A solute dissolves in a solvent to form a solution. The solute is usually the smaller component of the solution, and it forms a homogeneous mixture with the solvent. When a solute dissolves, it is soluble, and a material may be soluble in some solvents but not in others. Solubility measures how much of the solute dissolves, and it can vary with temperature and pressure. There can be more than one solute in a solution, and the solutes may react with each other or with the solvent to form new compounds.
TL;DR (Too Long; Didn't Read)
A solute is the material that dissolves in a solvent to form a homogeneous mixture called a solution. Solutes made up of polar molecules dissolve in polar solvents while non-polar solvents can dissolve non-polar solutes. Water, with polar molecules, is one of the strongest solvents because it can dissolve many materials, though not non-polar materials such as fats and oils. There can be several solutes in a solution, and they sometimes react with each other and with the solvent.
Types of Solutes
Solutes may be polar solutes, in which the molecules of the solute have positive and negative charges at opposite ends, or they may be non-polar with neutral molecules. Science in general and especially chemistry deal with both types while biology is mainly interested in non-polar organic solutes. The distinction is important because polar solvents will usually dissolve polar solutes while non-polar solutes dissolve only in non-polar solvents. The general rule for solvents and solutes is "like dissolves like."
Ionic compounds such a sodium chloride and polar covalently bonded molecules such as ammonia dissolve in polar solvents such as water. Non-polar molecules such as fats and oils dissolve in non-polar solvents such as carbon tetrachloride. Organic molecules such as oils will separate out when mixed with water while most polar solutes will not dissolve in organic solvents.
How a Solute Dissolves
Polar molecules have polar covalent or ionic bonds as do polar solvents. When polar solutes are mixed with polar solvents, new bonds are formed between the solute and solvent molecules, and they mix on a molecular level to form the solution.
For example, water is a polar solvent and sodium chloride is a polar compound with an ionic bond. When the two are mixed, the negative oxygen end of the water molecule attracts the positive sodium ion while the positive hydrogen end of water attracts the negative chlorine ion. These new bonds are strong enough to break the sodium-chlorine ionic bond, and the sodium chloride molecule dissolves.
When a non-polar molecule is placed in water, the water molecules stay attracted to each other and don't bond with the non-polar molecule, which can't dissolve as a result. But when the non-polar molecule is placed in a non-polar solvent, all the non-polar molecules form weak bonds and the non-polar solute can dissolve.
The Importance of Solutes
Solutes are important in chemistry and biology because many chemical reactions require solutions before they can proceed. When dissolved, solute molecules come into close contact with the molecules of the solvent or those of other solutes. Important chemical reactions such as acid-base, neutralization and precipitation reactions take place in solutions, and many biological processes and chemical reactions in living organisms are based on solutes in solution. Whether a material dissolves and can become a solute is often critical in determining its usefulness for a chemical process.
About the Author
Bert Markgraf is a freelance writer with a strong science and engineering background. He has written for scientific publications such as the HVDC Newsletter and the Energy and Automation Journal. Online he has written extensively on science-related topics in math, physics, chemistry and biology and has been published on sites such as Digital Landing and Reference.com He holds a Bachelor of Science degree from McGill University.