A true solution is a homogeneous mixture where one substance (the solute) dissolves completely into another (the solvent), creating a uniform composition throughout. If you’re trying to sort fact from fiction about solutions, the core truths come down to particle size, physical behavior, and how solutions differ from other mixtures like suspensions and colloids. Here are the statements that hold up.
Solute Particles Do Not Settle or Separate
In a true solution, the dissolved particles are incredibly small, typically individual molecules or ions. Because of their size, they never settle to the bottom of the container, no matter how long the solution sits undisturbed. This is one of the clearest distinctions between a solution and a suspension. In a suspension, particles are large enough that gravity pulls them down over time, and you can usually filter them out. In a true solution, standard filtration does nothing because the solute particles slip right through filter paper along with the solvent.
Solutions Are Homogeneous Mixtures
Every sample you take from a solution has the same composition as every other sample. Stir salt into water until it dissolves, and the concentration of salt is identical whether you test the top, middle, or bottom of the glass. This uniformity is what makes solutions “homogeneous,” and it’s a defining property. A suspension of sand in water, by contrast, is heterogeneous because the sand concentrates unevenly.
Solutions Can Exist in Any State of Matter
Most people picture a liquid when they think of a solution, but solutions also exist as solids and gases. The air you breathe is a gas-in-gas solution: roughly 78% nitrogen and 21% oxygen, with trace amounts of other gases mixed uniformly throughout. Brass is a solid solution (called an alloy) where zinc is dissolved into copper. Bronze combines copper and tin. Even 22-carat gold jewelry is a solid solution, with copper or silver dissolved into gold to make it harder and more scratch-resistant. Steel is an alloy of iron with carbon.
Light Passes Straight Through
When you shine a beam of light through a true solution, the light travels cleanly through without scattering. You won’t see the beam’s path. This is because the dissolved particles are too small to deflect light waves. In a colloidal mixture like fog or milk, the dispersed particles are large enough to scatter light in all directions, creating a visible beam. This scattering is called the Tyndall effect, and its absence is one of the classic tests for identifying a true solution.
Solutions Have a Solvent and at Least One Solute
Every solution has two roles: the solvent does the dissolving, and the solute gets dissolved. When the two components are in different physical states (like salt in water), the solvent is the one that keeps its state. When both are in the same state (like two liquids mixing), the solvent is typically whichever substance is present in a greater amount. A solution can contain more than one solute. Seawater, for example, has dozens of dissolved salts and gases in a single water solvent.
Adding Solute Changes Physical Properties
Dissolving a substance into a solvent changes several measurable physical properties, and these changes depend only on how many solute particles are present, not what those particles are. These are called colligative properties, and four of them matter most:
- Boiling point rises. A solution boils at a higher temperature than the pure solvent. This is why adding salt to water makes it boil at slightly above 100°C.
- Freezing point drops. A solution freezes at a lower temperature than the pure solvent. Road salt works by lowering the freezing point of water on pavement.
- Vapor pressure decreases. The solvent evaporates less readily when solute is dissolved in it, because fewer solvent molecules are at the surface to escape.
- Osmotic pressure increases. A solution draws pure solvent toward it through a semipermeable membrane. The more concentrated the solution, the stronger this pull.
All four effects trace back to the same mechanism: solute particles dilute the solvent, reducing the tendency of solvent molecules to escape into the gas phase or organize into a solid crystal.
Solutions Can Be Saturated, Unsaturated, or Supersaturated
An unsaturated solution still has room for more solute to dissolve. A saturated solution has reached its limit: the rate at which solute dissolves equals the rate at which dissolved particles come back out of solution, creating a dynamic equilibrium. You may see undissolved material sitting at the bottom. A supersaturated solution holds more dissolved solute than the saturated limit would normally allow. It’s unstable. Adding a single crystal or even a small vibration can trigger rapid crystallization, as the excess solute precipitates out all at once.
Some Solutions Conduct Electricity
Whether a solution conducts electricity depends on what’s dissolved in it. When an ionic compound like table salt dissolves in water, it splits into charged particles (ions) that carry electrical current. These solutions are called electrolytes. Strong electrolytes break apart completely and conduct well. Weak electrolytes only partially split, producing low conductivity.
Molecular compounds like sugar dissolve without forming ions. They stay as intact molecules in solution, which means the solution does not conduct electricity. These are nonelectrolytes. So the statement “all solutions conduct electricity” is false, but “some solutions conduct electricity” is true.
Temperature and Pressure Affect How Much Dissolves
For most solid solutes, raising the temperature increases solubility. Warm water dissolves more sugar than cold water. A few solids behave differently, but the general trend holds. For gases, the relationship flips: gases become less soluble as temperature rises, which is why a warm soda goes flat faster than a cold one.
Pressure mainly matters for gases dissolving in liquids. Higher pressure forces more gas into solution. This is the principle behind carbonated drinks: carbon dioxide is dissolved under high pressure, and when you open the cap, the pressure drops and the gas escapes as bubbles. For solids and liquids dissolving in liquids, pressure changes have very little effect on solubility.
Concentration Describes How Much Solute Is Present
Concentration is simply the amount of solute relative to the amount of solvent or solution. It can be expressed several ways. Molarity measures moles of solute per liter of solution. Molality measures moles of solute per kilogram of solvent. Mass fraction (or mass percent) compares the mass of the solute to the total mass of the solution. Mole fraction compares the number of solute particles to the total number of particles. Each unit serves a different purpose, but they all quantify the same basic idea: how “strong” or “weak” the solution is.