Required Practical 2: Osmosis

Question 1

Equipment list (11)

Answer 1

1. Paper towels
2. Four boiling tubes
3. Measuring cylinder
4. White tile
5. Measuring balance
6. Salt solutions (1%, 5%, and 10% concentration)
7. Scalpel
8. Cork borer
9. Potato
10. Ruler
11. Distilled water

Question 2

Method (9)

Answer 2

1. Use the cork borer to cut out 4 pieces of potato.
2. Use the ruler to measure that the pieces are all the same size.
3. Weigh the mass of each potato part.
4. Place each piece into 4 separate boiling tubes of different concentrations of salt solutions and water.
5. Wait overnight if possible (if not >10 minutes).
6. Take out each piece and dab away the moisture with paper towels.
7. Record the mass of each potato piece.
8. Calculate the change in mass.
9. Calculate the percentage change

Question 3

Independent variable

Answer 3

The concentration of the sugar or salt solutions

Question 4

Dependent variable.

Answer 4

Percentage change in mass

Question 5

Control variables (5)

Answer 5

1. Volume of the salt solutions and the solvent (water)
2. Mass od each piece of potato
3. Time left needs to be the same for all
4. Temperature
5. Type of potato

Question 6

State any risk and explain why it is dangerous and how to reduce the risk.

Answer 6

1. Knife/corer
   • could cut yourself
   • only use knife on the white tile
   • carry the knife safely

Question 7

State the results or trend (3)

Answer 7

1. Low salt concentration - potatoes gain mass
2. Medium salt concentration - little change in mass
3. High salt concentration - potatoes lose mass

Question 8

Explain the results (3)

Answer 8

1. Low salt concentration - higher concentration of water outside potato cells, so water moves into cells.
2. Medium salt concentration - same water concentration inside and outside cells.
3. High salt concentration - higher concentration of water inside potato cells, so water moves out if cells.

Question 9

Possible sources of error in method (3)

Answer 9

1. Potatoes not dried thoroughly, therefore increasing mass
2. Potatoes squeezed too much during drying, decreasing mass
3. Potatoes not same starting mass/surface area

Question 10

Ways of improving method to reduce sources of error (2)

Answer 10

1. Only dab potatoes lightly to remove surface moisture
2. Ensure potatoes have same mass and/or surface area

Question 11

Ways to modify the method to investigate different independent variables (3)

Answer 11

Could investigate…
1. Different types of vegetables
2. Wider range of concentrations
3. Different solutes (e.g., sugar)

Question 12

State the equation for percentage change in mass.

Answer 12

Percentage change in mass = (change in mass ÷ original mass) × 100

Question 13

Why is calculating percentage change in mass better than simply calculating the change in mass?

Answer 13

Allows better comparison if starting mass is not the same for all potatoes

Question 14

Define ‘osmosis’.

Answer 14

Osmosis is the net movement og eater from an area of higher water potential to an area of lower water potential through a partially permeable membrane.

Question 15

Explain why only the water molecules will move across the partially permeable membrane, and not the salt and sugar molecules. (2)

Answer 15

1. The water molecules were small enough to pass through the partially permeable membrane.
2. The sugar and salt molecules are too big to pass through the partially permeable membrane.

Question 16

Explain why this method would also work with other vegetables, fruit, aquatic organisms and even eggs.

Answer 16

These tissues all contain water and are surrounded by partially permeable membrane, so they would undergo osmosis if placed in solution.

Question 17

If you did perform an experiment with aquatic organisms, what would you need to consider beforehand (3)

Answer 17

1. Don’t use a concentration that would kill them
2. Need to use optimum temperature they are used to
3. Take care when measuring the mass so you don’t harm/kill them.

Question 18

How can you use your graph to estimate the concentration of the solution within the potato cells (3)

Answer 18

1. Plot a graph of concentration (x- axis) versus percentage change in mass (y- axis)
2. Draw the line best fit
3. Where the line of best fit crosses the x-axis, there is a percentage change of mass of zero- so this value of concentration is the same as inside the cells.

Question 19

Can you think of an application of this experiment in real life?

Answer 19

1. Having the correct salt concentration in an aquarium so that the fish don’t die, depending on whether they are freshwater or saltwater fish.
2. Plumping up dried fruit before baking a cake, by placing them in pure water.