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.
Solute, solvent, and solutionSaturated, unsaturated, and solubilitySolubility of solids and gasesDensity and relative densityMeasuring mass, volume, and density
Key terms
Solute
The substance that dissolves in a solvent; usually the solid component or the component present in smaller amount.
Solvent
The substance in which the solute dissolves; usually the liquid or the component present in larger amount.
Saturated solution
A solution in which no more solute can dissolve at a given temperature.
Solubility
The maximum amount of solute that can dissolve in a fixed quantity of solvent at a particular temperature.
Density
The mass present in a unit volume of a substance, calculated as mass divided by volume.
Relative density
The ratio of the density of a substance to the density of water at the same temperature, a number with no units.
Extra questions & answers
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In a solution of salt in water, what is the salt called?
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