Chemistry Essentials for Dummies

Chapter 10. A Salute to Solutions

In This Chapter

· Finding out about solutes, solvents, and solutions

· Working with the different kinds of solution concentration units

· Checking out the colligative properties of solutions

· Figuring out colloids

In this chapter, I show you some of the properties of solutions. I introduce you to the different ways chemists represent a solution’s concentration, and I tell you about the colligative properties of solutions (properties that depend on the number of solute particles) and relate them to ice creammaking and antifreeze. I also introduce you to colloids, close cousins of solutions, and discuss the importance of particle size. So sit back, sip on your solution of choice, and read all about solutions.

Mixing Things Up with Solutes, Solvents, and Solutions

A solution is a homogeneous mixture, meaning that it’s the same throughout. If you dissolve sugar in water and mix it really well, for example, your mixture is basically the same no matter where you sample it.

A solution is composed of a solvent and one or more solutes. The solvent is the substance that’s present in the largest amount, and the solute is the substance that’s present in the lesser amount. These definitions work most of the time, but there are a few cases of extremely soluble salts, such as lithium chloride, in which more than 5 grams of salt can be dissolved in 5 milliliters of water. However, water is still considered the solvent, because it’s the species that hasn’t changed state.

In addition, there can be more than one solute in a solution. You can dissolve salt in water to make a brine solution, and then you can dissolve some sugar in the same solution. You then have two solutes, salt and sugar, but you still have only one solvent — water.

Most people think of liquids when they think about solutions, but you can also have solutions of gases or solids. Earth’s atmosphere, for example, is a solution. Because air is almost 79 percent nitrogen, it’s considered the solvent, and the oxygen, carbon dioxide, and other gases are considered the solutes. Alloys are solutions of one metal in another metal. Brass is a solution of zinc in copper.

How dissolving happens

Why do some things dissolve in one solvent and not another? For example, oil and water don’t mix to form a solution, but oil does dissolve in gasoline.

REMEMBER. There’s a general rule of solubility that says like-dissolves-like in regards to polarity of both the solvent and solutes. A polar material is composed of covalent bonds with a positive and negative end of the molecule. (For a rousing discussion of water and its polar covalent bonds, see Chapter 6.) A polar substance such as water dissolves polar solutes, such as salts and alcohols. Oil, however, is composed of largely nonpolar bonds, so water doesn’t act as a suitable solvent for oil.

Concentration limits

Solubility is the maximum amount of solute that will dissolve in a given amount of a solvent at a specified temperature. You know from your own experiences, I’m sure, that there’s a limit to how much solute you can dissolve in a given amount of solvent. Most people have been guilty of putting far too much sugar in iced tea. No matter how much you stir, some undissolved sugar stays at the bottom of the glass.

REMEMBER. Solubility normally has the units of grams solute per 100 milliliters of solvent (g/100 mL), and it’s related to the temperature of the solvent:

Solids in liquids: For solids dissolving in liquids, solubility normally increases with increasing temperature. For instance, if you heat that iced tea, the sugar at the bottom will readily dissolve.

Gases in liquids: For gases dissolving in liquids, such as oxygen dissolving in lake water, the solubility goes down as the temperature increases. This is the basis of thermal pollution, the addition of heat to water that decreases the solubility of the oxygen and affects the aquatic life.

Saturated facts

A saturated solution contains the maximum amount of dissolved solute possible at a given temperature. If it has less than this amount, it’s called an unsaturated solution. Sometimes, under unusual circumstances, the solvent may actually dissolve more than its maximum amount and become supersaturated. This supersaturated solution is unstable, though, and sooner or later, the solute will precipitate (form a solid) until the saturation point has been reached.

If a solution is unsaturated, then the amount of solute that’s dissolved may vary over a wide range. A couple of rather nebulous terms describe the relative amount of solute and solvent that you can use:

Dilute: You can say that the solution is dilute, meaning that relatively speaking, there’s very little solute per given amount of solvent. If you dissolve 0.01 grams of sodium chloride in a liter of water, for example, the solution is dilute. (Another dilute solution? A $1 margarita, as one of my students once pointed out — that’s a lot of solvent [water] and very little solute [tequila].)

Concentrated: A concentrated solution contains a large amount of solute per the given amount of solvent. If you dissolve 200 grams of sodium chloride in a liter of water, for example, the solution is concentrated.

But suppose you dissolve 25 grams or 50 grams of sodium chloride in a liter of water. Is the solution dilute or concentrated? These terms don’t hold up very well for most cases.