Question

Three beakers contain clear, colorless liquids. One beaker contains pure water, another contains salt water, and another contains sugar water. How can you tell which beaker is which? (No tasting allowed!)

Short answer

Perform an electrical conductivity test with a conductivity tester. The beaker with a bright glow indicates saltwater, a dim glow indicates sugar water, and no glow indicates pure water. Additionally, conduct a precipitation test by adding silver nitrate solution to each liquid. Formation of a white precipitate indicates saltwater. Combining the results of both tests will help identify the contents of each beaker.

Step-by-step solution

List the distinguishing properties

Firstly, we need to identify properties that can help us distinguish between pure water, saltwater, and sugar water. Some properties to consider are density, boiling point, electrical conductivity, and solubility.

Test for electrical conductivity

One test that can be conducted is the electrical conductivity test. Pure water is a poor conductor of electricity, while saltwater (containing dissolved ions) is a good conductor, and sugar water is a moderate conductor due to the absence of free ions. To perform this test, dip two ends of a conductivity tester (which can be obtained from a science lab) into each beaker one by one. If the bulb glows brightly, it indicates that the solution has a high electrical conductivity like saltwater. If the bulb glows dimly, it suggests a moderate conductivity, which points to sugar water. If the bulb doesn't glow at all, the liquid is a poor conductor and likely pure water.

Test for forming a precipitate

To further confirm your findings from the conductivity test, we can perform a precipitation test. Adding a solution of silver nitrate (AgNO3) to the saltwater will form a white precipitate of silver chloride (AgCl), whereas this will not happen in the case of pure water or sugar water. In a test tube, add a few drops of silver nitrate solution to a small amount of each liquid. Observe the reaction. If a white precipitate forms, it is an indication of saltwater. If no reaction occurs, it can either be pure water or sugar water, and our choice can be based on the results of the electrical conductivity test in Step 2.

Conclusion

By combining the results from the electrical conductivity test (Step 2) and the precipitation test (Step 3), we can accurately identify which beaker contains pure water, saltwater, and sugar water without having to taste them.

Key concepts

Electrical Conductivity of Solutions

Understanding the electrical conductivity of solutions is crucial for differentiating substances dissolved in water. Electrical conductivity refers to a solution's ability to conduct an electric current. This property is directly linked to the presence of charged particles or ions in the solution.

When a substance such as table salt (sodium chloride, NaCl) dissolves in water, it dissociates into positive sodium ions (Na+) and negative chloride ions (Cl-). These ions are free to move in the solution and can carry an electric current, which results in a high electrical conductivity.

On the other hand, when sugar (such as sucrose) dissolves in water, it does not form ions. The molecules stay intact and do not conduct electricity well, resulting in a solution with low electrical conductivity. Pure water, without any dissolved substances, conducts electricity poorly because it lacks free ions.

To test for electrical conductivity, one might use a conductivity tester with two electrodes. If a solution allows the electric current to pass easily, it shines a light on the presence of ions, such as those found in saltwater. This is a practical and informative experiment that clearly distinguishes between ionic and molecular solutions.

Precipitation Reactions

Precipitation reactions are a tell-tale sign of certain ions present in a solution. When two solutions are mixed and an insoluble substance forms, this substance is known as a precipitate, and the reaction is termed a precipitation reaction.

To visualize this concept, let's consider the reaction between a solution of silver nitrate (AgNO₃) and saltwater containing sodium chloride (NaCl). When silver nitrate is added to saltwater, it reacts with the chloride ions (Cl-) to form silver chloride (AgCl), a white solid precipitate. This reaction can be represented by the chemical equation:
\[\begin{equation} AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq) \tag{precipitation reaction} \end{equation}\]
A precursor to understanding precipitation reactions is the concept of solubility rules, which dictate which ionic compounds are soluble in water and which are not. As silver chloride is not soluble in water, it precipitates out of the solution, making this test a powerful method to confirm the presence of chloride ions in saltwater.

Physical Properties of Solutions

The physical properties of solutions, such as density, boiling point, and the ability to conduct electricity, differ based on the substances dissolved in them. These properties allow us to differentiate between different types of solutions without the need to taste or visually identify them.

Density is a measure of how much mass is contained in a given volume. Saltwater is denser than both pure water and sugar water because the dissolved salt increases the mass without significantly increasing the volume.

The boiling point of a solution is another key property. A solution with a non-volatile solute, like salt or sugar, will boil at a higher temperature than pure water due to a phenomenon called boiling point elevation. The increase in boiling point is proportionate to the number of particles dissolved.

The aforementioned electrical conductivity test is an example of using the physical properties of solutions to distinguish among them. By understanding these properties, you can identify unknown substances or determine the presence of particular solutes, as demonstrated in the exercise with pure water, saltwater, and sugar water.