Learning Objectives A solution is another name for a homogeneous mixture. A mixture as a material composed of two or more substances. In a solution, the combination is so intimate that the different substances cannot be differentiated by sight, even with a microscope. Compare, for example, a mixture of salt and pepper and another mixture consisting of salt and water. In the first mixture, we can readily see individual grains of salt and the flecks of pepper. A mixture of salt and pepper is not a solution. However, in the second mixture, no matter how carefully we look, we cannot see two different substances. Salt dissolved in water is a solution. The major component of a solution, called the solvent, is typically the same phase as the solution itself. Each minor component of a solution (and there may be more than one) is called the solute. In most of the solutions we will describe in this textbook, there will be no ambiguity about whether a component is the solvent or the solute.) For example, in a solution of salt in water, the solute is salt, and solvent is water. Solutions come in all phases, and the solvent and the solute do not have to be in the same phase to form a solution (such as salt and water). For example, air is a gaseous solution of about 80% nitrogen and about 20% oxygen, with some other gases present in much smaller amounts. An alloy is a solid solution consisting of a metal (like iron) with some other metals or nonmetals dissolved in it. Steel, an alloy of iron and carbon and small amounts of other metals, is an example of a solid solution. Table \(\PageIndex{1}\) lists some common types of solutions, with examples of each. Table \(\PageIndex{1}\): Types of Solutions
What causes a solution to form? The simple answer is that the solvent and the solute must have similar intermolecular interactions. When this is the case, the individual particles of solvent and solute can easily mix so intimately that each particle of solute is surrounded by particles of solute, forming a solution. However, if two substances have very different intermolecular interactions, large amounts of energy are required to force their individual particles to mix intimately, so a solution does not form. This process leads to a simple rule of thumb: like dissolves like. Solvents that are very polar will dissolve solutes that are very polar or even ionic. Solvents that are nonpolar will dissolve nonpolar solutes. Thus water, being polar, is a good solvent for ionic compounds and polar solutes like ethanol (C2H5OH). However, water does not dissolve nonpolar solutes, such as many oils and greases (Figure \(\PageIndex{1}\)). Figure \(\PageIndex{1}\): A beaker holds water with blue food dye (upper liquid layer) and a much more dense perfluoroheptane (a fluorocarbon) lower liquid layer. The two fluids cannot mix and the dye cannot dissolve in fluorocarbon. A goldfish and a crab have been introduced into the water. The goldfish cannot penetrate the dense fluorocarbon. The crab floats at the liquid boundary with only parts of his legs penetrating the fluorocarbon fluid, unable to sink to the bottom of the beaker. Quarter coins rest on the bottom of the beaker. Animals were rescued from their predicament after the photo was taken. Figure used with permission from Wikipedia (Sbharris (Steven B. Harris)). We use the word soluble to describe a solute that dissolves in a particular solvent, and the word insoluble for a solute that does not dissolve in a solvent. Thus, we say that sodium chloride is soluble in water but insoluble in hexane (C6H14). If the solute and the solvent are both liquids and soluble in any proportion, we use the word miscible, and the word immiscible if they are not. Example \(\PageIndex{1}\) Water is considered a polar solvent. Which substances should dissolve in water?
Solution Because water is polar, substances that are polar or ionic will dissolve in it.
Exercise \(\PageIndex{1}\) Toluene (C6H5CH3) is widely used in industry as a nonpolar solvent. Which substances should dissolve in toluene?
Concept Review Exercises
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Is the solvent in seawater is salt?Ocean water is a homogenous mixture of salt in water. It contains about three percent sodium chloride. Ocean water is the type of mixture called a solution, because the salt is dissolved in the water. Water is the solvent, and sodium chloride is the solute.
What is the solute and solvent of brine solution?So a brine can actually be thought of as a salt water solution, in which the salt is dissolved in the water. The thing you have more of in a solution, in this case the water, is called the solvent, and the thing you have less of, in this case the salt, is called the solute.
What type of solution is water with salt?After the salt compounds are pulled apart, the sodium and chloride atoms are surrounded by water molecules, as this diagram shows. Once this happens, the salt is dissolved, resulting in a homogeneous solution.
Why is salt water a good solvent?At the molecular level, salt dissolves in water due to electrical charges and due to the fact that both water and salt compounds are polar, with positive and negative charges on opposite sides in the molecule. This allows the water molecule to become attracted to many other different types of molecules.
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