The Fundamental Unit of Life

5.1 How Can We Classify Mixtures?

Mixtures are combinations of two or more substances where each substance retains its own properties. They can be classified based on the uniformity of their composition into homogeneous and heterogeneous mixtures. A homogeneous mixture, also called a solution, has a uniform composition throughout. For example, a well-stirred mixture of sugar dissolved in water tastes equally sweet in every sip, indicating uniform distribution of sugar molecules. Other examples include vinegar (acetic acid in water) and aerated drinks like soda (carbon dioxide dissolved in water). Such mixtures do not show visible particles and remain uniform over time. In contrast, a heterogeneous mixture has a non-uniform composition. For instance, a mixture of sand and water shows visible sand particles that settle at the bottom over time. The mixture of oil and water is also heterogeneous as the two liquids do not mix and form separate layers. The classification is important because the method of separation depends on the type of mixture. To explore this, an activity involves preparing three mixtures: salt in water (homogeneous), chalk powder in water (heterogeneous), and milk in water (appears homogeneous but is a colloid). Observations include visibility of particles, light scattering (Tyndall effect), and filtration results. Salt solution remains clear with no residue on filtration, chalk suspension shows visible particles and residue, and milk shows light scattering but no residue, indicating different mixture types. This classification helps in understanding the nature of mixtures and guides the choice of separation techniques.

• Homogeneous mixtures have uniform composition and are called solutions.
• Heterogeneous mixtures have non-uniform composition with visible particles.
• Salt dissolved in water forms a homogeneous mixture.
• Sand in water forms a heterogeneous mixture with visible particles.
• Milk in water is a colloid showing properties between solutions and suspensions.
• Light scattering (Tyndall effect) helps distinguish colloids and suspensions from solutions.
• 📌 Homogeneous mixture: A mixture with uniform composition throughout.
• 📌 Heterogeneous mixture: A mixture with non-uniform composition and visible particles.
• 📌 Solution: A homogeneous mixture of solute dissolved in solvent.

5.2 Solutions

A solution is a homogeneous mixture formed when a solute dissolves in a solvent. The solute is the substance that gets dissolved, and the solvent is the substance that dissolves the solute. For example, in sugar dissolved in water, sugar is the solute and water is the solvent. Solutions have uniform composition and properties throughout. The concentration of a solution refers to the amount of solute present in a given quantity of solvent or solution. It is important to prepare solutions with precise concentrations in various fields such as medicine, agriculture, and food preparation. For example, Oral Rehydration Solution (ORS) is prepared by mixing specified amounts of salt and sugar in a fixed volume of water. Incorrect proportions can render the solution ineffective or harmful. Similarly, farmers must mix pesticides in correct proportions to avoid damage to crops or environment. Concentration can be expressed in different ways, including mass by mass percentage, mass by volume percentage, and volume by volume percentage. These expressions help quantify the amount of solute relative to the solvent or total solution, enabling accurate preparation and use of solutions in practical applications.

• Solutions are homogeneous mixtures of solute dissolved in solvent.
• Solute is the substance dissolved; solvent is the substance that dissolves the solute.
• Concentration indicates the amount of solute in a given amount of solvent or solution.
• Correct concentration is crucial in medicine, agriculture, and food industries.
• ORS preparation requires precise proportions of salt and sugar in water.
• Pesticide solutions must be prepared with correct concentration to be effective and safe.
• 📌 Solution: Homogeneous mixture of solute dissolved in solvent.
• 📌 Solute: Substance that dissolves in a solvent.
• 📌 Solvent: Substance that dissolves the solute.