KEY CONCEPT The Amount Of Solute That Dissolves Can Vary.

KEY CONCEPTThe amount of solutethat dissolves can vary.BEFORE, you learnedNOW, you will learn Solutions are a type of mixture A solution is made when asolute is dissolved in a solvent Solutes change the propertiesof solvents About the concentration ofa solution How a solute’s solubility canbe changed How solubility depends onmolecular structureVOCABULARYEXPLORE Solutions and Temperatureconcentration p. 117dilute p. 118saturated p. 118solubility p. 119How does temperature affect a solution?PROCEDURE1Pour cold soda water into one cup andwarm soda water into another cup.Record your observations.MATERIALS soda water 2 clear plasticcups2 After 5 minutes, observe both cups ofsoda water. Record your observations.WHAT DO YOU THINK? Which solution bubbled more at first? Which solution bubbled for a longer period of time?A solution with a high concentrationcontains a large amount of solute.MIND MAPRemember to use a mindmap to take notes on theconcentration of a solution.Think of water from the ocean and drinking water from a well.Water from the ocean tastes salty, but water from a well does not.The well water does contain salt, but in a concentration so low thatyou cannot taste it. A solution’s concentration depends on theamount of solute dissolved in a solvent at a particular temperature.A solution with only a small amount of dissolved solute, such as thesalt dissolved in well water, is said to have a low concentration.As more solute is dissolved, the concentration gets higher.If you have ever used a powdered mix to make lemonade, youprobably know that you can change the concentration of the drink byvarying the amount of mix you put into a certain amount of water.Two scoops of mix in a pitcher of water makes the lemonade strongerthan just one scoop. The lemonade with two scoops of mix has a higherconcentration of the mix than the lemonade made with one scoop.Chapter 4: Solutions 117DB

reading tipThe word dilute can beused as either an adjectiveor a verb. A dilute solutionhas a low concentration ofsolute. To dilute a solutionis to add more solvent to it,thus lowering the concentration of the solution. Degrees of ConcentrationA solution that has a low concentration of solute is called a dilutesolution. Salt dissolved in the drinking water from a well is a dilutesolution. The concentration of a solution can be even further reduced,or diluted, by adding more solvent. On the other hand, as more soluteis added to a solution, the solution becomes more concentrated.A concentrated solution has a large amount of solute.DilutesolventConcentratedsoluteLess solute is dissolved ina dilute solution.More solute is dissolved ina concentrated solution.Have you ever wondered how much sugar can be dissolved in a glassof iced tea? If you keep adding sugar to the tea, eventually no more sugarwill dissolve. The tea will contain as much dissolved sugar as it canhold at that temperature. Such a solution is called a saturated solutionbecause it contains the maximum amount of solute that can be dissolvedin the solvent at a given temperature. If a solution contains less solutethan this maximum amount, it is an unsaturated solution.Check Your ReadingVISUALIZATIONCLASSZONE.COMExplore supersaturatedsolutions and precipitation.DBHow are the terms dilute and saturated related to the conceptof concentration?Supersaturated SolutionsSometimes, a solution contains more dissolved solute than is normallypossible. This type of solution is said to be supersaturated. A saturatedsolution can become supersaturated if more solute is added while thetemperature is raised. Then if this solution is slowly cooled, the solutecan remain dissolved. This type of solution is very unstable, though.If the solution is disturbed, or more solute is added in the form of acrystal, the excess solute will quickly solidify and form a precipitate.This process is shown in the photographs on the top of page 119.118 Unit: Chemical Interactions

remindersolute crystalA precipitate is a solidsubstance that comes outof a solution.12A supersaturated solutioncontains more dissolved solutethan is normally possible.After a crystal of solute is added,or the solution is disturbed,a precipitate forms.One example of a supersaturated solution is a chemical heat packthat contains sodium acetate and water. The pack contains more sodiumacetate than can normally dissolve at room temperature, but when thepack is heated in a microwave oven, all of the sodium acetate dissolves.The solution inside the pack is supersaturated. The heat pack is activated by bending it. This disturbs the solution, solidifying the sodiumacetate and releasing a large amount of heat over a long period of time.SolubilityThe solubility (SAHL-yuh-BIHL-ih-tee) of a substance is the amountof that substance that will dissolve in a certain amount of solvent at agiven temperature. For example, consider household ammonia usedfor cleaning. This ammonia is not pure ammonia—it is a solutionof ammonia in water.Because a large amount of ammonia can dissolve in water, ammoniais said to have a high solubility in water. However, other substances donot dissolve in such large amounts in water. Only a small amount ofcarbon dioxide will dissolve in water, so carbon dioxide has a lowsolubility in water. Oils do not dissolve at all in water, so oils are saidto be insoluble in water.reading tipThe word solubility is relatedto the words solute andsolvent, and means “abilityto be dissolved.” A substance that is insoluble willnot dissolve.The amount of solute needed to make a saturated solution dependson the solubility of a solute in a particular solvent. If the solute is highly soluble, a saturated solution will bevery concentrated.If the solute has a low solubility, the saturated solution willbe dilute.In other words, a saturated solution can be either dilute or concentrated, depending on the solubility of a solute in a particular solvent.How does solubility affect a solution?Chapter 4: Solutions 119DB

The solubility of a solute can be changed.The solubility of a solute can be changed in two ways. Raising thetemperature is one way to change the solubility of the solute, becausemost solids are more soluble at higher temperatures. Another way tochange solubility when the solute is a gas is to change the pressure.The solubility of gases in a liquid solvent increases at high pressure.reminderAn increase in temperaturemeans an increase inparticle movement.Temperature and SolubilityAn increase in temperature has two effects on most solid solutes—theydissolve more quickly, and a greater amount of the solid dissolves in agiven amount of solvent. In general, solids are more soluble at highertemperatures, and they dissolve faster.The opposite is true of all gases—an increase in temperature makesa gas less soluble in water. You can see this by warming tap water in apan. As the water approaches its boiling point, any air that is dissolvedin the water comes out of solution. The air forms tiny bubbles thatrise to the surface.What effect does temperature have on most solid solutes? ongaseous solutes?SolubilityHow can you change solubility?Use what you know about solubility to design anexperiment that shows how a change in temperaturecan change the amount of table salt that will dissolvein water.PROCEDURE1Use the materials in the list to identify the relationshipbetween temperature and solubility.2 Write your procedure, identifying the constantsand variables.3 Perform your experiment and record your results.WHAT DO YOU THINK? Which variables did you change?What were your constants? Why? How do your results demonstrate theeffect of temperature on solubility?DB120 Unit: Chemical InteractionsSKILL FOCUSDesigningexperimentsMATERIALS clear plastic cupsthermometertap watertable saltbalanceplastic spoonhot-water bathcold-water bathTIME20 minutes

Think back to the earlierTemperature and Solubilitydiscussion of supersaturatedSolute IncreasedDecreasedTemperatureTemperaturesolutions. One way in which aSolidincrease indecrease insolution can become supersatusolubilitysolubilityrated is through a change inGasdecrease inincrease intemperature. For example, supsolubilitysolubilitypose that a solution is saturatedat 50 C (122 F), and is thenallowed to cool slowly. The solid is less soluble in the cooler solution, butthe excess solute may not form a precipitate. As a result, the solutioncontains more of the dissolved solute than would be possible undernormal conditions because of the change in temperature.A change in temperature can produce changes in solutions in theenvironment. For example, a factory located on the shore of a lake mayuse the lake water as a coolant and then return heated water to the lake.This increase in temperature decreases the solubility of oxygen in thelake water. As a result, less oxygen will remain dissolved in the water.A decrease in the oxygen concentration can harm plant and animallife in the lake.Changing Temperature Changes SolubilityMore sugar dissolves in hot water than in cold water.The solubility of most solids increaseswith a rise in temperature.About how much sugar will dissolve in 100 g of water at 70 C?Chapter 4: Solutions 121DB

Pressure and SolubilityA change in pressure does not usually change the solubility of solid orliquid solutes. However, the solubility of any gas increases at higherpressures and decreases at lower pressures.When manufacturers make carbonated beverages, such as soda,they add carbon dioxide gas at a pressure slightly greater than normalair pressure. When you open the can or bottle, the pressure decreasesand the carbon dioxide bubbles out of solution with a fizz.INFER If these divers arebreathing regular air, whymight they be looking attheir depth gauges?Another example is shown in the photograph on the left. When adiver’s tank contains regular air, about 79 percent of the air is nitrogen.People breathe air like this all the time without any problem,but the pressure underwater is much greater than on Earth’ssurface. The higher pressure increases the solubility of nitrogen in the diver’s blood.When a diver heads up to the surface too fast, the pressuredecreases, and so does the solubility of the nitrogen.The nitrogen comes out of solution, forming bubbles in thediver’s blood vessels. These bubbles can cause a painful andsometimes fatal condition called the bends.Pressure and Solidno effecton solubilityno effecton solubilityGasincrease insolubilitydecrease insolubilityDivers can avoid the bends in two ways. They can rise tothe surface very slowly, so that nitrogen bubbles stay smalland pass through the bloodstream more easily. They canalso breathe a different mixture of gases. Some professionaldivers breathe a mixture of oxygen and nitrogen that containsonly about 66 percent nitrogen. For very deep dives, themixture can also include helium because helium is less solublein blood than nitrogen.RHow does pressure affect the solubility of solids?of gases?Solubility depends on molecular structure.Everyone knows that oil and water do not mix. When a tanker spillsoil near shore, the oil floats on the water and pollutes the beaches.Why do oil and water not mix? The answer involves their differentmolecular structures.When a substance dissolves, its molecules or ions separate fromone another and become evenly mixed with molecules of the solvent.Recall that water contains polar covalent bonds. As a result, watermolecules have a negative region and a positive region. Watermolecules are said to be polar. The molecules of an oil are nonpolar—the molecules do not have positive and negative regions. Thisdifference makes oil insoluble in water.DB122 Unit: Chemical Interactions