Practical techniques and core practicals

Sample questions

Original practice questions written in the style of the exam.

1. Question 13 marks

   $1.0\,\text{cm}^3$ of DCPIP solution was decolourised by $0.50\,\text{cm}^3$ of a standard vitamin C solution with a concentration of $1.0\,\text{mg cm}^{-3}$.

   $1.0\,\text{cm}^3$ of the same DCPIP solution was decolourised by $2.0\,\text{cm}^3$ of orange juice.

   Calculate the concentration of vitamin C in the orange juice in $\text{mg cm}^{-3}$.

   Mark scheme

   • Mass of vitamin C needed to decolourise $1.0\,\text{cm}^3$ DCPIP $= 0.50 \times 1.0 = 0.50\,\text{mg}$ (1)
   • So $2.0$ of juice contains of vitamin C (1)
2. Question 24 marks

   A student prepares a squash of a garlic root tip to observe mitosis. In one field of view she counts 250 cells, of which 40 are in a stage of mitosis.

   (a) Explain why the root tip is placed in warm hydrochloric acid and then stained.

   (b) Calculate the mitotic index.

   Mark scheme

   • (a) Hydrochloric acid breaks down the middle lamella / bonds between cells, so the cells separate and can be squashed into a single layer (1)
   • (a) Hydrochloric acid also stops mitosis / fixes the cells at their current stage (1)
   • (a) The stain (e.g. toluidine blue or acetic orcein) binds to DNA so chromosomes become visible (1)
   • (b) Mitotic index $= 40 \div 250 = 0.16$ (1)