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Solutes and Solubility

Investigation 1 – Concept Day

 

 

 

 

 

 

 

 

 

 

 

 

 

Solutes and Solubility: Investigation 1

Concept Day

SLIDE SOL-1-1

Note: In this Investigation, we begin a three Investigation CELL on Solutes and Solubility. This is a relatively easy CELL both from a theoretical and Lab perspective. It will provide you with multiple opportunities to do chemical concentration calculations and make up solutions with different concentrations in the lab.

Along the way, you will learn what parameters increase or decrease the solubility of a solute in a solvent.

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SLIDE SOL-1-2

Note: You will have seen slides similar to this one earlier in the year in the Properties of Matter CELL.

  • All chemicals we know are composed of elements listed in the Periodic Table of the Elements. There is nothing mysterious about solutions and chemicals. They are simply combinations of known elements.
  • Recall the relationship between atoms, molecules, and compounds. We are using the example of the element oxygen in this slide. Oxygen can be found at the very top of column 16 of the Periodic Table. The simplest molecular form of oxygen is the gas, O2, which has two oxygen atoms in it. As shown in the example compound water, a compound is a molecule that contains more than one kind of atom or element. In the case of water, there are 2 hydrogen atoms and one oxygen atom. 

Note: Subatomic particles of oxygen are included for completeness but they will not figure directly into our discussion of solutes and solubility.

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SLIDE SOL-1-3

 

Note: This slide reviews what a chemical formula is. You will likely have some familiarity with chemical formulas from LabLearner elementary CELLs and from the first sixth-grade CELL of the year, Properties of Matter.

  • Although different compounds may be composed of the same type of elements or atoms, the relative amount of the atoms in the compound can have a profound influence on the properties of the compound.
  • In this example, both isopropyl alcohol (rubbing alcohol) and sucrose (table sugar) are composed of carbon, hydrogen, and oxygen atoms. However, the number and arrangement of the atoms are shown in their chemical formulas. The chemical formula for isopropyl alcohol is C3H8O. That is, it has 3 atoms of carbon, 8 atoms of hydrogen, and one atom of oxygen. The chemical formula for sucrose, on the other, hand is C12H22O11. It has 12 atoms of carbon, 22 atoms of hydrogen, and 11 atoms of oxygen. Sucrose is obviously a larger, more complicated molecule than isopropyl alcohol. While sucrose is a sweet-tasting solid at room temperature, isopropyl alcohol is a toxic liquid that can be used to kill microbes and prevent infections. Clearly, simply having the same type of constituent atoms does not necessarily cause compounds to have similar properties!

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SLIDE SOL-1-4

  • Here we introduce the terms solute and solvent on the left side of the slide. For all of the experiments in this CELL, the solute will be a solid and the solvent will be a liquid (water). However, solutes could be liquid as well and there are many other solvents than water. Isopropyl alcohol, for example, is a very good solvent in some cases.
  • Often times, one refers to the compound present in the least amount as the solute and the compound present in the greatest amount as the solvent.
  • The three beakers on the right represent some of the potential outcomes of mixing the solute and solvent on the left together. At the top, the solute is not able to dissolve in the solvent under these conditions. We would therefore say that the solute is insoluble in the solvent.
  • The center beaker on the right indicates a case where all of the added solute dissolves in the solvent, there is no residual solute left floating on top, suspended in the solvent, or sunk to the bottom of the beaker. We would therefore say that the solute was soluble or completely soluble in the solvent under these conditions.
  • Finally, the beaker at the lower right indicates yet another potential outcome of mixing the solute and solvent together. Judging by the color of the liquid, some of the solute was clearly soluble in the solvent. However, there is some residual solute sunk to the bottom of the beaker that did not “go into solution”. This is possibly because the solution has become saturated with the solute. The solvent simply cannot dissolve any more of the solute under these conditions.

Note: We will discuss and perform experiments regarding reasons for insolubility and saturation in Investigations 2 and 3. Also, you may find saturated solutions in your experiments in Investigation 1.

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SLIDE SOL-1-5

  • This slide is simply included to review concentrations and how to make solutions of different concentrations. As we move from the left beaker to the far-right beaker, the concentration of the solutions increase. We often speak of concentrations of solutions as grams of solute per milliliter of solvent, or grams per milliliter (g/ml).
  • Adding solute by mass to a fixed volume of solvent yields simple solutions. That is, a solution that has 250 grams of a solute in a total volume of 100ml is half as concentrated as one that has 500 grams. Adding 1,000 grams of solute to a total volume of 100ml yields a solution twice as concentrated as one with 500 grams (providing all 1,000 grams dissolve and the solution doesn’t become saturated).
  • Finally, notice that we can report the concentration as g/ml. Thus, 250 grams in 100ml water becomes 2.5g/ml (divide both 250g and 100ml by 100). Five hundred grams in 100ml becomes 5g/ml, and so on.

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SLIDE SOL-1-6

Note: The final two slides are included as a tool to prepare you for Investigation 1 Lab.

Note: Trials 1 through 4 will involve dissolving increasing amounts of the solute sodium bicarbonate (NaHCO3) in 100ml water. After making the solute additions, you will determine the solubility of the resulting solutions.

  • The molecular model or “structure” for sodium bicarbonate (baking soda) is shown to introduce you to the idea of molecular shape and bonding. You do not need to memorize these structures but you should be able to count the number of each kind of atom in the molecules.

Note: Notice that while the carbon, oxygen, and hydrogen are bonded together by solid bonds, the sodium is not shown attached to the rest of the bicarbonate portion of the molecule. That is because, in solution, sodium is a positively charged “ion” and the bicarbonate portion acts as a negatively charged ion. The opposite charges are all that hold these two parts of the molecule together.

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SLIDE SOL-1-7

  • This final slide shows the background information for Trials 5 through 8 of Investigation 1 Lab. Notice that each group will work with one of the two solutes (either salt or sugar). You will check for solute solubility at increasing solute concentrations.

Note: Notice that sodium chloride salt (NaCl) forms a positive and negative ion in solution, as did sodium bicarbonate. This is, in fact, fairly characteristic of salts in solution in general.

Note: Although none of the chemicals in Investigation 1 are harmful, you should nonetheless ware lab coats, gloves, and eye protection as always when working with any chemicals.