4—What Happens When Compounds Are Added to Water? Name: ______________________________________________ Date: ________________________________________________ Lab Day/Time: ____________________________________ Lab Partner: ________________________________________ Objectives •
•
•
•
Understand the difference between electrolyte and nonelectrolyte solutions
Learn to identify a compound as covalent or ionic and predict its conductivity in solution
Learn to write chemical equations for compounds that dissociate into ions in solution
Understand the relationship between moles of ions in solution and conductivity
Pre-­‐Laboratory Requirements •
•
•
Watch the video on Canvas titled “Using a Conductivity Meter”
Pre-lab questions (if required by your instructor)
Laboratory notebook—prepared before lab (if required by your instructor)
Safety Notes •
Eye protection must be worn at all times.
Discussion In this experiment, you will explore the conductivity of electrolyte and nonelectrolyte solutions. Compounds that
are electrolytes dissociate into ions when dissolved in water. When electrodes are immersed into an electrolyte
solution, the ions complete the electrical circuit and the solution conducts electricity. Nonelectrolytes do no
dissociate into ions and, therefore, these solutions do not conduct electricity. In this lab, the conductivity of
solutions will be measured with a Relative Conductivity Indicator.
Procedure Part I. What Happens When You Put Chemical Compounds in Water? One way you can explore the characteristics of different solutions is to measure whether or not they conduct
electricity and to what extent.
Using the Conductivity Tester: To measure the amount of electricity that your solution conducts, you will be
using a Conductivity Meter. When you are ready to make a measurement, press the on/off button on the meter and
place the probe in ~ 20 mL of deionized water in a beaker. Make sure the meter is reading close to zero (0-3 µS)
before you place it into your solutions. Record the conductivity reading and units (mS or µS) then remove the
probe from your solution. Thoroughly rinse the probe with deionized water and then place it back into the beaker
of deionized water to ensure the meter is reading close to zero between each measurement.
CAUTION: Do not touch any other buttons on the conductivity meter except the on/off button.
1 © 2015 James Madison University and JMU Board of Visitors
1. Investigate the properties of deionized water. Do NOT use regular tap water. Test the conductivity of the
deionized water with the conductivity meter, record the reading and then immediately remove the probe from
the water.
2. Prepare solutions of calcium chloride, dextrose, isopropyl alcohol, sodium chloride, and sucrose by adding
approximately 0.15 g of each compound in 30 mL of deionized water. As you put each of your compounds
into water, stir and carefully observe what happens. Record all of your observations. Be sure to label each of
your solutions.
3. Using the conductivity meter, investigate the electrical conductivity of each solution and record the reading
with units (mS or µS). Between each test, be sure to rinse the conductivity meter with deionized water.
Compound
Conductivity
reading
Chemical
Formula
Observations
Conductive?
Nonconductive?
Deionized Water
Calcium Chloride
Dextrose
Isopropyl Alcohol
Sodium Chloride
Sucrose
4. (Optional) Compile class data to investigate the reproducibility of the measurements. Do the trends
observed by your group correlate with the class data?
Compound
1
2
3
4
5
6
7
8
9
10
11
12
Deionized Water
Calcium Chloride
Dextrose
Isopropyl Alcohol
Sodium Chloride
Sucrose
2 © 2015 James Madison University and JMU Board of Visitors
AVG
Interpreting the Data Discuss your answers to the following questions with your lab partner(s) and record them:
1. What types of changes did you observe as you prepared your solutions?
2. How did the different solutions behave when you tested them with the conductivity meter? What relationship
did you observe between the chemical formulas and conductivity? Summarize your conclusions below.
3. How would you represent your observations with balanced chemical equations?
Compound + H2O
Balanced Chemical Equation
Calcium Chloride
Dextrose
Isopropyl Alcohol
Sodium Chloride
Sucrose
Class Discussion What happens to chemical compounds when you put them in water? Be prepared to explain your current thinking
and the observations that led you to your ideas. You may find it helpful to take notes about the discussion.
Part II. Predicting How New Systems Will Behave Predict which compound will produce the largest number of pulses, i.e. be the most conductive, when dissolved in
100.00 mL of water. Circle your prediction.
0.10 g NaCl
0.10 g NaI
0.10 g CaCl2
all will produce the same #
Provide a brief explanation for your prediction.
3 © 2015 James Madison University and JMU Board of Visitors
1. Prepare solutions of NaCl, NaI and CaCl2 by adding 0.10 g of each solid to a different 100.00 mL
volumetric flask. Fill each flask with deionized water such that the bottom of the meniscus touches the
mark denoting 100.00 mL. Place a stopper on the top of each flask and mix well by inverting the flask.
2. Pour a sample of each solution into a labeled beaker. Using the conductivity meter, test the conductivity
of each solution and record the reading. Between each test, be sure to rinse the conductivity meter with
deionized water.
Compound
Mass (g)
Moles (mol)
Volume (mL)
Conductivity
reading
Volume (L)
NaCl
NaI
CaCl2
3. (Optional) Compile class data to investigate the reproducibility of the measurements. Do the trends
observed by your group correlate with the class data?
Compound
1
2
3
4
5
6
7
8
9
10
11
12
AVG
NaCl
NaI
CaCl2
4. Calculate the molarity of each solution and the concentration of ions in each solution. Record your results
in the table below.
Compound
Conductivity Reading
(Group or Class AVG)
Concentration of salt solution
(M)
Concentration of ions in solution
(M)
NaCl
NaI
CaCl2
4 © 2015 James Madison University and JMU Board of Visitors
Sample Calculations: Interpreting the Data Discuss your answers to the following questions with your lab partners and record them:
1. Describe, in your own words, the difference between the concentration of the salt solution and the
concentration of ions in solution.
2. What determines the magnitude of the conductivity?
3. How did the conductivity values compare for these three solutions? How can you explain your
observations?
Class Discussion Have your observations in Part II changed your thinking compared to Part I? Be prepared to explain your
current thinking and the observations that support your ideas.
5 © 2015 James Madison University and JMU Board of Visitors
Part III. Why Does Your Source Of Water Matter? This lab has been very specific about the type of water you should use. It emphasized the importance of using
deionized water, rather than tap water, for all experiments. In the final portion of the lab, you will investigate
why this decision was made.
Predict what will happen if you use the conductivity probe with tap water. Explain the rationale behind your
prediction.
1. Test tap water with the conductivity indicator and record the reading.
Conductivity reading
Tap Water
Interpreting the Data 1. What do your results tell you about tap water?
2. Why did you need to use deionized water rather than tap water?
6 © 2015 James Madison University and JMU Board of Visitors