Concentration

In chemistry, concentration describes how much of a specific substance is present within a mixture. It is calculated by taking the abundance of a single constituent and dividing it by the total volume of the mixture. Most often, scientists use this term when discussing solutions. A solution is made of a solute, which is the substance being added, and a solvent, which is the liquid doing the mixing. Understanding concentration is vital because it allows scientists to measure exactly what is inside a liquid or gas.

There are several ways to change the concentration of a solution. To concentrate a solution, you must increase the amount of solute or reduce the amount of solvent. For example, adding more alcohol to a mixture would increase its concentration. Conversely, you can dilute a solution to lower its concentration. Dilution is the process of reducing concentration, usually by adding more solvent. You can also dilute a solution by reducing the amount of solute present. These actions change the ratio of the parts to the whole.

Scientists use four main mathematical descriptions to define concentration. The first is mass concentration, which is the mass of a constituent divided by the total volume. Its SI unit is kg/m³, though it can also be expressed as g/L. The second is molar concentration, or amount concentration. This is the amount of a constituent in moles divided by the volume. While the SI unit is mol/m³, scientists more commonly use mol/L. The third is number concentration, which counts the number of entities in a volume. Its SI unit is 1/m³. Finally, volume concentration is the volume of a constituent divided by the total volume. This is expressed as a number, such as 0.18 or 18%.

Sometimes, a solution reaches a physical limit known as saturation. A saturated solution occurs when no more solute can dissolve into the solvent. If you try to add more solute to a saturated solution, it will typically not dissolve. Instead, phase separation occurs. This means the substances exist in different phases, such as being completely separated or forming a suspension. In certain special circumstances, a solution may reach a state called supersaturation. The exact point at which a solution becomes saturated depends on many variables. These include the ambient temperature and the precise chemical nature of both the solute and the solvent.

Temperature plays a significant role in how concentration behaves. Concentration depends on how the volume of a solution changes with temperature. This happens because of thermal expansion, where substances expand as they get warmer. Because concentration is tied to volume, these temperature shifts change the concentration levels. This is why scientists must be careful when measuring substances in different environments. Even a small change in heat can alter the mathematical concentration of a mixture.

It is important to distinguish concentration from other related quantities. For example, molality is the amount of a constituent in moles divided by the mass of the solvent alone. Its unit is mol/kg. Another term is mole fraction, which is the amount of a constituent in moles divided by the total amount of all constituents. For very small amounts, scientists often use "parts-per" notation, such as ppm (parts per million) or ppb (parts per billion). Other related terms include the mole ratio, mass fraction, and mass ratio. While these describe the composition of a mixture, they are not technically called concentrations.

In everyday language, people often describe concentration qualitatively. They might use the word "dilute" for a solution with a low concentration. They might use the word "concentrated" for a solution with a high concentration. This is similar to how doctors might discuss medical levels. For instance, a person might have high serum levels of bilirubin in their blood serum. This means the concentration of bilirubin is higher than what is considered normal. By using these precise measurements, scientists can move from simple observations to exact mathematical truths.