Background

When you put a spoonful of sugar into a glass of iced tea, you probably begin to stir it up immediately. Why? If your reply is that the sugar will sink to the bottom and not easily dissolve if it is not stirred, you already understand an important factor about solution formation. Stirring is one of several factors that determines how fast a substance will dissolve and form a solution.

A solution consists of a solute, the material that is dissolved, and a solvent, the material that the solute is dissolved in. In this experiment, you will investigate the effects of stirring, temperature, and particle size on the rate of dissolution.

Goal

Observe the effect of particle size, degree of mixing, and temperature on the rate of dissolution.

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Equipment

safety goggles 1 wire gauze

7 large test tubes 1 gas burner

1 test tube rack 1 mortar and pestle

2 100-mL beakers 1 spatula

1 50-mL graduated cylinder 1 thermometer

1 ring stand 1 glass-marking pencil

1 ring support

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Materials

sodium chloride, NaCl paper towels

copper(II) sulfate pentahydrate, weighing paper

CuSO₄ 5H₂O distilled water

crushed ice

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Safety

• Note the Safety Symbols used here and in the Procedure section. Review safety information on pages 7-10.
• Always wear safety goggles when working in the lab.
• Copper compounds are toxic. Avoid contact with copper(II) sulfate pentahydrate.

Copy Tables 30.1 and 30.2 into your laboratory notebook. As you perform the experiment, record your observations in these tables.

Part A. Effects of Particle Size and Mixing

1. Label four large test tubes with the numbers 1-4. CAUTION: CuSO₄ . 5H₂O is toxic and irritation. Avoid contact with this material. Use a spatula to put four pea-sized crystals of copper(II) sulfate pentahydrate, CuSO₄ . 5H₂O, on a piece of weighing paper.

2. Put one crystal of CuSO₄ . 5H₂O into tube 1 and another crystal into tube 2. Crush a third crystal with the mortar and pestle and put the powder into tube 3. Crush the fourth crystal and put the powder into tube 4.

3. Fill each of the four test tubes about one-third full of water. Place tubes 1 and 3 in the test-tube rack without shaking them. Flick tubes 2 and 4. Note how long it takes for the contents of each of the four tubes to dissolve. Record your observations in Table 30.1.

4. Follow your teacherís instructions for proper disposal of the materials.

Part B. Effect of Temperature

5. Add 50 mL of distilled water to a 100-mL beaker. Using a gas burner, heat the water until it is almost boiling. While the water is heating, proceed to step 6.

6. Half-fill a 100-mL beaker with crushed ice and then add approximately 30 mL of distilled water to the beaker. While the water is chilling, proceed to step 7.

7. Label three large test tubes with the numbers 1-3. Add 5 g of sodium chloride to each test tube. Place the tubes in a test tube rack.

8. Fill tube 1 one-third full of ice-cold water (see step 6). Fill tube 2 one-third full with distilled water at room temperature. Fill tube 3 one-third full with hot water (see step 5). CAUTION: Pour the hot water from the beaker, using a paper towel handle as illustrated in Figure 30.1. Gently flick the contents of the test tubes. Note how long it takes for the contents of each tube to dissolve. Record your observations in Table 30.2.

9. Dispose of the contents of the three test tubes by pouring them down the drain.

Data Record
 

Table 30.1 Effects of Particle Size and Mixing on Solution Formation

Tube Particle Size Mixed? Time to Dissolve Observations

1

2

3

4

 
 
 
 
 
 

Table 30.2 Effects of Temperature on Solution Formation

Tube Temperature Time to Dissolve Observations

1

2

3

Conclusions

1. What effects does particles size appear to have on the rate at which a solute dissolves? Why should particle size make a difference in the rate of dissolving?

2. Does shaking the test tube affect the rate at which a solute dissolves? Explain your results.

3. Using kinetic theory, explain the effect of temperature on the dissolution rate of a solute.

Extensions

1. Devise an experiment to discover if there is a linear relationship between increasing temperature and the rate of dissolving. Use a temperature range from 0 ⁰C to 100⁰C. Test at least three different solutes.

2. Temperature affects not only the rate of dissolving, but also the amount of solute that will dissolve in a given volume of solvent. Design an experiment to quantitatively determine the solubilities of sodium chloride, sucrose, and potassium sulfate in water at O⁰C, 25⁰C, and 100⁰C.