Demonstrating density with a rainbow jar of liquids
Rainbow jars make density visible. Instead of calculating on a worksheet, learners watch coloured liquids occupy distinct horizontal bands, predict their order and explain how mass and volume determine the arrangement. The activity suits primary science, secondary materials studies and informal home learning.
Density is mass divided by volume, so equal volumes can have very different masses. In a settled mixture, denser liquid tends to sit below less dense liquid. Whether a boundary remains sharp also depends on miscibility: oil and water separate, whereas syrup and water gradually diffuse.
Many Australian schools use Australian Curriculum Science or closely aligned state and territory sequences to investigate the properties of materials. Teachers can scale the jar into a prediction activity, a measurement investigation or a model for explaining everyday sorting systems such as oil separation and waste recovery.
The demonstration needs careful preparation because ingredients, colours and viscosity affect what students see. It also offers a useful distinction between a temporary visual result and a scientific explanation. With safe materials, clear documentation and an age-appropriate task, the jar supports observation, measurement and evidence-based reasoning.
Why the layers form
The driving force is gravity acting on mass. A 50 mL sample of golden syrup has greater mass than 50 mL of vegetable oil, even though both occupy the same volume. When the liquids are poured slowly and left undisturbed, the denser sample moves toward the lower part of the jar.
Miscibility controls the boundary. Water and vegetable oil are immiscible and can form stable interfaces; golden syrup, dishwashing liquid and isopropyl alcohol can mix with water over time. The neat bands in an activity jar are therefore usually temporary, making gradual blurring another observation worth recording rather than hiding.
Preparing the rainbow jar
Use a tall, clear 500 mL jar, up to five small jugs, a spoon, labels and 50–70 mL of each liquid. Suitable choices include golden syrup, dishwashing liquid, coloured water, vegetable oil and, where school policy permits, 99% isopropyl alcohol. Never substitute methanol or an industrial solvent.
Pour from densest to least dense: syrup first, then dishwashing liquid, water, oil and alcohol. Rest the spoon against the jar wall or pour down its back so each liquid spreads across the surface instead of plunging through earlier layers. Leave headspace and do not stir after the first layer is placed.
Colour the ingredients separately before pouring, using compatible food colour or ready-tinted materials. Ordinary water-based colouring may clump in oil, so test a small amount first. School purchases should be classroom-safe rather than general novelty items from sites such as budget novelty suppliers, and every product should be checked for age suitability.
Comparing densities and behaviours
Measure density in grams per millilitre and keep temperature consistent. These are guide values because brands and concentrations differ. The relative order predicts where a liquid starts, while the final column reminds students that density does not prevent every pair from mixing.
| Liquid | Typical density (g/mL) | Initial place in the jar | Behaviour with water |
|---|---|---|---|
| Golden syrup | 1.32–1.40 | Lowest layer | Mixes slowly; highly viscous |
| Dishwashing liquid | 1.03–1.08 | Above syrup | Miscible; interface may blur |
| Water | 1.00 | Middle | Reference layer |
| Vegetable oil | 0.90–0.93 | Below water | Immiscible; boundary stays clear |
| 99% isopropyl alcohol | 0.78–0.80 | Highest layer | Miscible; diffuses into water |
Golden syrup is both dense and viscous, so it resists movement for longer than its density alone would predict. The alcohol is least dense, but because it is miscible with water, its lower edge becomes less distinct. Students can compare an ideal density ranking with the actual display and account for the difference.
Turning observation into investigation
Younger learners can match layer positions to coloured prediction cards, then use words such as denser, lighter, sink and float. Older students can record each layer before and after ten minutes, identify interfaces and explain why a thin band still has a measurable mass.
Volume skills can be rehearsed safely with dry media first. The activity measuring volume with rice shows how a measuring cup and known volume compare without relying on an already assembled rainbow. Students can then estimate the jar’s total volume and check their sum against the container markings.
For a quantitative extension, weigh equal 50 mL portions and calculate mass divided by volume. Small differences between samples, unrecorded colour or residue on the jug become useful sources of experimental error. When students graph mass against volume for several samples, the steepest line belongs to the densest liquid.
Using the jar in Australian settings
Many schools follow Australian Curriculum Science content on materials and their properties, while state and territory curricula and school scope-and-sequence documents determine the exact level of demand. A simple three-layer jar works well in lower primary classes; calculating density extends the same model for secondary students and homeschool families.
Metropolitan schools in Melbourne, Sydney and Brisbane may run the activity in shared science sessions or home-and-school workshops. Regional and remote teachers often value compact kits, reusable containers and activities that use pantry ingredients, so the same investigation can travel without specialist laboratory equipment.
Local supermarkets commonly stock golden syrup, vegetable oil and dishwashing liquid, while school suppliers serve the broader craft and manipulative market. Teachers can build a compact station from hands-on learning materials, then budget for consumables in Australian dollars and metric equipment rather than expensive glassware.
Safety, recycling and extension
Use eye protection, label every container and keep the jar on a tray. Food-coloured liquids are not drinks. Isopropyl alcohol is flammable and should be handled by an adult under the school’s chemical-risk procedure; relevant safety data sheets, state and territory work health and safety duties, and teacher supervision must take priority.
Consumer products bought for children should meet the Australian Consumer Law and any applicable mandatory safety standards, with clear age and use information. Check supplier labels and institutional purchasing policies. In warm weather, keep the jar shaded and out of a parked car, because temperature changes viscosity and can speed the layers’ movement.
Follow the school’s disposal procedure and local council guidance; do not pour oil, alcohol or coloured mixtures down the sink. Wash reusable equipment with suitable detergent, and clean empty glass jars may be accepted by council recycling programs, although lid and residue rules vary locally. The finished comparison connects property, measurement, safety and environmental responsibility in one vivid model.