What is a solution?
A solution is a mixture in which one substance is spread evenly through another, particle by particle. The substance that dissolves is the solute; the substance it dissolves in is thesolvent. Water dissolves so many substances that it is often called the universal solvent, and solutions in water are called aqueous solutions. Dissolving is a physical change: evaporate the water and the solute is left behind, and the total mass is conserved.
How dissolving works
Water molecules are polar: their oxygen end is slightly negative and their hydrogen ends slightly positive. When an ionic solid such as salt dissolves, water molecules attract the ions at the surface of the crystal, pull them out one by one, and surround them. Sodium chloride separates into Na⁺ and Cl⁻ ions. A molecular substance such as sugar dissolves differently: water surrounds whole sugar molecules, which stay intact. The particle view in the simulator shows both.
Unsaturated, saturated and supersaturated
A solution that can still dissolve more solute is unsaturated. When it holds the maximum amount that can stay dissolved at that temperature, it is saturated, and extra solid stays at the bottom. Even then, particles keep leaving the crystals while others rejoin them at the same rate: a dynamic equilibrium. Occasionally a solution holds more than its solubility. This supersaturatedsolution forms when a saturated solution is cooled (or loses water) with no crystals for the extra solute to build on. It is unstable: a single seed crystal makes the excess crystallize rapidly.
Solubility and solubility curves
Solubility is the maximum mass of solute that dissolves in a given amount of solvent at a given temperature, often given in grams per 100 g of water. It depends on the substance and on the conditions. A solubility curve plots it against temperature. Most solids dissolve better in hot water, but by very different amounts: potassium nitrate goes from about 32 g to 246 g per 100 g between 20 °C and 100 °C, while table salt barely changes (about 36 g to 39 g). A few, such as lithium sulfate, become slightlyless soluble as temperature rises. Gases always dissolve less in warm water, which is why fizzy drinks go flat faster when warm and why warm water holds less oxygen for fish. For gases, higher pressure increases solubility.
A point on the curve is a saturated solution, a point below it is unsaturated, and a point above it is supersaturated.
Measuring concentration
Concentration tells you how much solute is in a given amount of solution.
mass concentration (g/L) = mass of solute (g) ÷ volume of solution (L)molar concentration (mol/L) = moles of solute ÷ volume of solution (L)percent by mass = mass of solute ÷ mass of solution × 100A solution with a lot of solute is concentrated; one with little is dilute. Adding water dilutes a solution; evaporating water concentrates it. Neither changes the mass of solute.
Dissolving faster
Three things make a solid dissolve faster: stirring, which brings fresh solvent to the crystals; heating, which makes particles move faster; and crushing the solid, which gives it more surface area. Only temperature also changes the solubility. Stirring a saturated solution will not dissolve another gram.
Key terms
- Solute
- The substance that dissolves.
- Solvent
- The substance in which the solute dissolves.
- Solubility
- The maximum amount of solute that dissolves in a given amount of solvent at a given temperature.
- Saturated solution
- A solution that holds as much solute as it can at that temperature.
- Crystallization
- The formation of solid crystals as solute comes out of solution.
Real-world applications
- Medicine: standard intravenous saline contains 0.9 g of sodium chloride in every 100 mL, and doses of liquid medicines depend on their concentration.
- Purifying chemicals: recrystallization dissolves an impure solid in hot solvent and cools it, so pure crystals form.
- Sea salt is made by evaporating seawater until the salt crystallizes.
- Rock candy and fudge rely on supersaturated sugar solutions.
- Hot packs crystallize a supersaturated sodium acetate solution, releasing heat.
Common misconceptions
- “The solute disappears.” It is still there, spread out as tiny particles; the mass of the solution includes it.
- “Stirring makes more dissolve.” It only makes it dissolve faster.
- “Dissolving and melting are the same.” Melting needs heat alone; dissolving needs a solvent.
- “Everything dissolves better when hot.” Gases, and a few solids, do the opposite.
What the model simplifies
The solubility values come from standard data tables (rounded, with straight-line interpolation between listed temperatures); copper(II) sulfate is counted as the anhydrous salt, although blue crystals of copper sulfate actually contain water. Everything else is a teaching model. Dissolving and crystallizing speeds are illustrative, and the dots spread through the water far faster than real particles diffuse. Each 1 mL of water is treated as 1 g, and the molar concentration assumes the solution’s volume equals the water’s volume, although dissolving a solid changes the volume slightly. In real life a supersaturated solution may crystallize at any time when disturbed; here crystals form on their own only above 1.25 times the solubility, a simplified threshold. Solutions near 100 °C would also be boiling, which the model ignores.
Key takeaways
- A solution is a solute spread evenly through a solvent.
- Solubility is the maximum that can dissolve; it depends on the substance and the temperature.
- Unsaturated solutions can dissolve more; saturated ones cannot; supersaturated ones hold more than they should and are unstable.
- Concentration is the amount of solute per volume of solution, in g/L or mol/L.
- Stirring, heating and crushing make solids dissolve faster; only temperature (and, for gases, pressure) changes how much dissolves.