Gary wanted to find out if some food colourings contained a banned food dye. He put a drop of each food colouring and the banned food dye onto some special paper.
He hung the paper in a beaker of water.
After 10 minutes, the banned food dye and some of the dyes from the food colourings had moved up the paper.
Gary’s results are shown below.
(a) Gary wrote the labels on the paper in pencil. Why should he not write them in ink?