5 Solutions and Concentration
Learn how solutions form, how concentration and molarity describe their composition, and how solubility depends on substances and conditions.
Solutions and their components
A is a homogeneous mixture: its components are evenly distributed at the molecular or ionic level. The is the component that dissolves the others, and each substance dissolved in it is a . A can contain more than one .
In salt water, water is the and dissolved sodium chloride is the . Air is also a , with nitrogen as its main component and oxygen and other gases as solutes.
How substances dissolve
Dissolving does not necessarily mean that a substance reacts chemically. When sugar dissolves in water, its molecules spread throughout the water. When table salt dissolves, its ions separate and become surrounded by water molecules.
Whether substances mix well depends partly on the attractions between their particles. The guide “like dissolves like” means that polar substances tend to dissolve in polar solvents, while nonpolar substances tend to dissolve in nonpolar solvents. Water and oil do not form one uniform .
and
describes how much is present in a given amount of . A with little is called dilute, while one with more is called concentrated. These terms are comparisons rather than exact quantities unless a unit is given.
Two common ways to report are mass , the mass of per volume of , and , the number of moles of per liter of . Mass may, for example, be expressed in grams per liter, written .
The formula is:
For example, if of sugar is dissolved to make of , the is . The volume in this calculation is the total volume, not just the volume of .
and saturation
is the maximum of a substance that can dissolve in a particular under specified conditions, such as a given temperature and pressure. It describes an equilibrium limit, not how quickly dissolving occurs.
An contains less dissolved than this limit. A holds as much dissolved as it can at equilibrium under those conditions, so additional remains undissolved. A temporarily holds more dissolved than is normally stable at that temperature.
Factors that affect
The identities of the and affect because their particles must interact favorably for dissolving to occur. Polar substances often dissolve well in polar solvents, and nonpolar substances often dissolve better in nonpolar solvents.
Temperature affects in different ways for different kinds of solutes. For most solid solutes in water, increases as temperature rises, though there are exceptions. For gases dissolved in liquids, usually decreases as temperature rises; this is one reason a warm carbonated drink loses dissolved carbon dioxide more readily.
Pressure has its most noticeable effect on gases dissolved in liquids. At constant temperature, increasing the gas’s pressure above a liquid generally increases the amount of gas that dissolves. Lowering the pressure, as when a soda bottle is opened, lets some gas escape.
Dissolving speed and
Stirring and crushing a solid into smaller pieces can help it dissolve faster, but they do not usually change the maximum amount that can dissolve at equilibrium. They affect the rate of dissolving, not usually the limit. Temperature can affect both how quickly dissolving occurs and the itself.