Miscibility (Sugar Rainbows)

Useful information
Kit List: 

Food coloring - range of colors
measuring cylinder or something equivalently long and thin
Vegetable oil
Other containers for mixing

For ice part
Plastic measuring cylinder
Dyed ice cubes
Castor oil

Frequency of use: 

Here's How:

(Line up five glasses. Add 1 tablespoon (15 g) of sugar to the first glass, 2 tablespoons (30 g) of sugar to the second glass, 3 tablespoons of sugar (45 g) to the third glass, and 4 tablespoons of sugar (60 g) to the fourth glass. The fifth glass remains empty.
Add 3 tablespoons (45 ml) of water to each of the first 4 glasses. Stir each solution. If the sugar does not dissolve in any of the four glasses, then add one more tablespoon (15 ml) of water to each of the four glasses.
Add 2-3 drops of red food colouring to the first glass, yellow food colouring to the second glass, green food colouring to the third glass, and blue food colouring to the fourth glass. Stir each solution - might want to prepare the solutions before, depends how long this actually takes. If making or replenishing solutions while demonstrating, you can get children to stir (watch they don't eat it!).
Now let's make a rainbow using the different density solutions. Fill the last glass about one-fourth full of the blue sugar solution.
Carefully layer some green sugar solution above the blue liquid. Do this by putting a spoon in the glass, just above the blue layer, and pouring the green solution slowly over the back of the spoon. If you do this right, you won't disturb the blue solution much at all. Add green solution until the glass is about half full.
Now layer the yellow solution above the green liquid, using the back of the spoon. Fill the glass to three-quarters full.
Finally, layer the red solution above the yellow liquid. Fill the glass the rest of the way.
The sugar solutions are miscible, or mixable, so the colours will bleed into each other and eventually mix.
If you stir the rainbow, what will happen? Because this density column is made with different concentrations of the same chemical (sugar or sucrose), stirring would mix the solution. It would not un-mix, like you would see with oil and water.

Do they know about dissolving things? Especially if making the solutions with them, you might want to talk about salt in the sea and sugar in tea - how you can no longer see the bits of sugar but it's still there, just broken into smaller bits and surrounded by the water.

Explain that dissolving more sugar in the water leads to it being more dense. Ask if they know what that means, more mass per amount of volume, could say that adding more sugar to the water makes it have more mass but doesn't change how much space it takes up. Makes sense that the lighter ones will sit on top of the heavier ones.

It might be beneficial to show what happens if you add a more dense solution on top of a less dense one to see what happens (they just mix)!

Now repeat with oil and water, try stirring and see them separate out after a while. (You don't need a lot of oil to see this). See if they have any idea why we cannot mix the water and oil, hopefully some will same something like this is because they are unmixable. From here it will probably depend on audience how much you can explain but should try to explain what makes them not miscible, i.e. that one is polar and that the other is none polar, these terms will need explaining. MODEL FOR THIS.

Ask them to tell you what they can tell about the oil because it sits on top of the water. Encourage them to think about the sugar and water earlier. Hopefully some should be able to tell you that it is less dense.

Other ideas:
A measuring cylinder filled half with castor oil, half with water, with blue ice cubes floating on the top. The ice melts and the blue water drifts down through the castor oil to sit on the meniscus.

Try to avoid using gel food coloring. It is difficult to mix the gels into the solution. Darker colours e.g. black can also cause problems when they bleed into the other colours so you can't see the rainbow.

Risk Assessment
Date risk assesment last checked: 
Fri, 31/01/2020
Risk assesment checked by: 
Beatrix Huissoon
Date risk assesment double checked: 
Sun, 02/02/2020
Risk assesment double-checked by: 
Risk Assessment: 

Mix waters of different sugar concentrations, and so density, with different food colourings in and show they sit on top of each other in density order. Show water and sugar solution don’t go back if stirred whereas oil and water does separate.
Watching coloured ice melt and sink through castor oil.


Hazard Risk Affected person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Solutions Solutions getting in eyes. All 2 2 4 Pour solutions carefully so as to minimise splashing.
Call first aider in event of incident. Use eyewash to wash out of eyes if trained and confident to do so.
1 2 2
Solutions/ice cubes Drinking of solutions.
Children may try to eat ice cubes (choking hazard) or drink water/oil.
Public 3 3 9 Watch children carefully, don’t allow them to do experiment unsupervised.
Explain to children that they must not eat/drink anything used in any experiment.
Watch small children, and don't let them handle the ice.
Call first aider in event of ingestion.
1 3 3
Spillages Slip hazard from spillage. All 3 3 9 Keep solutions in secure locations. Mop up any spillages immediately. Use wet floor sign if necessary.
Call first aider in the event of an emergency.
1 3 3