Kitchen Science Experiment: Create a density rainbow

July 23rd, 2021

Are you feeling hungry for some science? In these edited extracts from The Curious World of Kitchen Science series, try your hand at creating a density rainbow, and learn some other cool facts from the worlds of physics, biology and maths!

A row of glasses filled with coloured water, arranged in the colours of the rainbow.


Are you looking for some science to sink your teeth into? Whether you’re famished for physics, craving some chemistry, bursting for some biology, or just have the munchies for some maths, The Curious World of Kitchen Science books have you covered! Released in 2020, these books will satisfy your hunger for at-home experiments. Explore the key principles and practice of everyday science by using analogies, experiments and research with food and drink in the home kitchen environment.

Cover of The Chemistry of Cola featuring a digital cartoon of cola in a bottle on an orange background.

The Chemistry of Cola by Jamie Gallagher

The below experiment can be found in Chapter 4: Food Chemistry of The Chemistry of Cola. You’ll need an adult to help with this one, but you’ll end up with a colourful result!


Can water float on… water? It might seem like a strange question, but the answer is yes. Water can be made denser by mixing it with other ingredients. In this experiment you will create a rainbow of sugar water, by mixing different densities of sugar solution.

Density is defined as mass divided by unit volume. It is given the symbol ρ and the unit kilogram per cubic metre (kg/m3). To work out the density of a substance, the following equation is used:

The formula for calculating density, shown in two ways. The first way shows that Density equals mass divided by volume. The formula is shown a second time using scientific symbols for each component of the formula: the Greek letter rho which is the symbol for density, and the letters m and v to represent mass and volume.






At room temperature, the density of water is 0.998 kg/m3, but that can be easily changed by adding sugar. When sugar is dissolved in water, the solution gains mass, but its volume isn’t increased. By creating different densities of water, you should be able to float different concentrations on top of each other.

An illustration of five glasses containing different coloured liquid, labelled as Step 1. Also included is an illustration of a single glass containing different layers of coloured liquid, labelled Step 7. An arrow points to the second illustration with the words, "Add slowly to see a rainbow."

Make sure you get an adult to help you!


You will need:

  • Tall glass
  • Food colouring (at least one, but ideally five colours)
  • Granulated sugar
  • Water
  • Kitchen scale
  • 5 small glasses
  • Spoon
  • Paper towel


What to do:

Step 1. Add 50 ml (3 tbsp) of tap water to each of your five small glasses, and add a few drops of food colouring to each. If you only have one colour, add it only to glasses one, three and five. If you have five colours, add a different colour to each glass.

Step 2. You won’t be adding anything else to glass one, but add 15 g (1 1/5 tbsp) of sugar to glass two, 30 g (2 2/3 tbsp) to glass three, 45 g (3 1/2 tbsp) to glass four and 60 g (4 2/3 tbsp) to glass five.

Step 3. Stir the glasses with the sugar. You should find that the sugar in glasses two and three dissolves quickly, while with four and five it will take longer. If after several minutes of stirring, solid sugar remains, heat the glasses in a microwave for short bursts (ten seconds at a time) to slightly warm the water.

Sugar water will warm rapidly and can burn your skin, so be careful not to overheat the water, and be careful not to splash it on your skin. Ask an adult to help.

Step 4. When you have dissolved as much sugar in glass five as possible, add it to the bottom of the tall glass.

Step 5. Cut a strip of paper towel approximately 3 cm (1 in) wide and 10 cm (4 in) long. Dip one end in glass four and then place the wet towel against the inside of the glass; it should stick there.

Step 6. Use the paper towel in glass four to guide the liquid gently into the tall glass by pouring the liquid slowly down the paper towel and into the glass, rather than pouring straight from one glass to another. The liquid from glass four should be less dense than the solution from glass five, and therefore it should float on top.

Step 7. Repeat steps five and six with glasses three, two and one (in that order).


What happens?

You should have a glass filled with layers of different colours and density, with water apparently floating on water.


Want to know some other cool science facts?

Cover of 'The Physics of Popcorn' featuring a cartoon of popcorn in a red and white striped container on a light blue background.

The Physics of Popcorn

Gravity and general relativity

Einstein’s theory of relativity states that gravity is not a force; it is a bending of space (and time). Earth travels around the sun because the sun is so massive it curves space around it. If the sun weren’t there, earth would continue to travel through space in a straight line. The curving of space makes Earth move in a big circle, and this makes it look like there is a gravitational force pulling Earth towards the sun. Is gravity a force or curved space? It can be seen as either.

From page 137 of The Physics of Popcorn by Aidan Randle-Conde.





Cover of The Biology of Bananas featuring a cartoon of a half-peeled banana on a light green background.

The Biology of Bananas

Why do onions make you cry?

Onions don’t smell strongly of anything when they’re whole, but once you cut them open and damage the cells, enzymes are released that react with other chemicals present in the onion to produce syn-Propanethial-S-oxide. This is a gas that travels through the air and, when it contacts the nerve cells in your eyes, causes a stinging sensation and makes your eyes water. The same compound is also responsible for the heat and burning sensation you get when eating raw onions.

From page 35 of The Biology of Bananas by Katie Steckles.





Cover of 'The Maths of Milkshakes' featuring a cartoon of a strawberry milkshake with whipped cream and a red-and-white straw on a pink background.

The Maths of Milkshakes

Infinity isn’t a number?

Mathematicians don’t consider infinity to be a number because it doesn’t behave like a number. For a finite number, if you add one (or any other number) to it, you end up with something different. But in this case, we can start off with infinitely many things, add extra things and still have infinitely many things, so it hadn’t changed.

From page 139 of The Maths of Milkshakes by Katie Steckles.







All four books in The Curious World of Kitchen Science series can be purchased individually or as a set from our website.

The experiment in this blog is from The Chemistry of Cola, written by Jamie Gallagher. Jamie has a degree in chemical physics and a PhD in chemistry and electrical engineering. He is an award-winning scientist and science communicator.