Chapter 18

Solutions and Their Behavior

18.1 Properties of SolutionsLesson ObjectivesThe 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

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

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Homogeneous MixturesA solution is a homogeneous mixture of substances (the prefix “homo-” means “same”), meaning that theproperties 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 particlelevel. 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, abrass coin seems like it is just one substance, but at a particle level two substances are present (copper andzinc). Now consider a handful of zinc filings and copper pieces. Is this now a homogeneous solution? Theproperties of any scoop of the mixture you are holding would not be consistent with any other scoop youremoved from the mixture, so the combination of zinc filings and copper pieces in a pile does not representa homogeneous mixture.The solvent and solute are the two basic parts of a solution. The solvent is the substance present in thegreatest amount. The solute, then, is the substance present in the least amount. When you are makinga cup of hot chocolate, you take a teaspoon of cocoa powder and dissolve it in a cup of hot water. Sincemuch 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 bevaried. If you were to dissolve a quarter teaspoon of table salt in one cup of water, a solution wouldform. You can, however, vary the composition of this solution to a point. If you were to add another halfteaspoon of salt to the cup of water, you would still make a solution, but the composition of this solutionwould be different from the last one. What would happen if you tried to dissolve half cup of salt in thewater? 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 thatcan be varied up to a point. There are, however, limits to the amount of substance that can be dissolvedinto another substance and still remain homogeneous.

Types of SolutionsThere are three states of matter: solid, liquid, and gas. If we think about solutions and the possibilities ofcombining these states together to form solutions, we have nine possibilities. Look at Table 18.1.

Table 18.1: Types of Solutions

Solid Solvent Liquid Solvent Gaseous SolventSolid Solute Solid in a Solid Solid in a Liquid Solid in a GasLiquid Solute Liquid in a Solid Liquid in a Liquid Liquid in a GasGaseous 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 solutionsinclude vinegar and the antifreeze/coolant used in cars. For a gas in a liquid solution, the most commonexample is in soft drinks: carbon dioxide is a gas dissolved in water. Another example is the ammoniasolution used for cleaning. Finally, to understand a gas in a gas solution, take a deep breath. Air is asolution made up of mostly oxygen gas and nitrogen gas. A solid in a solid solution would be an alloy likesteel or brass. The other types of solutions are still solutions, but they are less common in everyday lives.

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

1. salt water2. air

Solution:

1. solute = salt; solvent = water2. solute = oxygen; solvent = nitrogen

Colloids and SuspensionsUnlike solutions, colloids are mixtures in which the size of the particles is between 1×103 pm and 1×106 pm.In meters, these sizes translate to 1 × 10−9 m to 1 × 10−6 m. As a comparison, a small grain of sand has adiameter of 2× 10−5 m. A common example of a colloid is milk. One way to tell that milk is a colloid is bythe Tyndall effect. The Tyndall effect is the scattering of light by particles. This involves shining a lightthrough 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 truesolution, the particles are so small that they do not obstruct the light. For example, the solution vinegaris clear and allows light to pass through easily. In comparison, when light is passed through a colloid, thelarger particles act as an obstruction to the light, causing the light to scatter. The particles, however, arestill small enough so that they do not settle out of solution.Look at Table 18.2 for some common colloid products that are formed when different phase solutes andsolvents are mixed.

Table 18.2: Some Common Colloidal Materials

Solid Solvent Liquid Solvent Gaseous SolventSolid Solute Ruby, brass, steel Butter, cheese, mus-

tard, Jell-oMarshmallow

Liquid Solute Paint, milk of magnesia Milk, mayonnaise, facecreams

Whipping cream, shav-ing cream

Gaseous Solute Airborn viruses, car ex-haust, smoke

Fog, clouds Aerosol Sprays

Suspensions are mixtures where the particles settle to the bottom of the container. This means thatthe 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 asuspension is muddy water. If you had a beaker of water and stirred in a handful of fine dirt, when youlet 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 thesolvent is present in the greater amount (more than 50% of the solution).

• A solution is a mixture that has the same properties throughout.

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• Four types of solution can be classified as the most common in terms of everyday occurrence: solidin 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 × 103 pm and 1 × 106 pm.• The Tyndall effect involves shining a light through the mixture. If the light scatters, the mixture isa colloid or a suspension.

• Suspensions are mixtures in which the particles are large enough so that they settle to the bottomof the container and can be filtered using filter paper.

Further Reading / Supplemental LinksLecture slides on solution properties and their behavior are available at the following website.

• http://www.public.asu.edu/~jpbirk/CHM-113_BLB/Chpt13/sld002.htm

Review Questions1. What makes a solution homogeneous?2. Which of the following are homogeneous? Explain.

(a) gasoline(b) chocolate(c) blood(d) brass

3. Which of the following is a solution?(a) milk(b) blood(c) gold(d) air(e) sugar

4. Which of the following is not a true solution?(a) vinegar(b) sand and water(c) hard water, CaCO3(aq)(d) mercury alloy

5. Give an example of a homogeneous solution that is made from the following combinations:(a) a gas in a liquid(b) a solid in a solid(c) a solid in a liquid(d) 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?

(a) air(b) brass(c) milk(d) none of these

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10. Which is not an example of a colloid?(a) human body(b) mayonnaise(c) mustard(d) cloud

11. The biggest difference between a colloid and a suspension is that:(a) in colloids, the solute is permanently dissolved in the solvent.(b) in colloids, the particles eventually settle to the bottom.(c) in suspensions, the particles eventually settle to the bottom.(d) none of these are correct

12. Karen was working in the lab with an unknown solution. She noticed that there was no precipitatein the bottom of the beaker even after it had been on the lab bench for several days. She tested itwith a light and saw that light scattered as it passed through the solution. Karen concluded that theliquid was what type of a mixture?(a) colloid(b) suspension(c) homogeneous(d) heterogeneous

13. What are two good common examples of colloids?

18.2 Solution FormationLesson ObjectivesThe student will:

• define the terms miscible and immiscible.• explain why solutions form.• predict if a solution will form between two substances from their molecular formulas.• discuss the idea of water as the “universal solvent.”• explain the solvation of molecules in comparison to ionic solvation.• discuss the differences between electrolytes and non-electrolytes and give examples of each.• determine from the formula whether a compound is an electrolyte or a non-electrolyte.• identify electrolyte and non-electrolyte solutions by their properties.

Vocabulary• electrolyte• hydration• immiscible• miscible• non-electrolyte

IntroductionWe have learned that solutions can be formed in a variety of combinations using solids, liquids, andgases. We also know that solutions have constant compositions, which can be varied to a point. Not all

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