of the solutions, it may indicate the presence of \(\mathrm{Pb}^{2+}(\mathrm{aq})\) ions in the water.

Observe and Record Results

Record which solutions, if any, produced a solid precipitate in the lake water sample. This information will allow us to determine if lead ions are present.

Write Net Ionic Equations

Write the net ionic equation for each reaction that occurred in the experiment. The following net ionic equations represent the possible reactions between \(\mathrm{Pb}^{2+}(\mathrm{aq})\) ions and the added \(C l^{-}, B r^{-}, I^{-}\), and \(S O_{4}^{2-}\) ions: 1. \(\mathrm{Pb}^{2+}(\mathrm{aq}) + 2C l^{-}(\mathrm{aq}) \rightarrow \mathrm{PbCl}_2(\mathrm{s})\) 2. \(\mathrm{Pb}^{2+}(\mathrm{aq}) + 2B r^{-}(\mathrm{aq}) \rightarrow \mathrm{PbBr}_2(\mathrm{s})\) 3. \(\mathrm{Pb}^{2+}(\mathrm{aq}) + 2I^{-}(\mathrm{aq}) \rightarrow \mathrm{PbI}_2(\mathrm{s})\) 4. \(\mathrm{Pb}^{2+}(\mathrm{aq}) + S O_{4}^{2-}(\mathrm{aq}) \rightarrow \mathrm{PbSO}_4(\mathrm{s})\) These equations may be used to interpret the results of the experiment and confirm the presence of lead ions in the water.

Key concepts

Designing Scientific Experiments

The process of crafting a scientific experiment involves several critical steps, each of which contributes to ensuring that the findings are reliable and valid. To begin, a clear and testable hypothesis is essential, like the one formulated in the exercise about lead water contamination: 'The water in the lake near the school contains lead in the form of \(\mathrm{Pb}^{2+}(\mathrm{aq})\) ions.' The hypothesis should be based on observations or theoretical knowledge.

Once the hypothesis is set, you'll want to define the controlled, independent, and dependent variables. In this experiment, the independent variable is the type of anion introduced to the water sample. The dependent variable is whether a precipitate forms, indicating the presence of lead ions. Ensuring that other factors remain consistent across all test conditions is key to isolate the effect of the independent variable.

Furthermore, a rigorous experimental design requires repeatability. Other scientists should be able to replicate the process and obtain similar results. This is achieved by using precise quantities of reagents and documenting the procedure thoroughly. Lastly, it's crucial to avoid contamination and maintain a high level of care when collecting samples and reagents to guarantee the experiment's integrity.

Net Ionic Equations

Net ionic equations play a pivotal role in illustrating the occurrences at the molecular level during chemical reactions in aqueous solutions. These equations simplify reactions to show only the species that actually change during the reaction. To craft a net ionic equation, one must first write a balanced molecular equation and then convert it to a complete ionic equation by dissociating all strong electrolytes into their constituent ions.

Afterwards, spectator ions, which remain unchanged and do not participate directly in the reaction, are omitted, yielding the net ionic equation. For instance, when a solution containing \(\mathrm{Pb}^{2+}\) ions reacts with \(Cl^{-}\), the net ionic equation is: \[\mathrm{Pb}^{2+}(\mathrm{aq}) + 2\mathrm{Cl}^{-}(\mathrm{aq}) \rightarrow \mathrm{PbCl}_2(\mathrm{s})\]. This equation tells us that lead (II) ions combine with chloride ions to form a precipitate of lead (II) chloride.

Environmental Chemistry

Environmental chemistry is an interdisciplinary field focusing on the chemical processes affecting the environment's composition and quality. When investigating contaminants such as lead in water systems, environmental chemists apply their expertise to understand the implications for ecosystems as well as human health. Lead poisoning can have severe consequences, and its detection in water supplies is a cause for concern.

Lead's effects are evident in biological systems, and environmental chemists work on detecting and mitigating such pollutants. Through experiments like the one described in the exercise, scientists study the distribution and chemical behavior of substances in the water and develop methods to prevent or remediate contamination. Understanding the real-world outcomes of reactions, such as the formation of lead precipitates, is critical for assessing risks and implementing appropriate measures to protect public health and the environment.

Precipitation Reactions

Precipitation reactions are a cornerstone of analytic chemistry and are particularly pertinent when identifying the presence of certain ions in a solution. During these reactions, two soluble salts react in an aqueous solution to form an insoluble solid, known as a precipitate. The conditions of the reaction are defined by the solubility rules, which guide us in predicting whether a precipitate will form when two ionic compounds are mixed together.

For example, in the experiment where lead (II) ion presence is tested, adding a solution of a halide like \(\mathrm{Cl}^{-}\) to a solution containing \(\mathrm{Pb}^{2+}\) ions leads to the formation of lead (II) chloride, a solid precipitate. This outcome, as captured in the net ionic equations written earlier, enables us to conclude whether lead is present in the analyzed water sample. Understanding precipitation reactions not only helps in analytical methods but also in purifying substances and in treating environmental pollutants.