Required Practical 4: Osmosis (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Required practical 4: Investigating the effect of solute concentrations on the mass of plant tissue

• Aim:

  • To investigate the range of concentrations of salt or sugar solutions on the mass of plant tissue

• Procedure:

  • Prepare samples of potatoes (or other plant tissue) and place in different concentrations of sugar or sodium chloride (salt) solution

  • Make measurements of mass and length of your samples before and after soaking them in solutions

  • Calculate the percentage change in mass of plant tissue

  • Plot, draw and interpret appropriate graphs

• In this practical, you should take care to prepare your samples of potato carefully and record your measurements accurately

• This practical can be carried out with either salt or sucrose solutions of at least five different concentrations

• The length of time that the potato cylinders are left will vary

  • E.g. This experiment can be carried out in a water bath at 30 °C in 30 minutes 

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Measure out the volumes of solutions and potato lengths carefully using appropriate equipment; all measurements should be recorded

• Independent variable:

  • Concentration of salt or sucrose solution in mol dm³

• Dependent variable:

  • Mass and the length of each potato cylinder before and after it has been submerged in solution should be measured

  • Percentage change in mass and length will be calculated using these measurements

• Important control variables are:

  • type and volume of solute in solution

  • temperature

  • time

Analysing osmosis required practical

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A positive percentage change in mass indicates that the potato has gained water by osmosis (net movement of water from the solution into the potato) meaning the solution is more dilute, a negative percentage change suggests the opposite

Analysing osmosis graph results

• Where results lie above 0 on the X-axis, this means the potato has increased in mass and taken water in from the surrounding solution, via osmosis

• Where results lie below 0 on the X-axis, this means the potato has decreased in mass and water has been lost from the potato, via osmosis

• Where the line crosses the X-axis, this is the concentration of sugar/salt inside the potato and no water has been lost or gained at this point