Make a rainbow in a jar with salt and colour

A rainbow in a jar is a simple way to explore density, mixtures and careful observation. By preparing several salt solutions with different concentrations, children can stack coloured liquids into visible layers rather than watching them blend into one cloudy colour.

This hands-on activity suits a home learning afternoon, a primary classroom or a homeschool science session. It uses familiar materials, follows metric measurements used in Australia and gives learners a clear visual model of how heavier liquids settle below lighter ones.

The science behind the layers

Density describes how much matter is packed into a particular volume. When salt dissolves in water, the solution becomes denser than plain water. A strong salt solution contains more dissolved material in the same amount of liquid, so it has greater density and tends to sit beneath weaker solutions.

The colours do not create the layers. They make the layers easy to see. If the solutions are poured too quickly or stirred together, the boundary between them disappears. The activity therefore combines a science concept with fine motor control, patience and close observation.

Materials for the experiment

Use a tall, clear jar or a transparent plastic cup so the bands are easy to inspect. You will need warm water, table salt, five small cups, teaspoons, five colours of food colouring, a measuring spoon, a dropper or pipette, and a tray to catch spills.

For a classroom in Australia, 100 mL of water per cup is a convenient starting amount. Prepare five solutions using 0, 1, 2, 3 and 4 teaspoons of salt. The exact appearance can vary with the type of salt and the measuring spoon, so the important feature is the increasing salt concentration.

A ruler, labels and a sheet of paper are useful additions. Children can record the salt quantity, colour, layer position and their predictions before the liquids are combined.

Preparing the coloured solutions

Label the cups from lightest to heaviest. Add 100 mL of warm water to each cup, then stir the salt into the water until it has dissolved. Plain water belongs in the first cup, while the final cup should contain the greatest amount of salt.

Add one or two drops of a different food colour to each solution. Strong colours can make the layers harder to distinguish, so pale shades often work best. If the water is very hot, allow it to cool before children handle the cups; warm tap water is sufficient.

The solution with the most salt should be poured first because it needs to form the bottom layer. Follow with the next strongest mixture, finishing with plain water. Keeping the cups in order helps children connect the visible result with their measurements.

Building the rainbow in a jar

Place the jar on a level tray. Slowly pour the densest solution down the inside wall, or use a pipette to release it gently. Add the next solution in small amounts, directing it onto the side of the jar or onto the back of a spoon held just above the liquid surface.

Continue from the strongest mixture to the weakest. Pause between layers so the liquid settles. A tall jar gives the colours more room to separate, while a narrow container makes careful pouring easier. The finished density column may show four or five distinct bands.

A pipette is especially helpful for younger children or learners developing hand strength. Roylco’s classroom materials can complement this kind of practical investigation with tactile resources for sorting, measuring and recording observations.

Keeping the colours from mixing

The main cause of failure is turbulence. Pouring from a height pushes the new liquid through the layers below, and a fast stream creates swirls. Remind children to pour “slow and low”, using a small stream close to the surface.

If two colours mix slightly, leave the jar undisturbed for a few minutes. Some separation may become visible as the liquids settle. Avoid shaking the container, and do not add extra salt directly to the finished jar because undissolved grains can sink through several layers.

Different brands of food colouring may behave slightly differently. Clear cups, clean equipment and consistent water volumes make the result easier to interpret. In places with mineral-rich tap water, such as some regional areas, the layers may look a little different, but the density principle remains the same.

Questions and learning connections

Ask children which solution they think is heaviest and why. They can compare the number of teaspoons of salt with the height of each layer, then discuss what might happen if all five cups contained the same amount of salt.

The activity supports Australian Curriculum links across science inquiry, measurement and chemical sciences. In a Melbourne classroom, a teacher might connect it with a unit on materials and mixtures; in a regional school, students could compare the investigation with evaporation and salt production near places such as the Coorong or the salt lakes of Western Australia.

The experiment also builds language skills. Learners can describe a solution as concentrated, diluted, clear, cloudy, heavier or lighter, then write a short method using sequence words. Printable classroom resources and literacy activities can extend the vocabulary work without separating reading and writing from the practical science.

Safety, cleanup and extensions

Food colouring can stain benches, clothing and skin, so cover the work area and use an apron or an old shirt. Children should wash their hands after the activity and avoid tasting the solutions. Glass jars require close adult supervision; a sturdy clear plastic container is a safer option for early learners.

Pour the leftover mixtures down the sink with plenty of water, unless local water restrictions or school procedures require another method. In Australia, check the rules that apply to your state or territory, especially during dry periods when schools may be managing water carefully.

For an extension, make two jars with the same colours but different salt quantities and compare their stability. Students can also test whether sugar produces a similar density gradient, draw the jar as a labelled scientific diagram, or photograph the layers at intervals to observe how diffusion gradually softens the boundaries.