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A drop of dye in still jelly: how does it spread with nothing stirring it?
Diffusion lab · Last updated: 16 October 2026
You drop dye into still jelly, where nothing can stir it, and watch the colour slowly spread out by itself. They find out that tiny particles are always moving and bumping, and that this random jostling is enough to mix things. Diffusion is how smells fill a room and how oxygen reaches our blood.
What you'll discover
- The particles of the dye and the jelly are always moving, even when everything looks perfectly still.
- Random bumping spreads the dye from where it is crowded to where there is less of it.
- Diffusion is faster when it is warmer, and it gets slower the further the dye has to spread.
The science in plain words
All matter is made of tiny particles that are constantly moving. In liquids and gases they jiggle and wander about, bumping into each other billions of times a second. Each dye particle is knocked in a new, random direction at every bump, so it follows a zigzag path called a random walk.
No single particle has a reason to move outwards. But where the dye is concentrated there are many particles, and where it is thin there are few. Random movement carries more particles out of the crowded region than back into it, so overall the dye spreads from high concentration to low. This net movement is diffusion. It continues until the dye is spread evenly, after which particles still move but the colour no longer changes.
Jelly is used because it stops the liquid flowing and swirling, so the spreading you see is diffusion alone, not stirring or currents. Warmer particles move faster, so diffusion speeds up as the temperature rises. Diffusion is quick over tiny distances but slow over large ones: spreading twice as far takes roughly four times as long. That is why living things rely on it only over very short distances, such as oxygen passing from the lungs into the blood, and use blood flow to carry substances further.
Where it fits
- India, NCERT: Class 9 science, Chapter 1: Matter in Our Surroundings
- United States: Grades 6 to 8
- Next Generation Science Standards: MS-PS1-4
Common questions
What is diffusion in simple words?
Diffusion is the spreading of particles from a place where there are lots of them to places where there are fewer, caused by their own random movement. It is why the smell of food cooking fills a house and why a drop of ink slowly colours a glass of water.
Why does diffusion happen faster in hot water?
Particles move faster when they are warmer. In hot water, both the water particles and the dye particles jiggle and collide more vigorously, so the dye spreads out more quickly. In cold water the same spreading happens, but it takes noticeably longer.
Where does diffusion happen in the body?
Diffusion moves oxygen from the air sacs in the lungs into the blood and carbon dioxide back out. It also carries digested food from the small intestine into the blood and moves oxygen from the blood into body cells. All these happen across very thin surfaces.
Related labs
- Osmosis: Water moves through a thin skin towards the sugary side. Can it push itself uphill?
- States of Matter: Solid, liquid, gas: the same molecules, just jiggling harder.
- Solutions: How much can you dissolve in a glass of water, and does warming it help?
See all chemistry labs or every Taz science lab.
Try it yourself
The Diffusion lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.