In this lesson, we will explore the concept of solutions, which are homogeneous mixtures made up of two or more components. Solutions can exist in three physical states, depending on the state of the solvent. For example, an alloy is a solid solution, sea water is a liquid solution, and air is a gaseous solution.

There are nine types of solutions, ranging from gas-gas mixtures like the air we breathe to solid-solid mixtures like dental amalgam used for filling teeth. Liquid solutions are the most common type because water is a universal solvent. This leads to a wide variety of liquid solutions, from a single drop of rain to vast oceans. In fact, sea water contains 92 naturally occurring elements, making it a rich resource.

Throughout this lesson, we will define important terms such as solution, aqueous solution, solute, and solvent, and provide examples of each. We will also discuss the differences between saturated, unsaturated, and supersaturated solutions. You will learn how solutions are formed by mixing gases, liquids, and solids, as well as how to calculate the concentration and molarity of a solution. Additionally, we’ll cover how to prepare dilute solutions from concentrated ones and explore the principle of “like dissolves like” to predict solubility.

SOLUTION

A solution is a homogeneous mixture made up of two or more substances, where the boundaries between the components are indistinguishable. This means that a solution appears as a single phase. For example, the air we breathe is a mixture of several gases, and brass is a solid solution of zinc (Zn) and copper (Cu). When sugar dissolves in water, it creates a liquid solution.

One way to tell the difference between a solution and a pure liquid is through evaporation. A pure liquid will completely evaporate, leaving no residue, while a solution will leave behind a residue after evaporation. Alloys like brass or bronze are also considered homogeneous mixtures because they exhibit the properties of their individual components and have a variable composition.

Aqueous Solutions

An aqueous solution is formed when a substance dissolves in water, which is the solvent present in a larger amount. For instance, when sugar or table salt dissolves in water, we create an aqueous solution. These solutions are commonly used in laboratories. Water is often called the universal solvent because it can dissolve many compounds found in the Earth’s crust.

Solute and Solvent

In a solution, the component present in smaller quantity is known as the solute. The solute dissolves in the solvent to form the solution. For example, in a salt solution, salt is the solute, while water is the solvent. A solution can contain multiple solutes; for example, in soft drinks, water is the solvent, and sugar, salts, and carbon dioxide (CO₂) are the solutes.

The component of a solution that is present in larger quantity is called the solvent. The solvent dissolves the solutes, and when there are more than two substances in a solution, one acts as the solvent while the others serve as solutes.

SATURATED SOLUTION

Types of Solutions

When a small amount of solute is added to a solvent, it dissolves easily. However, if more solute is continuously added, there comes a point when the solvent can no longer dissolve any additional solute. At this stage, any extra solute will remain undissolved and settle at the bottom of the container. This is known as a saturated solution, which contains the maximum amount of solute that can dissolve at a given temperature. In a saturated solution, there is a dynamic equilibrium between the dissolved solute and the undissolved solute, meaning that while some solute continues to dissolve and crystallize, the overall amount of dissolved solute stays constant.

Unsaturated Solution

An unsaturated solution contains less solute than needed to reach saturation at a specific temperature. This type of solution can still dissolve more solute until it becomes saturated.

Supersaturated Solution

When a saturated solution is heated, it can dissolve more solute than usual, resulting in a supersaturated solution. This means it has more solute than required for saturation. Supersaturated solutions are unstable; they can easily lose excess solute. To create a supersaturated solution, one can heat a saturated solution, then cool it down slowly. For example, at 20 °C, a saturated solution of sodium thiosulfate (Na₂S₂O₃) contains 20.9 grams of salt per 100 cm³ of water. If you have less than this amount, it’s an unsaturated solution, while more than 20.9 grams would be a supersaturated solution.

Dilution of Solutions

Solutions are also classified as dilute or concentrated based on the amount of solute they contain. A dilute solution has a relatively small amount of dissolved solute, whereas a concentrated solution contains a large amount of solute. For example, brine is a concentrated solution of common salt in water. Diluting a solution involves adding more solvent, which decreases its concentration. The process of preparing dilute solutions from concentrated ones will be discussed further in later sections.

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