Unit of Chemistry Teaching EducationUniversity of Helsinki

Science Education CentreLUMA Centre Finland

Sugar-rainbowsFOCUS GROUP: Primary school students

DURATION: 30—45 min

IDEA: With this colourful experiment we want to introduce students to natural science and working in a laboratory. Students will learn how to use laboratory tools (balance, pipettes and test-tubes) and the basic principles of density.

LEARNING OBJECTIVE: The aim of this experiment is to inspire and to introduce the students to the world of chemistry. With older students it can also be used to explain the concept of density.

BACKGROUNDThe sugar-rainbow is based on density. The students will make solutions by dissolving different amounts of sugar in the same amount of water. The more sugar the solution contains, the heavier it is. This also means that the density of the solution increases with increasing amount of dissolved sugar. If the solutions are carefully stacked on top of each other, starting with the densest solution, they will not mix. A beautiful rainbow can be made if the solutions are coloured with food colours before stacking.

YOU’LL NEEDSmall sugar-rainbows:

Sugar Water Food colourings Pipettes Test-tubes Beakers Balance or tablespoons

Large sugar-rainbows: Sugar Water Food colourings A large measuring cylinder A tube A funnel Beakers Balance or tablespoons

SAFETY AND WASTE MANAGEMENTThe reagents used in this experiment are safe, however food colourings can dirty clothes. Therefore, it is recommended to wear a laboratory coat while working.

All solutions can be poured into the drain.

Unit of Chemistry Teaching EducationUniversity of Helsinki

Science Education CentreLUMA Centre Finland

QUESTIONSBefore the experiment is started:

1. You throw a rock and a piece of wood of the same size into water. What will happen and why?

During the experiment:1. Which solution is the heaviest? Why?2. Compare the volumes of the solutions. What can you notice and what might be the explanation

of it?3. In what order do you have to pipette the solutions in the test-tube in order to achieve separate

layers and form a rainbow?

INSTRUCTIONSSmall sugar-rainbows:Measure 50 ml of warm water in the beaker and colour it with food colourings. Dissolve the right amount of sugar in the water (see table below). Note that the amount of sugar can be measured using a balance or with tablespoons. If tablespoons are used, one should make sure that the spoonfuls that the student measure are the same size.

Colour InstructionsRed 50 ml water and 55 g or 5 tbs of sugarOrange 50 ml water and 44 g or 4 tbs of sugarYellow 50 ml water and 33 g or 3 tbs of sugarGreen 50 ml water and 22 g or 2 tbs of sugarBlue 50 ml water and 11 g or 1 tbs of sugarViolet 50 ml water and no sugar

Arrange the solutions from the heaviest to the lightest.

Carefully pipette a small amount of each solution in a test-tube.

Hint: It’s good to hold the test-tube in an angle and the pipette straight up when pipetting the solutions. This way the solutions will slide down the test-tube and are less likely to mix with the lower layers.

Unit of Chemistry Teaching EducationUniversity of Helsinki

Science Education CentreLUMA Centre Finland

Large sugar-rainbows:Prepare the sugar-solutions like described previously. Attach a funnel to one end of a tube and put the other end in the measuring cylinder. The tube should be long enough to reach the bottom of the measuring cylinder. Arrange the solutions from the lightest to the heaviest and pour them into the measuring cylinder using the funnel attached to the tube, starting with the lightest solution.

Hint: Make sure no air-bubbles get in the tube in between the solutions, because bubbles escaping the tube will mix the solution-layers! This can be done by adding the next solution in the funnel before the first solution has flown through it. If you squeeze the tube the solutions will flow at a slower speed.