Science 7–10 · Year 7
Brownian motion: smoke particles jostled by air molecules under a microscope
Solutions and mixtures
The idea
Invisible air molecules are moving fast and at random, because visible smoke specks they strike jiggle without any other cause.
Safety card
Hazards
- smouldering straw or cord is a burn and small-fire risk
- lamp becomes warm
- smoke irritates eyes and airways in quantity
- sunlight reflected up through the microscope, by its mirror or through the open stage aperture, can damage the eye
Controls
- adult lights and extinguishes the straw or cord over a heat-proof mat
- use the smallest puff of smoke that fills the cell
- ventilated room
- the microscope is set up away from direct sunlight, and the smoke cell is in place over the stage aperture before anyone looks through the eyepiece (the IOP warns that sunlight must not be reflected up through the microscope)
Note
No chemicals beyond a smouldering straw; heat source handled by an adult. Basic risk assessment per CSIS Section 1.7.
What you need
- Whitley smoke cell (glass cell with a small lamp and a glass rod lens) and a 12 V power supply
- compound microscope, low-power objective (about 10x) and a cover slip
- a paper drinking straw or a length of cord as the smoke source, dropping pipette
- black card to shield stray light
How to do it
- The adult lights the top of the straw (or cord) and blows it out so that it smoulders; draw smoke from it into the pipette, squeeze it into the cell and cover the cell with the cover slip at once.
- Place the cell on the microscope stage, connect the lamp to the 12 V supply and shield the cell with black card so the only light enters from the side.
- Start with the objective close to the cover slip and focus by moving it slowly away until tiny bright points appear; these are smoke specks lit from the side.
- Watch one speck for 30 seconds and sketch its path as a series of straight-line jumps. Note whether large or small specks move more.
- Repeat after leaving the cell for 5 minutes and record whether the motion stops.
What you should see
Bright specks jerk about in short random steps that never stop; smaller specks move more than large ones. The motion continues as long as the smoke is present and has no preferred direction; a slow drift of all the specks the same way is convection, a separate effect. Specks also vanish and reappear as they move up and down out of focus. The learner knows it worked when a single speck can be followed and its path drawn as a zig-zag with no preferred direction.
What changes
This activity lists no variables to change, measure and keep the same.
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- The specks move because the air in the cell is being blown or heated.
- What we see moving are the air molecules themselves.
- Particles come to rest when the substance is still.
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 7–10 Syllabus (2023)SC4-SOL-01SC4-WS-01
- Australian Curriculum v9AC9S7U05AC9S7I03
Sources
The pages the author read to write this activity.