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!)

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 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.

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.