Experiment: 
Factors Affecting Rates of Dissolving

Introduction:

Solutions are homogeneous mixtures which contain both a solvent and a solute.  The most common solutions contain water as the solvent.  These solutions are called aqueous solutions.  While many solutions are liquid (iced tea, coffee, sea water, Mr. Clean), solutions can also be in gas or solid phases.  The air around us is a gaseous solution.  That 14K gold necklace you wear is a solid solution.

You may have noticed that, for example, when you try to dissolve regular table sugar in a glass of iced tea, it is much harder to dissolve it as compared to dissolving table sugar in a mug of hot coffee.  There are different factors which influence the rate at which a solute dissolves in a solvent.  

What are some of these factors?  You will be examining the effect of three factors on the rate of dissolving.

• Agitation
• Temperature
• Surface area of solute particles

You will quantify your results in this experiment by calculating the rate at which the sodium chloride dissolves in grams per minute for each of the three factors you will test.

Materials:

• test tubes and rack
• 50 mL graduated cylinder
• stopwatch
• thermometer
• beakers
• glass stirring rod
• hotplate
• centigram balance
• weighing paper
• 3 sizes of sodium chloride crystals
• water
• ice

Procedure:

Agitation

1. Measure out TWO 1.0 g samples of the MEDIUM size of sodium chloride crystals on the weighing paper.
2. Measure 10 mL of water and pour it into a test tube.  Make another test tube like this.
3. Take one of the samples of NaCl and pour it into the test tube.  Just let the crystals sit but start timing right away (this is called your CONTROL group).
4. Now, for the second test tube, take the second sample of the NaCl and put in into the test tube.  Start your timer and then at the same time, use a glass stirring rod to dissolve the solute (NaCl)  into the solvent (H₂O).
5. Record how long it takes to dissolve the salt.
6. Did the control group sample dissolve as it just "sat" there?

Temperature

1. Heat about 150 mL of tap water in a beaker on a hotplate.  Heat to about 80^oC and then turn down the heat.
2. On the lab bench, fill up a beaker with ice and then add water to it to make an ice slurry.
3. On the lab bench, fill up a beaker with just tap water and let sit (this will be the room temperature water).
4. Measure out THREE 1.0 g samples of the MEDIUM size of sodium chloride crystals on the weighing paper.
5. Measure 10 mL of water and pour it into a test tube.  Make a set of 2 more test tubes like this.
6. Record the temperature of the hot water on your data table.
7. Record the temperature of the ice slurry on your data table.
8. Record the temperature of the room temperature water bath on your data table.
9. Add each of the NaCl samples to the test tubes.
10. Place one test tube in the hot water bath.  Start timing and stir gently with the glass stirring rod.
11. Repeat process for the ice slurry bath.
12. Repeat process for the room temperature water bath.
13. Record the time it takes for each sample to dissolve.

Surface area of solute:

1. Measure out a 1.0 g sample of each size of sodium chloride crystals.  Be sure to keep each crystal size separate from the other (label the weighing paper).
2. Measure 10 mL of water and pour it into a test tube.  Make a set of two more test tubes like this.
3. Add the first sample to a test tube.  Start stirring with the glass rod and begin timing.  Record the time when all of the sample is dissolved.
4. Repeat step #3 for the next two samples.  

Cleanup

1. Rinse out all test tubes.  Invert in racks.  
2. Return all materials (including racks) up to the front.

Data Table:

• Using Microsoft Word,  type your name, date and block period at the top of the page.  Then, RIGHT JUSTIFY!
• Then, copy/paste DATA TABLE BELOW from the website into your Word document in preparation for the lab.  After your have copy/pasted the table, make the table take up the whole page by clicking once on the table in the lower right corner and drag the corner to the bottom right of the page.  This way, you will have enough space to hand-write in your data in the experiment.  Make sure the data table uses the whole page!
• In the lab, be sure to use blue or black PEN (NO PENCIL) when you complete this data table.  NEVER WHITE-OUT data.  Enter data carefully so that you make no mistakes.  Print numbers and units neatly!

Analysis:

Word process your answers to these questions.  Staple to your completed data table and turn in on the due date.

1. Based on the results of this experiment, complete the following statement: Sodium chloride dissolves faster in water when.....
2. How does agitation affect the rate of dissolving?
3. How does temperature affect the rate of dissolving?
4. State the relationship between surface area and particle size.
5. How does surface area affect the rate of dissolving?
6. Dissolving sodium hydroxide (NaOH) in water is an exothermic process.  How do you think that placing pellets of sodium hydroxide in hot water would affect its rate of dissolving?