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Water What-ifs
Teacher
Notes,
Lesson Extensions and
Alternative Assessment Ideas
Note: Prior to Lesson 1, students should have been introduced to the concepts of acids and bases and have a basic understanding of the pH scale.
Lesson 1-Introductory Lesson-How is pH Determined?
1. Have available for the students six unknown substances labeled 1-6. Test each substance beforehand to determine the pH. Provide the students with a list of the uses of each substance. They will use this list to make an educated guess about the pH of each unknown.
2. Examples of substances to choose from:
- a weak NaOH solution will give a strong base reaction.
- a weak HCl solution will give a strong acid reaction. soda, such as Pepsi will give a weak acid reaction.
- vinegar or lemon juice will give a weak acid reaction.
- a baking soda solution will give a base reaction. The strength of the reaction will depend on how much baking soda is used.
- Distilled or deionized water will have a pH of approximately 7 and may be used as the control substance.
3. pH paper, a pH test kit or red cabbage juice may be used to determine pH. The pH test kit will provide increased accuracy. The red cabbage juice will be the least accurate and only determine if a substance is acid, base or neutral.
4. The recipe for red cabbage juice can be found at Beakman's World on the web at: http://www.beakman.com/acid/acid.html A color scale is also provided.
Lesson 2-Does a change in pH effect the growth and survival rate of aquatic plants? 1. Duckweed or elodea (anacharis) may be purchased from a pet supply store which specializes in aquariums or outdoor ponds.
2. Reagents
1% NaOH - To make 1 liter of this reagent, weigh out 1 gram of NaOH. Add the NaOH to 999 milliliters of distilled water. Store in a capped bottle. Invert and mix before use. Put in labeled dropper bottles for use in this lesson.
1% HCl - To make 1 liter of this reagent, add 1 milliliter of 1M HCl to 999 milliliters of distilled water. Store in a capped bottle in an acid cabinet. Put in labeled dropper bottles for use in this lesson.
Note: Safety goggles and aprons should be worn at all times when mixing reagents. When working with acids, reagents should be made in a exhaust hood.
Lesson 3-Determining the Effects of a pH Change on the Hatching and Development of Brine Shrimp Eggs. 1. Brine shrimp eggs may be purchase from a scientific supply company or at a local pet supply store.
2. Brine shrimp culture should be made 24 hours in advance and can be kept indefinitely, if fresh salt water is added on a continual basis.
3. The common brine shrimp (artemia) are closely related to zooplankton such as Daphnia and are often used as live food for aquariums. The artemia life cycle begins by the hatching of dormant cysts which are encased embryos that are metabolically inactive. The cysts can remain dormant for many years as long as they are kept dry. When the cysts are placed in salt water, they are rehydrated and resume their development.
After 15 or 20 hours at 25 degrees C (77 degrees F), the cysts burst and the embryo leaves the shell. For the first few hours, the embryo hangs beneath the cyst shell, still enclosed in the hatching membrane. The embryo will grow and progress through 15 molts before reaching adulthood in approximately 8 days. Adult artemia average about 8mm long, but can reach lengths of 20 mm under ideal conditions.
Other variables of importance are pH, light and oxygen. A pH of 7.5-8.5 is optimal, and can be lowered with muriatic acid (any type of acid may be used) or increased with baking soda. A minimum amount of light is necessary for hatching and is beneficial for increased adult growth.
Two liter soda bottles with the tops cut off and filled with tap water make great hatching containers. To the bottle filled with water add 10 to 20 grams of salt without iodine and a pinch of sodium bicarbonate (baking soda). Test the pH to make sure in stays in a range of 7.5-8.5. Aeration is necessary and can be accomplished by a small air pump with a length of tubing long enough to reach the bottom of the bottle. Aeration should be light, as the eggs will not hatch if aeration is too brisk.
Feeding the brine shrimp is necessary if the culture is to be used for several days. A solution of baker's yeast and fish tank water to form a milky solution is an ideal food for the growing brine shrimp. The brine shrimp culture only needs a few drops of the yeast solution as they are not big eaters and overfeeding can foul the culture. The yeast solution can be placed in a dropper bottle and stored in the refrigerator. 4. Reagents
1% NaOH - To make 1 liter of this reagent, weigh out 1 gram of NaOH. Add the NaOH to 999 milliliters of distilled water. Store in a capped bottle. Invert and mix before use. Put in labeled dropper bottles for use in this lesson.
1% HCl - To make 1 liter of this reagent, add 1 milliliter of 1M HCl to 999 milliliters of distilled water. Store in a capped bottle in an acid cabinet. Put in labeled dropper bottles for use in this lesson.
Note: Safety goggles and aprons should be worn at all times when mixing reagents. When working with acids, reagents should be made in a exhaust hood.
Lesson Extensions
1. Demonstrate how the pH is changed when an acid and a base are mixed together. Allow the students to test the pH of the new substance to determine if their hypothesis is correct. If the two substances are relatively equal in strength, they will neutralize each other and the new substance will have a pH of approximately 7.
2. Have students collect water samples from a lake or stream near their home and bring it to class to determine the pH. Create a database of all the pH values and the body of water where they were collected. Have students determine why the pH is different in different areas.
3. Have students collect rainfall and determine the pH. Allow the samples to sit overnight and check the pH again. Have the students hypothesize about a pH change.
4. Experiment with other organisms similar to brine shrimp to determine the optimal pH for growth and development.
Alternative Assessment Ideas
1. Have students assess the quality of several different water samples and determine how the numbers of plants and organisms will vary from sample to sample based on the pH.
2. Teach students to perform chemical tests. Assess their ability to conduct these tests accurately by providing known samples for them to analyze.
3. Set up several different aquatic ecosystems. Have the students keep a daily journal of the changes which take place over a period of time.
Water What-ifs Home | pH Lessons
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Last Modified: 7/27/00