2.4 Mixtures and Solutions

Key Concepts

1. Mixtures

A mixture is a combination of two or more substances that are not chemically bonded together. Mixtures can be separated into their individual components through physical means. There are two main types of mixtures: homogeneous and heterogeneous.

2. Solutions

A solution is a type of homogeneous mixture where one substance (the solute) is dissolved in another substance (the solvent). Solutions are uniform in composition and do not settle out over time.

3. Solubility

Solubility is the maximum amount of a solute that can be dissolved in a given amount of solvent at a specific temperature. Solubility varies with different solutes and solvents.

4. Concentration

Concentration refers to the amount of solute present in a given amount of solution. It can be expressed in various units, such as molarity, percentage by mass, and parts per million (ppm).

Detailed Explanation

Mixtures

Mixtures can be either homogeneous or heterogeneous. Homogeneous mixtures have a uniform composition throughout, while heterogeneous mixtures have visible differences in composition. Examples of homogeneous mixtures include saltwater and air, while examples of heterogeneous mixtures include sand and water, and oil and water.

Solutions

Solutions are homogeneous mixtures where the solute particles are evenly distributed throughout the solvent. The solute can be a solid, liquid, or gas, and the solvent is typically a liquid. Examples of solutions include sugar dissolved in water, carbon dioxide dissolved in soda, and alcohol in water.

Solubility

Solubility is influenced by temperature, pressure, and the nature of the solute and solvent. For example, sugar is highly soluble in water, while oil is not. Solubility curves show how the solubility of a substance changes with temperature. For instance, the solubility of potassium nitrate (KNO3) increases with temperature.

Concentration

Concentration can be expressed in several ways. Molarity (M) is the number of moles of solute per liter of solution. Percentage by mass is the mass of solute divided by the mass of the solution, multiplied by 100. Parts per million (ppm) is used for very dilute solutions and represents the number of parts of solute per million parts of solution. For example, a 1 M solution of sodium chloride (NaCl) contains 1 mole of NaCl per liter of solution.

Examples and Analogies

Example: Saltwater

Saltwater is a solution where salt (the solute) is dissolved in water (the solvent). The salt particles are evenly distributed throughout the water, making it a homogeneous mixture. You can taste the saltiness throughout the solution, indicating uniformity.

Analogy: Mixture as a Salad

Think of a salad as a heterogeneous mixture. It contains various ingredients like lettuce, tomatoes, and cucumbers, each with its own distinct properties. When mixed together, you can still see and separate the individual components.

Example: Carbonated Water

Carbonated water is a solution where carbon dioxide gas (the solute) is dissolved in water (the solvent). The concentration of carbon dioxide can vary, affecting the fizziness of the water.

Analogy: Concentration as Coffee Strength

Think of coffee as a solution. The strength of the coffee (concentration) depends on how much coffee grounds (solute) are dissolved in the water (solvent). A strong coffee has a high concentration of coffee grounds, while a weak coffee has a low concentration.