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

Chapter 9, 'The Amazing World of Solutes, Solvents, and Solutions', covers solutions (uniform mixtures), the definitions of solute and solvent, saturated and unsaturated solutions, solubility and how temperature affects it, solubility of gases, and the concept of density — including its formula, units, measurement methods, and how it explains why objects float or sink.

In a solid-liquid solution the solid component is the solute and the liquid is the solvent. When two liquids form a solution, the component present in smaller amount is the solute and the one in larger amount is the solvent.

An unsaturated solution is one in which more solute can still be dissolved at a given temperature. A saturated solution is one in which the maximum amount of solute has been dissolved and no more can dissolve at that temperature — any extra solute settles at the bottom.

Solubility is the maximum amount of solute that can be dissolved in a fixed quantity (100 mL) of a solvent or solution at a particular temperature.

For most solid substances, solubility increases with an increase in temperature — a saturated solution at a lower temperature becomes unsaturated if temperature is raised. For gases, the opposite is true: solubility of gases in liquids generally decreases as temperature increases. This is why cold water contains more dissolved oxygen than warm water.

Density is defined as the mass present in a unit volume of a substance. The formula is: Density = Mass ÷ Volume. The SI unit of density is kg/m³; for liquids, g/mL and g/cm³ are also used.

No. The density of a substance is independent of its shape or size. However, it does depend on temperature and pressure.

Water has a special property: its density is highest at 4 °C. When water cools further and freezes at 0 °C, the particles arrange themselves in a way that takes up more space (expansion). Because the same mass now occupies a larger volume, the density of ice is lower than that of liquid water, so ice floats.

Generally, the density of a substance decreases with heating and increases with cooling. When heated, particles spread out, increasing volume while mass stays the same; since Density = Mass/Volume, the density decreases. This is why hot air is less dense than cool air and rises — the principle behind hot-air balloons.

Use the water-displacement method: fill a measuring cylinder to a known volume (e.g., 50 mL), submerge the irregular object in it, and record the new water level (e.g., 55 mL). The difference (5 mL = 5 cm³) is the volume of the object.

When water is in a measuring cylinder, it forms a curved surface called the meniscus. For water and other colourless liquids, you read the mark that coincides with the bottom of the meniscus, with your eyes level with that mark. For coloured liquids, the mark should coincide with the top of the meniscus.

Oxygen dissolves in water only to a small extent, but even this tiny amount of dissolved oxygen sustains all aquatic life — fish, plants, and other organisms. Cold water holds more dissolved oxygen than warm water, which is why warming water bodies can threaten aquatic ecosystems.

Relative density of a substance with respect to water equals the density of that substance divided by the density of water at that temperature. For example, aluminium with a density of 2.7 g/cm³ has a relative density of 2.7. It is a dimensionless number (no units).

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