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17.1: Properties of Solutions

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Lesson Objectives

The student will:

  • define a solution.
  • describe the composition of solutions.
  • define the terms solute and solvent.
  • identify the solute and solvent in a solution.
  • describe the different types of solutions and give examples of each type.
  • define colloids and suspensions.
  • explain the differences among solutions, colloids, and suspensions.
  • list some common examples of colloids.

Vocabulary

  • colloid
  • solute
  • solution
  • solvent
  • suspension
  • Tyndall effect

Introduction

In this chapter, we begin our study of solution chemistry. We all might think that we know what a solution is, listing a drink like tea or soda as an example of a solution. What you might not have realized, however, is that the air or alloys such as brass are all classified as solutions. Why are these classified as solutions? Why wouldn’t milk be classified as a true solution? To answer these questions, we have to learn some specific properties of solutions. Let’s begin with the definition of a solution and look at some of the different types of solutions.

Homogeneous Mixtures

A solution is a homogeneous mixture of substances (the prefix “homo-” means “same”), meaning that the properties are the same throughout the solution. Take, for example, the vinegar that is used in cooking. Vinegar is approximately 5% acetic acid in water. This means that every teaspoon of vinegar contains 5% acetic acid and 95% water.

When a solution is said to have uniform properties, the definition is referring to properties at the particle level. What does this mean? Consider brass as an example, which is an alloy made from copper and zinc. An alloy is a homogeneous mixture formed when one solid is dissolved in another. To the naked eye, a brass coin seems like it is just one substance, but at a particle level two substances are present (copper and zinc). Now consider a handful of zinc filings and copper pieces. Is this now a homogeneous solution? The properties of any scoop of the mixture you are holding would not be consistent with any other scoop you removed from the mixture, so the combination of zinc filings and copper pieces in a pile does not represent a homogeneous mixture.

The solvent and solute are the two basic parts of a solution. The solvent is the substance present in the greatest amount. The solute, then, is the substance present in the least amount. When you are making a cup of hot chocolate, you take a teaspoon of cocoa powder and dissolve it in a cup of hot water. Since much less cocoa powder is used than water, the cocoa powder is the solute and the water is the solvent.

Just because solutions have the same composition throughout does not mean the composition cannot be varied. If you were to dissolve a quarter teaspoon of table salt in one cup of water, a solution would form. You can, however, vary the composition of this solution to a point. If you were to add another half teaspoon of salt to the cup of water, you would still make a solution, but the composition of this solution would be different from the last one. What would happen if you tried to dissolve half cup of salt in the water? At this point, the solution has passed its limit as to the amount of salt that can be dissolved in it, so it would no longer be a homogeneous solution. As a result, solutions have a constant composition that can be varied up to a point. There are, however, limits to the amount of substance that can be dissolved into another substance and still remain homogeneous.

Types of Solutions

There are three states of matter: solid, liquid, and gas. If we think about solutions and the possibilities of combining these states together to form solutions, we have nine possibilities. Look at Table below.

Types of Solutions
Solid Solvent Liquid Solvent Gaseous Solvent
Solid Solute Solid in a Solid Solid in a Liquid Solid in a Gas
Liquid Solute Liquid in a Solid Liquid in a Liquid Liquid in a Gas
Gaseous Solute Gas in a Solid Gas in a Liquid Gas in a Gas

Although there are nine possibilities, there are really only four common types of solutions. For example, a solid in a liquid solution can be anything from a sugar solution to seawater. Liquid in liquid solutions include vinegar and the antifreeze/coolant used in cars. For a gas in a liquid solution, the most common example is in soft drinks: carbon dioxide is a gas dissolved in water. Another example is the ammonia solution used for cleaning. Finally, to understand a gas in a gas solution, take a deep breath. Air is a solution made up of mostly oxygen gas and nitrogen gas. A solid in a solid solution would be an alloy like steel or brass. The other types of solutions are still solutions, but they are less common in everyday lives.

Example:

Name the solute and solvent in each of the following mixtures.

  1. salt water
  2. air

Solution:

  1. solute = salt; solvent = water
  2. solute = oxygen; solvent = nitrogen

Colloids and Suspensions

Unlike solutions, colloids are mixtures in which the size of the particles is between 1 \times 10^3 \ \mathrm{pm} and 1 \times 10^6 \ \mathrm{pm}. In meters, these sizes translate to 1 \times 10^{-9} \ \mathrm{m} to 1 \times 10^{-6} \ \mathrm{m}. As a comparison, a small grain of sand has a diameter of 2 \times 10^{-5} \ \mathrm{m}. A common example of a colloid is milk. One way to tell that milk is a colloid is by the Tyndall effect. The Tyndall effect is the scattering of light by particles. This involves shining a light through the mixture: when the light cannot pass through the mixture, the mixture is considered a colloid. This is why milk appears “cloudy” – or what we think of as “milky.” When light is passed through a true solution, the particles are so small that they do not obstruct the light. For example, the solution vinegar is clear and allows light to pass through easily. In comparison, when light is passed through a colloid, the larger particles act as an obstruction to the light, causing the light to scatter. The particles, however, are still small enough so that they do not settle out of solution.

Look at Table below for some common colloid products that are formed when different phase solutes and solvents are mixed.

Some Common Colloidal Materials
Solid Solvent Liquid Solvent Gaseous Solvent
Solid Solute Ruby, brass, steel Butter, cheese, mustard, Jell-o Marshmallow
Liquid Solute Paint, milk of magnesia Milk, mayonnaise, face creams Whipping cream, shaving cream
Gaseous Solute Airborn viruses, car exhaust, smoke Fog, clouds Aerosol Sprays

Suspensions are mixtures where the particles settle to the bottom of the container. This means that the particles in a suspension are large enough that gravity pulls them out of solution. With suspensions, filtration can usually be used to separate the excess particles from the solution. A common example of a suspension is muddy water. If you had a beaker of water and stirred in a handful of fine dirt, when you let the mixture stand, dirt would settle to the bottom.

Lesson Summary

  • In a solution, a solute is present in the least amount (less than 50% of the solution) whereas the solvent is present in the greater amount (more than 50% of the solution).
  • A solution is a mixture that has the same properties throughout.
  • Four types of solution can be classified as the most common in terms of everyday occurrence: solid in a liquid, liquid in a liquid, gas in a liquid, and gas in a gas. The other types are less common.
  • Colloids are mixtures in which the size of the particles is between 1 \times 10^3 \ \mathrm{pm} and 1 \times 10^6 \ \mathrm{pm}.
  • The Tyndall effect involves shining a light through the mixture. If the light scatters, the mixture is a colloid or a suspension.
  • Suspensions are mixtures in which the particles are large enough so that they settle to the bottom of the container and can be filtered using filter paper.

Further Reading / Supplemental Links

Lecture slides on solution properties and their behavior are available at the following website.

Review Questions

  1. What makes a solution homogeneous?
  2. Which of the following are homogeneous? Explain.
    1. gasoline
    2. chocolate
    3. blood
    4. brass
  3. Which of the following is a solution?
    1. milk
    2. blood
    3. gold
    4. air
    5. sugar
  4. Which of the following is not a true solution?
    1. vinegar
    2. sand and water
    3. hard water, \mathrm{CaCO}_{3(aq)}
    4. mercury alloy
  5. Give an example of a homogeneous solution that is made from the following combinations:
    1. a gas in a liquid
    2. a solid in a solid
    3. a solid in a liquid
    4. a gas in a gas
  6. Distinguish between a solution, a colloid, and a suspension.
  7. What is one true way to tell you have a colloid solution?
  8. Why do you think there is no example of a gas - gas colloid?
  9. Which is an example of a colloid?
    1. air
    2. brass
    3. milk
    4. none of these
  10. Which is not an example of a colloid?
    1. human body
    2. mayonnaise
    3. mustard
    4. cloud
  11. The biggest difference between a colloid and a suspension is that:
    1. in colloids, the solute is permanently dissolved in the solvent.
    2. in colloids, the particles eventually settle to the bottom.
    3. in suspensions, the particles eventually settle to the bottom.
    4. none of these are correct
  12. Karen was working in the lab with an unknown solution. She noticed that there was no precipitate in the bottom of the beaker even after it had been on the lab bench for several days. She tested it with a light and saw that light scattered as it passed through the solution. Karen concluded that the liquid was what type of a mixture?
    1. colloid
    2. suspension
    3. homogeneous
    4. heterogeneous
  13. What are two good common examples of colloids?

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