Solutions

A solution is a homogeneous mixture of two or more substances. In any solution, the component present in smaller quantity is called the solute, while the component present in larger quantity is the solvent. Solutions can exist in all three states of matter, with liquid solutions (specifically aqueous solutions, where water is the solvent) being the most common in board examinations. Depending on the amount of solute dissolved, solutions are classified as unsaturated, saturated, or supersaturated. A saturated solution contains the maximum amount of dissolved solute at a given temperature, establishing a dynamic equilibrium with undissolved solute.

Understanding concentration units is crucial for numerical problems in board exams. Students must master molarity ($M$), which is defined as the number of moles of solute dissolved per cubic decimeter ($dm^3$) or liter of solution. The formula $M = \frac{\text{moles of solute}}{\text{volume of solution in } dm^3}$ frequently appears in short and long numerical questions. Other ways to express concentration include percentage composition (mass/mass, mass/volume, and volume/volume) and parts per million (ppm), which is used for very dilute solutions like pollutants in water.

Solubility is heavily tested, particularly the famous rule "like dissolves like". Polar solutes dissolve in polar solvents (e.g., NaCl in water), while non-polar solutes dissolve in non-polar solvents (e.g., grease in benzene). Factors affecting solubility include temperature and the nature of the solute and solvent. Generally, the solubility of solid solutes increases with temperature, whereas the solubility of gases in liquids decreases with an increase in temperature (explained by Henry's Law).

Finally, the chapter distinguishes between solutions, suspensions, and colloids based on particle size. Solutions have particles smaller than $1\text{ nm}$ that do not settle out or scatter light (no Tyndall effect). Suspensions have large particles ($>100\text{ nm}$) that settle over time, while colloids have intermediate particle sizes ($1\text{ nm}$ to $100\text{ nm}$) and clearly exhibit the Tyndall effect. Board exams frequently ask for differentiating points or examples among these three types of mixtures.