CBSE Class 8 Annual Assessment

Chapter 9: The Amazing World of Solutes, Solvents, and Solutions

Summary

This chapter studies solutions and the related idea of density. A solution is a uniform mixture; when a solid dissolves in a liquid, the dissolving solid is the solute and the liquid is the solvent, and together they form the solution. When two liquids mix, the one present in smaller amount is the solute. A fixed amount of solvent can dissolve only a limited amount of solute: a solution that can dissolve more is unsaturated, while one that cannot dissolve any more at that temperature is saturated. The amount of solute in a fixed quantity is the concentration, described as dilute or concentrated, and the maximum that can dissolve is the solubility. For most solids, solubility increases with temperature, while for gases it generally decreases, which is why cold water holds more dissolved oxygen for aquatic life. The chapter then explains why objects float or sink using density, defined as mass per unit volume, with the SI unit kilogram per cubic metre. Density depends on temperature and, for gases, on pressure; relative density compares a substances density to that of water. Students learn to measure mass with a balance, volume of liquids with a measuring cylinder (reading the meniscus), volume of regular solids by calculation, and volume of irregular solids by water displacement, then combine these to calculate density. The floating of ice is explained by its lower density.

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