Science 7–10 · Year 7
Paper chromatography of the dyes on coloured sweets
Solutions and mixtures
The idea
A colour that looks single can be a mixture of dyes, and they separate because each is carried up the paper by water to a different extent.
What you need
- chromatography paper about 20 cm by 10 cm per group, as in the RSC method, pencil, ruler
- coloured sweets (at least four colours), a drop of water on a white tile per sweet
- 250 mL beaker with about 1 cm depth of water, and two plastic-coated paper clips to hold the paper as a cylinder, as in the RSC method
- fine paintbrush or capillary tube for spotting
How to do it
- Lay the paper with its longer side horizontal, draw a pencil line 1.5 cm from the bottom edge, and mark the first starting point about 2 cm from one end and each of the others about 2 cm from the one before, so the lanes cannot run into one another as the water rises.
- Wet each sweet on the tile so its dye runs into a drop; spot a small amount of each drop on its mark, letting it dry and repeating three times to make a concentrated spot.
- Roll the paper into a cylinder with the pencil line at the bottom, hold it with the two paper clips and stand it in the beaker so the bottom edge is in the water but the pencil line is above it; leave until the water has climbed to 1 cm from the top.
- Lift the cylinder out, mark the water front at once, unroll and dry the paper, and measure the distance each dye spot travelled and the distance the water travelled.
- Calculate the retention factor Rf = distance moved by dye divided by distance moved by water, for every spot.
What you should see
Several sweet colours separate into two or three dyes; brown and green sweets usually show more than one spot, and the same dye from two different sweets travels the same distance (same Rf). Every Rf lies between 0 and 1. The learner knows it worked when spots are round and clearly above the pencil line and the water front was marked before it dried.
What changes
- What you change
- sweet colour
- What you measure
- number of dye spots and their Rf values
- What you keep the same
- paper type
- solvent (water)
- starting line height
- temperature
- run stopped at the same front position
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A single colour is a single substance.
- The water carries every dye the same distance.
- Ink pen marks are fine for the starting line.
Safety card
Hazards
- none beyond ordinary glassware
Controls
- do not eat the sweets in the laboratory
Note
No hazardous chemicals. Basic risk assessment per CSIS Section 1.7.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC4-SOL-01SC4-WS-06
- Australian Curriculum v9AC9S7U06AC9S7I05
Sources
The pages the author read to write this activity.