¡ ¢ £ ¤ ¥ ¦ § KEY CONCEPT A solution is a type of mixture. BEFORE, you learned NOW, you will learn • Ionic or covalent bonds hold a • How a solution differs from compound together other types of mixtures • Chemical reactions produce • About the parts of a solution chemical changes • How properties of solutions • Chemical reactions alter the differ from properties of their arrangements of atoms separate components VOCABULARY EXPLORE Mixtures solution p. 111 Which substances dissolve in water? solute p. 112 solvent p. 112 PROCEDURE MATERIALS suspension p. 113 • tap water 1 Pour equal amounts of water into each cup. • 2 clear plastic cups 2 Pour one spoonful of table salt into one • plastic spoon of the cups. Stir. • table salt • flour 3 Pour one spoonful of flour into the other cup. Stir. 4 Record your observations. 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 A mixture is a combination of substances, such as a fruit salad. Remember to use the The ingredients of any mixture can be physically separated from each strategy of your choice. You might use a four square other because they are not chemically changed—they are still the same diagram for solution . 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 ¡ ¢ £ ¨ ¥ ¦ § Solutes and Solvents reading tip Like other mixtures, a solution has definite components. A solute The words solute and solvent (SAHL-yoot) is a substance that is dissolved to make a solution. are both related to the Latin When a solute dissolves, it separates into individual particles. A solvent word solvere, which means “to loosen.” 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 In some solutions, both the solute and the solvent are in the Air is oxygen same physical state. Vinegar, for example, is a solution of acetic and other gases dissolved in acid in water. In a solution of different liquids, it may be difficult nitrogen. 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 Solution Bronze consists solid solutions also exist. For example, bronze is a solid of tin dissolved solution in which tin is the solute and copper is the solvent. in copper. 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. Liquid Solution The air you breathe is a solution. Because nitrogen makes up Water often contains many the largest portion of air, it is the solvent. Other gases present, dissolved such as oxygen and carbon dioxide, are solutes. substances. When substances in a solution are in the same physical state, which is the solvent? DB 112 Unit: Chemical Interactions ¡ ¢ £ © ¥ ¦ § Solutions SKILL FOCUS How can you separate the parts of a solution? Observing PROCEDURE 1 Draw a solid black circular region 6 cm in diameter around the point of the filter. MATERIALS • black marker 2 Place the filter, point up, over the top of the bottle. • coffee filter • plastic bottle Squeeze several drops of water onto the point of the filter. 3 • eyedropper 4 Observe the filter once every minute for 10 minutes. • tap water Record your observations. TIME 15 minutes 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. 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 suspended particle solvent solvent Solution Dissolved particles cannot be identified as a substance different Suspension Particles that do not from the solvent. dissolve make a suspension look cloudy. Chapter 4: Solutions 113 DB ¡ ¢ £ ¥ ¦ § 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 (C 12 H22 O11 ) 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 covalent compound added to solvent added to solvent Ionic Compound Dissolved in 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 ¡ ¢ £ ¥ ¦ § 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 reminder water, the resulting solution has a freezing point below 0°C. In temperature measure- ments, C stands for Lowering the freezing point of water can be very useful in winter. “Celsius” and F stands Road crews spread salt on streets and highways during snowstorms for “Fahrenheit.” 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.