Project 1: Osmosis Lab Name: Potential Credits: /50 Project Goals: to gain and demonstrate a better understanding of osmosis Instructions Please read through the Unit 4 Lessons, paying particular attention to the osmosis notes and videos. Please read through the Procedure for Formal Lab Write-Ups and then complete the lab below. Osmosis Lab: BACKGROUND A shipwrecked sailor is stranded on a small desert island with no fresh water to drink. She knows she could last without food for up to a month, but if she didn't have water to drink she would be dead within a week. Hoping to postpone the inevitable, her thirst drove her to drink the salty seawater. She was dead in two days. Why do you think drinking seawater killed the sailor faster than not drinking any water at all? In this lab you will explore the cause of the sailor's death. You will prepare solutions of salt water to represent the seawater and then you will cut up slices of potato to represent. the sailor's cells as both the potato and the sailor are made up of cells PURPOSE To determine the cause of the sailor's death, you will determine the effects of salt water on slices of potato. The assumption is that potato cells will behave like the sailor's cells in her body. You will compare the potatoes in salt water versus fresh water to determine if there is an effect caused by the salt in the water. The fresh water acts as a control in this experiment. HYPOTHESIS Please include a hypothesis in your final report that involves "If/then" statements that predict what will happen to the length and mass of the potato slices in the various "seawater" solutions and which explains why you predict such an occurrence. Page 1 of 8
MATERIALS 1 or 2 large potatoes sharp knife salt water ruler food scale 1 clean 500 ml bottle or jar 3 clean 250-500 ml bottles or jars paper towel stove pot tape and pen for labelling 4 plastic or glass cups or mugs Saran wrap rubber bands to go around cups cork or apple corer (if available) PROCEDURE 1. To determine if the salt in the water has an effect on the potato cells, you first need to cut 4 equal sized slices of potato to represent the sailor. To do this: i. Take a large potato and carefully cut one end square, then measure 8 cm to the other end and cut it square. Now if you have a corer available, stand the potato on one end and push the corer through the potato to get 4 core samples. Align the four core samples and trim them to precisely the same length. ii. iii. If you do not have a corer you will need to use the knife to carefully cut off one edge so that the potato can sit flat on the cutting board and then proceed to cut four equally sized and shaped pieces from the potato (aim for 4 pieces that are 1 cm x 1cm x 8 cm) Once you have 4 potato slices that are the same shape and length (~8 cm) record their precise lengths and masses in Table 2 of the results section below. 2. Now make your "seawater" solutions. i. Heat 500 ml of water in a pot on the stove until it reaches a boil. Turn water off and start adding salt, 1 teaspoon at a time, stirring thoroughly after adding each teaspoon. Keep adding salt until it will no longer dissolve and there are salt crystals settling to the bottom of the pan after stirring. You have now made a saturated salt solution and this represents you 100% "seawater". Allow solution to cool to room temperature and then pour into a clean 500 ml bottle and label as "100% seawater". ii. Now use your "100% seawater" to make 100 ml of the following diluted solutions of sea water: "10% sea water" "5% sea water" "1% sea water" Note: It is fairly easy to do percentage based dilutions if you start with a 100% stock solution and make up 100 ml of the desired diluted solution. To do this use the following: Page 2 of 8
If desired solution = X % then add X ml of the 100% solution to enough ml of pure water to add up to 100 ml total volume For example to make a 75% solution you would mix 75 ml of the 100% solution with 25 ml of pure water to get a total of 100 ml of a 75% solution And to make up a 65% solution you would mix 65 ml of the 100% solution with 35 ml of pure water to get a total of 100 ml of a 65% solution. Please do the math and record the volumes needed to make up your 10%, 5% and 1% solutions in Table 1 of the results section below. 3. Label each of 4 cups as 10%, 5%, 1% and 0% (pure water), respectively. Then place 100 ml of the appropriate solution into the appropriately labeled cups. 4. Place one potato slice in each cup. Please be sure that you have precisely measured and recorded the lengths and masses of the potato slices into Table 1 of the results section below. There should not be any variances in lengths at this point and if there are any small variances in masses be sure to accurately record which mass of potato went into which % of solution. 5. Cover each cup with the saran wrap and elastic band and leave the cups in a safe place overnight. Why is it important to cover the cups with saran wrap? 6. After approximately 24 hours, remove potato from its solution, gently and quickly blot dry on paper towel and then measure and record its length and mass in Table 2 of the results section below. Use a + or sign to indicate increases or decreases, respectively, in mass and/or length (ex., if the potato increased in length by 2 mm record + 2 mm, if the potato decreased in mass by 0.25 g record -0.25 g). 7. Graph the Change is Mass data in Figure 1 of the results section. Keep in mind the independent variable is the variable that you are manipulating in the lab and should be placed on the x-axis while the dependent variable is the variable you are measuring in the lab and should be placed on the y-axis. The data points should be placed where the % solution and its respective change in mass intersect. Once all data points are recorded a line-of-best fit should be draw on the graph. Page 3 of 8
RESULTS Table 1. Creating Diluted "Seawater" Solutions. ( /3) Dilution of Solution Volume of "100% Seawater" (ml) Volume of Pure Water (ml) Total Volume of Diluted Solution (ml) 10% 100 5% 100 1% 100 0% (pure water) 0 100 100 Page 4 of 8
Table 2. Mass and Length of Potato Slice Before and After Being Left in Various "% Seawater" Solutions for 24 Hours. ( /8) 10% "Seawater" Solution 5% "Seawater" Solution 1% "Seawater" Solution 0% "Seawater" Solution (Pure Water) Initial Length of Potato (mm) Final Length of Potato (mm) Change in Potato Length (mm) Initial Mass Potato (g) Final Mass Potato (g) Change in Potato Mass (g) Turgidity of Potato (flaccid or crisp) Predicted Tonicity of Potato Compared to Solution (Hypertonic, Isotonic, or Hypotonic) Page 5 of 8
(+) (-) Figure 1. Mass Change of Potato Slices Left in Various "% Seawater" Solutions for 24 Hours. ( /3 proper axes titles including units and appropriate scales, line of best fit) Page 6 of 8
WRITE-UP Please prepare a formal lab write-up using the criteria found in the Formal Lab Write-Up section of this unit. Include the following sections for this lab: Title Introduction Purpose Hypothesis Materials Procedure Results (you can cut and paste from the lab handout) Discussion and Conclusion (numbered answers to the questions listed in the Discussion and Conclusions section below) DISCUSSION AND CONCLUSION In the discussion, you MUST address the following questions in numbered paragraphs. 1. What happened overnight to the potato slices? /2 2. Why did it happen? /4 3. Is diffusion or osmosis responsible for changes in length of the potato slices? Define both of these terms. /3 4. Which of the three solutions is isotonic to potato cells? How do you know this? /2 5. Using appropriate terms, explain why some potato cells gained mass and others lost mass. /2 6. Define hypertonic, isotonic, and hypotonic. /3 7. Using your graph to predict the mass of a 10 gram potato section placed in a sucrose solution of 0.1%, 2%, and 4.8%. /3 8. Discuss why wilted celery will become crisp when placed in a glass of pure water. /1 9. Why can road salt cause damage to street side plants? /1 10. Why did the sailor die more quickly drinking salt water than not drinking at all? What do you think killed the sailor? /2 Page 7 of 8
Marking Guide: Title /1 Introduction /5 Purpose /1 Hypothesis /4 Materials /1 Procedure /1 Results (you can cut and paste from the lab handout) /14 Discussion and Conclusion /23 Total = /50 Page 8 of 8