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Required Practical: Osmosis (AQA GCSE Biology: Combined Science)
Revision Note
Osmosis required practical
- 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
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
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
Examiner Tip
This is an extremely common exam question – you should be able to calculate the percentage change in mass and length and be able to plot a graph of the results.
You may also be presented with a graph to analyse so make sure you understand what this type of graph is showing.
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