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Unit 8: When Chemicals Meet Water—The Properties of Solutions

Section 2: What Is a Solution?

Whenever we combine two or more different substances, we create a mixture. As we saw in Unit 1 of this course, chemists used terms like mixture, compound, and element rather loosely through the 18th century, but later these words acquired more precise meanings. A mixture can be homogenous or heterogeneous. In a homogenous mixture, particles are dispersed uniformly throughout the solution. For example, in homogenized milk, naturally occurring clumps of fat are broken down into smaller sizes so that they will stay evenly dispersed throughout the milk. Without homogenization, the fat separates out, causing the cream to rise to the top. Unhomogenized milk is a heterogeneous mixture because the fat floats on top, not uniformly dispersed throughout the liquid.

A solution is more than just a homogenous mixture. It is a homogenous mixture down to the molecular level. So, even if it looks homogeneous to the human eye, there can still be groups of molecules clumped together that make it not a solution. Homogenized milk isn't a solution. Rather, it is a colloid—a mixture in which very small particles of one substance are distributed evenly throughout another substance. The clumps of fat in the milk, while small, are still in groups of fat molecules. If individual fat molecules were uniformly spread throughout the milk, homogenized milk would be a solution. Salad dressing can be made by mixing olive oil with vinegar. At first they don't mix; but if we shake them for a long time, they "look like" they make a solution. However, it is a colloid with very small oil droplets suspended in the dressing. Several hours later, it will separate back into the two layers.

Household rubbing alcohol is a good example of a solution. It contains a homogenous mixture of isopropanol (alcohol) and water. So, if we took one milliliter of liquid from the top of the bottle of rubbing alcohol and another milliliter from the bottom of the bottle, both of these samples would contain exactly the same components in the same proportion and would have identical properties: They would weigh the same, boil and freeze at the same temperature, taste the same, and so on. Uniformity in solution is extremely important in chemical experiments. We need to know exactly what is in the sample that we're working with, no matter where we take it from in the container.

Mixture vs. Solution

Figure 8-2. Mixture vs. Solution

© Left: Wikimedia Commons, Public Domain. Right: Wikimedia Commons, Creative Commons License 3.0. Author: Chris 73.

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Mixture vs. Solution

Figure 8-2. Mixture vs. Solution

Sand will not dissolve in the ocean, but the force of crushing waves may intermingle sand with water to form a mixture. Sugar and water, on the other hand, begin as a mixture until the sugar dissolves. Once the sugar evenly disperses throughout the water, it becomes a solution. This process is called "dissolution."

Pure water is not a solution because it is not a mixture of different types of molecules; rather, it contains just one type of molecule. But suppose we add a few tablespoons of sugar to the water. At first the sugar will settle at the bottom of the glass, so the mixture is not uniform—there will be less sugar in a milliliter of water from the top of the glass than in a milliliter from the bottom. After a while, however, the sugar molecules will dissolve and spread themselves uniformly throughout the water. At this point, when water molecules surround each individual sugar molecule, the mixture has become a solution. The process of molecules spreading out evenly through a mixture is called dissolution, and when it is complete the sugar is said to be dissolved in the water. (Figure 8-2)

Glossary

Colloid

A mixture in which very small particles of one substance are distributed evenly throughout another substance.

Dissolution

The process of creating a homogeneous solution of solute molecules in a solvent.

Heterogeneous

Describing a mixture consisting of ingredients with various physical and chemical properties, like a pepperoni pizza.

Homogenous

Describing a mixture having the same physical and chemical properties throughout, like salt water.

Solution

A mixture of substances in which one substance, the solute, distributes its molecules evenly throughout a second substance, the solvent.

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