Three words describe every solution, and they are easy to keep straight. The solute is whatever gets dissolved. The solvent is whatever does the dissolving.
The solution is the uniform mixture that results. Stir a spoonful of table salt into a glass of water and the salt is the solute, the water is the solvent, and the clear salty liquid is the solution. Nothing about that salt disappeared.
The mass of the solution equals the mass of the salt plus the mass of the water, and boiling the water away leaves the salt behind. Dissolved solute particles are far too small to see or to filter out, and they never settle to the bottom. That last point is what separates a true solution from a suspension like muddy water.
How do you tell which substance is the solute?
Two rules cover almost every case. First, if one substance changes state during mixing, that one is the solute, so solid salt entering liquid water is the solute. Second, when both components start in the same state, the one present in the smaller amount is the solute.
In rubbing alcohol that is 70 percent isopropanol and 30 percent water, the water is the solute. One convention overrides that rule: chemists call water the solvent in any water-based solution, even concentrated sulfuric acid, where the acid is the larger share. Solutes are not always solid.
Sugar is a solid solute, the ethanol in a tincture is a liquid solute, and the carbon dioxide that fizzes out of soda is a gaseous solute. Solutions do not have to be liquid either. Brass is a solid solution of zinc solute in copper solvent, and air is a gaseous solution in which oxygen and argon are solutes in nitrogen.
What actually happens when a solute dissolves?
Dissolving is a tug of war between forces. Solvent particles crowd the surface of the solute and pull individual particles away from the bulk. Each freed particle then ends up surrounded by a shell of solvent, a process called solvation, or hydration when the solvent is water.
Dissolving only proceeds when the solute-solvent attractions are strong enough to repay the energy spent pulling the solute apart, which is why oil will not dissolve in water. This is also why dissolving is not melting. Melting takes one pure substance and adds heat until the solid turns liquid.
Dissolving needs a second substance to do the work, and it happens far below the solute’s melting point. Table salt melts at 801 degrees Celsius, yet it dissolves readily in room-temperature water.
Electrolyte and nonelectrolyte solutes
Not every solute breaks apart the same way, and the difference matters. Ionic solutes such as sodium chloride dissociate: NaCl separates into Na+ and Cl- ions, so one formula unit yields two dissolved particles. The solution conducts electricity, and the solute is called an electrolyte.
Molecular solutes such as sucrose dissolve as whole, intact molecules. One sucrose molecule gives one dissolved particle, the solution does not conduct, and the solute is a nonelectrolyte. Strong acids and bases are electrolytes too, while weak acids like acetic acid only partly ionize.
How is solute concentration measured?
Molarity is the workhorse: M equals moles of solute divided by liters of solution. Read that wording carefully, because it is a classic exam trap. The denominator is the final volume of the whole solution, not the volume of solvent you started with.
Mass percent is the mass of solute divided by the mass of solution, times 100. Parts per million is that same ratio times one million, used for very dilute solutions like fluoride in drinking water. Worked example: dissolve 29.2 g of sodium chloride in enough water to reach a total volume of 500.0 mL.
The molar mass of NaCl is 58.44 g/mol, so moles = 29.2 g / 58.44 g/mol = 0.500 mol. Convert the volume, since 500.0 mL = 0.5000 L. Then M = 0.500 mol / 0.5000 L = 1.00 M.
Solubility and saturation
A solvent can only hold so much solute. About 36 grams of sodium chloride dissolve in 100 grams of water at room temperature, and anything beyond that sits on the bottom. A solution holding the maximum is saturated, one holding less is unsaturated, and one coaxed into holding more is supersaturated and unstable.
Heating usually raises the solubility of solid solutes but lowers it for gases, which is why warm soda goes flat fast.
Why does salt melt ice on roads?
Adding any solute lowers a solvent’s freezing point and raises its boiling point. These are colligative properties, meaning they depend on how many dissolved particles are present, not on what those particles are. For water, each mole of dissolved particles per kilogram of solvent lowers the freezing point by 1.86 degrees Celsius and raises the boiling point by 0.512 degrees Celsius.
Because sodium chloride splits into two ions, a 1-molal salt solution drops the freezing point by roughly 2 x 1.86 = 3.72 degrees Celsius, about twice the effect of 1-molal sugar. That is why road crews spread salt, not sugar, though it stops working well below about -9 degrees Celsius.