BDChapter 4: Solutions 111

VOCABULARY

solution p. 111

solute p. 112

solvent p. 112

suspension p. 113

BEFORE, you learned

• Ionic or covalent bonds hold acompound together

• Chemical reactions producechemical changes

• Chemical reactions alter thearrangements of atoms

NOW, you will learn

• How a solution differs fromother types of mixtures

• About the parts of a solution

• How properties of solutionsdiffer from properties of theirseparate components

KEY CONCEPT

A solution is a type of mixture.

EXPLORE Mixtures

Which substances dissolve in water?

PROCEDURE

Pour equal amounts of water into each cup.

Pour one spoonful of table salt into one

of the cups. Stir.

Pour one spoonful of flour into the other

cup. Stir.

Record your observations.

WHAT DO YOU THINK?• Did the salt dissolve? Did the flour dissolve?

• How can you tell?

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MATERIALS• tap water

• 2 clear plastic cups

• plastic spoon

• table salt

• flour

The parts of a solution are mixed evenly.

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 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.

solution

VOCABULARY

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

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reading tip

The words solute and solvent

are both related to the Latinword solvere, which means“to loosen.”

Solutes and Solvents

Like other mixtures, a solution has definite components. A

(SAHL-yoot) is a substance that is dissolved to make a solution.

When a solute dissolves, it separates into individual particles. A

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.

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.

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.

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 physicalstate, which is the solvent?

solvent

solute

112 Unit: Chemical Interactions

Liquid SolutionWater often contains manydissolved substances.

Solid Solution Bronze consistsof tin dissolvedin copper.

Gas SolutionAir is oxygenand other gasesdissolved innitrogen.

BD

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BDChapter 4: Solutions 113

How can you separate the parts of a solution?

PROCEDURE

Draw a solid black circular region 6 cm in diameter around the point of the filter.

Place the filter, point up, over the top of the bottle.

Squeeze several drops of water onto the point of the filter.

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.

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SolutionsSolutions

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 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.

suspension,

SKILL FOCUSObserving

MATERIALS• black marker• coffee filter• plastic bottle• eyedropper• tap water

TIME15 minutes

Solution Dissolved particles cannotbe identified as a substance differentfrom the solvent.

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

suspended particledissolved particle

solvent solvent

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BD

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.

114 Unit: Chemical Interactions

How Solutes Dissolve

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

Ionic Compound Dissolved in Solvent Covalent Compound Dissolved in Solvent

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

added to solvent added to solvent

ionic compound

covalent compound

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reminder

In temperature measure-ments, C stands for“Celsius” and F standsfor “Fahrenheit.”

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 effective-

ness 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?

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.

115

Adding salt tolower the freezingpoint of ice helpsto make ice cream.

Adding salt tolower the freezingpoint of ice helpsto make ice cream.

BD

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BD 116 Unit: Chemical Interactions

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.

Check Your Reading How does the boiling point of a solution depend on the amountof 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?

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 sub-

stance like flour have a similar

effect on the ice? Explain.

APPLY Why might theaddition of antifreeze tothe water in this car’sradiator have preventedthe car from overheating?

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