Observing Osmosis
1. Label the 8 cups at your station. Four should be labeled “Salt Water” with corresponding numbers,
and the other 4, “Fresh Water”. Also, label each of your cups with your group names.
2. Using table salt, tap water, beaker, graduated cylinder and a scale, prepare your solutions according
to the ppt (parts per thousand) of salinity that is appropriate for each cup. (Salt water has a salinity
of 35ppt and fresh water is 0ppt. To prepare the NaCl solution, use 33.03g of NaCl per liter of water.
Since you will not need an entire liter of water, scale down the ratios).
3. Obtain 4 grapes and 4 raisins. Using the balance, find the mass of each grape and raisin and record
it in the table. Place the grapes and raisins in their corresponding cups. Record observations on the
grapes and raisins.
4. Allow the grapes and raisins to sit in their corresponding cups for 24 hours.
5. After 24 hours, gently blot dry and mass each grape and raisin and record observations.
6. Fill in the table and create a graph to record your data. DON’T FORGET A TITLE!
TITLE: ____________________________________
Solution
Original
Mass
Final Mass
Change in
Mass
Average
change
OBSERVATIONS BEFORE
AND AFTER SOLUTION
Freshwater 1
Freshwater 2
Freshwater 4
Freshwater 3
Saltwater 1
Saltwater 2
Saltwater 3
Saltwater 4
Write a sentence for each scenario using the words “hypertonic”, “hypotonic” or “isotonic” as a comparison
of the solution to the item that is in each.
Freshwater/grape: ____________________________________________________________________
Saltwater/grape: ____________________________________________________________________
Freshwater/raisin: ___________________________________________________________________
Saltwater/raisin: _____________________________________________________________________
What are the major issues facing organisms that live in saltwater environments in regard to the difference
between their bodies and the saltwater? ______________________________________________________
_______________________________________________________________________________________
______________________________________________________________________________________
What are some of the ways that organisms deal with this issue? __________________________________
_______________________________________________________________________________________
_______________________________________________________________________________________
______________________________________________________________________________________
1
Observing the Effect of Changes in Density
1. Start with a 250 mL graduated cylinder filled with freshwater.
2. Drop a grape and time how long it takes to sink to the bottom.
3. Remove the grape (to get the grape out, pour the entire contents into a beaker CAREFULLY SO AS
NOT TO LOSE ANY VOLUME, pick out the grape, then carefully put the water back into the cylinder.)
4. Add 2.5 g (2.5 g/250 ml = 10 g/ 1000 ml = 10 ppt) of salt, stirring until the salt is dissolved.
5. Race the grape again and record the time (the same grape should be used every time or else
variable race times will result).
6. Continue adding 2.5 g of salt (increasing by 10 ppt) to your water sample and record the time it
takes the grape to hit the bottom.
7. Fill in the table and create a GRAPH to record your data. DON’T FORGET A TITLE!
TITLE: ____________________________________
Time to reach bottom
Solution in ppt
Trial 1
Trial 2
Trial 3
Average
Why does the grape sink at different rates?_______________________________________________
_____________________________________________________________________________________
How high is the salinity (ppt) that eventually prevented the grape from sinking? _____________________
What does this tell you about the grape and that salinity?_______________________________________
_____________________________________________________________________________________
How does this apply to organisms that exist in the ocean?________________________________________
_______________________________________________________________________________________
_______________________________________________________________________________________
(Attach your GRAPHS to this sheet)
2