KEY CONCEPT A solution is a type of mixture.

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KEY CONCEPT

A solution is a type of mixture.

BEFORE, you learned

? Ionic or covalent bonds hold a compound together

? Chemical reactions produce chemical changes

? Chemical reactions alter the arrangements of atoms

NOW, you will learn

? How a solution differs from other types of mixtures

? About the parts of a solution ? How properties of solutions

differ from properties of their separate components

VOCABULARY

solution p. 111 solute p. 112 solvent p. 112 suspension p. 113

EXPLORE Mixtures

Which substances dissolve in water?

PROCEDURE 1 Pour equal amounts of water into each cup.

2 Pour one spoonful of table salt into one of the cups. Stir.

3 Pour one spoonful of flour into the other cup. Stir.

4 Record your observations.

MATERIALS

? tap water ? 2 clear plastic cups ? plastic spoon ? table salt ? flour

WHAT DO YOU THINK? ? Did the salt dissolve? Did the flour dissolve? ? How can you tell?

The parts of a solution are mixed evenly.

VOCABULARY

Remember to use the strategy of your choice. You might use a four square diagram for solution.

A mixture is a combination of substances, such as a fruit salad. The ingredients of any mixture can be physically separated from each other because they are not chemically changed--they are still the same substances. Sometimes, however, a mixture is so completely blended that its ingredients cannot be identified as different substances. A solution is a type of mixture, called a homogeneous mixture, that is the same throughout. A solution can be physically separated, but all portions of a solution have the same properties.

If you stir sand into a glass of water, you can identify the sand as a separate substance that falls to the bottom of the glass. Sand in water is a mixture that is not a solution. If you stir sugar into a glass of water, you cannot identify the sugar as a separate substance. Sugar in water is a common solution, as are examples such as seawater, gasoline, and the liquid part of your blood.

Chapter 4: Solutions 111 DB

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Solutes and Solvents

reading tip

The words solute and solvent are both related to the Latin word solvere, which means "to loosen."

Like other mixtures, a solution has definite components. A solute (SAHL-yoot) is a substance that is dissolved to make a solution. When a solute dissolves, it separates into individual particles. A solvent is a substance that dissolves a solute. Because a solute dissolves into individual particles in a solvent, it is not possible to identify the solute and solvent as different substances when they form a solution.

In a solution of table salt and water, the salt is the solute and the water is the solvent. In the cells of your body, substances such as calcium ions and sugar are solutes, and water is the solvent. Water is the most common and important solvent, but other substances can also be solvents. For example, if you have ever used an oil-based paint you know that water will not clean the paintbrushes. Instead, a solvent like turpentine must be used.

Check Your Reading What is the difference between a solute and a solvent?

Types of Solutions

Many solutions are made of solids dissolved in liquids. However, solutes, solvents, and solutions can be gases, liquids, or solids. For example, oxygen, a gas, is dissolved in seawater. The bubbles in carbonated

drinks come from the release of carbon dioxide gas that was dissolved in the drink.

Gas Solution Air is oxygen and other gases dissolved in nitrogen.

In some solutions, both the solute and the solvent are in the same physical state. Vinegar, for example, is a solution of acetic acid in water. In a solution of different liquids, it may be difficult to say which substance is the solute and which is the solvent. In general, the substance present in the greater amount is the solvent. Since there is more water than acetic acid in vinegar, water is the solvent and acetic acid is the solute.

Solid Solution Bronze consists of tin dissolved in copper.

Although you may usually think of a solution as a liquid, solid solutions also exist. For example, bronze is a solid solution in which tin is the solute and copper is the solvent. Solid solutions are not formed as solids. Instead, the solvent metal is heated until it melts and becomes a liquid. Then the solute is added, and the substances are thoroughly mixed together. When the mixture cools, it is a solid solution.

Liquid Solution Water often contains many dissolved substances.

Solutions made of combinations of gases are also common. The air you breathe is a solution. Because nitrogen makes up the largest portion of air, it is the solvent. Other gases present, such as oxygen and carbon dioxide, are solutes.

When substances in a solution are in the same physical state, which is the solvent?

DB 112 Unit: Chemical Interactions

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Solutions

How can you separate the parts of a solution?

PROCEDURE 1 Draw a solid black circular region 6 cm in diameter around the point of the filter. 2 Place the filter, point up, over the top of the bottle. 3 Squeeze several drops of water onto the point of the filter. 4 Observe the filter once every minute for 10 minutes.

Record your observations.

WHAT DO YOU THINK? ? What happened to the ink on the filter? ? Identify, in general, the solutes and the solution in

this investigation.

CHALLENGE Relate your observations of the ink and water on the coffee filter to the properties of solutions.

SKILL FOCUS

Observing

MATERIALS

? black marker ? coffee filter ? plastic bottle ? eyedropper ? tap water

TIME

15 minutes

Suspensions

When you add flour to water, the mixture turns cloudy, and you cannot see through it. This mixture is not a solution but a suspension. In a suspension, the particles are larger than those found in a solution. Instead of dissolving, these larger particles turn the liquid cloudy. Sometimes you can separate the components of a suspension by filtering the mixture.

dissolved particle solvent

suspended particle solvent

Solution Dissolved particles cannot be identified as a substance different from the solvent.

Suspension Particles that do not dissolve make a suspension look cloudy.

Chapter 4: Solutions 113 DB

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Solvent and solute particles interact.

The parts of a solution--that is, the solute and the solvent--can be physically separated because they are not changed into new substances. However, individual particles of solute and solvent do interact. When a solid dissolves in a liquid, the particles of the solute are surrounded by particles of the liquid. The solute particles become evenly distributed through the solvent.

The way in which a solid compound dissolves in a liquid depends on the type of bonds in the compound. Ionic compounds such as table salt (NaCl) split apart into individual ions. When table salt dissolves in water, the sodium and chloride ions separate, and each ion is surrounded by water molecules. When a covalent compound such as table sugar (C12H22O11) dissolves, each molecule stays together and is surrounded by solvent molecules. The general processes that take place when ionic compounds dissolve and when covalent compounds dissolve are shown below.

How Solutes Dissolve

Ionic compounds separate into ions. Covalent compounds separate into individual molecules.

ionic compound

added to solvent Ionic Compound Dissolved in Solvent

covalent compound added to solvent

Covalent Compound Dissolved in Solvent

What difference between the two illustrations tells you whether a compound is ionic or covalent?

DB 114 Unit: Chemical Interactions

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Properties of solvents change in solutions.

In every solution--solid, liquid, and gas--solutes change the physical properties of a solvent. Therefore, a solution's physical properties differ from the physical properties of the pure solvent. The amount of solute in the solution determines how much the physical properties of the solvent are changed.

Lowering the Freezing Point

Recall that the freezing point is the temperature at which a liquid becomes a solid. The freezing point of a liquid solvent decreases--becomes lower--when a solute is dissolved in it. For example, pure water freezes at 0?C (32?F) under normal conditions. When a solute is dissolved in water, the resulting solution has a freezing point below 0?C.

Lowering the freezing point of water can be very useful in winter. Road crews spread salt on streets and highways during snowstorms because salt lowers the freezing point of water. When snow mixes with salt on the roads, a saltwater solution that does not freeze at 0?C is formed. The more salt that is used, the lower the freezing point of the solution.

Since salt dissolves in the small amount of water usually present on the surface of ice, it helps to melt any ice already present on the roads. However, there is a limit to salt's effectiveness because there is a limit to how much will dissolve. No matter how much salt is used, once the temperature goes below ?21?C (?6?F), the melted ice will freeze again.

Check Your Reading How does the freezing point of a solvent change when a solute is dissolved in it?

reminder

In temperature measurements, C stands for "Celsius" and F stands for "Fahrenheit."

Making ice cream also depends on lowering the freezing point of a solvent. Most hand-cranked ice cream makers hold the liquid ice cream ingredients in a canister surrounded by a mixture of salt and ice. The salt added to the ice lowers the freezing point of this mixture. This causes the ice to melt--absorbing heat from its surroundings, including the ice cream ingredients. The ice cream mix is chilled while its ingredients are constantly stirred. As a result, tiny ice crystals form all at once in the ice cream mixture instead of a few crystals forming and growing larger as the mix freezes. This whole process helps to make ice cream that is smooth and creamy.

Adding salt to lower the freezing point of ice helps to make ice cream.

115 DB

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Raising the Boiling Point

The boiling point of a liquid is the temperature at which the liquid forms bubbles in its interior and becomes a gas. Under normal conditions, a substance cannot exist as a liquid at a temperature greater than its

boiling point. However, the boiling point of a solution is higher than the boiling point of the pure solvent. Therefore, a solution can remain a liquid at a higher temperature than its pure solvent.

For example, the boiling point of pure water is 100?C (212?F) under normal conditions. Saltwater, however, can be a liquid at temperatures above 100?C because salt raises the boiling point of water. The amount of salt in the water determines how much the boiling point is increased. The more solute that is dissolved in a solution, the greater the increase in boiling point.

APPLY Why might the addition of antifreeze to the water in this car's radiator have prevented the car from overheating?

Check Your Reading How does the boiling point of a solution depend on the amount of solute in it?

A solute lowers the freezing point and raises the boiling point of the solvent in the solution. The result is that the solute extends the temperature range in which the solvent remains a liquid. One way in which both a decrease in freezing point and an increase in boiling point can be useful in the same solution involves a car's radiator. Antifreeze, which is mostly a chemical called ethylene glycol, is often added to the water in the radiator. This solution prevents the water from freezing in the winter and also keeps it from boiling in the summer.

KEY CONCEPTS 1. How is a solution different

from other mixtures? 2. Describe the two parts of a

solution. How can you tell them apart? 3. How does the boiling point of a solvent change when a solute is dissolved in it? How does the freezing point change?

DB 116 Unit: Chemical Interactions

CRITICAL THINKING

4. Compare Contrast the way in which an ionic compound, such as table salt, dissolves with the way in which a covalent compound, such as sugar, dissolves.

5. Infer Pure water freezes at 0?C and boils at 100?C. Would tap water likely freeze and boil at those exact temperatures? Why or why not?

CHALLENGE

6. Synthesize People often sprinkle salt on icy driveways and sidewalks. Would a substance like flour have a similar effect on the ice? Explain.

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