practical 3

• Calibration curve


  Graph used to determine an unknown concentration of a sample by comparing the unknown to a set of standard samples with known concentrations
• Dilution series


  A set of samples with known concentrations created to produce a calibration curve
• Water potential


  The tendency of water to diffuse from one area to another. Water molecules move from areas of high water potential to areas of low water potential by osmosis. The water potential is determined by the concentration of solutes.
• Equipment


  • Potato tuber
  • Cork borer
  • Scalpel
  • Ruler
  • Distilled water
  • Sucrose solution (1M)
  • Boiling tubes
  • Boiling tube rack
  • Timer
  • Digital balance
  • Paper towels
• Method


  1. Make a series of dilutions of 1M sucrose solution at 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0M

  2. Measure 5cm3 of each dilution into separate test tubes

  3. Use a cork borer to cut out six potato chips and cut down the sections into identically sized chips

  4. Dry each chip using a paper towel to remove excess water but do not squeeze

  5. Weigh each before the start of the experiment

  6. Place a potato chip in each test tube (one per sucrose concentration) and leave for 20 minutes

  7. Remove each potato chip, dry gently using paper towel, and weigh them in turn

  8. Calculate the percentage change in mass for each sucrose solution
• Hazards include cuts from scalpel and broken glass
• Graph


  1. Plot a graph of change in mass against concentration of sucrose solution

  2. The point at which the line of best fit crosses the x axis (zero change in mass) indicates the point at which the solution is isotonic
• Potato chips in lower concentrations of glucose solution will increase in mass, whilst those in the higher concentrations of glucose solution will decrease in mass
• In dilute glucose solutions

  The solution has a higher water potential than the potato, so water passively moves via osmosis to the area of lower water potential (the potato), causing the potato to increase in mass
• In concentrated glucose solutions


  Water will move out of the potato, thus the potato will decrease in mass