Science – using chromatography to separate pen ink

In Science, Year 5 have been developing their understanding of changing states. This has included some investigations around dissolving. More recently, they looked at Chromatography. Chromatography is an interesting process, which separates components of a mixture. It is used in lots of professional fields, including forensics to test for substances, and in food and medicine to test for purity.

Year 5 investigated the make-up of the ink compound found in the felt-tip pens, that they use every day in school. The children each took a cone of filter paper and added felt tip dots around the base. To see the separation, they placed the cones into a tray of very shallow water. Many of their predictions were that the ink would dissolve in the water and create an ink/water solution. What a lot of the children did not predict, was the way in which the ink travelled up the filter paper.

The classes observed the ink dot separating, as it travelled up the paper. This process was caused by the ink molecules dissolving in the water and being absorbed by the paper. The children were surprised to learn that the ink was made up of different coloured ink molecules, and the smaller molecules travelled further up the paper, separating from the bigger colours which stayed nearer the bottom. This left observable patterns on the filter paper.