Chemical solutions. Concept and concentration

Concentration of a chemical solution

The concentration of a chemical solution indicates the amount of solute present in a given amount of solvent or total solution. It is a way of expressing how “strong” or “diluted” a homogeneous mixture is, and is fundamental in many chemical processes, both in the laboratory and in industry. There are different ways to express the concentration of a chemical solution, among which the following stand out:

Percentage by mass (% m/m)

Indicates the grams of solute per 100 grams of solution. Example: a solution with 5 g of salt in 100 g of water has a concentration of 5% m/m.

Percentage by volume (% v/v)

Expresses the milliliters of solute per 100 mL of solution. Very common in liquid mixtures, such as alcohol in water.

Molarity (M)

Defined as the moles of solute (n) dissolved per liter of solution.

M = n / V

This unit is especially useful in stoichiometric calculations of chemical reactions.

Molality (m)

Moles of solute per kilogram of solvent. It is useful when precision is required in temperature variations.

Molar fraction (χ)

This is the ratio between the moles of a component and the total moles present in the solution. It is commonly used in physical chemistry.

The concentration of a solution directly affects its physical and chemical properties, such as:

• • • The boiling point (increases with concentration – ebullioscopic effect).
    • The freezing point (decreases with concentration – cryoscopic effect).
    • Osmotic pressure (important in biology and cellular processes).
    • Electrical conductivity, especially in electrolyte solutions.
    • These variations are known as colligative properties and depend on the number of solute particles present, not their chemical identity.

Preparation of a chemical solution

There are different ways of preparing a solution. The most common is the solid-liquid solution, where the solute is added to the solvent and stirred until it is completely dissolved. Liquid-liquid and gaseous-liquid solutions can also be prepared using techniques such as stirring, heating or using specific apparatus such as ultrasound.

Saturated and supersaturated solutions

Some solutions have special properties, such as saturated solutions, which contain the maximum amount of solute that can be dissolved in a given amount of solvent at a given temperature. If more solute is added, it will not dissolve and a precipitate will form. On the other hand, supersaturated solutions contain a larger amount of solute than can normally be dissolved, and are more unstable and can easily precipitate.

Importance of chemical solutions

Solutions are of great importance in chemistry and in many aspects of our daily lives. For example, salt water is a common solution found in the oceans and seas. Also, many medicines are solutions in which the active ingredients are dissolved in a suitable solvent.