Decay

• What is the purpose of the decay practical?

  Investigating the effect of temperature on the rate of decay of fresh milk by measuring pH change
• How do you set up the practical?

  Fill half a beaker with hot water

  Use a pipette to transfer 5cm^3 of lipase solution into a boiling tube and label as "lipase"

  Add 5 drops of Cresol red into another test tube and label as "milk" then transfer 5cm^3 of milk and 7cm^3 of sodium carbonate into the "milk" tube (the solution should be purple)

  Place the boiling tubes into the beaker of hot water and use a thermometer to check when the solutions in the boiling tube reach the same temp as the water

  transfer 1cm^3 of lipase from the lipase tube into the milk tube and immediately start the timer
• What are the independent, dependent and control variables?

  Independent - temperature

  dependent - time taken to change colour

  control - Volume of milk / other solutions
• Why do you repeat the experiment with different temperatures?

  To find the optimum temperature for decay to occur
• What happens in the practical?

  The colour of the solution will change from purple to yellow. You must record your results with the times and temperature in a table
• What are the sources of error?

  The colour change at the end point may be difficult to judge
• What are the safety precautions for this practical?

  Handling chemicals - rinse immediately after contact with lipase and sodium carbonate solution as they can cause irritations

  Glassware - if test tubes break inform a teacher

  pH indicator handling - Cresol red can stain skin and clothing

  Tie back hair and wear safety goggles and lab coats