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# Stirring Solution

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Source:
Author: Janice VanCleave

### Purpose

To determine how stirring affects the rate at which materials dissolve in a solution.

### Materials

• marker
• two 10-ounce (300-mL) transparent plastic cups
• 1-cup (250-mL) measuring cup
• tap water
• 2 sugar cubes
• timer
• spoon

### Procedure

1. Use the tape and marker to label the two cups "A" and "B."
2. Pour 1/2 cup (125 mL) of water into each plastic cup.
3. Add one sugar cube to each cup of water. Start the timer.
4. Using the spoon, gently stir the water in cup A until the sugar cube has completely dissolved and is no longer visible. Record the time as test 1 for cup A in a Dissolving Time Data table like the one shown.
5. Continue to let the timer run and observe cup B but do not stir the water.
6. When the sugar cube is completely dissolved so that no sugar is visible, record the time as test 1 for cup B.
7. Repeat steps 2 through 6 three or more times and average the results.

### Results

The sugar cube that is stirred (in cup A) dissolves faster.

### Why?

A solution is a homogenous mixture (a combination of two or more substances that is the same throughout). A solution is made of two parts: a solute that dissolves, meaning it breaks up into small particles—molecules or atoms—and moves throughout the second part, called the solvent. In this investigation, sugar is the solute and water the solvent. When the sugar is added to the water, it dissolves to form a solution. The dissolving of the sugar occurs because water molecules randomly move about, colliding with the surface of the sugar. While the cohesion (attraction between like molecules) between the molecules of the sugar is great enough to hold them together, the adhesion (attraction between unlike molecules) between the water molecules and sugar molecules is great enough to pull the sugar molecules off the sugar cube's surface. Because of the attraction of water molecules for the sugar molecules, each of the freed sugar molecules becomes completely surrounded by water molecules. This dissolving process by which water molecules surround a solute molecule is called hydration or solvation.

The rate of hydration can be increased by stirring. Stirring the mixture aids in the dissolving of the solute particles by moving hydrated solute particles away and bringing fresh portions of the solvent (water) in contact with the undissolved solute (sugar).

### For Further Investigation

As the temperature of a liquid increases, the motion of its molecules also increases. Would sugar dissolve faster in hot water? A project question might be, What effect does temperature have on the dissolving rate of sugar?

Repeat the original investigation with water at three different temperatures—hot, cool, and cold—but don't stir any of the cups. "Hot" can be hot tap water, "cool" can be water at room temperature, and "cold" can be tap water to which ice has been added to reduce the temperature. (Remove the ice before adding the sugar.) CAUTION: Ask an adult to fill the cups with hot water and to supervise handling of the cups of hot water.

### References and Project Books

Churchill, E. Richard. 365 Simple Science Experiments with Everyday Materials. New York: Black Dog & Leventhal,1997.

Gibson, Gary. Science for Fun Experiments. Brookfield, Conn.: Copper Beech Books, 1996.

Kenda, Margaret, and Phyllis S. Williams. Science Wizardry for Kids. Hauppauge, N.Y.: Barron's, 1992.

Nye, Bill. Bill Nye the Science Guy's Big Blast of Science. Reading, Mass.: Addison-Wesley, 1993.

Strauss, Michael. Where Puddles Go. Portsmouth, N.H.: Heinemann, 1995.

VanCleave, Janice A. Janice VanCleave's A + Projects in Chemistry. New York: Wiley, 1993.

Janice VanCleave's Chemistry for Every Kid. New York: Wiley, 1989.