Science and Technology K–6 · Year 5

Food dye in hot and cold water: particles move faster when warm

Science understanding: Chemical sciences

Practical, model not builtLow risk

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The idea

Particles in a liquid are always moving and move faster when warmer, so a drop of dye spreads through warm water sooner than through cold, though the fast swirling seen in a glass is mostly currents and true particle spreading is slow.

Safety card

Low riskAn adult supervises

Hazards

warm water

Controls

teacher pours the 45 degree water
dye stains, wear aprons

Note

Hot water above 50 degrees Celsius is poured by the teacher. Risk assessment before the lesson using the NSW Department of Education Chemical Safety in Schools package (CSIS 1.7 Risk Assessment) or Primary RiskAssess.

What you need

3 clear glasses of water: iced (about 5 degrees Celsius), room (about 20) and warm (about 45, teacher poured)
food colouring in a dropper
a thermometer
for the slow test, a small clear cup of plain agar or unflavoured gelatine jelly made up with water and set the day before
timer

How to do it

Let all three glasses stand still for 5 minutes so the water is not moving. Record each temperature.
Add one drop of dye to each at the same moment without touching the glass and time how long until the colour reaches the bottom and until the whole glass is evenly coloured.
Record the two times for each glass and rank them.
Slow test: put one drop of dye on the surface of the set jelly, mark the surface level on the cup and leave it overnight at room temperature. Measure how far the colour has crept down in the morning. The jelly stops the dye sinking and stops currents, so only particle motion can move it.
Compare the minutes in the glasses with the hours in the jelly and explain the difference.
Draw the particles in cold and warm water with arrows for their speed.

What you should see

The dye spreads fastest in the warm glass and slowest in the iced glass, over minutes. In the jelly the colour creeps only millimetres overnight: pure diffusion of a small dye molecule in water at 20 degrees Celsius has a diffusion coefficient of about 4 x 10^-10 square metres per second (Stokes-Einstein), so 1 cm takes about 32 hours, and a dilute jelly, which is mostly water, slows it only a little. The fast spreading in the glasses is carried by the dense drop sinking and by currents set up by the drop and by temperature differences; a drop placed on plain water in a straw would sink as a plume in the same way, which is why the slow test uses jelly. The jelly shows what particle motion alone does, and warmth speeds both.

What changes

What you change: water temperature
What you measure: time for the dye to colour the glass evenly
What you keep the same: water volume
one drop
no stirring
same dye

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The dye spreads because it is lighter than water (food colouring is slightly denser than water, which is why a drop sinks; sinking and currents carry it down, and particle motion spreads it).
Diffusion is what you see in the first ten seconds (that is mostly convection).
Particles stop moving in cold water.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST3-6MW-S ST3-1WS-S
Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST3-PQU-01
Australian Curriculum v9AC9S5U04AC9S5I01AC9S5I03AC9S5I05

Sources

The pages the author read to write this activity.

All Concept Studio activities