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It can be really exciting to explore a big underground cave like the one in this picture. Do you know how caves form? Believe it or not, water is the answer. Water slowly dissolves rocks, especially certain types of rocks such as limestone. When rocks or other substances dissolve in water, they form a solution.

What Is a Solution?

A solution is a mixture of two or more substances, but it’s not just any mixture. A solution is a homogeneous mixture. In a homogeneous mixture, the dissolved particles are spread evenly through the mixture. The particles of the solution are also too small to be seen or to settle out of the mixture. To put solutions in context as a type of mixture, watch the video at this URL: http://www.youtube.com/watch?v=z2vM-G5I92U&feature=related

Parts of a Solution

All solutions have two parts: the solute and the solvent. The solute is the substance that dissolves, and the solvent is the substance that dissolves the solute. Particles of solvent pull apart particles of solute, and the solute particles spread throughout the solvent. Salt water, such as the ocean water in the Figure below , is an example of a solution. In a saltwater solution, salt is the solute and water is the solvent.

Q: A scientist obtained a sample of water from the Atlantic Ocean and determined that the sample was about 3.5 percent dissolved salt. Predict the percent of dissolved salt in a sample of water from the Pacific Ocean.

A: As a solution, ocean water is a homogeneous mixture. Therefore, no matter where the water sample is obtained, its composition will be about 3.5 percent dissolved salt.

Soluble or Insoluble?

Not only salt, but many other solutes can dissolve in water. In fact, so many solutes can dissolve in water that water has been called the universal solvent. Even rocks can dissolve in water, which explains the cave that opened this article. A solute that can dissolve in a given solvent, such as water, is said to be soluble in that solvent. Conversely, a solute that cannot dissolve in a given solvent is said to be insoluble in that solvent.

Although most solutes can dissolve in water, some solutes are insoluble in water. Oil is an example. Did you ever try to mix oil with water? No matter how well you mix the oil into the water, after the mixture stands for a while, the oil separates from the water and rises to the top. You can see how oil floats on ocean water in the Figure below .

Oil from an oil spill floats on ocean water near shore.

Solutions and States of Matter

Like salt water in the ocean, many solutions are normally in the liquid state. However, matter in any state can form a solution. An alloy, which is a mixture of a metal with one or more other substances, is a solid solution at room temperature. For example, the alloy bronze is a solution of copper and tin. Matter in the gaseous state can also form solutions.

Q: What is an example of a gaseous solution?

A: Air in the atmosphere is a gaseous solution. It is a mixture that contains mainly nitrogen and oxygen gases, with very small amounts of several other gases. The circle graph in the Figure below shows the composition of air. Because air is a solution, it is homogeneous. In other words, no matter where you go, the air always contains the same proportion of gases that are shown in the graph.

Summary

  • A solution is a homogeneous mixture of two or more substances in which the dissolved particles are too small to be seen or to settle out of the mixture.
  • In a solution, the substance that dissolves is the solute, and the substance that dissolves the solute is the solvent.
  • A solute that can dissolve in a given solvent is said to be soluble in that solvent. A solute that cannot dissolve in a given solvent is said to be insoluble in that solvent.
  • Solutions may be liquids such as salt water, solids such as alloys, or gases such as air.

Vocabulary

  • insoluble : Unable to dissolve in a given solvent.
  • soluble : Able to dissolve in a given solvent.
  • solution : Homogeneous mixture in which particles are too small to reflect light and too small to settle or be filtered out of the mixture.

Practice

Listen to the solutions song at the following URL, and then fill in the blanks in the sentences below. http://www.youtube.com/watch?NR=1&feature=endscreen&v=3G472AA3SEs

  1. The best-mixed mixtures of two or more substances are __________.
  2. The term that means the same throughout is __________.
  3. A solution in which the solvent is water is called a(n) __________ solution.
  4. The lesser portion of a solution is the __________.
  5. The greater portion of a solution is the __________.

Review

  1. What is a solution?
  2. Identify and describe the two parts of a solution.
  3. Give an example of a substance that is soluble in water and a substance that is insoluble in water.
  4. Steel is an alloy of iron and carbon. What can you infer about steel based on the fact that it is a solution?
  5. Which of the following statements about solutions is false?
    1. All solutions are mixtures.
    2. All solutions are homogeneous.
    3. All solutions contain two or more compounds.
    4. All solutions contain at least two substances.

Vocabulary

insoluble

insoluble

Unable to dissolve in a given solvent.
soluble

soluble

Able to dissolve in a given solvent.
solution

solution

Homogeneous mixture in which particles are too small to reflect light and too small to settle or be filtered out of the mixture.

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Description

Difficulty Level:

At Grade

Concept Nodes:

Grades:

7 , 8

Date Created:

Nov 14, 2012

Last Modified:

Nov 12, 2014
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