Ideas for Research Challenge #1:
Ideas for Experiments
The following are a few ideas that may help you in designing an experiment. Remember, there is not just one right answer -- for each question, there are many possible experimental designs.
Varying the pH
One possibility is to prepare a dilute solution of sulfuric acid by mixing 1 ml of 1M H2SO4 into 2 L distilled water. The pH of this initial stock solution will be around 3. Each time you dilute this stock solution by a factor of ten, the pH will increase by about one unit. Keeping all other factors constant, you could use this series of H2SO4 dilutions to test the affects of acidity on lettuce seed germination and growth.
Amount of Solution per Dish
Directions for the Reference Toxicity Test call for adding 2 ml of test solution to each Petri dish, but perhaps this is not the optimal amount for lettuce seed germination and growth. Design an experiment in which all other variables are constant, and only the amount of test solution is varied.
a Minimal Salts Solution
In the Reference Toxicity Test, distilled water is used as the control. Ideally, the control should promote optimal lettuce seed germination and growth so that any inhibition caused by toxic chemicals can be compared to what the growth would be under the best possible conditions.
If you have tried using lettuce seed bioassays to test tap water, you may have found that the seeds grew better in tap water than they did in the distilled water control. Does this mean it would be better to use tap water as the control when you are running bioassays on toxic chemicals? What disadvantages can you think of to this approach?
For consistency from experiment to experiment, scientists try to control everything other than the test variable (the independent variable). Since you do not know the exact chemical make-up of your tap water, you have no way of guaranteeing that it will remain the same from one experiment to the next. If students in other towns want to replicate your experiment, they also will have no way of knowing how their tap water compares to yours.
A better approach would be to start with deionized or distilled water, then add known amounts of whatever compounds help to promote lettuce seed germination and growth. To determine what these compounds might be, and in what concentrations, a number of different experiments are possible. You might want to try adding a mineral compound that is commonly found in tap water, such as MgCO3 or CaCO3. Another possibility would be to test the effects of supplying a nutrient such as NH4NO3 or K2HPO4. See if you can come up with a recipe for a solution that promotes better lettuce seed growth than you get in deionized or distilled water.
If you want to be able to compare results from one experiment to another, it is important to keep everything the same except for the test variable. Temperature is one of the factors that usually is kept as constant as possible. You might want to experiment with this, though. If you make temperature your test variable, you will be able to find out how sensitive the lettuce seed bioassay is to heat or cold. In this case, you will want to use the same solution in all of your petri dishes but incubate them at different temperatures. You could make a simple incubator using a light or heating pad enclosed in a box. For cold temperatures, you could try a refrigerator or chilly windowsill.
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