3.5 Separating Solutions 67 NEL Separating Solutions Have you ever been swimming in the ocean (Figure 1)? Once you dry off, you may notice that your skin feels strange. Maybe you licked your fingers and tasted salt. Where did the salt come from? e ocean is a solution of salt and other solutes in water. Aſter you swim in the ocean, the water on your skin evaporates. is means that the water on your skin is heated by the warm skin and changes from the liquid state to the gas state. As a gas, the water dissolves in the air and “disappears.” e salt does not evaporate, so it gets leſt behind on your skin. What does this have to do with separating a solution? As you know, solutions are mixtures in which the particles of the different components are so completely mixed that the mixture appears to be a single pure substance. How can you separate a solution into its components? To answer this question, you need to think about how the particles of a solution behave. Particles of a Solution Figure 2 shows sugar and water being mixed together. What happens to the sugar particles when they are mixed with the water particles? ink back to the model of dissolved particles in a solution (Figure 6 in Section 1.6). Remember that the particles of a solution are evenly mixed. e sugar particles are still in the solution. ey have not changed into something different. If you were to make a solution of sugar and water in your kitchen at home, you could tell that the sugar is still there because the solution would taste sweet. evaporate: change from a liquid to a gas 3.5 Figure 2 How can we get the sugar back from a sugar and water solution? Figure 1 Ocean water contains salt. The salt dries on your skin when the water evaporates.
Transcript
3.5 Separating Solutions 67NEL
Separating SolutionsHave you ever been swimming in the ocean (Figure 1)? Once you dry off , you may notice that your skin feels strange. Maybe you licked your fi ngers and tasted salt. Where did the salt come from?
Th e ocean is a solution of salt and other solutes in water. Aft er you swim in the ocean, the water on your skin evaporates. Th is means that the water on your skin is heated by the warm skin and changes from the liquid state to the gas state. As a gas, the water dissolves in the air and “disappears.” Th e salt does not evaporate, so it gets left behind on your skin. What does this have to do with separating a solution?
As you know, solutions are mixtures in which the particles of the diff erent components are so completely mixed that the mixture appears to be a single pure substance. How can you separate a solution into its components? To answer this question, you need to think about how the particles of a solution behave.
Particles of a SolutionFigure 2 shows sugar and water being mixed together. What happens to the sugar particles when they are mixed with the water particles? Th ink back to the model of dissolved particles in a solution (Figure 6 in Section 1.6). Remember that the particles of a solution are evenly mixed.
Th e sugar particles are still in the solution. Th ey have not changed into something diff erent. If you were to make a solution of sugar and water in your kitchen at home, you could tell that the sugar is still there because the solution would taste sweet.
evaporate: change from a liquid to a gas
3.5
Figure 2 How can we get the sugar back from a sugar and water solution?
Figure 1 Ocean water contains salt. The salt dries on your skin when the water evaporates.
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Separating Solutions by EvaporationHow could you reclaim the sugar from a water-and-sugar solution? Th ink about some of the diff erences between sugar and water. When you heat liquid water, it evaporates quite easily. Sugar does not evaporate so easily. You can use this diff erence to remove the water from the sugar-and-water solution, leaving the sugar behind. Evaporation occurs when a liquid changes into a gas. Th e solid (or a more concentrated solution) is left behind.
Evaporation is oft en used to remove the liquid from a solution made of a liquid and a solid. Th is is how maple syrup is made from maple sap. Th e process of making maple syrup only releases water into the air, so it is not directly damaging to the environment. However, burning wood or natural gas to heat the sap releases gases that cause pollution.
evaporation: the process by which a sample of matter changes from a liquid to a gas
TRY THIS: Make a Stalactite
Look at the cave in Figure 3. When water evaporates, it leaves its dissolved solutes behind. You can use this fact to make your own cave formations!
Equipment and Materials: apron; large bowl; spoon; piece of cardboard; 2 clear plastic cups; warm water; Epsom salt; 50 cm of cotton string or wool yarn
Put on your apron. Pour about 400 mL of warm water into 1. the large bowl. Add large spoonfuls of Epsom salt and stir until you get a saturated solution (until no more will dissolve).
Choose a place in your classroom or home where the 2. experiment can stay for about a week. Put the cardboard on a fl at surface.
Pour half of the solution into each cup. Place one cup at 3. each side of the piece of cardboard, about 15 cm apart.
Rinse the string in tap water. Next, soak the string in the 4. saturated solution for a few seconds.
Hang the string between the two cups as shown (Figure 4). 5. Let the string sag in the middle, but do not let it touch the cardboard.
Wait for several days. Be careful not to touch your 6. experiment.
Was your experiment successful? What happened?A.
Explain your observations.B.
Explain how the stalactites you made are similar to, or C. different from, the rock formations in Figure 3.
Figure 3 Dripping water evaporates, leaving behind the mineral solutes. Over thousands of years, the minerals form stalactites hanging from the roof and stalagmites on the ground below.
stringcup
cardboard
Epsom salt solution
Figure 4 Creating stalactites
68 Chapter 3 • Separating Mixtures NEL
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3.5 Separating Solutions 69NEL
Separating Solutions by DistillationIs there a way of separating two liquids that are mixed together without losing either of them? Th ere is a way if one of the liquids evaporates more easily than the other. Distillation is the process of heating a solution of two or more liquids until one liquid evaporates into a gas. Th e gas is then trapped and cooled until it becomes a liquid again (Figure 5).
If you had a liquid solution with more than two liquid components, you could repeat the distillation process to separate all the liquid parts of the solution. You would have to use a diff erent collecting fl ask for each component that evaporated and then cooled to become a liquid again.
distillation: the process of separating liquids in a solution by heating the solution, trapping and cooling the gas, and collecting the resulting pure liquid
thermometer
solution of water and ethanol
hot plate and hot water bath
glass tube
ethanol gas
cold water flows in and out
condenser
ethanol
Figure 5 To separate ethanol and water using distillation, the solution is heated to just below 78 °C. The ethanol evaporates rapidly at this temperature. As a gas, it travels to the condenser where cold water cools it. The ethanol then cools to become a liquid again. As a liquid, it drips into the collecting fl ask. Most of the water stays behind in the round-bottomed fl ask.
How can you use information about separating solutions as you work on the Unit Task? Can you use evaporation or distillation?Unit Task
Locating InformationThink about the concept map you created in Section 3.2. What connections can you make between concepts learned in that section and in this one (Section 3.5)? What two additional methods of separation can you now add to your concept map?
L I N K I N G T O L I T E R A C Y
To learn more about evaporation and distillation, Go to Nelson ScienceGo to Nelson Science
Describe one way to separate sugar from a sugar-and-water 1. mixture.
Explain, using a labelled diagram, how the process of 2. distillation works.
The sap from a sugar maple tree is a mixture of sugar 3. dissolved in water.
(a) What method is used to make maple syrup? (b) When you boil maple sap, something leaves the mixture.
What is it? Is this process likely to be harmful to the environment?
CHECK YOUR LEARNING
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