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Factors Affecting Solubility (page 2)

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Author: Beth Touchette

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.

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