Convection Currents in Water
Drop some color into water at two temperatures and watch heat move all by itself — no stirring, no pump, just physics.
Use hot tap water only — never boiling water, to avoid burns and cracked glass.
Materials
- A tall clear glass or jar
- Hot tap water (not boiling)
- Cold water with ice
- Red and blue food coloring
- Two small droppers or spoons
- A white paper background
Procedure
Fill the tank
Fill the glass three-quarters full with room-temperature water. Set it against the white paper so the colors show clearly.
Add hot red water at the bottom
Mix a few drops of red coloring into a spoonful of hot tap water. Gently release it at the very bottom of the glass with the dropper.
Watch it rise
Watch the red plume climb. Sketch the path it takes — does it rise straight or curl?
Add cold blue water at the top
Now mix blue coloring into cold water and release it gently at the surface. Watch the blue ribbon sink.
Trace the full loop
Keep watching for 5 minutes. Describe the full loop the colors trace — that loop is the convection cell.
What to observe
Hot colored water released at the bottom rises in a plume.
Cold colored water released at the top sinks in a ribbon.
After a few minutes the colors mix as the loop keeps turning.
Why it happens
Heating water makes its molecules jiggle faster and spread slightly apart, so hot water is less dense than cold. Buoyancy then does the rest: the lighter hot parcel floats upward while denser cold water slides down to replace it, creating a rolling loop called a convection cell. This same loop drives ocean currents, moves heat through the atmosphere, and circulates water in a home radiator — all without any pump.
Try variations
Put the glass in the fridge for 10 minutes first — does the loop slow down?
Try salt water versus fresh water at the same temperature. Does density from salt drive a current too?