Solutions: Introduction

Relevance of the Topic

Solutions are one of the main pillars of chemistry, used in a wide range of applications, from medications to paints. They play a crucial role in our daily lives and in the industry. Understanding the components and characteristics of solutions is essential for all subfields of chemistry and is a fundamental skill for any chemistry student.

Contextualization

Solutions are a natural extension of the study of elements and compounds, introduced in the chemistry curriculum in early high school. They are homogeneous mixtures, meaning that all their components are intimately mixed on a molecular scale. This atomic union allows us to explore the basic principles of chemistry, such as diffusion, vapor pressure, and intermolecular collaboration.

Studying solutions also leads us directly to the concepts of concentration and molarity, fundamental tools for problem-solving in chemistry. Furthermore, understanding dilute, concentrated, saturated, and supersaturated solutions prepares us for the study of chemical equilibria in advanced disciplines.

Theoretical Development

• Solution Components: A solution is composed of a solute (what is dissolved) and a solvent (the medium in which the substance is dissolved). The ratio of solute to solvent gives rise to what we call the solution's concentration.

• Homogeneous Mixtures: Solutions are examples of homogeneous mixtures, which means their properties (such as color, density, and taste) are uniform throughout the mixture. Each component of the solution is distributed uniformly.

• Dilution and Concentration: Dilution and concentration of a solution are reversible processes. Dilution occurs when more solvent is added to the solution, decreasing the solute concentration. Concentration, on the other hand, occurs when the amount of solute in the solution is increased, raising the concentration.

• Types of Solutions: Solutions can be classified as dilute solutions (small amount of solute relative to the solvent), concentrated solutions (large amount of solute relative to the solvent), saturated solutions (maximum amount of solute the solvent can dissolve at a given temperature), and supersaturated solutions (contain more dissolved solute than allowed by a saturated solution, which can precipitate the extra solute with an external stimulus).

• Colligative Properties: Solutions exhibit colligative properties, which depend on the number of solute particles in the solution and not on their identity. The main colligative properties are: lowering of vapor pressure, elevation of boiling point, depression of freezing point, and osmotic pressure.

Key Terms

• Solution: A homogeneous mixture of two or more substances, in which the solute (what is dissolved) is uniformly dispersed in the solvent (the substance that dissolves the solute).

• Solute: The substances that are dissolved in a solution. It can be a solid, liquid, or gas.

• Solvent: The substance that dissolves, that is, the component of the solution that is present in greater quantity.

• Concentration: The amount of solute dissolved per unit volume of solvent. Concentration is usually expressed in grams of solute per liter of solution.

• Molality: The ratio between the amount of solute (in moles) and the mass of the solvent (in kg). Molality is mainly used in calculations of colligative properties.

Examples and Cases

• Example 1 - Solubility of Table Salt: The solubility of table salt (NaCl) in water is approximately 35g/100ml at 25°C. Therefore, if we add 35g of salt to 100ml of water at 25°C, we will have a saturated solution.

• Example 2 - Dilution of Powdered Juice: If we add 1 tablespoon of powdered juice to 200ml of water, we will have a diluted solution. However, if we add the same tablespoon of powdered juice to 100ml of water, the solution will be concentrated.

• Case 1 - Freezing of a Lake: In very cold climates, lakes can freeze completely because the addition of solute (salt) to the lake water lowers its freezing point, thanks to the depression of the freezing point, a colligative property.

• Case 2 - Boiling Water with Salt: If we add salt to water, the boiling point will be higher than 100°C, being an example of the elevation of the boiling point, another colligative property. This is exploited when cooking fruits in syrup, for example. Here, water and sugar form a solution, and the addition of sugar raises the boiling point of water, causing the fruit to cook at a temperature higher than 100°C.

Detailed Summary

Key Points

• Solutions are homogeneous mixtures, characterized by the presence of a solute dissolved in a solvent.
• Dilution and concentration are reversible processes that alter the balance between solute and solvent in the solution.
• Solubility is the maximum amount of solute a solvent can dissolve at a given temperature, determining whether a solution will be diluted, concentrated, or saturated.
• Solutions can be classified as dilute solutions (small amount of solute relative to the solvent), concentrated solutions (large amount of solute relative to the solvent), saturated solutions (maximum amount of solute the solvent can dissolve at a given temperature), and supersaturated solutions (more dissolved solute than allowed by a saturated solution, which can precipitate the extra solute with an external stimulus).
• Solutions exhibit colligative properties, which are dependent on the number of solute particles present and not on their identity.

Conclusions

• Understanding the concept of solutions, their characteristics, and properties is essential for the study of chemistry and for various practical applications.
• The ability to determine the concentration of a solution and perform calculations related to the amount of solute and solvent in a solution is an essential skill.
• The colligative properties of solutions, which depend on the number of solute particles present, provide additional tools for analyzing the nature of solutions.

Exercises

1. Exercise 1: Create a comparative table to differentiate between dilute, concentrated, saturated, and supersaturated solutions, presenting a practical example for each of these solutions.

2. Exercise 2: If a solution contains 10 grams of salt in 200ml of water, what is the concentration of the solution in g/L? And if this solution is diluted to 500ml without adding more solute, what will be the new concentration of the solution in g/L?

3. Exercise 3: Explain why adding salt (NaCl) to a pot of boiling water causes the water to take longer to evaporate, using the concept of the colligative property of elevation of the boiling point.