A. OBJECTIVES The purpose of this lab is to show the amounts of chemicals that are dissolved to produce significant concentrations in solution. Comparisons are then
made with concentration limits that are set for toxic chemicals in our water. B. BACKGROUND Water molecules have the special quality of being polar. The structure of
two smaller hydrogen atoms attached to the side of a large oxygen atom is asymmetrical. Consequently, the positive charge of hydrogen is not completely balanced by the
negative charge of oxygen, and the molecule has left-over charge. Many compounds are held together by ionic bonds. They are composed of oppositely charged ions (atoms
that have gained or lost electrons). When ionic compounds are exposed to water, the positive and negative charges of the water molecules pull the ions out of the
structure and encase them in envelopes of water. In this way, many ionic compounds dissolve in water (“the universal solvent”). When they are dissolved, the ions are
present in the water but are unable to join together. The more ions that are dissolved in water, the less healthy it is for humans. Some chemicals dissolved by water
are toxic when present in even a small amount. Others only affect health when large amounts are dissolved. The mass of a compound that is dissolved in one liter of
water is termed the “concentration” and is expressed as milligrams/liter (mg/1) or parts per million (pip). These two units are interchangeable. C. EQUIPMENT You will
need a scale capable of weighing in grams or in fractions of an ounce (1 ounce = 28.3 grams; 1 new clean copper penny weighs 2.5 grams). If you cannot find or afford a
scale, use the alternative method of measuring the appropriate volume of salt in each step. You will also need a spoon, table salt, a tooth pick, and an empty 2-liter
soft drink bottle (or a large measuring cup marked in milliliters or liters). D. PROCEDURES 1. Produce a concentration of saltwater that is close to the salinity of
sea water (35,000 mg/1). a. Measure approximately 35 grams (or ½ ounce) of table salt. This mass will fill a small match box (e.g., Red Top matches) (5cm x 3.5 cm x
1.5 cm) rounded at the top.
b. 2.
Measure 1 liter of water, and stir in the 35 grams of salt.
Experiment with the nature of saltwater compared to that of freshwater. a. Put an ice cube in freshwater and another one in your saltwater mixture, and see which one
melts first. • The heat transferring characteristics of saltwater are modified. b. Fill one portion of an ice cube tray with saltwater and another portion with
freshwater. Put the tray in your freezer, and check it every hour to see what happens. Produce a concentration close to that of tap water. a. Measure approximately 1
gram of salt onto a piece of paper. This amount will almost but not quite cover the bottom of the match box tray. b. Dissolve this amount of salt in a liter of water.
This is the maximum amount of dissolved material acceptable for drinking water in most states. Can you taste this amount of salt? Produce a concentration close to the
maximum concentration allowable for many chemicals in drinking water. a. Ten salt grains weigh approximately 1 milligram. How many milligrams are in a gram? Therefore,
how many grains of salt are in a gram of salt? b. Separate 10 grains of salt from one of your leftover piles of salt, using a toothpick. Drop the 10 salt grains into a
liter of tap water. This is the same concentration c. as 1 mg/l (or 1 ppm). d. Taste the water, and see if you can detect any saltiness. e. On the next page is a list
of the maximum concentration limits established for the drinking water standards of most states. There are additional toxic materials that are dangerous at
concentration levels of 1 part per billion or even several parts per trillion when taken in water for a long period of time. Seeing how dangerous some materials are,
think twice before you throw chemicals, batteries, and pesticides in the trash or down the drain. You may be contributing to the pollution of our life support system.
Optional: If you have some left-over garden seeds or if you have some seeds for sprouts, start some seeds in a moist paper towel or in potting soil.
Table 13-1 Federal Drinking Water Standards
ELEMENT (Recommended Limit) Total Dissolved Solids Chlorides Sulfate Nitrate Iron Manganese Copper Zinc Boron Hydrogen Sulfide (Maximum Permissible Limit) Arsenic
Barium Cadmium Chromium Selenium Lead Mercury Silver Fluorine E. RESULTS AND CONCLUSIONS
CONCENTRATION (mg/1) 500 250 250 45 0.3 0.05 1.0 5.0 1.0 0.05
0.05 1.0 0.01 0.05 0.01 0.05 0.002 0.05 1.4-2.4
Report your observations and answers to questions in a conclusion statement. F. REVIEW AND PRACTICE 1. Check some labels of items in your garage or cleaning closet,
and see if you have any of the items listed in the table. Think of some ways that saltwater is used in our outdoor environment. What bad effects might this have? If
normal spring water has 500 mg / l of dissolved solids, what is that concentration expressed in ppm? A water sample from the lead and zinc mining area of northeast
Oklahoma has the following concentrations: 1.0 mg / l each of zinc, lead, and sulfate. Is this water in violation of the Federal Drinking Water Standards? If so, why?
laboratory report should contain the following sections: (1) Hypothesis, (2) Procedures,
(3) Observations and Results, and (4) Conclusions. Make certain you include all four headings with at least a short paragraph for each. In addition, tables, graphs,
and answers to questions may be necessary in the latter two sections.
HYPOTHESIS
Scientific research should contain a preliminary statement of the expected outcome of the experiment. This can include predictions of the specific experiment or the
general anticipated result. If you are merely doing an observation and have no idea of the outcome, you cannot make an actual hypothesis. Instead, make a short
statement of the purpose of the observation. However, if you have preconceived ideas of the outcome, include them in this section, and then see how they compare to
the results.
PROCEDURES
Even though you are told what to do, write a paragraph of the specific steps you actually took in doing the experiment or observation. Because you are coming up with
your own equipment, your procedures will be of particular interest.
OBSERVATIONS AND RESULTS
This is where you should make a detailed statement of the outcome of your experiment. Record all your pertinent observations in a clear, readable form. Arrange your
data in tables (such as measurements and calculations you make). Answer any questions asked in this Study Guide, marking these clearly so that they can be easily
found.
CONCLUSIONS
Your conclusions should include a comparison between the outcome of the experiment and your initial predictions made in the hypothesis. In cases where you are
attempting to recreate a physical constant, compare your number to the accepted value, using the formula for experimental error:
Experimental Error Equation
If you find a large difference in your results from the expected value or if your anticipated observations are not the same as your actual observations, try to
identify possible sources of error or reasons for the difference in the hypothesis and results
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