Solutes Solvents and Solutions

Solvent: Definition

Let's start with the definition of a solvent.

For example, if you added some cocoa powder to a glass of milk and stirred, the cocoa powder would dissolve in the solvent, which is milk in this case!

$$ \text{Solute (Cocoa powder) + Solvent (Milk) = Solution (Chocolate milk) } $$

Now, a solvent's ability to dissolve another substance depends on its molecular structure. The three types of molecular structures of solvents are polar protic solvents, dipolar aprotic solvents, and non-polar solvents.

Polar protic solvents consist of a molecule containing a polar OH group and a non-polar tail. Its structure is represented by the formula R-OH. Some common polar protic solvents include water (H₂O), methanol (CH₃OH), Ethanol (CH₃CH₂OH), and acetic acid (CH₃COOH).

• Only polar compounds are soluble in polar protic solvents. H₂O however, can dissolve nonpolar substances too!

Dipolar aprotic solvents are usually molecules with a large bond dipole moment. They do not have an, OH, group. Acetone ((CH₃)₂C=O) is a common example of a dipolar aprotic solvent.

Non-polar solvents are immiscible in water, and they are considered lipophilic. In other words, they dissolve non-polar substances like oils and fats. Examples of non-polar solvents include Carbon tetrachloride (CCl₄), diethyl ether (CH₃CH₂OCH₂CH₃), and benzene (C₆H₆).

Solvent: Examples

While water (H₂O) is the most important inorganic solvent, there are many other solvents that can be used to dissolve solutes and form solutions. Some examples of inorganic solvents are concentrated sulfuric acid (H₂SO₄), and liquid ammonia (NH₃).

For instance, zinc carbonate (ZnCO₃) can be dissolved in sulfuric acid (H₂SO₄) to form zinc sulfate (ZnSO₄), water (H₂O) and Carbon dioxide (CO₂) as products (figure 1)!

Figure 1. Chemical reaction between zinc carbonate and sulfuric acid, Isadora Santos - StudySmarter Originals.

What about organic solvents? Organic solvents can be oxygenated, hydrocarbon, or halogenated solvents. As the name suggests, oxygenated solvents are that contain oxygen. These solvents have many applications including dissolving paints! Examples of oxygenated solvents are alcohols, ketones, and esters.

Hydrocarbon solvents contain only hydrogen and carbon atoms. Hexane, gasoline, and kerosene are examples of hydrocarbon solvents.

Halogenated solvents are organic solvents that have halogen atoms present. Halogens atoms are those found in group 17 on the Periodic Table, such as chlorine (Cl), fluorine (F), bromine (Br), and iodine (I). Examples include trichloroethylene (ClCH-CCl₂), chloroform (CHCl₃), tetrafluoromethane (CF₄), bromomethane (CH₂Br), and iodoethane (C₂H₅I)

Solute: Definition

Now, let's dive into solutes. The definition of solute is shown below.

Think about air, for example. Air is a gaseous solution in which Nitrogen is the solvent and oxygen and all the other gases are the solutes! Another example is carbonated water. In carbonated water, carbon dioxide (CO₂) gas is the solute and H₂O is the solvent.

Solubility

When dealing with solutes and solvents, there is a very important term that you need to be familiar with: Solubility. To be soluble, the attractive forces that are formed between solute and solvent need to be comparable to those bonds broken in the solute and in the solvent.

Solubility depends on three things: type of solute and solvent, temperature, and pressure (for gases).

• The solutes that dissolve in polar solvents are polar molecules, while the solutes that dissolve in non-polar solvents are non-polar molecules. Like dissolves like.
• As temperature increases, solids become more soluble and gases become less soluble. When added to hot water, sugar, for example, dissolves much better than when added to cold water!
• Gases are more soluble at higher pressures.

Solution: Definition

Now that we know that solutes dissolve in solvents to make solutions, let's look at the definition of a solution.

$$ \text{Solute + Solvent = Solution } $$

The process of formation a solution occurs in three steps (figure 2). First, the attractive forces of the solute particle break, causing the separation of solute particles. Then, the separation of solvent particle occurs in the same way. Lastly, attractive forces are formed between the solute and solvent particles.

Now, let's explore the different types of solution that can be formed. Solid-liquids solutions are the most common type of solution. Here, a solid is dissolved in a liquid.

Although it might sound strange, solid-solid solutions also exist. These solutions can be formed when a solid gets dissolved in another solid. Alloys are the best example of solid-solid solutions.

• An alloy is a combination of two one more metals, or metals with nonmetallic elements. Steel is an alloy of iron with a very small amount of carbon.

Gas-liquid solutions are solutions resulting from a gas dissolving in a liquid. Carbonated soda is an example of a gas-liquid solution.

When a gas dissolves in another gas, gas-gas solutions are formed. Air is an example of gas-gas solution!

Lastly, we have liquid-liquid solutions. These solutions are formed when a liquid is dissolved in another liquid.

Solute and Solution: Examples

Depending on the amount of solute added to a solvent, we can have either saturated, unsaturated, or supersaturated solutions. So, let's talk about what these solutions are and look at some examples!

A saturated solution is a solution in which no more solute can be dissolved in it. In other words, it is a solution where the maximum amount of solute has dissolved in the solvent. For example, if you added sodium chloride (NaCl) to a glass of water until no more of the salt dissolves in the water, you have a saturated solution.

On the other hand, we have unsaturated solutions. An unsaturated solution is a solution that has the ability to dissolve more solute. Unsaturated solutions contain less than the maximum amount of solute possible. So, you added more solute to it, it would dissolve.

Now, if a solution holds more solute than is normally possible, it becomes a supersaturated solution. This type of solution usually forms from a saturated solution when heated to high temperatures. If all the material in the saturated solution is dissolved by heating and allowed to cool it will often remain a homogenous solution; no precipitate will form. If a crystal of the pure solute is added to the cooled homogenous supersaturated solution a precipitate of this solute will form. This technique is often used in an Organic Chemistry lab to obtain pure compounds.

Molarity

When mixing a solution, there are two main things chemists need to know: the amount of solute and solvent to use, and the Concentration of the solution.

To calculate concentration, we can use the formula for Molarity (M) as concentration is often measured in units of Molarity. The equation for Molarity is as follows:

$$Molarity\,(M\,or\,mol/L)= \frac{moles\,of\,solute\,(mol)}{liters\,of,solution\,(L)}$$

Difference between Solute and Solution

To finish off, let's look at the differences difference between solvent, solute, and solution.

Now, I hope that you feel more confident in your understanding of solutes and solutions!