SA: vol ratio practical

• Preparing agar pieces


  1. Place block of agar on white tile

  2. Cut block to produce 2 cubes 10mmx10mmx10mm

  3. Keep one as large piece

  4. Cut other cube in half to produce 2 smaller pieces 10mmx10mmx5mm

  5. Keep one as small piece
• Setting up experiment


  1. Stand beaker on white tile

  2. Pour dilute HCl into beaker until half full

  3. Carefully place large agar piece and one smaller piece into beaker

  4. Start timer

  5. Ensure pieces do not touch
• Observe colour change as acid penetrates agar
• Time in seconds how long it takes each piece to change colour completely from red to orange and record results
• you were told to cut your pieces of agar from the same block suggest why this was important?

  so that the concentration of dye in the agar was the same
• you did not add a fixed volume of acid the the beaker - why wasn't it necessary?

  bc the volume of acid doesn't affect the concentration - only need the acid to cover the agar
• told to make sure the pieces of agar did not touch each other when in the acid - why important?

  would have changed the surface areas in contact with the acid - to make sure all sides are in contact with the acid
• during this time in the acid, the pieces of agar may have had one face in contact with the beaker which could have affected you results - suggest how the method could have been modified to reduce problem?

  swirl regularly to keep agar suspended

  use a larger beaker so they spread out more
• when you placed the pieces of agar in the acid - the indicator gradually changes colour from red to orange - explain why?

  indicator shows a different colour in a neutral environ, as the acid diffuses into the cube it changes colour as it causes the pH of the bock to fall