Practical: Investigating Diffusion and Osmosis

Section 1 of 8Practical 1: diffusion in agar cubes

The idea

• The idea Agar is a clear jelly.
  • When made with sodium hydroxide and phenolphthalein indicator, it is pink, because phenolphthalein is pink in alkaline conditions.
• When dilute hydrochloric acid diffuses into the jelly it neutralises the alkali, and the pink colour disappears.
  • The time this takes shows how fast the acid has reached the middle.

Variables

• Variables Independent variable: the size of the cube, which changes the surface area to volume ratio.
• Dependent variablethe time taken for the cube to turn colourless.
• Control variablesconcentration and volume of acid, temperature, and the agar batch.

Safety

• Safety Wear eye protection. Dilute hydrochloric acid and the alkaline agar are irritants.
• Cut the agar on a white tile and cut away from your body, because the scalpel is sharp.

Section 2 of 8Method: agar cubes

1

Use a scalpel and ruler to cut cubes of pink agar with sides of 0.5 cm, 1 cm and 2 cm.

2

Calculate the surface area, volume and surface area to volume ratio of each cube.

3

Put the same volume of dilute hydrochloric acid, for example 50 cm³, into a boiling tube for each cube.

4

Use forceps to drop a cube into the acid and start the stopwatch immediately.

5

Stop the stopwatch when the cube has become completely colourless and record the time.

6

Repeat at least three times for every size and calculate a mean.

Section 3 of 8Results and evaluation: diffusion

Cube side (cm)	SA : V	Expected result
0.5	12 : 1	Fastest to turn colourless
1	6 : 1	Intermediate time
2	3 : 1	Slowest to turn colourless

Section 4 of 8Practical 2: osmosis in potato

1

Prepare sucrose solutions of several concentrations from 0 mol/dm³ (distilled water) up to 1 mol/dm³, and pour the same volume of each into a labelled test tube.

2

Use a cork borer to cut potato cylinders, then trim them to the same length.

3

Blot each cylinder dry with paper towel and find its initial mass on a balance.

4

Place one cylinder in each tube and leave for a set time, for example several hours.

5

Remove the cylinders, blot them dry in the same way and find their final mass.

6

Calculate the percentage change in mass: (final mass − initial mass) ÷ initial mass × 100.

Section 5 of 8Interpreting the osmosis results

Potato in sucrose

Distilled water or dilute solution

• Distilled water or dilute solution Water moves into the potato cells by osmosis, so the mass increases.
  • The cells become firm, or turgid.

Concentrated solution

• Concentrated solution Water moves out of the cells, so the mass decreases.
  • The cylinders feel soft, and the cell membranes can pull away from the wall (plasmolysis).

Zero change

• Zero change Where the line on a graph of percentage change crosses zero, the solution has the same water concentration as the potato cells.
  • There is no net movement.

Section 6 of 8Limitations and improvements

Agar cubes

• Agar cubes Judging the exact moment a cube turns colourless is subjective.
  • Measure instead how far (in mm) the acid has travelled after a fixed time.
• Cubes are hard to cut accurately, so use a sharp scalpel and a ruler.

Potato cylinders

• Potato cylinders Different potatoes may have different solute concentrations, so use cylinders from the same potato.
• Blot with the same amount of pressure each time, otherwise unequal surface water affects the mass.

Section 7 of 8Key terms

Term	Meaning
Diffusion	The net movement of particles from a region of higher concentration to a region of lower concentration.
Osmosis	The diffusion of water across a partially permeable membrane from a dilute solution to a more concentrated solution.
Percentage change in mass	The change in mass divided by the starting mass, multiplied by 100.
Turgid	Describes a plant cell that is swollen and firm because it is full of water.
Plasmolysis	When a plant cell loses so much water that its membrane pulls away from the cell wall.
Partially permeable membrane	A membrane that lets some substances pass through but not others.

Section 8 of 8Exam technique

1

Saying that the large agar cube diffuses 'more slowly'. The rate through the jelly is the same; the larger cube takes longer because the acid must travel further and has less surface area per unit of volume.

2

Comparing the final masses of potato cylinders rather than the percentage change, which allows for different starting masses.

3

Forgetting to blot the potato before weighing, so liquid on the surface adds to the mass.

4

Describing osmosis as the movement of sugar. In these experiments it is water that moves, from dilute to concentrated solution.