8.2: Lab - Osmosis and Types of Solutions
- Page ID
- 438412
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Equipment and Materials
Prepare 12 sets of equipment and materials for 24 students per class section. Include a few more for backup if needed. Each set should include
Equipment
- Computers and internet access
- Balance
- Potato cutter, knives
- Five 150 or 250 mL beakers
- Paper towels
- Glass stirring rods
Materials
- Potatoes (~2-3 cm diameter, 0.3-1 cm thickness)
- Potatoes should be provided by lab staff.
- Instructors will either pre-cut into disks of proper size or will instruct students in proper cutting technique.
- Each group will need 5 potato disks.
- Deionized water
- Salt solutions: 0.5% (w/v), 2% (w/v), 5% (w/v) and 10% (w/v) \(\ce{NaCl}\)
- Can just prepare stocks of these. Students will need approximately 50 mL of each solution per group.
- Five colloids, suspensions or solutions placed into large containers in lab for observation. Some good options are:
- Kaopectate
- Muddy water
- Orange juice with pulp
- Mayonnaise
- Gatorade
Waste Disposal
- Solutions, colloids, and suspensions provided should be kept in or returned to their containers for future use.
- \(\ce{NaCl}\) solutions can go down the drain.
- Used potatoes can go in the trash.
- Recognize a solution and its composition of a solute and solvent.
- Define the process of osmosis and express its importance to biological systems.
- Distinguish homogeneous colloidal solutions from heterogeneous suspension mixtures.
- Differentiate and identify isotonic, hypertonic, and hypotonic solutions.
- Apply osmosis to red blood cells and understand the impact that various substances have on the human body when introduced via intravenous (IV) solution delivery.
Laboratory Skills
- Measure mass quantities of affected substances.
- Use Excel to plot experimental data.
Equipment and Materials
- Potatoes
- Potato cutter, knives
- Deionized water
- Salt solutions: 0.5% (w/v), 2% (w/v), 5% (w/v) \(\ce{NaCl}\), and 10% (w/v) \(\ce{NaCl}\)
- Balance
- 5 150-mL or 250-mL beakers
- Stations of labeled mixtures for identification
Safety and Hazard Information
N/A
Background Information
Osmosis and Solutions
In order to understand osmosis, you must first be able to define and understand the concepts of a solution, solvent, and solute. A solution is a homogeneous mixture of a solvent and one or more solutes. A solvent is the compound that dissolves or surrounds the solute, and is found in the greatest amount. A solute is the component(s) that is in the lesser quantity. Osmosis is the movement of solvent molecules from a region of low solute concentration to a region of higher solute concentration through a semi-permeable membrane. Semi-permeable means certain substances are allowed to go through, and certain substances are not. Within the human body, cells live within a fluid environment that can contain a combination of dissolved particles, such as sodium and potassium salts. Cells have a semi-permeable membrane that separates the intracellular fluid (inside the cell) from the extracellular fluid (outside the cell). Osmosis between intracellular and extracellular fluids depend on the number and types of dissolved particles inside and outside of the cell.
In the experiment you will do here, the cell walls of a potato will behave as a semi-permeable membrane. You can assume for this lab that the semi-permeable membrane on the potato will behave exactly like the semi-permeable membranes on the cell walls in your body.
At some point in time, the solvent will no longer freely travel only in one direction (towards the higher solute concentration side). This is due to osmotic pressure. Osmotic pressure is the pressure applied to a solution that prevents flow of solvent across a semi-permeable membrane. Osmotic pressure is the basis of filtering by reverse osmosis, a process commonly used to purify water. The water to be purified is placed in a chamber and put under an amount of pressure greater than the osmotic pressure that is exerted by the water with solutes (“impurities”) dissolved in it. This allows the water molecules, but not the solute particles to pass through a membrane. Reverse osmosis works so well to purify water, it can produce fresh water from salty ocean water!
You will observe the effects of placing a potato (representing a biological cell) into solutions containing different solute concentrations. Since the concentration of salt (\(\ce{NaCl}\)) in a biological cell is 0.90%, solutions with both higher and lower \(\ce{NaCl}\) concentrations than this baseline will be utilized. These solutions are categorized as hypotonic, hypertonic, or isotonic. A hypotonic solution is a dilute solution that has a salt concentration lower than that of the cell. Since the solvent will travel towards the area that has the higher solute concentration, the cell will gain water through osmosis and will, therefore, swell up (hemolysis). A hypertonic solution is a concentrated solution that has a salt concentration higher than that of the cell. The cell placed in this solution will lose water through osmosis and the cell will shrivel (crenation). An isotonic solution is a solution that has exactly the same salt concentration as the cell. There will be no net movement of water across the cell membrane and the cell will experience no change in size. Figure \(\PageIndex{1}\) (below) depicts these various solutions as they relate to animal and plant cells.
You will measure the mass of the potatoes both before and after allowing them to soak in the solutions. After the experiment, you will evaluate the percent change in the mass, as in Equation \ref{1}.
\[ \% \text{ change} = \frac{\text{final mass } - \text{ original mass}}{\text{original mass}} \times 100 \label{1}\]
Please note that if the potato decreases in size, the percent change will be negative. Likewise, if the potato increases in size, the percent change will be positive.
Types of Mixtures
A mixture is a combination of two or more pure components. You are very used to one type of mixture, solutions, which was discussed in the previous section. In a solution, very small solute particles are dissolved in the solvent (usually water) to give a homogeneous solution. There are, however, many other mixtures that you encounter everyday where the solute and solvent are distinguishable. These are colloids (blood, Mayonnaise, and hairsprays) or suspensions (Kaopectate, antacids and liquid penicillin). Colloids are similar to solutions in that they are homogeneous and they do not separate or settle out. The difference is that the size of solutes in colloids is larger than that of particles in a solution. Suspensions, on the other hand, are heterogeneous mixtures with particles so large that gravity causes the particles to settle out in a solution. All of these types of mixtures have important applications in chemistry, industrial and clinical settings.
In this lab, you will observe five mixtures that you will identify as solutions, colloids, or suspensions. You will base your designation on observations about particle size and settling.
Special Instructions (if any)
All samples (solids and liquids) in this experiment should be retained for reuse, except water.
Procedure
Part \(\PageIndex{A}\): Osmosis with Potato
1. Cut five potato disks using the tools provided for you. For best results, the disks should be circular, 2-3 cm in diameter, and 0.3-1 cm in height.
2. Place each potato disk on a paper towel labeled with the salt concentration (0, 0.5, 2, 5, or 10 %) to avoid mixing up the disks.
3. Record observations about the appearance and texture of each disk in Data Table \(\PageIndex{1}\).
4. Measure the mass of each disk and record in Data Table \(\PageIndex{1}\).
5. Place each disk in a beaker and add the desired concentration of \(\ce{NaCl}\) solution until the disk is completely covered. Note that one of the solutions is 0% \(\ce{NaCl}\), which is deionized water.
6. Allow the potato slices to soak for 45 minutes. Make sure the disks are always completely covered with the solution. If disks are floating, you may put a glass stirring rod on the disks to hold them under the liquid.
(While waiting, this is the time to start working on Part \(\PageIndex{B}\))
7. After soaking, record observations about appearance and texture in Data Table \(\PageIndex{2}\).
8. Blot each disk with a paper towel to remove excess liquid. Measure the mass of each disk and record in Data Table \(\PageIndex{2}\).
9. Copy masses before and after soaking into Data Table \(\PageIndex{3}\).
10. Calculate the percent change for each potato disk using Equation \ref{1}. Show your work.
11. Prepare a graph showing % change in mass as a function of % \(\ce{NaCl}\).
Part \(\PageIndex{B}\): Identifying Mixtures
1. There are five stations in the lab each with a set of mixtures labeled 1 to 5. Move to each station, observe each mixture and fill in Data Table \(\PageIndex{4}\) with the name, type of particles (small, medium, or large), and whether the particles settle (yes or no). Using these observations, classify each mixture as a suspension, colloid, or solution.
Please note that a good time to do this section is while you are waiting for your potatoes to soak in Part \(\PageIndex{A}\), Step 6.
Experimental Report
Part \(\PageIndex{A}\): Osmosis with Potato
Data Table \(\PageIndex{1}\): Potato Masses BEFORE Soaking in Solutions
| Mass (g) | Observations | |
|---|---|---|
| Deionized Water (0.0 % w/v \(\ce{NaCl}\)) | ||
| 0.50% w/v \(\ce{NaCl}\) | ||
| 2.0 % w/v \(\ce{NaCl}\) | ||
| 5.0 % w/v \(\ce{NaCl}\) | ||
| 10. % w/v \(\ce{NaCl}\) |
Data Table \(\PageIndex{2}\): Potato Masses AFTER Soaking in Solutions
| Mass (g) | Observations | |
|---|---|---|
| Deionized Water (0% w/v \(\ce{NaCl}\)) | ||
| 0.50% w/v \(\ce{NaCl}\) | ||
| 2% w/v \(\ce{NaCl}\) | ||
| 5% w/v \(\ce{NaCl}\) | ||
| 10% w/v \(\ce{NaCl}\) |
*** Copy the measured masses from Data Tables \(\PageIndex{1}\) and \(\PageIndex{2}\) into Data Table \(\PageIndex{3}\). ***
Data Table \(\PageIndex{3}\): Percent change by MASS
| Mass BEFORE Soaking | Mass AFTER Soaking | Percent Change (%) | |
|---|---|---|---|
| Deionized Water (0% w/v \(\ce{NaCl}\)) | |||
| 0.50% w/v \(\ce{NaCl}\) | |||
| 2% w/v \(\ce{NaCl}\) | |||
| 5% w/v \(\ce{NaCl}\) | |||
| 10% w/v \(\ce{NaCl}\) |
Calculate the percent change by mass for each of the potato disks. Show all work. Then copy your results into Data Table \(\PageIndex{3}\) above.
Make an XY-scatter plot using Excel showing % change in mass (\(y\)-axis) as a function of % \(\ce{NaCl}\) (\(x\)-axis). Use the computer program to graph your data and calculate the line of best fit through your five data points. Where the line of best fit crosses the horizontal zero line is the point at which the potato is isotonic with its surroundings, and is therefore the estimated salt concentration of the potato. Draw or submit your plot in the space below, or submit your Excel file, as instructed by your professor.
Isotonic point of the potato: ___________________% w/v \(\ce{NaCl}\)
Part \(\PageIndex{B}\): Identifying Mixtures
Data Table \(\PageIndex{4}\): Data Collection and Analysis
| Mixtures | Name | Key Observations | Classify Mixture | |
| Types of Particles | Settling | |||
| 1 | ||||
| 2 | ||||
| 3 | ||||
| 4 | ||||
| 5 | ||||
Follow-up questions
Part \(\PageIndex{A}\): Osmosis with Potato
Is deionized water a hypertonic, hypotonic, or isotonic solution when in the human body? _______________________
Describe what you would theoretically expect to happen to the potatoes in each of the three solutions, assuming potatoes mimic the salt concentration of red blood cells (0.90 % \(\ce{NaCl}\)).
a. 20% \(\ce{NaCl}\) _________________________________________________________________
b. 0.90% \(\ce{NaCl}\) ________________________________________________________
c. Deionized water ________________________________________________________
Describe the effects of each of the below solutions on the textures and masses of the potato disks. In each case, explain what happened to the cells in the potatoes and in which direction the water flowed. Feel free to include drawings to aid in your explanation.
a. Deionized water (0% w/v \(\ce{NaCl}\))
a. 10% w/v \(\ce{NaCl}\)
Part \(\PageIndex{B}\): Identifying Mixtures
Write a summary of what you observed for this part of the experiment (Part B), and what you have learned about how to classify mixtures.
Practical Applications – Critical Thinking Questions
A patient has been admitted to the hospital with severe dehydration due to constant vomiting and diarrhea for the past 3 days. The first thing the hospital staff needs to do for this patient is administer fluids intravenously, but the patient is demanding he only be given “pure” water, and is refusing any other IV fluids. You are responsible for patient education at this hospital. In order to get the patient to agree to be administered the 0.9% saline IV fluids he needs to rectify his dehydration, you must first educate him on what will happen if he exposes his cells to “pure” water. Explain to this patient why he should not receive pure water intravenously, using the concepts from this lab.
Kidney dialysis machines use osmosis to take over the filtering function of the kidneys. Dialysis machines use a semi-permeable membrane through which some small molecules can pass (such as water, salts, metabolites, and some toxins) but through which larger objects (such as proteins and blood cells) cannot. Using the concepts learned in this lab, explain how dialysis could be used to get toxins out of the blood and replace the filtering function of the kidneys.