Factors Affecting Solubility

Problem

learn about solutions!

Materials

• Distilled water (this type of water has absolutely no minerals dissolved in it)
• White granulated sugar
• Teaspoon
• Three straws
• Thermometer 
• Clear glass

Procedure A: How Sweet It Is!

1. Add two teaspoons of sugar to one cup of the distilled water.
2. Stir.
3. You are now going to see if the concentration of sugar varies in different parts of a sugar solution. Luckily, you have your own sugar detector: your tongue! You are going to draw samples of the sugar solution from the top, middle, and bottom of the cup of the sugar solution. Which location do you think will taste the sweetest? Why?
4. Dip the straw into the bottom of the cup. When some of the sugar water has entered the straw, put your finger over the top of straw and lift carefully out of cup. Taste the liquid at the bottom of the straw.
5. Using a fresh straw each time, repeat step 4, sampling at the middle and top of the sugar solution, respectively.
6. If you are unsure of your results, try again, or have a friend repeat the experiment with you.

Procedure B: How Sweet Can it Get?

1. Measure one cup of room temperature distilled water into a clear glass. 
2. Add 1 teaspoon of sugar.  Stir.
3. Continue adding sugar to the water, 1 teaspoon at a time, stirring after each addition. Make to keep track of how many teaspoons of sugar you add.
4. Keep adding sugar to the solution, until the solution reaches a point where sugar no longer dissolves and instead sinks to the bottom of the glass.
5. At this point, you will have made a saturated solution. A saturated solution is a solution that holds the maximum amount of that particular solute (in this case sugar) for that particular solvent (in this case water) at that particular temperature (room temperature).

Procedure C: What about Hot and Cold?

1. Heat up one cup of distilled water until hot. Add sugar one teaspoon at a time, stirring after each addition, until you have added the same amount of sugar that you added to the cup filled with room temperature water. Observe.
2. Fill a second cup with chilled distilled water. Add sugar one teaspoon at a time, stirring after each addition, until you have added the same amount of sugar that you added to the cup filled with room temperature water. Observe.

Results

For Procedure A, the water will be uniformly sweet—it should have made no difference whether the water came from the top, middle, or bottom of the cup. For Procedure B, your results will vary depending on the exact temperature and volume of water in your cup. For Procedure C, the hot water should not have become saturated when the sugar was added to it. The cold water will have become saturated before all the sugar was added.

Why?

The reason you did not notice a difference in sweetness when you sampled the solution at different levels is because the solute and solvent were uniformly distributed. You might have thought the solution would taste sweeter near the bottom of the glass—but that happens with suspensions (like orange juice), not solutions. Of course, if your sugar solution was saturated, it would taste sweeter at the bottom—but we intentionally had you make an unsaturated solution for this part of the experiment. Speaking of saturated solutions, the reason your solution could only dissolve so much sugar is that eventually there are not enough water molecules around to surround each sugar molecule, so some of the sugar molecules begin to clump together and fall to the bottom. In many solutions in which solids are dissolved in liquids, the warmer the solvent is, the more solution is able to dissolve: temperature is one of the key factors affecting solubility. Cooler liquid solvents are often capable of holding less solute.  

Solutions don’t always involve a solid dissolved in a liquid. For instance, soda has both solid sugar and carbon dioxide gas dissolved in water. The air we breathe is a solution of several gases, including nitrogen, oxygen, and carbon dioxide. Solids can even be dissolved in solids! Brass is a solid solution (or alloy, because the component materials are metals) of zinc and copper. The combination of the two metals makes brass stronger and more durable than either zinc or copper alone. Bronze is an alloy of copper and tin. Pure gold (24 karat) is too soft and expensive for most jewelry, so it is made it into alloys with stronger, cheaper metals. 18 karat gold earrings are 75% gold and 25% other metals.

Going Further

For a yummy extension, make rock candy by making a supersaturated sugar solution.