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Solutes and Solubility

Investigation 3 – Concept Day

 

 

 

 

 

 

 

Solutes and Solubility: Investigation 3

Concept Day

SLIDE SOL-3-1

Note: In this final Investigation of the CELL Solutes and Solubility, we will introduce another method of increasing molecular collisions and increase the solubility of a solute in a solvent.

This is once again a relatively short Investigation, so we will take the opportunity to end the presentation with an easy method for making solutions of any desired concentration. We will also include several practice calculations for the class to do together with the teacher.

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SLIDE SOL-3-2

  • This slide is a modification of an earlier slide in Solutes and Solubility. Once again, we see the three potential outcomes of the mixing of a solute (in this case NaCl) and a solvent (in this case water).
  • The top beaker on the right shows a condition of insolubility. In the case of NaCl, this would not actually happen in water. However, if solvent molecules do not interact with solute molecules, then no solubility will occur.
  • The middle beaker on the right shows the condition of solubility. Notice how the solvent molecules surround and interact with the solute molecules.
  • The bottom beaker on the right shows a saturated NaCl solution. In a case like this, the water molecules will have surrounded all of the sodium and chloride ions they can but there are simply not enough solvent molecules (water) to surround and interact with all of the solute molecules (NaCl) present. As a result, some of the salt is soluble and is suspended in solution, but undissolved, insoluble clumps of solute are also present and sink to the bottom of the beaker.

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SLIDE SOL-3-3

  • This slide shows common table sugar, sucrose. Sucrose will serve as the solute in the next couple of slides.
  • The chemical formula for sucrose is C12H22O11. It is soluble in water but, like other solutes, its solubility can be saturated. An example of how sucrose sugar can come “out of solution” and become insoluble will be presented in a later slide.

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SLIDE SOL-3-4

  • This slide shows sucrose as a solid solute on the left followed by the solvent; water, in the center illustration. When the solute and solvent are mixed together, a soluble sucrose solution may be achieved (right illustration).
  • A saturated solution is one in which no more solute can be dissolved in it. There are simply not enough solvent molecules available to surround the added solute molecules.
  • Consider what would happen if a saturated solution is allowed to evaporate. When evaporation of a solution occurs, the liquid solvent molecules transition to a gas phase and leave the solution. However, the solute molecules do not change phases and stay behind. As the volume of the solvent decreases due to evaporation, the dissolved solute becomes more concentrated (in this example, less water but the same amount of sugar). At some point, there will not be enough solvent molecules to keep the dissolved solute in solution and the solute will become more and more insoluble as the solvent continues to slowly evaporate.
  • What do you think happens if a nearly saturated sucrose solution begins to evaporate?

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SLIDE SOL-3-5

  • This slide shows rock candy being made from a sucrose solution in colored water. Large amounts of sucrose are dissolved in boiling water to a point of near saturation. Food coloring and flavoring can be added at this point.
  • The hot sucrose solution is poured into a container and a stick or string is hung from the top to just above the bottom of the container. The solution is then left for a number of days.
  • As cooling and evaporation takes place, the sucrose solution becomes more concentrated. Some of the soluble sucrose starts to become insoluble and comes out of solution. As it does, it solidifies and crystallizes on the added surfaces (in this case, wooden sticks). The crystals continue to grow as the sucrose in solution becomes more and more concentrated as solvent molecules are lost to evaporation.

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SLIDE SOL-3-6

  • This slide reminds us that in order for solubilization or any type of chemical reaction to occur, molecular collisions must take place.
  • We used a similar slide in Investigation 2 and asked how kinetic energy can be increased. In Investigation 2, we followed up on one of the potential answers to this question, namely stirring or mixing. We will now consider another way of increasing the kinetic energy of a solution, increasing its temperature.

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SLIDE SOL-3-7

  • This slide shows two different ways of increasing the kinetic energy of a solution.
  • On the left, the solution in the beaker is being stirred. Whether one uses a magnetic spin bar and hotplate stirrer like shown here or a simple glass rod or even a spoon, mechanical energy is applied and gets the solute and solvent molecules moving in relation to each other, thus increasing their kinetic energy. This causes increased molecular collisions that can lead to solubility. It also serves to increase the rate of many other types of chemical reactions as well. The analogy of the pool break (this is a 4-second time-lapse photo) to mechanical stirring is a good one. Movement (kinetic energy) is transferred from outside the solution and directly causes the molecules in the solution to move.
  • On the right, we consider the effect of temperature on solubility. At low temperature (pale blue box) molecules move very slowly or not at all. Heat is a form of kinetic energy. Therefore, as the solution is heated, its molecules will receive more kinetic energy and they will move more. More molecular movement causes more molecular collisions between solute and solvent molecules and solubility is increased. It is for this same reason (increased molecular motion and collisions) that warming up most reactions increases the rate of the reaction.

Note: The picture of the iced tea and hot tea is instructive. From your own experience, which is easier to get sugar to dissolve or become soluble in, hot tea or iced tea? The answer proves the point of the discussion and leads directly to the experiments in Investigation 3 Lab.

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SLIDE SOL-3-8

Note: This slide introduces you to a very simple way of calculating how much solute and solvent to add for different solution concentrations. It works on the principle of ratios and can be very useful for a number of different kinds of calculations in science.

 

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SLIDE SOL-3-9

 

Note: This slide gives another step-by-step example calculation of making up a specific concentration of a solution.

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SLIDE SOL-3-10

Note: This final slide gives three additional practice calculations using the ratio method.